mengsc report - establishing a particle image velocimetry ... · master of engineering science...

MECH9010 – Mechanical Engineering Project

THE UNIVERSITY OFNEW SOUTH WALES

SCHOOL OF MECHANICALAND MANUFACTURING

ENGINEERING

Master Of Engineering Science (MEngSc) Project Report

Establishing a Particle ImageVelocimetry (PIV) System

forStudying Heat Transfer

Enhancements using DimpledSurfaces

by

Patrick Sean Coray – 3109350

February 2005

Supervisors: Prof. Eddie Leonardi

Dr. Tracie Barber

Abstract

Abstract

A particle image velocimetry (PIV) system was developed to aid the analysis of two-

dimensional flowfields on dimpled surfaces and other structures.

The work included setting up a pulsed laser, programming a laser control application,

aligning beam combining and frequency doubling optics, designing a suitable light

sheet made of low cost components and synchronising with an image recording system.

Furthermore the generation of suitable seeding particles and the requirements for

successful imaging and evaluation of the flowfield has beenaddressed with particular

attention towards dimple specific flow issues.

Acquiring a good theoretical and practical understanding of the processes involved in

generating and imaging laser light, as well as gaining experience in handling lasers and

optics were found to be a key issue in the development of the PIV system.

The PIV system was completed and successfully applied on a small test windtunnel and

a larger tunnel used with dimples. Remaining issues includefurther optimisation of

flow seeding.

i

Statement of Originality

Statement of Originality

I hereby declare that this submission is my own work and to thebest of my knowledge

it contains no material previously published or written by another person, nor material

which to a substantial extent has been accepted for the awardof any other degree or

diploma at UNSW or any other educational institution, except where due

acknowledgement is made in the thesis. Any contribution made to the research by

others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in

the thesis.

I also declare that the intellectual content of this thesis is the product of my own work,

except to the extent that assistance from others in the project's design and conception or

in style, presentation and linguistic expression is acknowledged.

Patrick Coray

9 Feb 2005

ii

Acknowledgements

Acknowledgements

The realisation of this project during the past five months would have not been possible

without the contribution of a number of lecturers, staff / technicians, peers and industry

professionals to which I would hereby like to express my gratitude.

Firstly, I would like to thank my supervisors Prof. Eddie Leonardi and Dr. Tracie

Barber for giving me the opportunity to work on this excitingand interesting project;

with further sincere thanks for their support and guidance and for believing in my

ability of getting a running system despite the many pitfalls encountered.

A special acknowledgement and thank you goes to Alex Litvak,Fluids Lab manager

and laser safety officer, for his efforts in assisting me with the many administrative and

technical matters as well as for taking time to discuss problems whenever I got stuck.

I also especially acknowledge the help of the following UNSW staff:

• Tim Anderson, IC Engines and Heat Transfer laboratory manager for the support

during the measurements with the school's ILA PIV system andfor the always

helpful assistance on several other occasions.

• The help of George Otvos in electrical matters.

• Graeme Louk and Andrew Higley for occasional assistance, John Barron for the

manufacturing of necessary parts and Terry Flynn from the Aero Lab for feedback on

previous experience in using the PIV laser.

• Sharon Turnbull, coursework program advisor for her generally uncomplicated help

in administrative matters related to both this project as well as my Master by

Coursework degree.

Due to the initial condition of the laser, the work would havenot been possible without

the always very professional, efficient and - most notably -prompt support of Irmantas

Mikulskas from Ekspla Experimental Lasers Ltd., to whom I amtherefore deeply

indebted.

The help and support of the following students is also greatly appreciated:

• Chris Beves and Sam Diasinos, with their help on lab and otherproject related

iii

Acknowledgements

matters.

• Indra Budihardjo for introduction to the ILA PIV system.

• Soo Too Yen Chean with whom I had to share the time available for the ILA PIV

system.

• Ekim Özkan, who kindly agreed to print and bind the final version of this report as I

already had to leave Australia.

Finally I would like to make use of this opportunity to thank ABB Switzerland / ABB

University Marketing for their Master scholarship award. Without their generous

financial assistance studying in Australia would have been difficult for me.

iv

Table of Contents

Table of Contents

Abstract...............................................................................................................................i

Statement of Originality....................................................................................................ii

Acknowledgements..........................................................................................................iii

Table of Contents...............................................................................................................v

Notation...........................................................................................................................xii

1. Introduction...................................................................................................................1

1.1 Background and Aim..............................................................................................1

1.2 Initial Situation and Approach................................................................................1

2. Flow Over Dimpled Surfaces........................................................................................3

2.1 Introduction.............................................................................................................3

2.2 Dimple Geometry....................................................................................................3

2.3 Important Characteristic Numbers..........................................................................5

2.3.1 Introduction.....................................................................................................5

2.3.2 Independent Characteristic Numbers..............................................................5

2.3.3 Dependent Characteristic Numbers.................................................................6

2.4 Flow Features and Further Important Aspects........................................................7

3. The PIV Method..........................................................................................................10

3.1 Introduction...........................................................................................................10

3.2 PIV – The Basic Principle....................................................................................10

3.2.1 Velocity = Displacement per Time...............................................................10

3.2.2 Introduction to Estimating the Displacements..............................................11

3.2.3 A Typical ('Standard') PIV Measurement Setup...........................................13

3.2.4 More General PIV Setups.............................................................................14

3.2.5 Summarised Approach..................................................................................15

3.2.6 Further Properties of the PIV Method...........................................................15

3.3 A Short Note on 3 Component (Stereoscopic) PIV..............................................16

3.4 Distinction between PIV and other Methods such as PTV...................................18

3.5 Correlation Analysis.............................................................................................20

3.5.1 Introduction...................................................................................................20

v

Table of Contents

3.5.2 Cross-Correlation..........................................................................................20

3.5.3 Auto-Correlation...........................................................................................24

3.5.4 Increasing the Evaluation Speed by FFT......................................................25

3.5.5 Subpixel Interpolation...................................................................................26

3.5.6 Resolution, Accuracy and Dynamic Range..................................................28

3.5.7 Further Improvements – Adaptive Correlation.............................................28

3.6 Post-processing.....................................................................................................30

3.6.1 Introduction...................................................................................................30

3.6.2 Finding Outliers (Erroneous Vectors)...........................................................31

3.6.3 Reconstucting Missing Vectors.....................................................................33

3.6.4 Further Processing of Data............................................................................33

3.7 Pitfalls and Errors.................................................................................................34

3.7.1 Introduction...................................................................................................34

3.7.2 2-C vs 3-C PIV: A Perspective Error............................................................35

3.7.3 Velocity Gradients .......................................................................................37

3.7.4 Particles Moving Out of Bounds and Laser Flash Separation......................38

3.7.5 Pulse Duration...............................................................................................39

3.7.6 Pixel (Peak) Locking and Particle Image Diameter......................................39

3.7.7 Seeding Particle Density...............................................................................43

3.7.8 Quantisation Levels (n-bit Pictures).............................................................44

3.7.9 Light Sheet Coincidence...............................................................................45

3.7.10 Image Intensity Distribution.......................................................................45

3.7.11 Other Image Quality Aspects......................................................................46

3.7.12 Computational Scheme (Subpixel, Adaptive etc.)......................................46

3.7.13 First Order Approximation of the Particle - Not Flow - Velocity..............46

4. The Laser.....................................................................................................................48

4.1 Introduction...........................................................................................................48

4.2 Working Principle of a PIV Nd-YAG Laser.........................................................49

4.2.1 Basic Laser Principle.....................................................................................49

4.2.2 Pumping with Flashlamps.............................................................................51

4.2.3 Introducing a Q-Switch.................................................................................52

vi

Table of Contents

4.2.4 Varying and Maximising the Pulse Energy..................................................54

4.2.5 Combining Two Lasers.................................................................................55

4.2.6 Frequency Doubling: Invisible IR→ Visible Green.....................................55

4.2.7 Further Nd-YAG Properties..........................................................................56

4.3 Laser Safety..........................................................................................................57

4.3.1 Introduction...................................................................................................57

4.3.2 Potential Sources of Eye Damage.................................................................58

4.3.3 Improving the Safety by Appropriate Shielding...........................................60

4.3.4 Some Theory on Laser Goggles....................................................................61

4.3.5 To See Or Not To See: On Handling Laser Goggles....................................63

4.3.6 Further Tips and Comments on How to Prevent Potential Accidents..........64

4.4 Ekspla Nd-YAG Laser – Operation......................................................................64

4.4.1 Introduction...................................................................................................64

4.4.2 Connections...................................................................................................64

4.4.3 Powering Up – Making the Laser Ready......................................................65

4.4.4 Using the Control Pad...................................................................................66

4.5 Ekspla Nd-YAG Laser – Maintenance.................................................................69

4.5.1 Introduction...................................................................................................69

4.5.2 Changing the Cooling Water.........................................................................69

4.5.3 Laser Head View...........................................................................................70

4.5.4 Changing the Flashlamps..............................................................................71

4.5.5 Visualising Infrared.......................................................................................71

4.5.6 Cleaning Optical Components......................................................................72

4.5.7 Optimising the Beam Overlap and Beam Path..............................................72

4.5.8 Storing the KTP Crystal................................................................................74

4.5.9 Optimising the KTP-Crystal Alignment.......................................................74

4.5.10 Measuring the High Voltage at the Pockels Cell........................................76

4.5.11 Measuring the Laser Pulse Energy..............................................................78

4.5.12 Optimising the High Voltage Circuit..........................................................79

4.5.13 Optimising the Q-Switch Timing................................................................80

4.5.14 Potential Problems and Troubleshooting....................................................80

vii

Table of Contents

4.6 Labview Laser Control.........................................................................................82

4.6.1 Introduction...................................................................................................82

4.6.2 Program Usage / Interface.............................................................................83

4.6.3 Program Structure.........................................................................................85

4.6.4 Effect of the Energy% Setting.......................................................................86

5. The Light Sheet...........................................................................................................88

5.1 Introduction...........................................................................................................88

5.2 Principles in Designing a PIV Lightsheet.............................................................89

5.2.1 Introduction...................................................................................................89

5.2.2 Lens Material Damage Thresholds...............................................................89

5.2.3 Do's and Don'ts: Back Reflections................................................................90

5.2.4 Do's and Don'ts: Power Density....................................................................90

5.2.5 From Beam to Lightsheet: The Basic Principle............................................91

5.2.6 On the Importance of the Light Sheet Intensity Distribution........................93

5.2.7 From Beam to Lightsheet: A More Advanced System.................................94

5.3 Designing a Lightsheet (LS) from Pre-Made Parts..............................................95

5.3.1 Parts Available from Edmund Optics...........................................................95

5.3.2 Mathcad Program for Evaluating Possible LS Configurations.....................96

5.3.3 The Resulting Light Sheet.............................................................................97

5.3.4 Mounting the Parts and Using the Final Light Sheet....................................98

5.4 Designing a Beam Redirection Mirror and LS Mount.........................................99

5.4.1 Introduction...................................................................................................99

5.4.2 Choosing a Suitable Mirror...........................................................................99

5.4.3 Mount Design and Alignment.....................................................................100

5.5 Possible Future Extensions.................................................................................100

6. Camera, Lens and Imaging........................................................................................102

6.1 Introduction.........................................................................................................102

6.2 Basic Principles...................................................................................................102

6.2.1 Components of a PIV Lens / Imaging System............................................102

6.2.2 Optical Bandpass Filter...............................................................................104

6.2.3 Focus...........................................................................................................105

viii

Table of Contents

6.2.4 Zoom (Scale) and Field of View.................................................................105

6.2.5 The Effect of Aperture................................................................................106

6.3 Properties of CCD Chips used for PIV...............................................................108

6.3.1 CCD Chips used for PIV.............................................................................108

6.3.2 CCD Damage (Overexposure)....................................................................110

6.4 'Safe' Approach to Getting a Picture...................................................................111

6.5 Dealing with Reflections....................................................................................113

6.6 Imaging Particles................................................................................................113

6.7 Calibrating the Image..........................................................................................117

6.8 Viewing at an Angle – The Scheimplug Correction...........................................118

6.9 Using the Redlake Megaplus ES1.0 Camera......................................................120

6.9.1 General Overview.......................................................................................120

6.9.2 Mounting on the ILA Scheimpflug Adaptor...............................................120

7. Synchronising the Components.................................................................................121

7.1 Introduction.........................................................................................................121

7.2 Triggered Camera Control..................................................................................121

7.2.1 Redlake ES1.0.............................................................................................121

7.2.2 PCO Sensicam Timing for Comparison......................................................123

7.3 PIV Laser and Camera Synchronisation.............................................................124

7.3.1 Introduction.................................................................................................124

7.3.2 The Common Approach..............................................................................124

7.3.3 Ekspla Laser................................................................................................124

7.4 Laser Sync-Out Signals......................................................................................124

7.4.1 Introduction.................................................................................................124

7.4.2 Measurement Setup.....................................................................................125

7.4.3 Laser Back Panel Outputs...........................................................................126

7.4.4 Laser Front Panel Sync Out........................................................................127

7.5 Trigger Level Conversion, Back Panel Sync Out...............................................127

7.5.1 Introduction.................................................................................................127

7.5.2 TTL Trigger Level Conversion...................................................................127

7.5.3 Directly Triggering the Camera from the Back Panel Sync Out................129

ix

Table of Contents

7.6 The Final Camera Synchronisation Method.......................................................130

7.6.1 The Trigger Signal Generation....................................................................130

7.6.2 Setting up the Function Generator..............................................................131

8. Seeding......................................................................................................................132

8.1 Introduction.........................................................................................................132

8.2 Seeding Particle Requirements...........................................................................132

8.2.1 Scattering Intensity and Resulting Image Size...........................................132

8.2.2 Ability to Follow the Flow..........................................................................133

8.2.3 Particle Distribution....................................................................................134

8.2.4 Health, Pollution and Other Issues..............................................................134

8.3 Seeding Liquid Flows.........................................................................................134

8.4 Seeding Gas Flows..............................................................................................135

8.5 Own Experiments................................................................................................136

8.5.1 Introduction.................................................................................................136

8.5.2 Fogger ('Smoke').........................................................................................136

8.5.3 Hollow Glass Spheres.................................................................................137

8.5.4 Vegetable Oil..............................................................................................138

8.5.5 Comparison Table.......................................................................................138

8.6 The Laskin Based Seeding Generator at UNSW................................................139

8.6.1 Introduction.................................................................................................139

8.6.2 Working Principle.......................................................................................139

8.6.3 Health Issues...............................................................................................141

8.6.4 Seeding Liquids...........................................................................................142

9. PIV Evaluation using VidPIV...................................................................................144

9.1 Introduction.........................................................................................................144

9.2 A Brief Overview of the Steps Involved in using VidPIV.................................144

9.2.1 General........................................................................................................144

9.2.2 Importing Image Pairs.................................................................................145

9.3 Preparing n-bit Images for the 8-bit Importer.....................................................146

9.3.1 Introduction.................................................................................................146

9.3.2 The Matlab Program...................................................................................147

x

Table of Contents

10. The Test Rig............................................................................................................148

10.1 Introduction.......................................................................................................148

10.2 The Test-Windtunnel........................................................................................148

10.3 The Windtunnel in Operation...........................................................................149

11. System Usage Summary..........................................................................................151

11.1 Introduction.......................................................................................................151

11.2 Steps when using the System............................................................................151

12. Conclusion...............................................................................................................153

References.....................................................................................................................156

General Literature.....................................................................................................156

Publications on Dimples...........................................................................................158

Literature on PIV, Laser and Optics.........................................................................160

Homepages Used for General (Product) Information...............................................164

Appendices....................................................................................................................166

xi

Notation

Notation

Abbreviations and Acronyms

2C-PIV Two component PIV; The vectors have only 2 components in space (x, y).

2D-PIV Two dimensional PIV; Covers both 2C and 3C vectors determined in a plane.

3C-PIV Three component PIV; The vectors have 3 components in space (x, y, z).

3D-PIVThree dimensional PIV; The vector field extends into three dimensions. Inpractice stereoscopic (3C-PIV) is often referred to as 3D-PIV.

CCD Charge coupled device.

CTA /CTV1

Constant Temperature Anemometry / Velocimetry: Pointwise velocitymeasurement method based on heat transfer from a hot wire.

FL Flashlamp.

HV High voltage.

IR Infrared.

LDA /LDV 1

Laser Doppler Anemometry / Velocimetry: Pointwise velocity measurementmethod based on interferometric / Doppler shift effects.

LS Light sheet.

LSV Laser Speckle Velocimetry.

Nd-YAG

Neodym-Yttrium-Aluminium-Garnet (Laser based on Nd3+ ions in a YAG crystal).

PIV Particle Image Velocimetry.

PTV Particle Tracking Velocimetry.

QE Quantum efficiency – likelihood of a photon to release an electron.

SH,SHG

Second harmonic / second harmonic generation.

SNR Signal to noise ratio.

vi Virtual instrument, i.e. a Labview (sub) program.

1 Velocimetry is mostly used in the US, Australia and parts of Asia while anemometry is more commonin the UK and the rest of Europe. Note that PIV is universally used (PIA is not synonymous to PIV).

xii

Notation

Physical and Mathematical Symbols

Latin:

A area, m2

bflback focal length, m (distance from the last lens of a system to the focal point)

cp specific heat at constant pressure, J/(kg·K)

Cs scattering cross section, m2

d, D diameter or length, m

Dp dimple print diameter, m

E energy, J; either pulse energy or energy level (state) of an atom

f (1)friction factor, - (dimensionless);

sometimes also denoted as λ.typically

f = p

12⋅⋅v2⋅lengthfactor

f (2) function e.g. f(x)

f (3) focal length, m

f# f-number (focal length normalised by aperture diameter)

h heat transfer coefficient, W/(m2·K); some authors use α instead

H channel height, m

I intensity (unscaled)

i, j discrete coordinates (like x, y)

J effectiveness of heat transfer augmentation, - (dimensionless),

k thermal conductivity, W/(m·K); some authors use λ instead

L (characteristic) length, m

M magnification factor, -

N number; often number of pixels or particles (compare NI and NpixI)

NI number of particles in an interrogation area; chosen to comply with [Raf98]

NpixI (explicitly) pixels per side of an interrogation area (i.e. NpixI x NpixI pixels)

Nu Nusselt number, - (2-4)

O[...] on the order of

OD optical density, - (dimensionless)

p, ∆p pressure / pressure drop, Pa

Pr Prandtl number, - (2-3)

xiii

Notation

r radius, m

Re Reynolds number, -; (2-2)

s length or distance, m

t time, s or thickness, m

t1, t2, ∆ttime of the first and second laser flash (light pulses);

time between pulses ∆t = t2 – t1

v velocity, m/s

x,y,z coordinates or lengths, m

z0 image – lens distance, m

Z0 object – lens distance, m

Greek:

δ dimple depth, m

ε error

λ wavelength of a photon (i.e. wavelength of light)

ρ density, kg/m3

τ transmission ratio (ratio of transmitted to total power impinging on a surface)

Symbolic:

[...] Matrix or functional bracket

() functional brackets or coordinates

xiv

Chapter 1 - Introduction

1. Introduction

1.1 Background and Aim

Concave dimpled surfaces are known to increase heat transfer rates whilst keeping the

associated increase in pressure drop (compared to a flat wall) lower than with other heat

transfer enhancement methods such as ribs and fins, thus allowing higher energy

efficiencies to be achieved ([Leo04a]). Current research at the UNSW School of

Mechanical Engineering aims at gaining further understanding of the physical

mechanisms causing the favourable performance of dimpled surfaces. This involves

studying the flow structures in and around dimples using both computational and

experimental methods, with the experimental methods playing a major role in validating

the numerical results.

The aim of the project presented in this report is therefore to establish a two-

dimensional particle image velocimetry (PIV) system suited for measuring flow across

dimples as well as for use in other applications. PIV involves creating two short flashes

of light distributed in a plane. Particles added to a flow areilluminated during the laser

light flashes and recorded using a special double frame camera. The flow velocity is

then estimated by dividing the displacement of the particles between the two flashes by

the elapsed time (Chapter 3).

1.2 Initial Situation and Approach

To obtain some background information on the main topics thefirst step involved

acquiring some knowledge on flow around dimples and on the PIV method. Both topics

are briefly addressed in Chapters 2 and 3.

Starting point for the PIV system was a pulsed laser (Appendix 1), which (among

others) previously was used in the PhD of Stephen Hall ([Hall01a]) and the work of

Ivan Tan ([Iv02]), both investigating the flow over a backward facing step. The laser

turned out to be defect and had to be sent to Lithuania for repairs. Setting the repaired

laser back up again involved obtaining further in depth laser knowledge which is

1

Chapter 1 - Introduction

covered in Chapter 4.

The previously used light sheet delivery system as designed by Stephen Hall ([Hall01a])

was no longer available hence an alternative had to be found.Initially it was planned to

purchase a custom designed light sheet or alternatively a more standard one as designed

by manufactures of commercial PIV systems such as Dantec andILA ([Dan02],

[ILA03b]). However these turned out to be too expensive, hence it was decided to

design a cheaper light sheet based on standard components available from Edmund

Optics (www.edmundoptics.com). This is covered in Chapter 5.

A new PIV camera together with a suitable frame grabber was acquired as well. Both

worked out of the box without causing further difficulties.Their use together with

further information on imaging is given in Chapter 6. The synchronisation between

camera and laser is covered in Chapter 7.

Proper seeding of the flow with reflective particles is one of the most critical issues for

successfully applying the PIV method. Important issues such as light scattering, ability

to follow the flow and health hazards are discussed in Chapter 8.

Finally different types of seeding particles have been tested using the school's ILA PIV

system as well as the PIV system built during this project, with the results being

discussed in Chapter 8 and the test setup presented in Chapter 10.

2

Chapter 2 - Flow Over Dimpled Surfaces

2. Flow Over Dimpled Surfaces

2.1 Introduction

Knowledge of the measurement apparatus / method, which in this case is PIV, alone

will not guarantee that the actually measured results are useful. Instead one also has to

be aware of any simplifications made during the measurements, what the experimental

boundary conditions are (e.g. laminar or turbulent flow) and to what kind of practical

real world conditions or numerical boundary conditions these experimental results

should compare to.

A common way of describing fluid- and thermodynamic problems in order to allow

comparison with other situations is by using characteristic dimensionless numbers.

Ideally all these non-dimensional parameters occurring inan experiment should be the

same as in the real world, thus complete similarity would be fulfilled. However, this is

not usually the case and therefore the experimentalist has to be aware of the

simplifications made and the potential consequences thereof. It is also helpful to

develop an understanding of the problem investigated by drawing from the results and

conclusions of others.

In this respect it is the purpose of the following subchapters to summarise potentially

helpful information found in literature to raise the awareness of what might have to be

considered for PIV measurements on flow across dimples. It is not intended however to

cover all the previous work and findings as this can already be found in [Leo04a] and

the references therein.

2.2 Dimple Geometry

A single dimple is typically a spherical and concave cavity in a surface characterised by

radius of curvature, print diameterDp and dimple depthδ as shown in Figure 2.1. It

follows from geometric relations that the curvature radiusis directly related to the

parametersDp and δ, and thus the dimple is commonly characterised by the ratio of

dimple depth to dimple print diameter δ/Dp.

3


An approach to further reduce the complexity of initial measurements and CFD

calculations is to replace the three-dimensional dimple with a two-dimensional concave

cylindrical channel as used in initial testing of the current PIV system. Due to the

strongly three-dimensional nature of the vortices described in Chapter 2.4, this

simplification is mainly useful to gain experience in doingthe measurements and

calculations, while it seems likely that results from such atwo-dimensional approach

won't be realistic enough to be of direct use.

Series of spherical dimples can be arranged on a diverse number of surfaces ranging

from simple rectangular or cylindrical channels as in references [Burg03] and [Bunk03],

to more complex objects such as gas turbine blade cooling passages ([Khal01]), as

shown in Figure 2.2.

(a) Rectangular Channel (b) Circular Channel(c) Turbine Blade

Figure 2.2 Possible applications of dimples (overview).(part c from reference [Khal01], Fig. 1; parts a and b similar to references [Grif03] and [Bunk03])

It appears that there is a vast (infinite) number of possibilities for the number, size and

geometrical arrangement of the dimples relative to each other. In order not to further

4

Figure 2.1 Schematic of a dimple. Dp: print diameter; δ: dimple depth &r: dimple radius (compare [Burg03], Fig. 3)

δ

Dp

r r=

2

D p2

8⋅


complicate things these are not further discussed at this place. Among the more

important geometrical scales are a characteristic global length, typically the channel

height or hydraulic diameter and the normalised dimple depth δ/Dp.

2.3 Important Characteristic Numbers

2.3.1 Introduction

As indicated in Chapter 2.1 characteristic numbers are usedto describe the flow in a

way that allows comparison between different models (different in scale, fluid

properties, ...). They are typically divided into independent and dependent

dimensionless numbers. The dependent dimensionless numbers can be expressed as a

function of the independent ones. In the current case of forced convection (forced flow

over a dimpled surface), the Nusselt number characterisingthe heat transfer rate

(Equation (2-4)) is – for a given geometry – predominantly a function of Reynolds (2-2)

and Prandtl (2-3) numbers as shown in Equation (2-1). Dimplerelated practical

applications can be found in the work of Vicente et al [Vic02b] for example where this

relation (2-1) was used in their study on dimpled tubes.

Nu=NuRe,Pr ,Geometry Equation (2-1) - Nusselt number dependencies(compare with the equations used in [Inc02], Chapter 8)

Further information as well as more in depth explanation of the dimensionless numbers

and relations presented can be found in standard literatureon heat transfer such as

[Inc02] or [Kr99], of which both have been used for preparingthis chapter. Note that as

the focus is more on giving an overview not all potentially useful dimensionless

numbers are covered.

2.3.2 Independent Characteristic Numbers

The problem geometry as discussed in Chapter 2.2 can be described by normalising all

lengths with a suitable reference length scale L. In case of heat transfer across the walls

of a channel the characteristic length is typically chosen as the channel (hydraulic)

diameter or height as in references [Burg03] and [Grif03]. Anumber of other

normalisations can be useful as well such as the ratio of dimple depth to dimple print

5


diameter δ/Dp mentioned in Chapter 2.2.

Arguably one of the most important non-dimensional numbersdescribing the flow

condition is the Reynolds number (2-2), which is the ratio of inertia and viscous forces.

Re=⋅v⋅L

Equation (2-2) - Reynolds number(compare with [Leo04b] and [Inc02], Table 6.2)

In many cases the flow conditions are predominately dependent of the Reynolds

number. This is especially true for subsonic flows and whereno further addition or

removal of energy from or to the flow occurs (i.e. by means of mass-, momentum-, and

heat transfer). As heat transfer is involved in the study of dimpled surfaces, the effect of

adding or removing thermal energy would strictly seen have to be accounted for as well.

By assuming that the effect thermal energy has on the flow is low compared to the

Reynolds number, initial measurements to study the effect of dimple geometry on the

flow can still be made without having to impose a heat flux. Isaev and Leont'ev [Isa03]

for example made a similar simplification in their numerical analyses by first solving

the dynamic problem and then solving the energy equation forthe pre calculated

velocity field.

Pr=

k /cp

Equation (2-3) - Prandtl number(compare with [Leo04b] and [Inc02], Table 6.2)

The Prandtl number (2-3), which is solely dependent of the fluid material properties,

can be interpreted as the ratio between momentum diffusion and diffusion of heat.

Studies investigating the effect of Prandtl numbers on the heat transfer in dimpled tubes

have been made by Vincente et al. ([Vic02a] & [Vic02b])

2.3.3 Dependent Characteristic Numbers

The Nusselt number (2-4) and the effectivenessJ (2-5) are two dependent non-

dimensional results of special interest, with the former allowing the calculation of the

heat transfer rate and the latter allowing the relative advantage of an augmented over a

smooth surface to be judged. While non of the two are a direct result from flow velocity

measurements, it is of interest to relate their behaviour toand partially explain them

with the observed flow structures.

6


The Nusselt number, Equation (2-4), can be interpreted as the total convective heat

transfer rate across a given temperature difference normalised by a hypothetical

conductive heat transfer rate through the same medium and same temperature drop.

Nusselt numbers can be given locally (i.e. at a certain location in space) or as a global

average.

Nu=h

k /LEquation (2-4): Nusselt number

(compare with [Leo04b] and [Inc02], Table 6.2)

The parameter J as described in Equation (2-5) is used to showthe increase in heat

transfer in relation to the increase in friction factor, with higher values indicating better

performance. Chen et al. [Chen01] have pointed out that J is (at least in part) dependent

of the Reynolds number (J=J(Re,...) ), which thus must be taken into account for

comparisons.

J=ha/hs

f a/ f s

Equation (2-5): effectiveness of heat transfer augmentation(from [Leo04a], page 30; a = augmented & s=smooth; also compare [Chen01])

2.4 Flow Features and Further Important Aspects

It is known that flow across dimples is an unsteady phenomenawith vortex shedding

occurring [Leo04a]. This infers that not only the time-averaged but also the

instantaneous flow-field can give valuable information for the analysis. Mahmood /

Ligrani et al. [Mah01] for example paid particular attention to how the augmentation of

local Nusselt numbers relates to the (instantaneous) localflow structure caused by

dimples.

One can expect that an important feature to be measured will be the secondary flow

occurring in and around dimples as shown in Figure 2.3. A feature of these secondary

flows is that they "promote mixing with the bulk of the flow" and thus enhance heat

transfer ([Leo04a], page 31). It follows that the measurement region is not only around

but also inside the dimples.

7


Figure 2.3 Plane view (a) and side view (b) of the flow in a dimple.(from [Leo04a], Figure 1)

The three-dimensional sketch of the flow structure around adimple based on

instantaneous flow visualisations with smoke, as made by Mahmood / Ligrani et al

[Mah01], further illustrates just how highly three-dimensional the flow structure is

(Figure 2.4). It also becomes clear that not just the flow in the main flow direction but

also the flow in the plane perpendicular to it is of central importance. Mahmood /

Ligrani et al. [Mah01] further state that these vortices areof particular interest as their

fluid mass transport together with the associated thermal transport produces heat

transfer augmentation noticeable by the increase of local Nusselt numbers. It was

further found that the vortex shedding occurred periodically and the importance of the

reattachment of the shear layer forming across the top of each dimple was mentioned

([Lig01]). More detailed description of the vortices observed by Ligrani et al. can be

found in [Mah01] and [Lig01].

Figure 2.5 shows a conceptualised description and overviewof the flow features

together with their effect on heat transfer enhancement as outlined by Griffith et al.

8

Figure 2.4 Sketch of a three-dimensional flow structure around a dimple.(from [Mah01], Fig. 4)


[Grif03]. The dimple will first cause the flow to separate, resulting in a recirculation

zone, where heat transfer is low and a flow reattachment zone, where heat transfer is

high. The flow leaving the dimple will then induce (periodical) shedding of vortex pairs

and cause a large upwash region. Flow in the upwash region will mix with the cold core

flow and the shedded vortex pairs will further facilitate heat transfer in the downstream

region.

9

Figure 2.5 Conceptualised description of secondary flow effects.(from [Grif03], Fig. 4)

Chapter 3 - The PIV Method

3. The PIV Method

3.1 Introduction

The aim of this chapter is to give an overview of how a 2-component (2-C) PIV system

works and to give an insight in some of the more detailed but nevertheless important

aspects. Apart from addressing the possibilities and limits of the method it is

additionally intended to show the fundamentals in a way thatwill allow to an

understanding on how to optimise and judge the quality of PIV results.

While the basic principle covered in Chapter 3.2 is fairly straightforward to understand,

achieving proper results and exploitation of the method's capabilities requires further in

depth knowledge. Owing to the complexity of the details, literature sources such as

[Raf98], [West93] and [West97] tend to cover the important information in an very

mathematical way which is not always easy to understand, especially for readers having

only little previous experience in using the applied (mathematical) methods. With this

in mind the intent of the following chapters is to present theimportant information

about PIV in a simple and understandable way.

Note that as the PIV system designed during this project is a 2-C PIV system the

emphasis is set on 2-C evaluation methods. Although 3-C (stereoscopic) PIV is briefly

mentioned in Chapter 3.3 and many methods can be used in 2-C aswell as in 3-C,

several aspects – among others the uncertainty and errors – are not addressed in this

place. Further information on 3-C PIV can be found in [Raf98]as a starting point.

Given the ongoing improvements of stereoscopic PIV, newer papers should also give

further information. Additionally manuals from PIV manufacturers such as Dantec

([Dan00b]) and LaVision ([LaV02a] - Chapter 13) provide valuable theoretical as well

as practical information on stereoscopic PIV.

3.2 PIV – The Basic Principle

3.2.1 Velocity = Displacement per Time

Figure 3.1 shows the schematic of flow in a rectangular channel seeded with tracer

10


particles. The fundamental principle underlying PIV is to image the flow at two

different times (t1 and t2), with a time difference small enough that most of the tracer

particles are on the image at both times. By assuming that theflow velocities are

approximately equal to the particle velocities, the flow velocity field can be determined

by the displacement of the particles over time.

3.2.2 Introduction to Estimating the Displacements

By moving out of bounds some particles get lost and new ones appear between the two

recordings (times t1 and t2 on Figure 3.1). This moving out of bounds gets more likely

the higher the time between the two recordings is and hence the time has to be kept low.

Shorter times between the recordings however result in smaller displacements and thus

higher relative uncertainties of the displacements.

It follows that if an automated procedure were to concentrate on single particles alone,

the evaluation of the displacement would often not work (lost or new particles) and

have a high uncertainty. To overcome this the PIV method estimates the displacement

of particle groups contained in interrogation regions. These interrogation regions, as

indicated in Figure 3.2 and Figure 3.3, ideally contain about ten particles (compare

Chapter 3.7.7). This allows the determination of a statistical mean displacement and is

far less sensitive to the occurrence of lost and added particles.

With the exception of more sophisticated photographic methods or special situations

11

Figure 3.1 Imaging seeded flow.

xy

x

y

t

t1

t2

⇒

Camera

Seeding

seeded flowat time t1

seeded flowat time t

2


(like very slow flow) it originally only was possible to record both instances of the flow

(t1, t2) on one single frame as shown in Figure 3.2. This still allowed to determine

magnitude and direction (albeit not the sign) of the velocities to be determined

statistically by the use of autocorrelation explained in Chapter 3.5. This single frame –

double exposure approach was further improved by image shifting methods ([Raf98],

Chapter 4.3) allowing to determine the sign of the velocities. Further variations include

making n-exposures (i.e. t1, t2, t3, t4, ...) on one frame.

At time of writing the single frame – double exposure approach has become very

unusual and PIV recording nowadays make use of the double frame – single exposure

method. As shown in Figure 3.3 the two instances in time (t1 and t2) are recorded on

separate frames. This method has the advantage that the signof the velocity is clear,

that it is less susceptible to noise in the image and that it asa consequence allows higher

resolution of the velocities ([Raf98], Chapter 4.4). Because this approach is now

predominant the remainder of this report will assume the single frame – double

exposure method is used expect stated otherwise.

Once the displacements have been determined the velocity vector field can be

calculated by dividing the displacement vectors by the timebetween the exposures

(Figure 3.4). Even in good quality PIV recordings it is likely that a few vectors will be

erroneous which requires the validation and further post-processing of the vector field

12

Figure 3.2 Single frame, double exposure (both instances t1 and t2 on the same frame);Statistical evaluation of ∆x and ∆y by autocorrelation.

x

y

displacement ∆x and ∆yparticle at

time t1 particle at

time t2

one double exposed picture

interrogation area


as described in Chapter 3.6.

Figure 3.3 Double frame, single exposure (each instance t1 and t2 on a separate frame);Statistical evaluation of Dx and Dy by cross-correlation.

Figure 3.4 Determining vector velocity fields.

3.2.3 A Typical ('Standard') PIV Measurement Setup

A typical PIV system as shown in Figure 3.5 consists of two powerful lasers (usually

Nd-YAG) allowing two pulses of light to be created with a freely selectable time

separation. These light pulses are made coincident and passed through a light sheet

optics, generating a plane sheet of light illuminating particles in the flow. The short

light duration on the order of 5 nanoseconds generates a 'frozen' image of the flow.

Using a special double frame PIV camera allows the two illuminations of the flow to be

stored on two separate frames. Camera and laser are synchronised using a

13

x

y

two single exposed pictures

interrogation area

particles at time t

1

particles at time t

2

displacement ∆x and ∆y

x

y

→ vector fieldvx

vy≈ x

y⋅1

t1−t2


synchronisation unit either available as a computer card oras an external unit. The

synchronisation unit (and in turn laser and camera) is controlled from a computer which

usually also runs some sort of PIV software allowing the evaluation of the recorded

images.

Figure 3.5 Typical (2D) PIV setup.(Idea originating from [Raf98], Fig 1.4 and [Dan00a], Fig 4.1.)

3.2.4 More General PIV Setups

Basically any method resulting in pictures which can be evaluated by the PIV method as

described in the previous chapters could be called a PIV system. In the past PIV

systems have consisted of analog (film) cameras, standard consumer video devices,

continuous wave (not pulsed) lasers, flashlamps and several other alternative devices.

Interestingly even the actual evaluation can be done without using computers. Before

reasonably (for PIV) powerful computers were readily available the PIV evaluation was

14

2 Laser Heads

Beam Combining Optics

Laser PowerSupply

Light Sheet Optics

Double FramePIV Camera

Seeded Flow

Computer with Internal or External Synchronisation Hardware and

PIV Evaluation Software

Light Sheet Illuminated

Particles


often based on optical rather than computational methods (compare [West93] pages 17-

19). Once again more detailed information on this topic can be found in Raffel et al.

([Raf98]).

3.2.5 Summarised Approach

When using a 'standard' PIV setup as described in Chapter 3.2.3 the necessary steps

involved summarised are as follows (compare with reference [Dan00a], Chapter 4-2):

• Seeding of the flow.

• Illumination of the seeded flow (laser, light sheet optics).

• Imaging of the seeded flow (camera, lens).

• Synchronisation of camera, light source and other devices (e.g. phase of an engine).

• Calibration of the imaged length scales (pixel ↔ meters).

• Optimisation of image quality (seeding density, particle size, preconditioning), pulse

separation time (t1, t2 → ∆t) and other parameters.

• Cross-Correlation analyses.

• Data validation.

• Further postprocessing (interpolation, averaging, calculation of vorticity, data

visualisation, ...).

3.2.6 Further Properties of the PIV Method

Following some of the more important properties of the PIV method are listed (except

where noted the list is based on [Raf98], Chapter 1.2):

• PIV is a non-intrusive technique. Unlike measurement methods based on pitot tubes

and hot wire techniques, no physical bodies are present in the flow field.

• PIV is an indirect measurement method as not the actual flow but the seeding

particles transported with the flow are measured .

• Seeding plays an important role in accuracy ([Bol99], page 4); compare Chapter 3.7.

• PIV is a whole field technique. While other methods such as LDA/LDV, hotwire and

pitot tube can only measure at one point, PIV allows a whole flowfield to be captured

at an instance in time.

• The temporal resolution of PIV is very low. The achievable double frame rate is

15


typically on the order of 4-20 Hz, which usually is not sufficient resolve unsteady

flow over time ([Bol99], page 3). However more recent developments resulting in

faster lasers and cameras do allow frame rates on the order of4000 frames per

second, and so called time resolved PIV systems for flows of lower velocity are now

available from manufacturers such as Dantec and LaVision

(www.dantecdynamics.com, www.lavision.de).

• The raw PIV data is stored in form of a picture before further processing. In contrast

to LDA/LDV and CTA/CTV methods, which commonly evaluate theraw signal

'instantly', many variations in the PIV evaluation parameters can be applied without

having to repeat the experiment.

3.3 A Short Note on 3 Component (Stereoscopic) PIV

There are a number of methods to measure the third velocity component in PIV. The

more common ones are dual plane and stereoscopic PIV. Stereoscopic PIV always

involves the use of two cameras to resolve the third component, while dual plane PIV

setups can range from using one up to four cameras, with the third component being

determined by use of a second light sheet.

The author found the dual plane method a bit less simple to understand when going into

details, especially when considering the many options available, ranging from using just

one camera and two light sheets to using up to four cameras, then allowing to determine

vorticity in three dimensions (not possible with 3C-PIV in a2D plane). It appears that

stereoscopic PIV is more commonly used and being aware of itsadvantages gives a

better understanding of the difference to the 2C-PIV systemdeveloped during this

project. Readers interested in dual plane PIV are thereforereferred to literature such as

references [Raf98] (Chapters 7.2 and 8.4) and [LaV03a].

The following introduction on stereoscopic PIV is based on information found in

[Raf98] (Chapters 7.1 and 8.3), [LaV02a] (Chapter 13) and [Dan00b].

As shown in Figure 3.6, stereoscopic PIV consists of having two cameras look at the

illuminated area from different perspectives. Due to the not perpendicular viewing angle

achieving a focused image requires tilting the camera lens relative to the camera axis by

16


following the Scheimpflug correction method explained in Chapter 6.8.

Having images from two different perspectives allows a location in space to be

reconstructed from the two projections on the camera plane.While the necessary

relations for three-dimensional reconstruction could be calculated theoretically it is

more common to generate a mapping function based on images ofa 3D calibration

target.

To prevent the particles from moving out of the plane and to allow movement in the

third dimension to be imaged the light sheet thickness can now range up to 10mm

instead of the 0.5-1mm used in 2C-PIV ([Dan00b], Chapter 4.2.2). As a consequence

higher power lasers are required.

Figure 3.6 Two component vs three component (stereoscopic) PIV setup.(from [LaV02a], Fig. 13.1)

In operation standard 2C- PIV images are recorded and the vector fields thereof

calculated. Each of these vector fields will have a perspective error but using the

mapping function to combine them eventually results in a theoretically correct field of

three-dimensional vectors in a plane as indicated in Figure 3.7.

17


Figure 3.7 Stereoscopic PIV evaluation.(from [LaV02a], Fig. 13.9)

3.4 Distinction between PIV and other Methods such as PTV

Newbies to the PIV method and less experienced users are often unsure of what

characterises an image suitable to be evaluated by the PIV method. Given that the

concept of determining displacements by comparing two images taken at the same

spatial location but at different times is straightforwardto understand, it is tempting to

assume that the PIV method will allow determining the displacement of almost any

moving pattern. In theory and practice however the suitability of PIV to determine

displacements and the accuracy of results is highly dependent of seeding density and

particle size (both discussed in Chapter 3.7), with an example of of a good quality PIV

image shown in Figure 3.8.

Figure 3.8 PIV Image (single exposure).(from [LaV02a], Fig. 4.1)

18


As indicated in Figures 3.9a and 3.10a the seeding density used for PIV measurements

is somewhere between the two extremes of very scarce and very dense seeding.

Once the number of particles per interrogation area falls below unity the particle

tracking velocimetry (PTV) method can be used to track the movement of single,

'isolated' particles. Unlike PIV, PTV velocity vectors arenot evenly distributed but

occur randomly at the location of each particle. As stated byWesterweel [West93], this

has got the consequence that PTV "cannot fully resolve the displacement field" which is

a disadvantage compared to PIV. Other more practical issuesare that evaluation

algorithms will tend to fail as soon as there is more than one particle in an interrogation

area or when particles move out of or in bounds (get lost or appear) between the two

illuminations. The PTV method is thus only applicable in special situations where the

control over the seeding density and image quality is excellent.

Figure 3.9 The three modes of particle density as described in [Raf98], Fig 1.5.(a) low density – PTV; (b) medium density – PIV; (c) high density – LSV

At very high densities coherent (laser) light reflected from a particle will overlap with

light from other particles resulting in random interference patterns, called speckles

(speckling) hence the name laser speckle velocimetry (LSV). According to [West93]

(page 14), LSV images can in theory be evaluated in the same way as PIV images but,

as Bolinder [Bol99] points out, LSV will increase the rms error. Additionally the author

has found that in practice the evaluation of overly dense images tends to predominately

results in erroneous vectors except when the contrast between different regions is high.

It also appears that the examples shown in literature seem topredominately consist of

medium density PIV data as shown in Figures 3.9b and 3.10b andLSV is not directly

19


mentioned in the manuals of PIV manufacturers. This indicates that very densely seeded

images are considerably less suited for the evaluation of flowfields.

Figure 3.10 The three modes of particle density as described in ref. [West93], Fig 1.4.The circle indicates the size of the interrogation area.

3.5 Correlation Analysis

3.5.1 Introduction

An outmost central part of a PIV analysis is determining the displacement of a group or

pattern of particles in an interrogation region (compare Chapter 3.2.2). It is desired to

achieve accurate results calculated in an time efficient way.

Dedicated literature ([West93], [West97], [Raf98]) typically addresses the correlation

analysis in a mathematical fashion which for the inexperienced is not straightforward to

understand. Even references keeping the mathematics a bit simpler ([Bol99] for

example) require some more in depth understanding of the mathematical approach. To

gain a better understanding it was therefore found that moregeneral sources on signal

analysis not directly related to PIV such as [Rand04] can be very helpful.

Giving answers on limits of the PIV method such a accuracy andbiases requires a

minimal understanding of the correlation method. The following chapters intend to

present this in a very simple way whilst still showing some ofthe simpler mathematical

relations.

3.5.2 Cross-Correlation

In the cross-correlation analysis the seeding particles attwo instances in time are

20


available as separate images. If these were printed on transparencies the two images

could be moved relative to each other until qualitatively a best possible overlap is

found. The relative position of the images to each other is then directly related to the

displacement of the particles as shown in Figure 3.11.

Figure 3.11 Cross Correlation: Finding the best overlap of the two images.

With digital camera recordings the image data is stored in matrix form ([I 1], [I 2] in

Figure 3.11). Mathematically the overlapping can be done bymoving the indexes of the

two matrices relative to each other by a discrete (pixel) amount (∆i,∆j); e.g. [I1](i, j)

↔ [Ι2](i+∆i, j+∆j). A good overlap will have the feature that pixels of high intensity will be

very likely to overlap with pixels of high intensity. Multiplying those pixel intensity

values will then yield a high value. A poor overlap will cause a pixel of high intensity to

overlap with a random intensity value. Multiplication can then yield anything from low

to high intensity values. Summing up this multiplication over all pixels will in case of a

poor overlap result in a very low value while a good overlap will result in very high

correlation values. The mathematical expression of this isgiven by Equation (3-1)

(compare [Raf98] (5.1); [Käh04] (2.4) and [Bol99] (5)). To find the most likely

displacement R(∆i, ∆j) has to be maximised.

21

∆i∆ j

ji

∆i

∆ j

Image 1, Intensity Matrix [I1]

displacement∆i, ∆j

∆i'∆j

'

partial overlapdisplacement ∆i', ∆j'

'best fit' overlap→ displacement ∆i, ∆j

⇒

Image 2, Intensity Matrix [I2]


R i , j = ∑ i=−N

N

∑ j=−N

N

I 1i , j ⋅I 2i i , j j Equation (3-1)

Cross-Correlation function

Figures 3.12 through 3.14 visualise this whole procedure in one dimension.

Figure 3.12 Particles moving in one dimension.

Figure 3.13 Cross-Correlation in one dimension.

22

i

I1(i)

Image 1,intensity I

1 at location i

i

I2(i)

Image 2,intensity I

2 at location i

∆i

i

particle displacement ∆i between images

particles at t1 particles at t

2

i

I1(i)

Image 1,intensity I

1 at

location i

i

I2(i+∆i')

Image 2,shifted ∆i' backwards

i

I12

(i, ∆i')=I1(i) · I

2(i+∆i')

partial overlap→ sum of intensities is low (Σ

i I

12 = 17)

mul

tiply

i

I1(i)

Image 1,intensity I

1 at

location i

i

I2(i+∆i)

Image 2,shifted ∆i backwards

i

'best' overlap→ sum of intensities

is maximal (Σ

i I

12 = 419)

mul

tiply

I12

(i, ∆i)=I1(i) · I

2(i+∆i)


As shown in Figure 3.13 shifting image I2 by an amount∆i' and multiplying with image

I1 results in very low intensity2 values when the overlap is poor and in very high values

when the overlap is good. The sum of intensity2 values over all pixel locations i will

give a good indication of the overlap agreement. Calculating this sum over all possible

displacements∆i gives a function as shown in Figure 3.14 with a maxima occurring at

the most likely displacement.

Figure 3.14 Correlation function vs displacement; the most likelydisplacement will have the highest peak.

Performing the same calculations in a plane (Equation (3-1)) will result in a two-

dimensional function as shown in Figure 3.15, which is the equivalent of Figure 3.14.

Figure 3.15 Cross-correlation function.(from [Käh04], Fig. 2.12)

As stated previously this introduction to cross correlation is only a very simple

description. Most of the information is based on more detailed material which can be

found in references [Käh04], [Raf98], [LaV02a], [Dan00a] to name a few.

23

0 4 8 12 16 20 24 28 320

200

400

∆i

∑i

I12 i , i

highest peak → most likely displacement


3.5.3 Auto-Correlation

When it is not possible to generate separate images single frames with multiple

exposures have to be made. The displacement can be determined using the auto-

correlation function, which essentially is a cross-correlation of the multiply exposed

image with itself ([I1] = [I 2]). As a consequence of 'merging' two images to one some

information is lost, which expresses itself by a loss of the sign, i.e. it is no longer

possible to determine whether the displacement is positiveor negative except when

special tricks such as image shifting are applied ([Raf98], Chapter 4.3).

Figure 3.16 Auto-correlation of a double exposed image.

The reason for the loss of sign becomes apparent from Figure 3.16. Again using the

transparency analogy it becomes apparent that the best agreement of the two now

identical (!) transparencies is obviously when no displacement occurs at all. Next to this

super-maxima two smaller maxima appear when moving the transparencies in either

direction. This situation with three peaks is shown in Figure 3.17.

While in the early days of PIV analysis auto-correlation wasoften the only option

available it has now become increasingly obsolete. Reasonsfor this are not only the

problem of directional ambiguity but also the increase in noise and decrease in

24

∆i

∆j

ji

∆i

∆j

Image 1 Intensity Matrix [I1] = Image 2 Intensity Matrix [I2] ([I1]=[I2])

displacement∆i, ∆j

best agreement:image

overlapping itself

⇒

−∆i

−∆j

first maxima'backwards' second maxima

'forwards'


resolution ([Dan00a], Chapter 4.4.1.3). More informationon auto-correlation of PIV

images can be found in [Raf98], [Dan00a] and [Bol99].

Figure 3.17 Auto-correlation function.(adapted from [Raf98], Fig. 3.9)

3.5.4 Increasing the Evaluation Speed by FFT

Using Equation (3-1) to calculate the correlation functionfor an image region sized

NxN pixels (e.g. 32x32) requires on the order of N4 summation operations, which when

performed over a larger number of pictures can become very time consuming even with

modern computers. A neat trick which can be applied to drastically reduce the number

of operations uses theWiener-Khinchin relationship, which states that the Fourier

transform of the correlation function results in the complex conjugate multiplication of the

Fourier transforms of the two individual images ([Rand04],Unit 7). If the fast Fourier

transform is used as shown in Equation (3-2), the calculation becomes very efficient and the

number of operations reduces from O[N4] to O[N2·log2(N)] ([Raf98], Chapter 5.4). For an

interrogation region sized 32x32 pixel this adds up to a reduction from 1'048'576 to merely

5'120 operations. Although a single FFT operation is likelyto take a bit longer than a

simple summation the time saving can in some situations evenreach the order of a factor of

1000 ([LaV03b], Chapter 3.4).

25


R i , j = ∑ i=−N

N

∑ j=−N

N

I 1i , j ⋅I 2i i , j j =ReFFT−1FFT I 1*⋅FFT I 2

Equation (3-2) Cross-Correlation function (Compare [Bol99], equation (6))

Nothing's free, also when using FFT and so there are a number of tradeoffs in applying

the FFT transform. These tradeoffs are due to the underlyingassumption of the FFT

that the transformed data is periodic, which strictly seen is not true for the theoretically

random seeded PIV images. If not dealt with properly the FFT transform will introduce

a number of unwanted 'errors' most notably high frequency noise (aliasing etc.), and a

bias towards lower velocities. As readers familiar with signal processing will know

these problems can be addressed by using window functions overlaid with the image, a

process which in turn will have new consequences and therefore is not addressed here

(interested readers are referred to [West93], [Raf98], [LaV03b], [Dan00a]).

Despite these pitfalls the massive advantage in speed makesit worth using the FFT

approach and one can assume that dedicated software packages will handle the

calculations appropriately.

In a final note and as mentioned previously it is interestingto know that the Fourier

transform of PIV images can also be carried out using opticalmethods as described by

Raffel et. al in [Raf98], Chapter 5.3.

3.5.5 Subpixel Interpolation

As a digital picture consists of discrete pixels, the correlation function as shown in

Figures 3.14 and 3.15 has got discrete maxima. What before digitisation was a

continuous function with a real value peak has been reduced to integer values.

Neglecting other uncertainties the actual location of the maxima could be anywhere on

the pixel, which leads to an associated uncertainty of ±0.5 pixels ([Raf98] Chapter 5.4.4

and [Hua97], Chapter 2.2).

As in PIV there is more than one particle going into the correlation analysis and as a

particle ideally will span more than just one pixel on the image, there are a number of

effects which eventually allow the displacement to be determined to subpixel accuracy.

Firstly, having more than one particle on the image will result in a kind of averaging of

26


the correlation function. Raffel et al ([Raf98], page 130) describe this in an example of

an interrogation region where 5 particles are displaced by 2and another 5 by 3 pixels,

with the resulting average being 2.5. Secondly a particle spanning more than one pixel

will result in a gradual ('stepped') rather than a sudden increase of the correlation

function. Particles spanning more pixels will result in broader correlation peaks, with

the correlation peaks themselves spanning more than just one pixel. In this case a

suitable continuous function (typically a Gaussian curve)can be fitted to the discrete

correlation function as shown in Figure 3.18, allowing the displacement to be

determined to subpixel accuracy.

Figure 3.18 Subpixel interpolation of the correlation peak.(compare with Figure 4-31 in reference [Dan00a])

According to reference [Raf98], Chapter 5.4.4, subpixel interpolation allows the

displacement to be determined to an accuracy of 0.05-0.1 pixels for optimal particle

sizes of 2-3 pixels and 8-bit images with good contrast. Larger particles are said to

produce a broader correlation peak, increasing the uncertainty of the maxima, while

particles smaller than 1.5 pixels result in a poor fit as the resulting narrower correlation

peak will cause the values around the peak to be overshadowedby noise. A further

discussion in this respect is given in Chapter 3.7.6 and morein depth information on

subpixel interpolation can be found in [Raf98], Chapter 5.4.4 and [Dan00a], Chapter

4.7.2.

27

4 2 0 2 4 6 8 10 12 14 16 18 200

0.25

0.5

0.75

1

displacement, pixels

no

rma

lise

d c

orr

ela

tion

noise

discrete correlation peak

(6 pixels)

interpolated correlation peak

(6.4 pixels)

curve fitted continuous function


3.5.6 Resolution, Accuracy and Dynamic Range

The topics addressed in this subchapter are closely relatedto subpixel interpolation

described in Chapter 3.5.5 and the information given in Chapter 3.7.

Both resolution and accuracy of PIV measurements are given by the goodness of the

subpixel interpolation described in Chapter 3.5.5. As previously mentioned most

sources claim realistic accuracies on the order of 0.05-0.1pixels to be achievable when

image and evaluation parameters are optimised ([LaV02a], Chapter 4.5.7). The

important image parameters are the particle diameter (2-3 pixels, compare Chapter 3.5.5

), particle density (Chapters 3.4 & 3.7.7) and contrast.

The dynamic range is a measure of the smallest to the largest displacement (velocity)

which can be resolved with reasonable accuracy. In theory the upper limit would be half

the side of the interrogation area NpixI/2 (e.g. 16 pixels for a 32x32 area), in practice

however a quarter of the interrogation area, NpixI/4, is more realistic (i.e. only 8 pixels

for a 32x32 area; compare [Bol99], page 12 and [Dan00a] Chapter 4.7.2.2). The lower

limit is given by the accuracy of the subpixel interpolation.

It follows that without subpixel interpolation the dynamicrange is NpixI/4 : 0.5 =

NpixI/2 : 1 (0.5 pixels is the accuracy without further interpolation). Image qualities

allowing subpixel interpolation to accuracies of 0.1 pixelfurther increase the dynamic

range to NpixI/4 : 0.1 = 2.5·NpixI : 1.

In this respect the author suggests that the dynamic range achievable with PIV is, as a

very crude rule of thumb, at least the size of the interrogation area NpixI : 1. Note that

increasing the interrogation area will decrease the numberof vectors extracted from the

image and vice versa.

3.5.7 Further Improvements – Adaptive Correlation

When keeping the location of the interrogation areas fixed between the two recordings a

number of particles will appear or disappear by crossing theboundaries as shown in

Figure 3.19. The actual cross-correlation function as given by Equation (3-1)

fundamentally already takes this into account by shifting the first instance of the

correlation area relative to the second as in Figure 3.20. One of the artefacts of

28


calculating the correlation function in the Fourier transform domain however is that it

assumes the interrogation area to be periodic in space, shifting around is thus obsolete

as the data in theory would repeat itself. However when considering Figure 3.19 where

the effect of random seeding particle location can be seen, this is strictly not the case.

Figure 3.19 Loss of correlation due to particles moving out of bounds.

By properly handling the FFT approach using window functions it is still possible to

achieve very good results without having to shift interrogation windows as stated in

Chapter 3.5.4. By first calculating an estimation of the displacement on a fixed

interrogation area and then shifting the two images relative to each other according to

the estimated displacements (Figure 3.20) the noise will begreatly reduced and the

measurement accuracy increased. Several different schemes for applying this method

can be used as outlined in [Raf98] Chapter 5.4.3, [LaV02a] Chapter 4.5.3 and [Bol99]

pp. 10-11.

Note that users of PIV software such as VidPIV have to be awareof the possibility of

increasing the accuracy by adaptive window shifting as the software will not usually do

the shifting process in standard cross-correlation analyses. Further information can be

found in the respective software manuals.

29

ji

Image 1; [I1]

⇒

Image 2; [I2]

particles moving out of bounds

('disappearing')

particles moving into bounds('appearing')

only few overlaps – poor correlation


Figure 3.20 Increasing the correlation by shifting the interrogation areas.

3.6 Post-processing

3.6.1 Introduction

While raw PIV images obtained from the correlation analysisas in the example from

Figure 3.21 usually already contain a lot of interesting information on the flow there is

still a need for further processing. Raffel et al ([Raf98], Chapter 6) group post-

processing into roughly six procedures: Validation of raw data, replacing incorrect data,

reducing data, analysing the information and bringing the results into a presentable

form.

Figure 3.21 'Raw' PIV measurement result showing spurious vectors.(from [West93], Fig. 4.1)

30

trick: offsetting (shifting) the two

interrogation areas relative to each other

⇒good overlap; excellent correlation

Image 1; [I1] Image 2; [I2]

interrogation area shift


3.6.2 Finding Outliers (Erroneous Vectors)

In real world situations if is not always possible to create flawless PIV recordings and

so chances are that spurious vectors such as the odd ones in Figure 3.21 can appear.

These outliers occur when correlation peaks originating from effects such as noise and

poor seeding as shown Figure 3.15 become higher than the actual displacement

correlation peaks.

There are a number of filtering methods suited to eliminate spurious vectors and only

the most basic concepts will be presented here. Filters can roughly be grouped into two

categories: Global filters spanning all interrogation regions and local filters spanning

just a few of the neighbouring cells.

Figure 3.22 2-D global velocity histogram.

The global filter is often applied for a first coarse separation of highly erroneous

vectors. Typically all vectors falling out of an allowable range of x- and y velocities are

filtered out. Determining an allowable range of plausible data can be done by using

two-dimensional velocity histograms as shown in Figure 3.22. The limits of the global

velocity filter can be defined using statistical parameters (i.e. confidence limits),

allowing the global velocity filter to adapt to different PIV results automatically. Note

that not all histograms look similar to Figure 3.22. Flow discontinuities can cause two

regions of dense vector accumulation to exist and accordingto [Raf98] certain sources

31


of noise can appear in distinctive regions of the global velocity histogram (compare

[Raf98], Chapter 6.1.1 and [ILA03a], page 62).

Once the global velocity filters have been applied local velocity filters can be used.

Local filters compare vectors with their neighbours in terms of magnitude and / or

direction to filter out further spurious vectors. The evaluation is again based on statistics

and a number of methods such as mean and median tests exist, with many literature

sources addressing the details ([Raf98] Chapter 6, [Bol99]page 10 and [West93]

Chapter 4.3). It is also helpful to address the manuals of theused PIV software for more

information on the algorithms used.

Another measure of the quality of local vectors is the ratio of the highest to the second

highest correlation peak (P1/P2 shown in Figure 3.23), giving an indication of the signal

to noise ratio. It is pointed out in Chapter 6.3 of reference [LaV02a] however that this

ratio of correlation peaks is "unspecific" and that it is prone to remove many good

vectors as well. Reason for this is that a local increase in noise might give give a bad

SNR but the result can still be in good agreement with its neighbours. Using local

velocity filters based on comparison with neighbours is thus preferable.

Figure 3.23 Signal to noise as a quality criteria.(from [LaV02a], Fig. 6.1.)

For all the mentioned methods there is still a finite probability that valid vectors are

filtered out and special care has to be taken in situations with sudden discontinuities,

namely shocks ([Raf98]).

32


3.6.3 Reconstucting Missing Vectors

Once the filters have been applied the PIV images will contain a number of missing

vectors (gaps). Certain operations such as discrete differentiation will fail when gaps are

occurring hence the need to reconstruct the missing data ([Raf98] Chapter 6.1).

Algorithms to reconstruct missing velocity data are usually based on interpolation of

information from neighbouring vectors and thus are somewhat similar to local velocity

filters. For the filters to work it is important that the datafrom neighbouring cells

suffices to yield reasonable estimations and in case of frequent gaps it may be necessary

to take the data from more distant neighbours into account aswell as explained in

references [Raf98] Chapter 6.2, [Bol99] page 11 and [ILA03a] page 63.

It has to be kept in mind that several algorithms will also manage to interpolate

seemingly sensible looking vectors into regions where gapspredominate and the data is

not sufficient for accurate reconstruction. Checking the filtered PIV vectors for such

regions of poor data prior to interpolation is thus essential for achieving solid results

(compare with the comments in [LaV02a], Chapter 6.4).

3.6.4 Further Processing of Data

Once the complete vector fields have been obtained the PIV data can further be

processed to aid the analysis and interpretation of the flow phenomena.

This can include reducing the data by calculating time average flow fields, comparison

with CFD results, calculation of vorticity generation of animations and further

operations.

Among the further operations is the possibility of smoothing the data, which typically is

done by applying some Gaussian function. While this can result in a nicer looking flow-

field, it will not result in higher accuracy but instead smear out a lot of potentially

useful information. According to [LaV02a] smoothing can beuseful when applied prior

to calculation of derivatives (vorticity for example) as these can react very sensitive to

unsteadiness. Further information can again be found in [Raf98] Chapter 6 and [Bol99]

page 11.

33


3.7 Pitfalls and Errors

3.7.1 Introduction

The intention of this Chapter is the raise the awareness of potential errors and pitfalls

which can be encountered when using PIV. It is also intended to present the issues in a

way that will allow to develop a rough idea of the order of the measurement uncertainty

associated with a certain measurement setup. Basic knowledge of these issues will help

to determine the quality of the measurements, which in turn is essential for justifying

results and showing that a serious and professional approach was taken.

There are numerous sources of error (i.e. deviations from the 'real world' situation) in

PIV. In a first broad categorisation the following three groups are made:

• All modifications necessary to facilitate PIV measurements at the test rig such as the

introduction of seeding particles and optical windows can potentially influence the

visualised flow.

• Errors introduced by the image recording process. This can be due to projection of

the particles, image noise and particle slip.

• Errors introduced by the image evaluation and post-processing methods.

According to Raffel et al. ([Raf98], Chapter 5.5) errors canbe grouped into systematic

errors εsys, which are predictable and residual errorsεres which are random and not

predictable. Separatingεsys andεres in practice is not always possible and thus the total

error is divided into a bias errorεbias and a random errorεrms, of which the latter could be

called the actual measurement uncertainty.

tot=sysresid.=biasrmsEquation (3-3) measurement error (uncertainty)

(from [Raf98], Equations (5.7) and (5.8))

While most types of uncertainties have been shown to be estimateable by theoretical

means ([Raf98]), doing so for an entire problem appears to bea very tedious and, given

the time ranges available in many projects, often impractical task. Nevertheless the

following subchapters will still allow an uncertainty estimation (albeit a coarse one) to

be made by considering what each of the listed effects implies for a particular PIV

application.

34


3.7.2 2-C vs 3-C PIV: A Perspective Error

A central limitation of two-component PIV is that a three-dimensional displacement

will be projected onto a two-dimensional plane. As shown in Figure 3.24 this is not a

problem as long as the particle moving perpendicular is on or close to the optical axis. If

the perpendicular movement is further away from the opticalaxis however the a

perpendicular displacement in the light sheet plane will beprojected onto the camera's

CCD chip as an in-plane displacement, which strictly is not the case.

Figure 3.24 Perspective error – example sketch.(based on combining the written example in [Dan00a] Chapter 4.7.6 with Fig. 2.28 in [Raf98])

A worst case error estimation can be made by assuming that a particle moves across the

whole thickness of the light sheet in the outer region of the visible plane. The resulting

projection error can be calculated by using geometrical relations as shown in Equation

(3-4). As an example: For a camera with d1=1000 pixels at a viewing distance of

L=500mm and a light sheet thickness of 0.5mm a particle moving perpendicular across

the light sheet would already be projected as an in plane displacement on the order of ½

a pixel! This shows that for 2-component PIV measurements itis of outmost importance

35

movement normal to the light sheet→ zero in-plane displacement

displacements normal to the light sheet are zero on the optical axis only

perceived in-plane displacement

→ projection error εp

t

t: light sheet (LS) thicknessL: distance LS-CCD plane

L

d 1 d 1

x2 x1

d1: viewfield lengthd2: CCD width

x1, x2: spacing to the lens principal point


that the main flow velocity lies in the plane and not perpendicular to it.

proj.max=d1

2⋅1−

1

1tL⋅d1

d2

1≈d1

2⋅

ttL

Equation (3-4) Worst caseperspective error for perpendicularviewing; conventions as in Figure3.24

Equation (3-4) assumes a situation where the camera has a perpendicular view on the

light sheet plane. Another situation with worse consequences arises when viewing at an

angle considerably smaller than 90°. Irrespective of special configurations such as

applying the Scheimpflug correction (Chapter 6), the dominant perspective error when

viewing at an angle can be estimated by a simple trigonometric cotangent function as

shown in Figure 3.25 and Equation (3-5). Unlike the perspective error for perpendicular

viewing which varies across the image (no error in the centrewith increasing error

towards the edges) the predominant part of the projection error when viewing at an

angle persists throughout the image.

Figure 3.25 Consequence of viewing at an angle.(from [LaV02a], Fig. 12.1)

proj.angle=dz⋅cotEquation (3-5) Perspective error for viewing at anangle; conventions as in Figure 3.24

Due to the vortices in dimples, which have a significantly large velocity component

perpendicular to the main flow and the necessity to view inside the dimples by tilting

the camera, projection errors are of particular importancefor two-component PIV

measurements on dimpled surfaces.

36


More general equations and further information on the projection error can be found in

references [Dan00a] Chapter 4.7.6, [Raf98] Chapter 2.4.3 and [LaV02a].

3.7.3 Velocity Gradients

Having widely varying velocities within an interrogation region will cause the slower

particles to (in proportion) occur more frequently than thefaster particles. This will

cause the resulting velocity to be biased towards lower values ([Raf98] Chapter 5.5.7

and [Dan00a] Chapter 4.4.3).

Estimating the effect of a particular velocity gradient canbe done by using Monte-Carlo

simulations to create artificial images of known velocity.If own Monte-Carlo

simulations are not an option it is still possible to judge the error by comparing with

simulation results from other authors such as references [Bol99] (Fig. 12) and [Raf98]

(Fig. 5.32; shown in Figure 3.26).

Figure 3.26 Influence of velocity gradients determined by a Monte-Carlo simulation.Conditions: 2pixel particle diameter, 8bit image, no noise – idealised conditions.

NI: number of particles per interrogation area; (from [Raf98], Fig 5.33).

Decreasing the gradient error can be done by reducing the size of the interrogation area,

which requires an appropriate seeding density (number of particles in the image) and

image quality.

37


3.7.4 Particles Moving Out of Bounds and Laser Flash Separation

With increasing time separation between the two light pulses (images) the number of

particles moving out of and into bounds increases. In case offixed interrogation areas

(or volumes respectively) this causes the correlation to decrease, which in turn increases

the uncertainty.

For in plane displacements the time separation should in theory be chosen so that the

particle displacements will at least lie in a range of 0.1 pixels to a quarter of the

interrogation area size NpixI/4 as stated in Chapter 3.5.6 (compare [Bol99] page 16 and

[LaV02a] Chapter 4.7). According to [LaV03c] one should aimfor a displacement of

approximately 5 pixels as a basis for further optimisation.

From the nature of the PIV method the in plane correlation loss will result in a bias

towards lower velocities which can be corrected by appropriate weighting functions.

One can assume these functions to be implemented into any serious PIV evaluation

software without requiring further user intervention (→ [Raf98] Chapter 5.5.3). Next to

the bias error, which can be compensated for, there still is anon-correctable effect of

the particle displacement on the RMS-uncertainty shown in Figure 3.27. Note the

increase of the uncertainty for displacements higher than 2-3 pixels. It is also interesting

that the larger particles tend have a higher uncertainty.

Figure 3.27 Measurement uncertainty due to in plane particle displacements.(from [Raf98] Fig. 5.26; obtained by Monte Carlo simulations; dτ: particle size in pixels)

38


While it is possible to compensate for the out of bound correlation loss by adaptive

window shifting (Chapter 3.5.7) other effects such non-linear particle paths and the

subsequently addressed third displacement component impose further limits on the

allowable particle separation.

For 2-C PIV particle displacements normal to the plane can bemore problematic than

large in plane displacements. Next to causing a perspectiveerror as shown in Chapter

3.7.2 they also strongly limit the allowable normal component and according to

Bolinder, [Bol99] page 15, the displacement perpendicularto the plane must be no more

that ¼ of the light sheet thickness. In strongly three-dimensional flow this will restrict

the time separation between the laser flashes, which can cause the in-plane

displacement to be low, in turn resulting in high relative uncertainties.

Due to these effects on the uncertainty and correlation quality optimising the pulse

separation is an essential step for good PIV results.

3.7.5 Pulse Duration

According to [Raf98], page 6, the duration of the illumination must be short enough to

freeze the particle motion. The picture should neither look blurry nor contain streaks.

While this is not really an issue with the typically used Nd-YAG lasers having a pulse

duration on the order of 5ns (compare Chapter 4 and [Bol99] page 2), which is short

enough to freeze in particle images even at very high velocities, the problem has to be

kept in mind when using other light sources such as flashlamps.

3.7.6 Pixel (Peak) Locking and Particle Image Diameter

Displacements are determined to subpixel accuracy by meansof fitting a suitable

function to the discrete correlation peak as explained in Chapter 3.5.5. For this fit to be

reliable the correlation peak must be wide enough to ensure that there is more than just

one component exceeding the noise level. In practice this isfulfilled for particle image

sizes exceeding 1.5 pixels as shown in Figure 3.28. If the correlation peak gets so

narrow that there is just one discrete component exceeding the noise level left the fitting

function will 'lock' to this peak resulting in integer displacement values.

39


Figure 3.28 Subpixel interpolation requires wide correlation peaks.(adapted from [LaV02a], Fig. 4.16)

A measure for this effect can be found by analysing pixel valued displacement

histograms as shown in Figure 3.29. Pixel locking manifestsitself by accumulation of

the measured displacements around integer values giving a mountain-valley like view.

Figure 3.29 Displacement histogram - pixel locking: biased towards integer values(adapted from [Raf98], Fig. 5.25)

While in theory one could try to find a better subpixel estimator the more common

approach is to try and increase the particle image diameter by means of image pre-

conditioning or variation in seeding. A trick to increase the particle diameter is by

slightly defocussing the lens resulting in a blurred image (compare with the

recommendations in [Dan00a] Chapter 4.4.1.5.4, [LaV02a] Chapter 4.7, and [Raf98]

Chapter 5.5.2). Note that due to the small size of the seedingparticles the physical

particle diameter is not the only parameter determining theimaged particle diameter but

a series of effects associated with laser diffraction causeother parameters such as the

camera lens aperture to be of importance as well (compare with the comments in

Chapter 6.6).

40


Estimating the effect of pixel locking for a particular situation can again be made by

Monte Carlo simulations as shown in Figure 3.30

Figure 3.30 Pixel locking bias: the smaller the imaged diameter dτ, the larger the error(idealised 8bit image: no noise, optimum exposure, ...)

(from [Raf98], Fig. 5.24)

Most sources recommend the imaged particle diameter to be greater than 2 pixels

(compare for example with [Dan00a], page 55). While a too small particle image will

result in pixel locking a too big particle size will increasethe uncertainty as shown in

Figures 3.31 and 3.32. Exploiting the evaluation parameters and methods allows to

improve the uncertainty and allowable diameter range, which explains why Bolinder

[Bol99] recommends to limit the particle size to to 2-4.5 pixels in accordance to Figure

3.32 while Raffel et al. ([Raf98]) and LaVision ([LaV02a]) suggest an optimal range

between 2-3 pixels.

41


Figure 3.31 RMS uncertainty due to the particle image diameter.Idealised image quality conditions (from [Raf98], Fig. 5.23a)

Figure 3.32 Uncertainty due to the particle image diameter and the effect ofcomputational schemes; 16 particles in a 32x32 area; zero background noise

(from [Bol99], Fig. 8)

42


3.7.7 Seeding Particle Density

It is known that the quality and distinctiveness of the correlation peak and thus the

probability of having a valid result increases with the number of particles in the

interrogation region ([Raf98] Chapter 5.5.4). Most sources recommend to aim for a

seeding density of at least ten particles per interrogationarea ([Hart00], [Bol99]) a too

densely seeded flow however can result in an image of poor contrast and laser speckling

(compare with Chapter 3.4).

Information on the influence of the seeding density on the detection probablility can be

found in references [Raf98] Fig. 5.28 and [Bol99] page 4, Fig. 13. A measure for the

associated uncertainty is given by Figures 3.33 and 3.34.

Note that next to the seeding density it is desired to have theparticles distributed both

homogeneously and random ([Raf98] Chapter 1.2).

Figure 3.33 Influence of the seeding density on the RMS uncertainty.Idealised noise-free conditions; 32x32 pixels; particle size 2.5 pixels.

(after [Bol99])

43


Figure 3.34 Influence of the seeding density NI on the measurement uncertainty.32x32 pixel, 8 bit, 2.2 pixel particle size, idealised conditions: no noise etc.

(from [Raf98] Fig 5.29)

3.7.8 Quantisation Levels (n-bit Pictures)

Raffel et al. concluded that the effect of image quantisation is of minor importance, as

shown in Figure 3.35 where reduction down to a level of 4 bits had little consequence.

Figure 3.35 Effect of image quantisation level. Idealised background noise free conditions; NI=10.2 particles; 32x32 pixel area.

(from [Raf98] Fig 5.30)

44


One limitation of this comparison however is that Figure 3.35 is based on a background

noise free simulation. If another consequence of reducing the quantisation level were a

decreased signal to noise level this would have serious consequences on the accuracy of

the results.

While the quantisation effect turned out to be small when using the PIV evaluation

method it is noted by Raffel et al. [Raf98] that the accuracy of methods relying on

single particles such as PTV tends to improve with increasing quantisation levels.

3.7.9 Light Sheet Coincidence

In the commonly used PIV setups the two flashes of light originate from two separate

laser heads. For the system to work the two beams must overlapas good as possible or

the two flashes of light will occur in different planes thus illuminating particles which

are completely uncorrelated with each other. One way of doing this is to compare the

beams without the light sheet optics installed as explainedin Chapter 4. Another way of

checking the beam overlap is described in [LaV03c] where theflow at low velocities is

imaged with very short inter-flash delays. If the beam alignment (and seeding) is good

the resulting velocities should be zero.

A poor beam coincidence will require one of the two beams to berealigned by using the

mirrors in the laser head. Doing this with the Ekspla laser used for this project is

described in Chapter 4.5.7.

3.7.10 Image Intensity Distribution

Another potential source of bias is a varying image intensity distribution (compare

[Raf98] Chapter 3.2). Particles moving from regions of highto low intensity would tend

to have a reasonably lower correlation peak (which is a result of multiplying particle

intensities) than particles moving in regions of similar intensities, thus biasing the result

towards particles of more even illumination.

A much greater problem occurs when the intensity on one part of the image turns out

very high as frequently happens due to reflections. In ordernot to harm the camera it is

necessary to limit the peak intensity, which can lead to the situations where the particle

intensity in other parts of the image drops below the dynamicrange of the camera (i.e.

45


below the lowest resolvable intensity). Without sufficiently illuminated particles the

correlation analysis and in turn the PIV evaluation method will fail.

3.7.11 Other Image Quality Aspects

Further image quality aspects are often linked with increasing the signal to noise ratio

(SNR). One of these aspects is the image background noise level, which is influenced

by the camera dark current noise, ambient light, CCD exposure time and potential

sources of reflections. It is desired to keep the backgroundnoise level as low and as

uniformly distributed as possible. A good contrast with a high particle image intensity is

also desired ([Bol99]). In this respect it was shown by Raffel et al. ([Raf98] - Chapter

5.5.6) that minor background noise up to 10% of the particle intensity level tends to be

of lesser if not neglectable effect. In practice however situations such as strong

reflections from walls can cause the noise levels to get unfavourably close to the peak

particle intensities, in turn resulting in a reduction of the measurement accuracy and

reliability (increase in false vectors).

3.7.12 Computational Scheme (Subpixel, Adaptive etc.)

Although already mentioned in the previous chapters it should not be forgotten that the

accuracy of the results can be further improved by optimising the evaluation steps.

Instead of performing just one run of correlation calculations on a fixed grid of

interrogation areas, modern PIV evaluation software usually allows to implement

several steps into the evaluation process beginning with a coarse calculation of the

displacements on large interrogation areas which then are further refined and combined

with adaptive window shifting techniques (compare [Raf98] Chapter 5.4.3).

3.7.13 First Order Approximation of the Particle - Not Flow - Velocity

Despite the name 'particle' image velocimetry the fact thatthe velocity of the particle is

measured is prone to be forgotten. Due to forces – most notably inertial forces – acting

on the particle its velocity and path will never exactly match the path of the flow as

outlined in further detail in Chapter 8.

Next to the deviation from the flow the PIV method gives a linear approximation of a

typically non-linear particle path. Reason being is that, with the exception of time

46


resolved PIV (Chapter 3.2.6), it is usually not possible to capture images of more than

two coherent (i.e. correlateable) instances of the flow in time. The discrete

approximation of the displacement in time is thus confined to be a first order method as

shown in Figure 3.36 ([Raf98] page 120 and [West97]).

Figure 3.36 First order approximation of the particle path.(after [West97] Fig. 1)

47

Chapter 4 - The Laser

4. The Laser

4.1 Introduction

In PIV it is desired to illuminate a plane with a light-flash of sufficiently short duration

allowing to obtain a 'frozen', non-smeared particle image.White light sources such as

flashlamps are less suited because of the difficulty in collimating their light and the

relatively long flash duration which can make additional shutters necessary. Due to their

ability of emitting light at discrete wavelengths bundled in a defined direction lasers

tend to be more advantageous for PIV applications. With their lower intensity

continuous wave (cw) lasers are usually limited to low lightapplications such as slow

water flows. As they emit continuously cw lasers further have to be combined with an

additional shutter or scanning mechanism ([Dan00a] Chapter 4.6.3). Pulsed lasers, most

notably the Nd-YAG types, are more commonly used in PIV application because of

their ability to produce very powerful pulses of light lasting just a few nanoseconds

([Dan00a] Chapter 4.6 and [Raf98] Chapter 2.2).

A Nd-YAG type twin head laser (i.e. two lasers) model NL301-2G manufactured by

Ekspla Experimental Lasers Ltd. in Lithuania was used in this project. Unfortunately

the laser had to be sent to Lithuania for repairs at the beginning of the project due to a

broken pockels cell caused by a leak in one laser head and in order to realign the

mirrors of the other laser head. Once the laser was returned anumber of operations had

to be done by the author in order to get the laser up and running again.

Due to the number of maintenance operations a substantial part of the project consisted

of getting the laser running again and thus a major share of this chapter is about

describing the necessary operations involved. As a prefaceto the maintenance

operations theoretical laser background is explained and the topic of laser safety

addressed. Finally a laser control program which was written to allow convenient

operation of the laser from a computer is presented.

48


4.2 Working Principle of a PIV Nd-YAG Laser

References used in this chapter: [Her02] Chapter 6.5.4, [Raf98] Chapter 2.2.1,

[LaV02b] Chapter 3.1, [Dan00a] Chapter 4.6.2 and input fromIrmantas Mikulskas -

Senior Service Engineer Ekspla Ltd.

4.2.1 Basic Laser Principle

A laser consists of a lasing material which is excited by an energy source. In the case of

a Nd-YAG laser used for PIV this is typically white light originating from a flashlamp

exciting neodym ions (Nd3+) embedded in a cylindrical yttrium-aluminium-garnet rod as

indicated in Figure 4.1. The Nd3+ ions (i.e. their electrons respectively) can be in various

energy states as shown in Figure 4.2.

Figure 4.1 Basic laser configuation. (adapted from [Raf98], Fig. 2.11)

Without further addition of energy the electrons will predominately be in the lowest

state indicated as E1. Normally photons are only able to bring an electron onto a higher

level if their energy matches the difference between two discrete states. A special

property of the Nd3+ ions is that their upper energy levels indicated as the rangeE4* are

continuous instead of discrete, which unlike other lasing materials allows them to

absorb photons of various energy levels contained in white light. Once the Nd3+ has

been excited to E4* it will first make a transition to the lower energy state E3. In state E3

it can either spontaneously emit a photon or be stimulated toemit a photon by an

impinging photon. In case of stimulated emission the emitted photon will be in phase

with the impinging photon. Note that the impinging photon will still exist so now there

49


are two photons which can stimulate further excited atoms causing a so called

population inversion in a 'chain reaction' like manner as shown in Figure 4.3. This

population inversion will continue as long as there are excited atoms and can be in a

steady state condition if the pumping energy is added continuously.

Figure 4.2 Nd-YAG laser energy bands.(based on [Raf98] Fig. 2.13 and [LaV02b] Fig. 3.3)

Figure 4.3 Population inversion ('chain reaction').

50

E1

E2

E3

E4*

Energy level

excitation by flashlampphotons E

1→E

4

Nd-YAG: continuous upper energy level→ can absorb more than just one wavelength λ

nonradiativetransition

nonradiativetransition

radiative transition→ photon at λ=1064nm

⇒

⇒

⇒

⇒

⇒

⇒

⇒

⇒

⇒

⇒

⇒

⇒

⇒

photon stimulates emission of a second photon from an excited atom

now two photons stimulate emission from further atoms ... → population inversion


Adding mirrors at each end of the lasing material will cause the emitted photons to pass

the lasing material several times, thus amplifying the effect of emission. In addition this

resonator-like configuration will result in standing waves of photons at discrete

resonance frequencies (i.e. wavelengths). Making one of the two mirrors partly

transparent will allow a part of this laser light to pass out of the oscillator. It is of

importance that the mirrors are perfectly aligned to each other or lasing will not occur.

4.2.2 Pumping with Flashlamps

In order to achieve high powers high excitation intensitiesare needed as well. As

continuous excitation at such high power would result in overheating and in turn failure

of conventional white light sources, Nd-YAG lasers are commonly pumped

intermittently by one or more krypton flashlamps, which usually are immersed in

deionised or distilled water pumped through the laser head by an external pump /

cooling circuit combination.

Figure 4.4 Flashlamp vs laser output.

Without further modification using flashlamps to pump a system as shown in Figure 4.1

would result in lasing as long as the flashlamp intensity is above the lasing threshold

level as indicated in Figure 4.4.

For thermal stability it is important that the flashlamps are operated at a stable

51

flashlamp envelope (intensity)

tlaser outputpower(relatively low)

laser output

t

lasing thresholdlevel


frequency. In case of the used Ekspla laser the components are optimised for flashlamps

flashing at 20Hz and the laser output won't be activated before a five seconds warm up

([Eks97], [Raf98]). In operation it was observed that it is best to let the flashlamps flash

for several minutes until the laser head reaches a state of near thermal equilibrium with

roughly constant beam properties. Without this warm up subsequent laser flashes tended

to be of varying intensity both locally and globally, which results in an uneven

illumination of the flowfield.

4.2.3 Introducing a Q-Switch

Figure 4.5 shows the configuration of the Ekspla laser used in the current project.

Compared to Figure 4.1 it is apparent that the main difference lies in two additional

components located between the mirrors. These are a pockelscell (PC) and a polariser

plate (P), which together form the so called Q-switch.

Figure 4.5 Ekspla Nd-YAG laser configuration. (from [Eks97], Fig. 10)Legend: HV – high voltage supply; SYNC: synchronisation signals switching the HV;

M: mirrors; PC: pockels cell; P: polariser plate

In a very simple analogy the Q-switch could be regarded as "a mechanism that quickly

opens and closes the cavity" ([Dan00a] Chapter 4.6.2.1). Inorder to be able to analyse

problems however a somewhat more thorough understanding is needed:

Without any high voltage applied to the pockels cell (PC) it more or less acts as a

transparent media and the laser would lase during flashlamppulses as shown in Figure

52


4.4. Applying high voltage to the pockels cell will cause it to turn the polarisation of

any incoming light by 90°. This alone wouldn't stop the laserfrom lasing but when

combined with a polariser plate (P) the result is a switch with basically no inertia.

Reason for this is that the polariser plate acts as a filter which only lets light at a defined

polarisation angle pass. If the light after the polariser plate is turned by another 90° the

component normal to the polariser direction will be zero and the light cannot pass.

Figure 4.6 Effect of the Q-switch.

53

a) flashlamp envelope (intensity)

tb) excitedNd atoms(stored energy)

c) HV @ the pockels cellO(5kV)

~190µs

flashlamp starts→ laser energy builds up

HV @ the pockels cell → no lasing

HV pulse → sudden population inversion→ powerful laser pulse

no HV-pulse → laser energy would decay again; no lasing occurs

t

t

d) laser outputO(MW)

t~5ns

high power laser flashtriggered by HV pulse


Figure 4.6 shows how the effect of the Q-switch can be exploited to control the laser

output between no output at all to creating a powerful laser flash of short duration. The

topmost graph (Figure 4.6a) shows the flashlamp envelope which is a measure for the

amount of excitation energy being pumped into the Nd-YAG laser rod. As long as HV

is applied to the pockels cell the light from the back mirror will be rejected from the

polariser plate, as it is 90° out of phase and thus can't get back into the lasing material,

and so lasing does not take place. As indicated in Figure 4.6bthis causes the excitation

energy stored in the Nd3+ ions to build up and decay again if the high voltage is left at

its high state. Suddenly inverting the applied HV will allowthe light from the back

mirror to get back into the laser rod, causing immediate laser oscillation and sudden

release of the excitation energy stored in the Nd-YAG material.

When adjusted optimally the pulse duration of the Ekspla laser will be around 5ns

which for a pulse energy of 100mJ is equivalent to a power of 20MW averaged over the

flash duration!

Note that for the Q-switch to work properly the pockels cell and polariser must be

aligned perfectly within the two mirrors for otherwise the light passing the PC won't

turn out to be in perfect anti-phase with the polariser plate.

4.2.4 Varying and Maximising the Pulse Energy

There are a number of ways to change the pulse energy passed into the light sheet:

• By placing an optical attenuator after the laser: New Wave (www.new-wave.com)

offers such mechanically driven optical attenuators allowing to vary the output

power as an option for their lasers. The advantage is that thelaser can be operated

under optimal conditions as the attenuation is independent of the beam generation.

• By changing the flashlamp (FL) voltage and thus the pumping energy: This is

possible with the Ekspla laser which allows to vary the FL voltage between 600V

and 1600V, but many lasers (New Wave PIV lasers for example) often won't have

the option for continuous flashlamp voltage adjustment forreasons of thermal

stability.

• By adjusting the Q-switch timing: In principle the Q-switchcan be operated at any

time in the flashlamp cycle. The resulting pulse energy willdepend on the energy

54


stored in the Nd-YAG rod at time of operation.

4.2.5 Combining Two Lasers

As the repetition rate of a single Nd-YAG laser usually is toosmall to result in

reasonably short delays between the two flashes it is necessary to combine two lasers

operating at the same repetition rate but out of phase to eachother. This requires joining

the two beam paths so that both beams become coincident, which is done by mirrors as

explained in Chapter 4.5.7 (compare with the sketch on beam combining optics in

Figure 3.5).

The flashes are controlled by TTL signals generated by a timing unit as explained in

Chapter 7.

4.2.6 Frequency Doubling: Invisible IR→→→→ Visible Green

The light emitted by an Nd-YAG laser at conventional operating temperatures has got a

wavelength of 1064nm ([Raf98] Chapter 2.2.2). Such infrared light is unsuited to be

detected by common CCD chips and thus it is run through a so called second harmonic

(SH) generator which doubles the frequency of a part of the incident light to 532nm

(green). After frequency doubling the green light is usually separated from the infrared

by either using a prism harmonic separator ([Raf98]) or in the case of the Ekspla laser a

dichroic mirror which has maximum reflection for 1064nm butlets the predominant

part of the green 532nm light pass.

Commonly used second harmonic (SH) generators are KDP, KD*Pand KTP type

crystals. KTP crystals as used in the Ekspla laser tend to have higher conversion

efficiencies on the order of 60% (compare with data sheets asavailable from MolTech

GmbH – www.mt-berlin.com or from [CAS99]).

When dealing with SH crystals the following points are of importance:

• The commonly used crystals are hygroscopic and would dissolve when absorbing

moisture. In case of many lasers such as the Ekspla laser thisis prevented by heating

them, which also increases the damage threshold ([CAS99]).It is of outmost

importance not to turn off the heatingfor longer periods of time (hours).

Unfortunately this requires the Ekspla laser to remain plugged in to the mains power

55


supply. SH crystals are transported in sealed containers containing moisture

absorbing silica gel pads.

• The conversion efficiency is dependent of the angle of incidence relative to the laser

beams and follows a sin2(x)/x function ([CAS99]). Replacing the crystal requires

realignment as explained in Chapter 4.5.9.

• SH crystals are a potential source of back reflections and must be tilted in order to

avoid potentially fatal (for the laser) back reflections into the laser cavity.

Next to frequency doubling frequency tripling and quadrupling are often used in PIV

applications exploiting additional information from fluorescent tracer particles.

Further information on SH crystals and using them in lasers can be found in [Eks97],

[CAS99]) and [Raf98].

4.2.7 Further Nd-YAG Properties

In the following list some of the Nd-YAG laser properties of more direct importance for

everyday use will be listed. More detailed information can be found in the respective

laser manuals, on the manufacturers homepages (www.ekspla.com, www.new-

wave.com) and in [Raf98] (Table 2.4).

• Nd-YAG rods are water cooled. The water must be of high purity(distilled /

deionised) and has to be replaced from time to time. Some lasers also require

replacing an additional water filter located in the cooling circuit from time to time.

• The laser power supplies are either air-cooled or, as in the case of the Ekspla laser,

require additional water cooling.

• Typical repetition rates of a single laser are on the order of10..20Hz with some

specialised models allowing operation in the kHz range.

• The Q-switches and flashlamps can often be controlled by externally (user) supplied

TTL signals. Control of the Ekspla laser is more limited as described in Chapter 7.

• Power supply is usually possible via a convenient standard single phase mains plug,

500W-1000W.

• Beam diameters 2.5mm – 10mm, depending on the laser model.

• Energy stability ~ O(5%) for a good laser, i.e. the image intensity will vary on the

56


order of 5%. These values are usually only achieved after a sufficiently long warmup

period.

• Size and weight: Modern PIV lasers offer twin-laser heads weighing less than ten

kilograms and sized on the order of a shoe box, with the power supply sized in the

range of a conventional midi-tower computer. In this respect the Ekspla laser can be

considered as relatively large.

• Sensitivity to dirt and vibrations: A laser is a sensitive device and should be handled

with care as if holding an expensive glass sculpture. Most parts of the laser are

protected from dirt by covers and unprotected optical components should remain free

of dust and (especially) grease.

4.3 Laser Safety

4.3.1 Introduction

Next to the possibility of skin burns handling Nd-YAG lasersbear the very real danger

of suffering permanent blindness, be it partial or complete.

Due to the highly directional laser beam which is concentrated on a relatively small area

even low power lasers on the order of >5mW (continuous power)have the potential of

permanently damaging the eyesight if directed straight on the human eye. Using such

lasers thus underlies safety guidelines imposed by variousinstitutions such as

governmental workplace safety agencies.

The high power Nd-YAG lasers with peak intensities of more than 20MW (!) pose an

even greater threat which leads to the situation that insurers and legislators often request

strict compliance with laser safety guidelines. Consequently PIV users are usually

obliged to follow laser safety guidelines, which in a perfect world would make perfect

sense.

Once it comes to the application of the safety guidelines however novice users of laser

devices may find it often impractical and sometimes very difficult to impossible to fully

comply with the strict interpretation of laser safety rules, as was experienced by the

author himself. To make things worse many experienced userstend not to follow many

of the strict rules at all, which after an initial phase of great respect inspires novices to

57


follow suite. – Not an ideal situation for laser safety.

Reasons for this are manifold. One is that laser safety regulations are often not written

with PIV in mind and in particular not with setting up a PIV system, which can require

operations such as alignment of an unshielded beam. Also safety regulators appear to

have the tendency to try and completely abolish any risk of harm from laser use. In its

most extreme form this can mean complete separation of laserand user or even result in

not using a laser at all. Workplace supervisors who have to adapt these rules into PIV

guidelines potentially face the threat of possible legal issues if neglecting some of the

stricter guidelines, which again can result in unpractical guidelines.

Experienced users neglecting certain guidelines in turn may only face little risk of

severe harm by laser irradiation if handling the laser in an appropriate way, which

however can require experience and know-how obtained over several years.

Due to this situation novice users may find it difficult to obtain appropriate information

allowing them to work in a safe yet productive way. For this reason it is the intention of

this chapter to provide tips on handling PIV laser equipmentin a safe way based on a

combination of information found in literature from PIV manufacturers ([Dan00a] &

[LaV02b]), obtained by the author from various industry professionals (who are, despite

not being able to name them all, greatly acknowledged) and based on the author's own

experience with PIV and laser hardware.

Note that despite the aforementioned pitfalls laser safetyguidelines are nevertheless

worth reading and recommended for all readers interested inlaser safety. Many

guidelines provided by PIV manufacturers ([Dan00a], [LaV02b]) contain useful

information on the do's and don'ts of laser operation. Furthermore a good overview in

the nature of how lasers cause damage to the eye can be found in reference [Lit03].

4.3.2 Potential Sources of Eye Damage

Figure 4.7 shows three potential paths for hazardous laser light to reach the human eye.

These are: A direct hit by the 'unaltered' laser beam, a direct hit by a coherently

reflected laser beam and exposure to diffusely reflected light.

Installing a light sheet will additionally increase the range of the laser beam, while the

58


intensity level (power per area) due to the high Nd-YAG powerwill still suffice to

cause severe eye damage. Distributing 20MW peak power alonga 1m long light sheet

of 1mm thickness will result in an average intensity of 20 GW/m2, 20 million times

more than the typical intensity of the sun when hitting the earth (1kW/m2; from [Lit03]).

That hurts (blinds)!

Figure 4.7 Sources of laser induced eye damage.

Chances of accidentally looking directly into the laser beam or sheet as in Figure 4.7a

are real. One of the first preventive measures is to appropriately shield the beam as

described in Chapter 4.3.3. Next to proper shielding the nature of human reactions itself

poses a threat. Despite being informed about the danger humans are prone for actions

such as going to 'take a look' when something doesn't work or changing something in

the test region while someone else is working with the laser,which can cause exactly

such a direct hit situation. Although this might sound unlikely many accidents happen

due to the occurrence of seemingly less likely events, whichis what makes them

dangerous.

Indirect hits through reflections pose another greater threat. All reflective surfaces must

be treated as a potential hazard. This includes watches (total reflection on glass),

jewellery, windows (e.g. pieces of acrylic or glass), metallic surfaces (e.g. steel rulers,

chrome-plated surfaces). Preventive measures include removing watches and other

reflective materials and inspecting all objects in the beams path before setting up the

laser. Note that not only 'total' reflection but also the case of partial reflection, which

occurs when light hits a transparent media is a risk. As shownin Equation (4-1) on the

order of 4% of the intensity of a laser beam impinging a transparent media such as glass

will be reflected (4% of 20MW=800kW).

59

a) direct hit

b) indirect hit through'coherent' reflection c) indirect hit through

diffusive reflection


ref=n−1n1

2

=1.5−11.51

2

≈4%

Equation (4-1) Percentage of reflected lightintensity upon perpendicular incidence on glass.(compare [MelG02] - Coating theory; n: index ofrefraction)

Diffuse reflection (Figure 4.7c) from a surface often appears comparably harmless. In

fact many PIV users will have experienced that while diffusereflections can appear

partly blinding at first, especially if the rest of the room is not well lit, the actual

damaged caused is not more than what occurs after leaving a rather dim building on a

sunny day. This causes the dangerous situation of underestimating the potential of

diffuse reflections. If lets say 20MW were diffusely reflected from a surface, which

could occur for a circular laser beam hitting the surface without having a light sheet

optics installed, one could assume a Lambert's law intensity distribution ([Her02] eq. (6-

62)). At a viewing angle of 45°, 1m distance from the surface,the intensity will still be

around 4.5MW/m2 (Equation (4-2)).

I 45°=1A⋅P0

⋅cos45° ⋅

A

r 2=20MW⋅cos45°⋅ 1

1 m2

≈4.5MW

m2

Equation (4-2) Estimation of the intensity of a diffuse 20MW (P0) source in 1mdistance (r) at a viewing angle of 45°.

(based on [Her02] eq. (6-62) and (6-60) combined with the integration of (6-62) found in [Schä99])

Of course a direct comparison with the 1kW/m2 of continuous sunlight is limited as the

duration of the laser flash is only around 5ns but nevertheless the powers are enormous

and pose a serious threat to the eyesight when not handled properly.

4.3.3 Improving the Safety by Appropriate Shielding

The aim of shielding is to minimise the laser radiation goingbeyond the measurement

region and can include limiting access to the path of the laser before the measurement

region.

One part of shielding is to prevent people not involved in themeasurement process of

getting exposed to the laser, which can be done by working in closed rooms with

restricted access or by using non-transparent curtains andother materials to separate the

laser workplace from the rest of the room. Using signs informing about the laser hazard,

60


running warning lamps while working and installing interlocks shutting down the laser

in case someone accidentally enters the room are just a few ofthe often compulsory

measures (consult [Lit03] for more conformation).

Next to shielding other people from the laser hazards it is good practice to install further

shielding inside the workspace as (in the authors opinion) just using safety goggles is

not enough, especially as there is no guarantee that they will be worn at all times.

Such shielding inside the workspace includes preventing the laser from reaching beyond

the measurement area, which in case of transparent walls canbe done by masking with

black paper, cardboard or other similar materials. It is further useful to mask regions

where stronger reflections occur, a measure which in addition can lead to improved

camera background noise. The possibilities of masking are limited however as camera,

laser and very often the operators require some minimum optical accessibility to the

measurement region.

In many cases it is also useful to restrict access to the lightsheet / laser beam before the

measurement region, further minimising potential regionsof exposure. In situations

where the lightsheet optics cannot directly be installed onthe laser housing, laser

guiding arms are particularly useful.

In a final note shielding materials should not consist of easily inflammable materials. A

sheet of quality paper (80gm/m2; preferably black) should suffice however don't mark it

with pencils as the graphite will ignite (noticeable by crackling), especially when

irradiated by 1064nm infrared.

4.3.4 Some Theory on Laser Goggles

Laser goggles are designed to protect the eyes from high power laser light occurring at

specific wavelengths. Lasers with different wavelengths and powers thus require

different goggles.

=I transmitted

I impinging

=10−ODEquation (4-3) - Transmission ratio of laser light impingingon a filter as a function of optical density. (based on definitions

found in the photonics dictionary www.photonics.com/dictionary)

The ability of protective eyeware to block incoming laser light is characterised by

61


optical density (OD) which can be converted to a transmission ratio by using Equation

(4-3). A typical OD characteristic for Nd-YAG protective goggles is shown in Figure

4.8, where it can be seen that the critical wavelengths of 1064nm, 532nm (doubling),

355nm (tripling) and 266nm (quadrupling) are shielded by anoptical density of more

than 7. In case of a direct hit with a 6mm diameter 20MW beam (intensity 7.1e11

W/m2) the fraction of 10-7 would pass the goggles, resulting in a remaining intensity of

71 kW/m2 or 71 times the intensity of sunlight on an average sunny day,which sounds a

lot better than the MW/m2 obtained in other examples without goggles.

Despite this apparent increase in safety goggles are in the first place made to shield the

user from disturbing diffusely scattered laser light and toprevent serious eye damage in

case of accidentally caused reflections (diffuse or directed). Goggles are however

absolutely no excuse for not taking proper shielding measures and working in a zone

where parts of the body are directly exposed to high intensity (reflected) laser light!

Figure 4.8 Example filter characteristic for Nd-YAG protective eyewear.(from Glendale product specifications, glendale-laser.com)

In an important note it must be said that next to the strong goggles blocking almost all

laser light, alignment goggles, which have an optical density of just OD5 @ 532nm

(glendale-laser.com product specs), will no longer protect from direct high intensity

laser impact. Several types of goggles are used at the UNSW School of Mechanical

Engineering and users must be aware of the difference. This consists of first confirming

the wavelength range (Nd-YAG, Argon-Ion or He-Ne laser light) and the strength of

attenuation (weak - alignment versus strong).

62


4.3.5 To See Or Not To See: On Handling Laser Goggles

Due to the destructive potential of PIV Nd-YAG lasers as outlined in Chapter 4.3.2

practically all safety regulations require to permanently use protective eye-wear.

It then often comes as a first surprise for novices that many professionals don't seem to

wear goggles at all. This can be observed during conferencesand courses where PIV

manufacturers present their products to potential customers as well as when giving

instructions on how to use their PIV system.

While there are a number of reasons for this the most apparentis that the laser beam

will become completely invisible when wearing fully protective laser goggles. While

there are special adjustment goggles allowing to see the more intense reflections these

goggles won't provide the 'full' safety any more and still are often less convenient for

alignment. Many professionals will argue that seeing a laser beam will be safer than

having no idea where it is. Wearing goggles will also reduce the contrast which is

especially annoying when monitoring instant PIV images on a computer screen.

While it must be stressed that not wearing laser eye protection while using PIV lasers is

a breach of safety regulations the following list nevertheless provides tips on working

safer and productively both with and without goggles:

• Use fluorescent paper to visualise laser light when wearinggoggles. Orange

textmarkers such as the Staedler Textsurfer colour number 4on white paper give

excellent results and even scattered laser light of very lowpower can be detected

with ease. – Excellent when checking for dangerous reflections and to get a better

contrast when aligning.

• Keep your goggles close by when having to take them off (around the neck or

forehead for example; from [Dan00a], Chapter 4.6.2.2).

• Only work at low powers when aligning lasers / lightsheets.

• Only work at low powers when not wearing goggles.

• Always keep the laser power as low as the application allows.

63


4.3.6 Further Tips and Comments on How to Prevent Potential Accidents

• Don't rush – calmly reflect about and plan the next step.

• Be aware of what can pose a threat: Are there reflecting objects? Where is the beam

path? Where will the beam go if I accidentally flip this mirror? ...

• Avoid setting up any laser beam systems on eye level (compare [Dan00a] 4.6.2.2).

• Restrict access.

• Make sure the laser is disabled when working in directly exposed zones. Turn off the

power supply or use at least one mechanical shutter (you can't really do enough).

Restrict other people from being able to turn the laser back on while you are

working.

• Use goggles but don't regard them as an absolute safety guarantee.

• Know your devices. Incorrectly mounting the lightsheet, being unsure how to operate

the laser etc. is dangerous.

• Keep laser powers to the necessary minimum.

4.4 Ekspla Nd-YAG Laser – Operation

4.4.1 Introduction

This section will explain how to operate the Ekspla NL301-2GNd-YAG laser. While

the information can more or less also be 'assembled' from reading through the laser

manual ([Eks97]) it is the aim to give the instructions in a more straightforward way.

Most of this chapter is thus held in a checklist like form.

4.4.2 Connections

• Water: Connect the inflow (upper) and outflow (lower) cooling water supply on the

back panel (Figure 4.10). The inlet temperature should be lower than 20°C although

25°C will still work. Don't worry if the water won't flow oncepressure is applied as

the cooling unit has a valve which opens and closes on demand.

• Mains power: Always keep the laser connected to mains power as it is needed to heat

the KTP crystal which otherwise would dissolve after several days.

64


• Remote: This connects to the control pad.

• Sync Out: This output is used to synchronise with the camera via a delay unit.

• RS232: Connect this to a computer running the control software (Chapter 4.6).

Figure 4.9 Ekspla Laser Front Panel. (adapted from [Eks97], Fig. 6)

4.4.3 Powering Up – Making the Laser Ready

All abbreviations refer to Figure 4.9.

• Turn the power key (K1).

• First turn on the power of the cooling unit (switch S5).

65


• Then set the desired cooling temperature: By default the cooling unit displays the

current water temperature. Pressing the button B7 (Measure/ Set) will cause the

display to show the temperature where the valve of the cooling circuit will open.

This 'valve-trigger-level' temperature can be set with the potentiometer P7.

• Next turn on the power supply (switch S4).

• Set the triggering mode switch S3 to external. The repetition rate and delay

potentiometers P5 and P6 will then be obsolete. This may first sound unlogical but

external in this respect means that the power supply is triggered by the control unit

('Control Unit & HV') and not by the power supply's internal clock. The control and

power supply units can be viewed as two separate devices.

• Set the oscillator (laser 1) flashlamp voltage according tothe desired maximal pulse

energy (Figure 4.11) using potentiometer P3.

• Turn on the amplifier (laser 2) using switch S2 and set the flashlamp voltage using

potentiometer P4.

• Press the button B5 triggering. This activates the high voltage at the pockels cell.

• If there are no error LEDs appearing and all ready / on LEDs arealight the laser is

ready for operation (note that some LED heads lie a bit hiddenpartially inside the

cover but can still be seen).

• Finally don't forget to open the shutter on the laser head or an interlock error will

occur.

4.4.4 Using the Control Pad

Next the the control pad the laser remote control software (Chapter 4.6) can also be

used. The control pad however can be very practical for operations like aligning and

other situations where a computer is too big and too far away.It also allows changing

certain system constants used for maintenance / initial tuning which cannot be

addressed via the RS232 interface.

Using the control pad is not overly intuitive and only a rudimentary explanation

covering just a few of the available options will be given at this place. More details can

be found in the laser manual ([Eks97]).

The control pad has got 4 buttons (Figure 4.9) which work as in Table 4-1:

66


OP Operate. Turns the Q-switch / flashlamps on or off.

Warning: Pressing operate works at all menu levels and will initiate the laser if

triggering is on (Figure 4.9 button B5)!

SEL Select: Changes the position of the cursor (blinking) orworks like an enter key

to switch to the next submenu option (i.e. option within the same menu but on a

new line).

∧ / ∨ Changes the value at the cursor.

SEL+

∧ / ∨

Hold select; then press ∧ or ∨ to switch to a different menu option.

Table 4-1 Control pad controls

Table 4-2 shows a listing of the more useful remote control settings and symbols. These

can be found by browsing through the different menu options by pressing SEL+∧ / ∨

and SEL as explained in Table 4-1.

/ Stop / Operate: Symbol shown in the energy settings menu whenthe

laser is turned on or off. Note that this is not displayed in all other

menus although pressing OP will still turn the laser on and off.

Of/Ad/Ma Laser energy settings: Of – flashlamps will flash but no lasing; Ad –

lasing at ~ 10% energy; Ma – lasing at 100% energy. Note that the

flashlamps will flash for 5s before lasing begins.

DLxx Delay between the two laser flashes in 250ns steps.

E.g: DL00 – no delay; DL100 25µs delay.

F Div XX Frequency division. Ratio of flashlamp flashes toQ-switch operations.

I.e. laser flashing frequency. E.g. F Div 1 – 20Hz; F Div 2 – 10Hz; ...

Special section (Regime 8):

RegimeXX Operating regime; 0 – normal; 8 – laser servicing.

EO ON/OFF Electrooptics on / off. If set to OFF no high voltage will be applied to

the pockels cell. The laser will run in free running mode and lase during

flashlamp flashes as shown in Figure 4.4.

67


O-E1 XX Delay between flashlamp and Q-switch trigger in 250ns units.

Set as follows: O-E1 760 – 'press SEL' – O-E2 760 – 'press SEL'.The

delay is now set up 190µs (~optimal).Table 4-2 Control pad options (incomplete)

Figure 4.10 Ekspla Laser Back Panel (adapted from [Eks97], Fig. 7)

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Figure 4.11 Pulse energy as of December 02, 2004.(Note that this will decay with ageing flashlamps.)

4.5 Ekspla Nd-YAG Laser – Maintenance

4.5.1 Introduction

This chapter informs about performing laser maintenance and troubleshooting

operations on the Ekspla laser. It is mainly restricted to operations that had to be

performed by the author but this probably already covers more than the majority of

typical cases encountered by PIV laser users.

4.5.2 Changing the Cooling Water

The externally supplied cooling water cools an internal cooling circuit consisting of

deionised or distilled water. This internal cooling water flows through the laser head

and gets in direct contact with the flashlamps (and possiblythe laser rod), which is why

it must be of high purity as deposits on those components have a devastating effect.

While the Ekspla manual suggests changing the water every three months ([Eks97]) the

water can probably left in the laser for longer periods of time. Still it should not be

forgotten to change the water.

69

1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 16000

25

50

75

100

125

150Laser 1 (OSC), measuredLaser 2 (AMP), measuredLaser 1, fit (2nd order poly)Laser 2, fit (2nd order poly)

Pulse Energy vs FL Voltage

Flashlamp Voltage, V

Max

imal

Pul

se E

nerg

y, m

J


To change the water the cooling unit has to be removed from thelaser rack by first

removing four screws from the front panel (Figure 4.9) and then pulling out the cooling

unit while carefully watching that none of the four attachedhoses get stuck. The

additional earthing cable might also have to be removed but it is useful to keep the

interlock and power supply cables connected, allowing to test the unit before building it

back into the laser.

Once the cooling unit is outside of the rack it is helpful to remove the top steel cover to

get a better view of the inside. Then the water has to be drained via the drain port

(Figure 4.10) and the cylindrical tank refilled from an inlet on the top. The tank has got

two or three level transducers (switches) mounted on one of the two ends of the cylinder

which is transparent. The water level should be between the lowest and highest

transducer heads. Once the water has been refilled the cooling unit (not the power unit)

should be turned on and the pump activated so that the water can circulate for a while to

remove accumulations of air from the system. It is further useful to check the interior of

the cooling unit for leaks.

4.5.3 Laser Head View

The (twin) laser head as shown in Figure 4.12 consists of two lasers (Laser 1 & 2),

capacitors, a unit distributing the high voltage (HV) and trigger signals, beam

combining optics, a frequency doubling crystal (SH), a dichroic mirror separating the

green from the infrared and a beam dump (BS) for the unused infrared beam.

The Q-switch consists of a high voltage switching/driver part accessible via the small

cover plate on the top part of each laser and the pockels cell /polariser plate accessible

via the small covers located on the side of the laser heads. The flashlamp is contained in

a block called laser chamber (looks a bit like a big cover plate) mounted into each laser

head. The laser chamber is also noticeable by the thick high current cables needed to

drive the flashlamp.

Combining the beams is done as follows: The vertically polarised light from laser 1 is

directed onto the polariser plate P1 using the steering mirrors SM1 and SM2. Polariser

plate P1 will reject (i.e. reflect) all vertically polarised light and thus acts as a mirror for

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laser 1. Before reaching the polariser plate P1 laser 2, which is vertically polarised as

well, is passed through the phase retardation plate PRP1 which turns the polarisation by

90°, now making beam 2 horizontally polarised. The polariser plate P1 won't reject

horizontally polarised light and thus is transparent for laser 2. If the mirrors SM3 and

SM4 are aligned appropriately laser 1 and laser 2 are now coincident. Before the second

harmonic generator SH the laser beam polarisation is converted to circular by phase

retardation plate PRP2 (from [Eks97] Chapter 5.3).

Figure 4.12 Ekspla laser head view. Legend: SM – steering mirror; SH: second harmonic crystal; P: polariser plate;

PRP – phase retardation plate (from [Eks97], Fig. 4)

4.5.4 Changing the Flashlamps

The maximum allowable flashlamp voltage is 1600V. If at thisvoltage the output

energy is less than 100mJ (rough indicator) or if a flashlampdoesn't work at all it is

time to change the flashlamps. Note that usually both flashlamps are changed at the

same time to ensure that both laser heads are in a similar condition.

Due to space restrictions instructions and tips on how to change the flashlamps are

given in Appendix 2.

4.5.5 Visualising Infrared

The primary wavelength of a Nd-YAG laser (1064nm) is in the infrared (IR), invisible

for the human eye. While the predominant part of the infraredis separated by the

dichroic mirror and won't get outside the PIV laser head working in the beam

71


combining region will expose the IR beam. Although IR is invisible it will still

penetrate the human eye and cause serious damage ([Lit03]).Being invisible

additionally makes it more dangerous as there is only little warning of its presence.

Detecting infrared can be done with an infrared detector card which is about the size of

a credit card and contains an IR sensitive strip which will glow greenish when hit by an

IR beam. Care has to be taken not to use the card at high powers as this will burn a hole

in the detector. IR will also ignite graphite from pencils resulting in electric discharge

like crackling sounds.

4.5.6 Cleaning Optical Components

It is important to keep all optical components free from dustand grime / grease as local

heat development around pollutants can damage the optical surface due to the very high

instant laser powers.

If an optical surface does get dirty it must be cleaned. To avoid scratches it is necessary

to first blow the dust away using a clean air spray can (alwayshold them vertically or a

nasty sticky liquid will be expelled as well, or better: use filtered dry nitrogen).

Common pressurised air should not be used as this usually is dirty and can damage the

optics. More stubborn dust can be wiped away using a clean lens brush. The surface can

then be made free of grease and grime using reagent grade isopropyl-alcohol or reagent

grade ethanol and special optical grade wipes. More detailed information on cleaning is

given in reference [Ed05].

When handling light sheet optics it is usually sufficient tojust blow the dust away. Any

grease originating from seeding particles or fingerprints should be removed as well.

4.5.7 Optimising the Beam Overlap and Beam Path

For the lightsheets generated by laser 1 and laser 2 to lie in the same plane the beams

must be coincident. This requires the beams to overlap in thenear, middle and far field

or the two lightsheets will only overlap partially, resulting in poor correlation (weird

looking PIV vectors).

Another requirement is that the beam axis is roughly coaxialwith the optical axis of the

72


lightsheet, which while setting up the laser caused some problems. Initially the beams

were aligned to be coaxial with the laser output hole shown inFigure 4.12.

Unexpectedly the laser bench as used by Stephen Hall ([Hall01a]), which serves as an

interface between laser and lightsheet optics as there are no suitable mounting holes to

directly mount the optics on the laser, turned out to be roughly 4mm off axis with the

laser output hole. As the lightsheet optics described in Chapter 5 was designed to be

able to correct for a beam which is no more than 0.5-1mm off axis, the laser beam axis

had to be corrected inside the laser. The resulting beam axiswas a compromise as the

lower position of the laser beam is limited by the KTP crystal tilt mount.

To align the beam path it is necessary to remove the laser front cover and override the

interlock switch located close to the output (tape the switch down). The infrared beam is

now exposed! Work should only be performed at low powers justabove the lasing

threshold(<1-3mJ) and the laser level (height) kept below eyesight. Note that a small

fraction of the green light will be reflected by the dichroicmirror and can pass

sidewards out of the laser.

The first step is to position beam 1 to the desired location relative to the laser output

hole. This step is not necessary for correction of the beam overlap and it should be

avoided otherwise. Beam 1 can be positioned by using steering mirrors SM1 and SM2

(Figure 4.12). The infrared beam has to be visualised using an infrared detector card.

One should try to keep the beam roughly horizontal to the laser base and in the centre of

SM2. It might be necessary to slightly move the KTP crystal incase it partially blocks

the beam (consult Chapter 4.5.9).

The next step is to get beam 2 coincident with beam 1 by using the steering mirrors

SM3 and SM4. SM3 (and only SM3!) is used to get beam 2 to overlapbeam 1 in the

near field. Then SM4 is used to do the same in the far field, at least 2 meters from the

laser. This step must then be repeated until the beams coincide. Note that if after

correcting the near field the far field is misaligned again and vice versa, slightly

overshooting the target overlap can speed up alignment.

Finally it must be checked that the infrared beam is still directed into the beam dump

73


BS. If necessary the last steering mirror can be used for correction. Note that the

infrared beam will burn objects if not dumped appropriatelyand at higher laser powers

can even leave pits on metal surfaces such as aluminium.

Tips, Hints and Notes:

• Wear safety goggles, especially in the far field where the beam goes free across the

workspace.

• Colour a piece of white paper with an orange textmarker and mark a black dot in it.

Viewing the green beam on the orange colour with the strong laser goggles gives an

excellent contrast and the dot makes a good target. This willalso allow to see ring-

like patterns of the laser beam intensity distribution.

• Use these ring-like patterns occurring across the laser beam as a help for alignment.

• Using the control pad (Chapter 4.4.4) to turn the laser on/off is convenient for this

operation.

• Don't touch any optical components such as the mirrors! If itdoes happen the mirrors

must be cleaned from grease and dust as outlined in Chapter 4.5.6.

4.5.8 Storing the KTP Crystal

If the laser has to be disconnected from the mains power supply for a longer time (more

than a day) it is necessary to remove the KTP crystal from the laser and store it at a dry

place.

The KTP crystal is mounted in a housing (Figure 4.13) which also contains its heater

and an electrical connector. It can be removed from its housing mount shown in Figure

4.14 by using two thumbscrews. Storing the crystal can be done by putting its housing

in a plastic bag which is then put in another plastic bag containing silica gel pads

sucking up moisture. After sealing the plastic bag it is bestto wrap some padded foil or

similar protection around the sensitive crystal.

When mounting the crystal back into the laser again it can be necessary to slightly

correct its alignment as explained in Chapter 4.5.9.

4.5.9 Optimising the KTP-Crystal Alignment

As stated in Chapter 4.2.6 the efficiency in converting infrared to green laser light by

74


using a KTP crystal is dependent of the angle of indicence andfollows a sin2(x)/x

function.

Figure 4.13 KTP housing.Figure 4.14 KTP housing mount / tilt.

The KTP used with the Ekspla laser has a maximal conversion efficiency of 60%

(according to Ekspla) which in case of total misalignment reduces itself to 0%, that is

no green light will be visible. The Ekspla KTP is only sensitive to rotation around the

vertical axis and insensitive to rotation around the horizontal axis. To prevent back

reflections into the laser cavity (would destroy the laser)the crystal must be tilted

forward around the horizontal axis. The meaning of rotating around vertical / horizontal,

which can be a bit confusing, is further illustrated in Figure 4.14 with knob (a) rotating

the crystal around the horizontal and (b) around the vertical axis.

The crystal can be aligned as follows:

• First of all note that using laser goggles will make life difficult. The orange looking

532nm goggles will make it impossible to see the green glow of the detector card and

the greenish looking IR-only protection goggles will not protect against the 532nm

light.

• Shield the side of the laser against reflections from inside(especially the small

fraction of green not passing through the dichroic mirror).

• Don't place the laser on eye level.

75


• In case of strong misalignments such as after strongly changing the path of the beam

or after completely removing the tiltable KTP holder (Figure 4.14) it is best to begin

with loose base screws (don't unscrew the screws though), allowing to move around

the crystal parallel to the laser base plate.

• Make sure the KTP crystal is tilted forward (Figure 4.14 knob(a)) so that no

reflections will get back into the laser.

• Increase the flashlamp voltage until the IR is just visible on the IR detection card; do

not work at higher FL voltages.

• Move the crystal around by hand. Notice the in- and decrease of green light intensity

when rotating it around the vertical axis.

• Roughly find the maxima (qualitatively or quantitatively by using the energy meter).

→ Make sure the entire laser beam cross section passes through the crystal.

• Tighten the tiltable KTP mount to the laser base plate.

• Further optimise the maxima by using the fine adjustment screw (Figure 4.14 knob

(b)) and reading the pulse energy from the energy meter (Chapter 4.5.11).

→ Note that at the low laser powers the pulse energy tends to be unstable. Waiting

for a while will help stabilising this a bit but some engineering judgement of the

maxima's location will still be necessary.

4.5.10 Measuring the High Voltage at the Pockels Cell

As explained in Chapter 4.2.3 applying high voltage to the pockels cell will prevent

population inversion and thus the laser from lasing. Suddenly inverting the high voltage

during the flashlamp pulse will in turn result in a powerful flash.

The reason for measuring the high voltage was because after the laser returned from

repairs in Lithuania it initially only produced very weak flashes as if no high voltage

were applied to the pockels cell. Occasionally however (~ 1 in 100 flashes) a brighter

flash could be observed. To analyse the problem it was necessary to measure the high

voltage and flashlamp pulses.

Figure 4.15 Std. oscilloscope probe converted to a HV antenna.

76


An electric current flowing through a cable will cause a magnetic field around the cable.

Changing currents will cause a change of the magnetic flux which in turn will induce a

current proportional to the time rate of change of this magnetic flux in any cable close

enough to the magnetic field (compare literature on physicssuch as [Her02]). This can

be used to pick up the flashlamp and pockels cell signal usingby a simple antenna

connected to an oscilloscope, without having to make any cumbersome direct

connections to high voltage cables.

Constructing a suitable antenna to pick up the pockels cell high voltage signal at first

wasn't simple as the magnetic field induced by the strong flashlamp current tends to

predominate. However together with some suggestions from Ekspla in Lithuania a

solution was found by using an ordinary oscilloscope probe,which is shielded except

for its needle like tip. Placing this tip close under the pockels cell allowed to pick up an

excellent Q-switch signal. A good place to for the probe was ahex socket of a screw as

shown in Figures 4.16 and 4.17. Note that the tip of the probe had to be covered with

masking tape shown in Figure 4.15 as direct electrical contact with the screw would

have resulted in picking up the flashlamp current again.

Figure 4.16 HV measurement location. Figure 4.17 HV "antenna" installed.

Picking up the flashlamp signal was done by a winding up a wireto a coil and placing it

next to the flashlamp (Figure 4.18), away from the second flashlamp which would have

disturbed the signal. Note that the cable between the end of the wire and the

oscilloscope must be shielded or other disturbing signals will be picked up again.

As the change of the magnetic flux is measured, the flashlampand HV pulse picked up

by the oscilloscope as shown in Figures 4.19 and 4.20 show theunscaled time

derivative of the signals described in Figure 4.6.

77


It is important that the flashlamp and Q-switch (HV) pulse occur at the same time and

same frequency. The initial problem which occurred after the laser was returned from

repairs was that the triggering mode switch (Figure 4.9, S3)was set to internal, which

means that the internal clock in the power supply triggers the flashlamps while the HV

pulse is triggered by the control unit. On the oscilloscope both signals moved relative to

each other and thus never correctly agreed. This revealed that the two clocks are not

synchronised and setting switch S3 (Figure 4.9) solved the problem.

Figure 4.19 HV vs FL pulse, no lasing. Figure 4.20 HV vs FL, lasing.

4.5.11 Measuring the Laser Pulse Energy

To check and / or optimise the laser's performance it may be necessary to measure the

pulse energy. Sensors based on pyroelectric crystals, which build up an electric

potential due to the development of heat when absorbing laser light, are suited to

measure the energy of pulsed lasers such as the Nd-YAG types used for PIV.

78

Figure 4.18 HV and flashlamp measurement setup.


Figure 4.21 Measuring the infrared pulse energy.

A pyroelectric head together with a display from Ophir was used to measure the laser

pulse energy of the green 532nm and infrared 1064nm light. The laser light had to be

attenuated by a special filter to avoid destroying the pyroelectric head. Users should

take care not to mistake the pyroelectric with the photodiode and other heads as these

are not suited for the Nd-YAG laser and would be destroyed. Further information can

be found on Ophir's website (www.ophiropt.com) and in [Oph03].

4.5.12 Optimising the High Voltage Circuit

To get an optimally powerful laser flash it is a requisite that there are no leaks. Having a

leak means that the laser emits infrared light, perhaps evenbelow the conversion

threshold to green light, while the flashlamps are flashingbut without inverting the high

voltage at the pockels cell (laser in 'off' mode).

After fixing the synchronisation problem it was found that the laser in off mode (Q-

switch off) would begin to emit infrared light at flashlamp voltages around 1400V. By

adjusting the HV trim potentiometer shown in Figures 4.22 and 4.23 the high voltage

level was adjusted until no infrared was emitted up to (and including) the maximal

applicable flashlamp voltage of 1600V.

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Figure 4.22 Laser control unit (beneath thepower supply cover).

Figure 4.23 Potentiometer to tune the Q-switch HV signal.

4.5.13 Optimising the Q-Switch Timing

As explained in Chapter 4.2.3 and shown in Figure 4.6 the optimal time to invert the

high voltage at the pockels cell ('switch the Q-switch') is when the population in the

lasing material is maximal and thus the laser energy will be maximal too.

This Q-switch timing, which usually is relative to the beginning of the flashlamp pulse

or flashlamp trigger, can be adjusted and in fact used to change the laser output power

without changing the flashlamp voltage. The difference between the 'Ma' and 'Ad'

setting on the Eskpla laser control pad lies in the Q-switch timing and by using the

RS232 interface controllable via a computer the pulse energy can be adjusted between

10% ('Ad') and 100% ('Ma') in integer steps (Chapter 4.6).

The time between flashlamp trigger and maximum laser power can be programmed into

the Ekspla laser via the control pad (only) by setting the 'O-E1' and 'O-E2' value while

in regime 8 – servicing mode as outlined in Table 4-2. By tuning with the energy meter

(Chapter 4.5.11) it was found that the optimal delay value isaround 190µs (or

760x250ns steps).

4.5.14 Potential Problems and Troubleshooting

There are a number of issues which can keep the Ekspla laser from operating correctly.

This chapter will address some of the encountered problems addressed and additionally

a more complete listing can be found on pages 30 through 32 of the laser manual

[Eks97]. Further troubleshooting support can also be foundby asking the laser

manufacturer ([email protected] – Irmantas Mikulskas).

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– Potential problems –

No lasing but flashlamps flashing:

• The following assumes that the lasing mode is not off and the flashlamp voltage is

above the lasing threshold (but not higher than 1600V).

• Check if infrared is emitted (this can be checked before the KTP crystal using the IR

detection card – Chapter 4.5.5).

→ If IR is emitted and no green light can be seen after the KTP crystal then the

crystal could be misaligned or dissolved due to insufficient or no heating.

→ If no IR is emitted then turn off the electro-optics (no HV at the pockels cell); see

Table 4-2. If still no IR can be seen then first check if the interior of the laser is

still clean, i.e. no leaks occurred. This can either be done by opening the

flashlamp cavity or by illuminating the interior via a flashlamp at the front end of

the laser and viewing with a mirror at the back end of the laser. If there is good

transparency without any dirt it is likely that the laser mirrors are misaligned.

Alignment of the mirrors is a 'complicated' procedure whichrequires a He-Ne

laser and usually has to be done by a professional with the right equipment and

training.

Weak lasing while flashlamps flashing and laser off:

• Most likely the high voltage is not correctly applied to the pockels cell.

→ Check that the EO is on via the control pad (Table 4-2).

→ Check the HV cables and HV / trigger signal unit / box (Figure 4.12). There could

be a bad contact.

• Another possibility could be misalignment of the pockels cell, which would allow a

part of the light to still pass the polariser plate.

→ This is not so simple to solve, contact Ekspla ([email protected]).

Poor beam quality (ugly looking lightsheet):

• Possibly dirt (apart from other damage); consult Chapter 4.5.6 for more information.

• Make sure this is not caused by additional windows. They mustbe clean and the

materials transparent without leaving streaks.

Varying light sheet intensities:

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• This is usually due to thermal stability. Run the flashlampsfor a long enough time

(several minutes) to warm up the laser. It is not necessary toactually lase and

keeping the flashlamps flashing for short 1-3 minute interruptions while just turning

of the Q-switching will further help stabilising.

Interlock:

• There are several: Laser front shutter, at the back of the power supply, when opening

the laser covers, ... .

Error LED's:

• Turning the laser on/off again can help.

Water overheat:

• Adjust the set temperature using button B7 in Figure 4.9.

4.6 Labview Laser Control

4.6.1 Introduction

While the Ekspla laser control pad shown in Figure 4.9 is small and handy, its

compactness is also its greatest disadvantage. Users may tend to find that for laser

maintenance and alignment tasks where the main operation isturning the laser on and

off the control pad allows a great degree of freedom. For PIV use on the other hand the

control pad makes it very difficult to keep track of the different settings such as delay

between lasers and repetition rate which are all hidden under different menus. It also

gives no real feedback in case something doesn't work and it only allows two laser

power settings (10% and 100%).

Another option for controlling the Ekspla laser is by sending serial commands via a

computer's RS232 port. The RS232 commands cover all optionsavailable with the

control pad in regime 0 and additionally allow to set the delay between flashlamp

trigger and Q-switch pulse at levels not just correspondingto 10% and 100% pulse

energy but at freely chooseable integer values between 10 and 100 (%). Although it is

not possible to access any laser servicing options (controlpad regime 8), this is not

overly relevant for normal laser use either.

82


As no prewritten software was available it was decided to write a control interface using

the National Instruments Labview software (www.ni.com). Labview is a graphical

programming environment. This means that contrary to more conventional (or classical)

programming languages where the code is text based, a Labview program, called a vi or

virtual instrument, is written by linking ('wiring') graphical building blocks together.

These building blocks can be almost anything from a simple addition operator to a full

blown sub-vi (i.e. subprogram).

4.6.2 Program Usage / Interface

Starting the program requires opening it in Labview, connecting the computers COM

port to the lasers RS232 interface, verifying the serial port selection (pulldown menu on

the right of the program in Figure 4.24 ) and clicking the run button (file – run).

Figure 4.24 Laser control program interface.

Upon running the program it will first try to connect to the laser by sending a series of

commands and then wait for an answer from the laser. A successful activation of the

laser can be seen from the command history table as shown in the bottom part of Figure

4.24. Failure to connect typically occurs when not turning on the laser or when

83


choosing the wrong COM port setting. Once the laser has been activated for RS232 use

the control pad will no longer work until the laser has been turned on an off again.

Note that despite only few steps being involved users not familiar with Labview may

find it difficult at first to actually get the program running. In this case it can be useful

to ask a more frequent Labview user on how to run a program and on how to change

settings in pulldown menus before running it. Once the program is running however, the

use should be pretty straightforward, similar to most other Windows programs.

On the upper right side of the program the user can set the Q-switch frequency (number

of double flashes per second), which for the camera used during this project is 10Hz.

Right beneath it is the control to set the delay between laser1 and laser 2 in

microseconds.

Note that after every new command or when changing a setting the command history –

shown in the lower half of the program window – gives feedbackover last few

commands sent to the laser together with the laser's response and an interpretation

thereof. This provides the user with information about whether the laser is actually

performing the requested tasks.

In the centre upper part of the program two controls to set thelaser energy from 10% to

100% are located. These controls change the time between flashlamp trigger and Q-

switching, which in turn influences the amount of flashlampenergy actually converted

to laser light. It is good practice to begin at low pulse energies and gradually increase

them until the necessary light intensity is reached. To ensure that the laser will already

lase at the 10% setting, the flashlamp voltage must be chosenhigh enough (at least

30mJ @ 100%). The advantage of these energy controls is that the flash intensities can

be fine tuned from the same computer where the camera image isrecorded, thus

liberating the user from the time-consuming hassle of having to go to the laser power

supply every time a slight adjustment is to be made.

The upper left part of the program hosts a number of command buttons. Clicking 'Laser

1' or 'Laser 2' will start the Q-switching of the respective laser and if the flashlamps

don't flash yet turn on the flashlamps (mind the 5 second delay after turning on the

84


flashlamps). Clicking the 'Laser 1' / 'Laser 2' button againwill stop Q-switching but still

leave the flashlamps on, allowing to maintain more stationary thermal conditions and

thus a more stable beam. The 'Stop Both' button will stop the Q-switching and the

flashlamps of both lasers. Note that the reason why the flashlamps and Q-switches aren't

started independently grew historically due to the available serial commands which

themselves don't accommodate a special command to start the flashlamps.

Finally the 'Request Status' button can be used to get information about any possible

problems such as interlocks keeping the laser from operating. The more common error

replies such as shutter closed are directly interpreted. Some hints on error codes not

encountered while testing the program might be found on page20 of the laser manual

[Eks97].

Clicking quit in the upper right corner will as the name implies quit the program after

sending a stop command to the laser.

4.6.3 Program Structure

Starting point for the program structure and interface was the documentation of the

available RS232 commands as shown in the laser manual pages 18 through 20. For

simplicity and in accordance with good Labview programmingpractices the program is

built on writing simple vi's (virtual instrument, i.e. Labview program) which eventually

are combined in a comparably complex interface vi shown in Figure 4.24. Each vi has

got a user interface and can be run 'independently' (i.e. it might need some of the other

vi's to be able to run). This allows controlling the laser without having to access the

main interface vi as shown in Figure 4.24.

Due to the graphical programming technique printing the Labview program on paper

requires large formats, which seemed unpractical for the report. The documentation of

the vi's as produced by Labview have thus been made available on the web:

http://www.yaroc.ch/unsw/

Alternatively it is also possible to request the vi documentation from the author

([email protected]).

The first vi, called Activate_Ekspla.vi, as its name implies does nothing but activate the

85


laser. Activation success is indicated by a boolean value.

The second vi, called Send_a_Command.vi, is used to send strings to the laser and read

the lasers reply. Any kind of string can be sent so the user is relatively free in

experimenting with commands.

After experimenting with the different commands and the laser's replies, of which the

latter differed a bit from the laser manual, the Ekspla_Serial_commands.vi was written.

Here a set of user-selectable pre-written commands are stored in a pull-down menu and

an interpretation of the more common laser replies is made as well.

Finally all sub-vi's were combined in the main vi named Ekspla_Commander.vi having

the interface shown in Figure 4.24. The graphical depencendies of the different program

components are shown in Figure 4.25.

Figure 4.25 Labview laser control program dependencies.

4.6.4 Effect of the Energy% Setting

Reducing the energy channel 1 / 2 setting adjustable with theLabview laser control

leads to a linear increase in the delay time between flashlamp and Q-switch trigger as

shown in Equation (4-4), which was made by a linear regression of delay time

measurements. The corresponding decay in pulse energy is not exactly linearly related

to the linear increase in time and is also dependent of the applied flashlamp voltage but

according to the Ekspla laser manual the resulting laser pulse energy at the 10% energy

86


setting will be roughly 10% of the laser energy at the 100% setting.

t FL.Qswitch≈190µ s100−E%⋅131µ sEquation (4-4) Approximate delaybetween FL and Q-switch trigger.

87

Chapter 5 - The Light Sheet

5. The Light Sheet

5.1 Introduction

As the lightsheet designed by Stephen Hall ([Hall01a]) and originally used with the

Ekspla laser was no longer available an alternative had to befound. It was first intended

to buy a custom- or commercially made lightsheet from such manufacturers as Dantec,

ILA-Oxford lasers and LaVision, together with a specially made mirror system allowing

to use the lightsheet downwards and sidewards of the laser. All offers turned out to be

too expensive compared with what could be afforded from the available budget and so

another solution had to be found.

It was decided to try and design a suitable lightsheet opticsmade from standard pre-

made parts available from optics suppliers. Because the possibility to get parts

manufactured in workshops was highly limited at the time as many standardised ready-

made parts as possible had to be used.

This chapter will therefore first describe some basic principles in designing a lightsheet,

then address a design based on standard parts from Edmund Optics, and finally discuss

mounting this light sheet to the Ekspla laser by using a mirror based beam redirection

system.

References used for designing the light sheet and this chapter are first of all basic

knowledge on optical lens systems as can be found in literature on basic physics such as

[Her02], [Schä99] and most importantly the Melles Griot Optics Guide ([MelG02]). In

addition the light sheet specifications from LaVision ([LaV03d]) and Dantec ([Dan02]),

which contained some more practical details about their lenses were found to be useful.

Finally some information found in [Raf98] (Chapter 2.3) and[Wer02] (Lecture 3) was

used as well.

88


5.2 Principles in Designing a PIV Lightsheet

5.2.1 Introduction

Light sheet optics consist of a lens system turning a cylindrical beam in to a planar

sheet of light. Due to space restrictions basic lens theory as described in [MelG02] will

not be covered in this report. As it was aimed to keep the descriptions shown in this

report simple, readers having only a basic idea of what a lens does should still be able to

follow.

First of all a number of do's and don'ts when using lenses in a high power laser beam

path will be addressed. Then creating an actual light sheet is described by a succession

of simple lens combining steps.

5.2.2 Lens Material Damage Thresholds

In Appendix 4 it was estimated that the peak power densities (intensities) of the Ekspla

laser will be on the order of 100 MW/cm2. Due to these very high intensities the quality

and durability of the used optical materials becomes very important as wrongly chosen

optical components or poorly manufactured parts can end up being shattered, dimmed,

hazy, that is simply in an unusable condition.

Selecting suitable lenses depends on choosing the right type of glass, glass quality

(purity), type and quality of coatings and on the quality of surface polishing. There are

roughly two parameters determining the damage threshold: The durability of the

(ideally pure) base material itself and any disturbances caused by impurities or surface

roughness. Disturbances can cause self focussing of or absorb laser light, both leading

to local heat development resulting in damage.

A more thorough discussion on selecting suitable lens materials and coatings is made in

Appendix 4 on pages two and three . It was decided to base all lens materials on BK7

glass, combined with an MgF2 coating where available. It was further decided to only

choose lenses of highest quality as obtainable from Thorlabs, Edmund Optics or Melles

Griot.

The MgF2 coating reduces the amount of reflected 532nm laser light per surface from

89


4.3% to 1.5% (compare Appendix 4). Note that it is possible tomake wavelength

specific coatings allowing a transmission ratio of more than 99% per surface but such

lenses usually are made on order only and cost more than on-stock material.

5.2.3 Do's and Don'ts: Back Reflections

Whenever light passes from one surface to another having a finite difference in the

index of refraction a certain fraction of light will be reflected. While the fraction of

reflected light can be reduced by choosing an appropriate surface coating, it will still be

on the order of 1% per surface, which is a lot.

As laser light reflected back into the laser can cause serious damage it is of outmost

importance to arrange the lenses in a way that the incoming laser light will impinge on a

curved and not on a plane surface as shown in Figure 5.1. This will cause the reflected

light to scatter in multiple directions and not straight back into the laser.

Figure 5.1 Avoiding back reflections.(adapted from [Raf98], Fig. 2.21)

5.2.4 Do's and Don'ts: Power Density

Great care has to be taken whenever a lens with a positive focal length leading to a

converging beam is used. Reducing the effective beam cross section will lead to an

increase in intensity (power per area or power density) which can result in the following

problems:

• High beam intensity above the lens damage threshold.

• High beam intensity causing even stronger reflections.

• Increase of power density to the point that the air is ionised, noticeable by the noise

of the resulting electric discharges. This is especially a problem when removing the

diverging cylindrical lens from light sheet optics.

90


Situations where the beam is focussed onto a small spot or strip are shown in Figures

5.2 and 5.3 . It is apparent that the entire beam power will be concentrated at that spot

leading to unacceptably high power densities. Distributing the beam power across a thin

strip of a light sheet as shown in Figure 5.5 in contrast is usually not problematic.

Note that there are several examples of light-sheets suitable for low power cw lasers in

literature (in particular Figures 2.19 and 2.20 of [Raf98])which result in relatively high

power densities when used with Nd-YAG lasers and thus should not be applied.

5.2.5 From Beam to Lightsheet: The Basic Principle

Placing a single, one-dimensional (cylindrical) lens withpositive focal length in the

path of a laser beam gives a converging-diverging beam cross-section of constant width

as shown in Figure 5.2. As stated in Chapter 5.2.4 this will result in high intensities at

the focal point (which is of finite extension) and should be avoided.

Figure 5.2 Converging 1-D lens.

A similar situation as in Figure 5.2 is shown in Figure 5.3 with a 2-D lens of positive

focal length. The main difference is that the converging-diverging beam cross-section

now remains circular and that the intensity at the focal point is even higher.

Figure 5.3 Effect of a 2-D beam converging lens.

91

peak power concentrated in a small strip, intensity potentially a bit too high

converging 1D lens

focal length f

elliptical cross section, of constant width and varying height (thickness)

focussing lens→ converging beam

peak power in a very small area → too high intensity

focal length fcircular cross section,axially varying diameter


Placing a one-dimensional lens with negative focal length in the laser beam path results

in a diverging beam with an elliptical cross section of constant height (thickness) and

increasing width as shown in Figure 5.4. A two-dimensional lens of negative focal

length would also result in a diverging beam but with a circular cross-section of

increasing radius.

Figure 5.4 Effect of a 1-D beam diverging lens.

For three-component PIV applications, where a certain thickness is necessary

(→Chapter 3.3), the simple lens shown in Figure 5.4 can alreadymake a useable light

sheet although it is not possible to vary the thickness. In most applications of two-

component PIV however it is desired to have a thinner light sheet in order to reach

reasonably high intensities. This can be achieved by combining a lens having a positive

focal length with a 1-D cylindrical lens having a negative focal length as shown in

Figures 5.5 and 5.6.

Figure 5.5 Combining a 2-D converging and 1-D diverging lens.

Next to the overall positive focal length f perpendicular tothe light sheet plane and the

92

diverging 1-D lens

elliptical cross-section of constant thickness and increasing width

additional diverging1D lens

peak intensity in a thin but stretched ellipse

converging2-D lens

elliptical cross section of axially varying thickness and width

back focal

length bfl

focal length f


overall negative focal length in the light sheet plane, the back focal length (bfl) is

another important parameter for PIV users. The back focal length specifies the distance

from the last lens in a lens system to the focal point, which isimportant for positioning

the laser and lightsheet optics as it usually is desired to have the focal point in the centre

of the camera image. The positive focal length determines the thickness of the light

sheet, which is important for the intensity distribution, and the negative focal length

determines the light sheet's angle of divergence.

Figure 5.6 Combining a 1-D converging and 1-D diverging lens.

5.2.6 On the Importance of the Light Sheet Intensity Distribution

It is apparent from Figures 5.5 and 5.6 that the resulting light sheet is of varying

thickness and width, resulting in a variation of cross section (area) and thus beam

intensity. The more even the light sheet intensity distribution however, the better the

conditions for a successful PIV correlation analysis as outlined in Chapter 3.7.10.

During the design of the light sheet used in this project it turned out that for the

available sheet angles the intensity variation due to increase in width is a lesser problem

than the variation due the varying sheet thickness.

t LS.waist∞f

dbeambeforelensEquation (5-1) - light sheet thickness proportionality

The thickness of the light sheet at its waist, i.e. at the focal point, is inverse proportional

to the laser beam diameter before the light sheet lens systemand proportional to the

overall focal length f. If the overall focal length turns outto be very small this can result

in a light sheet of rapidly varying axial intensity distribution. Note that having a large

93

diverging 1D lenson the other axis

converging 1D lenson one axis

focal length f

back focal

length bflelliptical cross section of axially varying thickness and width


back focal length (bfl) can still result in an unfavourably short focal length.

5.2.7 From Beam to Lightsheet: A More Advanced System

The focal lengths of the light-sheets shown in Figures 5.5 and 5.6 are fixed, so changing

the focal length would require changing a lens. A fixed focallength also doesn't allow

fine tuning the position of the beam waist except when makinga fixture allowing to

move the single focussing lens or when moving the laser, withboth solutions not being

overly practical. The much more elegant approach commonly made ([LaV03d],

[Dan02]) is to combine a diverging and a converging lens withvariable spacing

between the lenses.

Figure 5.7 Basic variable beam focussing system.

Such a diverging / converging lens system, which resembles an inverted Galileian

telescope, is shown in Figure 5.7 with two different lens spacings s. The closer the two

lenses are together the larger the (back) focal length and for lens systems where the

negative focal length is larger than the positive one it is possible to let the maximum

focal length go up to infinity, allowing to cover a very largefocal length range with just

two different lenses. A major drawback of this system is thata decreasing back focal

length leads to an over-proportional decrease of the resulting focal length, which in turn

results in a disadvantageous intensity distribution. In Figure 5.7 this becomes apparent

94

f2 (>0)f1 (<0) s(a)

f2 (>0)f1 (<0) s(b) (s(b)>s(a))

bfl(a)

f(a)

f(b)

bfl(b)

diverging lens

converging lens

a) small lens spacing → large bfl

b) large lens spacing → small bfl


from the ratio between the focal length f and the back focal length bfl (f/bfl), which in

case b) is considerably smaller than in case a). As this behaviour was especially

pronounced with the beam from the Ekspla laser special measures as discussed in

Chapter 5.3.3 had to be taken.

It is further possible to design a lens system allowing a variable light sheet angle but in

practice is is more common to just use a set of exchangeable cylindrical lenses

([LaV03d], [Dan02]) allowing to change the angle in steps asfine tuning is rarely

needed.

Creating a light sheet of constant width is a slightly more complex task no longer

allowing a small and compact lens system as some sort of big lens or mirror has to be

employed after a lens combination as in Figures 5.5 / 5.6. Such a system has been

designed and successfully applied in the PhD of Stephen Hall([Hall01a], [Hall01b]). In

practice however such constant width systems are rarely seen and most commercially

available PIV systems come with diverging light-sheets.

Note that designing a light sheet system as in Figure 5.7 is possible with 2-D as well as

with 1-D (cylindrical) lenses. Using 1-D lenses will have the advantage that the light

sheet divergence (angle) is independent of the focal lengthdetermining the light sheet

thickness, which among other things simplifies calculations. Varying the spacing

between the lenses in turn will no longer be possible by a simple threaded tube system

as with 2-D lenses, for the 1-D lenses must not be rotated relative to each other.

5.3 Designing a Lightsheet (LS) from Pre-Made Parts

5.3.1 Parts Available from Edmund Optics

Once it was decided to design an own light sheet the products available from the three

manufacturers Edmund Optics, Thorlabs and Melles Griot were evaluated for their

suitability. It was found that all three manufacturers had awide range of lenses which

could be used on optical benches or in custom manufactured lens holders / lens systems.

However manufacturing own lens holders was restricted as the schools workshop had a

strongly limited capacity during the period of the project hence another solution had to

be found. It turned out that Edmund Optics has got a number of modular parts

95


consisting of lens holders, focussing tubes, spacing ringsand other items actually

intended to be used as a lens prototyping system. While Thorlabs and Melles Griot also

offer a number of parts which can potentially be used in a light sheet optics only the

prototyping system from Edmund Optics allowed to constructa complete light sheet

without having to manufacture additional parts.

5.3.2 Mathcad Program for Evaluating Possible LS Configurations

It is possible to design an entire light sheet system by hand using a few simple

equations, roughly estimating a pair of suitable lenses, calculating the optimal lens

spacings and constructing a suitable housing. As pre-made Edmund Optics parts were

used the possibilities to house the different lenses were limited and adaptations were

only possible in relatively broad steps. Furthermore it wasalso found that

experimenting with a lot of different lens variations couldget very time consuming so

first a Mathcad sheet, titled 'Thin Lens Combination Calculator' was written. This

simple Mathcad sheet, given in Appendix 5, allowed to enter adifferent number of

lenses, focal lengths and lens spacings and would automatically plot the resulting beam

path.

Figure 5.8 Example light sheet intensity distribution.

Next to the simple Mathcad sheet a more complete one titled 'Light Sheet Optics

Calculation' given in Appendix 4 was written as well. Here the full range of calculations

96


necessary to design a light sheet were made. Next to calculating the beam path in a

similar fashion to Appendix 5 but now with the full detail of thick lenses, the damage

threshold, the maximal power density, the light sheet thickness and an estimation of the

light sheet intensity distribution are calculated.

Next to the damage threshold calculations, the estimation of the light sheet intensity

distribution (shown in Figure 5.8) was found to be especially useful as it allowed to

compare the quality of light-sheets at similar back focal lengths.

5.3.3 The Resulting Light Sheet

Being limited to pre-manufactured parts it turned out that it making a single lens

combination which would have allowed to work in a back focal length range from

500mm to 3000mm to infinity would in turn have resulted in an rather poor light sheet

intensity distribution at short distances around 500mm from the lens. The focussing

system and situation would have turned out to be similar to Figure 5.8b with the

relatively large beam diameter of the Ekspla laser (6mm compared to the 4.5mm of a

New Wave laser) further contributing to a rather thin beam waist (Equation (5-1)).

Figure 5.9 The applied focussing system.

As many applications were expected to be relatively close tothe laser an additional lens

was added at the beginning of the optics , resulting in an overall focal length which now

closely matched the back focal length, f≈bfl as shown in Figure 5.9 (this is also shown

in the plot on page 9 of Appendix 4). The cost of this measure isthat in its basic

configuration the working distance (i.e. back focal length) is in the limited range of

roughly 450mm-800mm. It is still possible to cover working distances from 450mm-

3000mm-infinity though, but it is necessary to swap a lens, which can be done quite

97

f ≈ bfl

additional converging lensf2 (>0)f1 (<0) s

f3 (>0)

resulting longer focal length


easily as the lenses are mounted in exchangeable threaded tubular lens holders.

The light sheet angle was limited to the only two suitable cylindrical lenses available

from Edmund Optics. At a working distance of 500mm the maximal light sheet width is

roughly on the order of 125mm which might be a bit small for some applications but

still gives a reasonably large field of view considering thedistance is just 500mm from

the laser. Alternatively it is possible to buy a stronger lens from Thorlabs or Melles

Griot but this in turn requires to manufacture a lens holder,not possible at the time the

light sheet was designed.

5.3.4 Mounting the Parts and Using the Final Light Sheet

A drawing of the resulting light-sheet is shown in Figure 5.10. All parts are from the

Edmund Optics catalogue, with further details on the different possible lens

configurations and the resulting light sheet dimensions given in Appendix 6.

Figure 5.10 Modular light sheet optics.

Mounting the lenses was relatively straightforward, with the main tools used being

plastic tweezers to place the lenses into the lens holders and spanner wrenches to

tighten the ring fixing the lenses. Great care was taken not to make the lenses dirty by

wearing gloves and working in a relatively clean environment.

When changing parts of the modular light sheet optics it is advisable to avoid heavier

dirt getting inside the lens holders and on the lenses. Dust can and should be blown

away using clean pressurised air (best: dry nitrogen) from aspray can (don't tilt them to

98


avoid spilling out liquid). Further cleaning instructions are given in reference [Ed05].

A final note when using the lightsheet - there are two critical points:

• The rotationable module, which is a bit unstable (can cause the light sheet to fall). It

is intended for rotating the light sheet, which in the unsensitive range of the focus

can also be done by turning the focussing tube.

• The light sheet focus as after 25mm it will unscrew completely.

5.4 Designing a Beam Redirection Mirror and LS Mount

5.4.1 Introduction

As the Ekspla laser must be mounted horizontal (more or less)it was decided that a

suitable light sheet mount would enable pointing the sheet sidewards and downwards of

the laser as straightahead would be a rare option. This required a design with a mirror to

redirect the beam and allowing to compensate for small misalignments. To avoid having

to manufacture a great number of parts the laser bench designed by Stephen Hall

([Hall01a]) was used as a starting point to mount the light sheet.

5.4.2 Choosing a Suitable Mirror

It may sound surprising but mirrors with standard metallic coatings (even the very high

quality ones) will only reflect around 85%...95% of the incoming 532nm laser light at

45° angle of incidence (compare with Edmund Optics and Melles Griot mirror

datasheets). The remaining 5% to 15% will predominately be absorbed in the base

material, what in case of high power laser light would resultin damage of the mirror

and eventually make the mirror useless as also was noted in [Hall01b].

If the incident light consists of a single wavelength it is possible to optimise the coating

to maximise reflection for that particular wavelength. Forthe current application such a

special laser line mirror reflecting more than 99.8% of the incident laser light was

chosen from Edmund Optics.

99


5.4.3 Mount Design and Alignment

The basis for the light sheet mount and mirror holder are three plates connected together

in a corner like configuration. These plates connect to the laser bench, mirror holder and

light sheet adaptor ring as shown in Figure 5.11.

Figure 5.11 Light sheet mount and beam redirection mirror.

Full assembly and alignment instructions are given in Appendix 7. Basically the

alignment is done by first mounting a tube with two aperturesabout the size of the laser

beam cross section at each end. The beam cross section passing through this tube is then

maximised by optimising the position of the mirror.

5.5 Possible Future Extensions

There are a number of possible extensions and enhancements which could be made with

the light sheet system.:

• By using a different cylindrical lens the light sheet angle (width) could be increased.

This requires manufacturing a lens adaptor as outlined in Part 6 of Appendix 6.

• A more stable version of the light sheet rotation ring (located at the back end of the

light sheet) could be designed and manufactured.

100


• Strictly seen the mirror used to align the light sheet does not suffice as an additional

dimension of translation is necessary. Designing the lightsheet holder so that it can

be translated normal to the plane perpendicular to the laserbeam would solve this

detriment.

• Finally it is also possible to completely redesign the lightsheet so that it can be used

at working distances between 500mm and infinity but as already stated this will

require manufacturing specially adapted lens holders and focussing tubes.

101

Chapter 6 - Camera, Lens and Imaging

6. Camera, Lens and Imaging

6.1 Introduction

Unlike conventional consumer cameras where settings such as focus, aperture, exposure

etc. are set automatically, PIV cameras and lenses require making these settings

manually. Consequently the PIV operator should know and understand some of the

basic issues about camera, lens and imaging systems as addressed in this chapter. The

topic itself is more far more complex than what is covered here and can fill several

books. More interested readers are thus again referred to [Raf98], PIV manuals

([Dan00a], [LaV02b]) and a the vast amount literature available on photography such as

the 'freely' available [Merk90].

The topics addressed includes basic lens principles, properties of CCD chips,

reflections, imaging of particles, and viewing at different angles. An additional

overview on the Redlake Megaplus ES1.0 camera used in this project is given as well.

6.2 Basic Principles

6.2.1 Components of a PIV Lens / Imaging System

Figure 6.1 Imaging a seeding particle.

102

image – lens distance z

0object – lens distance: Z0

image plane(CCD chip, film, ...)

aperture, diameter D

a

Da

bandpass filter(e.g. 532±10nm),

illuminated object(particle in lightsheet)

object plane(lightsheet)

lens system(actually several lenses)

ambient light rejected

532nm laser light passes

obj

ect

scal

e H 0

imag

e sc

ale

h 0field of view(FOV)

y

z

y 0

Y0

depth of focus DOF

lens principal plane


Figure 6.1 shows a schematic of the seeding particle image recording process. Light

reflected from a particle will be collected by a lens and imaged onto the image plane

containing the CCD chip (alternatively CMOS, film, ...). A good quality lens will also

accommodate a variable aperture, which in brief words allows to opt between better

image quality (sharpness, closed aperture) or more light (higher sensitivity, open

aperture) as explained in more detail in Chapter 6.2.5. Additionally a good lens will

allow to mount a filter and often optical bandpass filters mainly allowing 'only' the laser

light to pass are used. In order to get a sharp image the image-lens distance z0 must be

variable and especially when imaging close objects the lensquality and suitability

(macro lens) becomes very important.

Figure 6.2 Example of a quality macro lens.(from www.europe-nikon.com, PC-55-2_8.jpg)

Figure 6.3 Example of a zoom lens.(from www.europe-nikon.com, NZ-35-200.jpg)

An example of a real world lens commonly used for PIV applications is shown in

103


Figure 6.2. Such a lens has got a fixed focal length and thus doesn't allow zooming. In

situations where the camera cannot be moved to adapt to the field of view a zoom lens

allowing to vary the focal length as shown in Figure 6.3 can be very useful.

6.2.2 Optical Bandpass Filter

The working principle of most PIV CCD cameras requires the exposure time of the

second double frame picture to be on the order of 30ms as will be outlined in Chapter

6.3. In comparison the laser flash duration of 5ns is more than six orders of magnitude

lower so any background light occurring during the 30ms exposure time will be

recorded and can deteriorate the resulting image to the point where it is no longer useful

for PIV.

Figure 6.4 Spectral transmission of a 532nm bandpass filter @10nm bandwidth.(adapted from [LaV02b], Fig. 12.2)

To avoid having to work in the dark optical bandpass filters are used which mainly let

the PIV laser wavelength pass. They are characterised by their central wavelength

(532nm for PIV), the 50% bandwidth (typically 10nm or 3nm) and the maximum

transmission at 0° angle of incidence (~65%). A typical spectral transmission plot of a

532nm bandpass filter with 10nm bandwidth is shown in Figure6.4. The bandpass

filters typically have got a shiny mirror like surface on oneside and a less reflective one

on the other. Although they work in both directions Edmund Optics (FAQ on Optics –

www.edmundoptics.com) recommends orienting the mirror like side towards the

104


source.

(Based on [LaV02b] Chapter 12 and [Dan00a] Fig. 4-42 / Chapter 4.7.7.)

6.2.3 Focus

In order to get a sharp image the object has to be in focus, which means that the

relationship between the object position, image position and focal length must follow

Equation (6-1). A camera lens will (and must) allow to vary the image position z0 either

by a manual adjustment ring or automatically (autofocus). For PIV and generally in

technical photography it is more common to focus by hand as the object – camera

distance is usually fixed and conventional auto-focussingmethods often fail and focus

on something else.

1f=

1z0

1Z0

Equation (6-1) Relationship between focal length f, objectposition Z0 and image position z0 ([MelG02], [Her02]).

The range of the image position is an important parameter as this determines how close

the camera can be placed to the object – the larger z0 the closer the object. It is possible

to use extension rings to increase z0 and thus get closer to the object but this decreases

the amount of light collected and will also result in a poorerquality image, especially

when low quality lenses are used. Lenses allowing to focus very close to an object while

still resulting in a good image quality are called macro lenses.

On camera lenses as shown in Figures 6.2 and 6.3 the object – lens distance is marked.

While these markings are only valid for the camera type and model the lens was mainly

designed for and doesn't apply to most PIV cameras, the distances could be recalculated

and / or measured experimentally. Note that in lens systems with multiple lenses (i.e. a

common photographic lens) the lens principal plane (Figure6.1 - reference plane for f,

z0 and Z0) can also lie outside the actual lens system.

6.2.4 Zoom (Scale) and Field of View

By using simple laws of geometric similarity the magnification factor, which by

definition is the ratio between the size of the projection onthe image plane h0 and the

object itself H0, can be written as shown in Equation (6-2).

105


M=z0

Z0

=h0

H 0

Equation (6-2) Lens magnification factor.([MelG02], [Her02], [Raf98]).

The field of view, which is the size of the object plane imagedonto the CCD chip, is

dependent of the magnification factor and the size of the CCD.

Combined with Equation (6-1) (6-2) can be rewritten as Equation (6-3), which shows

that to increase the magnification factor while keeping theobject distance Z0 constant

(zooming in) the focal length f has to be increased. Increasing the magnification factor

(focal length f) will thus decrease the field of view. If in turn one wants to increase the

field of view (zoom out) the focal length has to be decreased.Note that the ability to

focus the image will be limited by the range of the image distance z0.

M=z0

Z0

=h0

H 0

=...=f

Z0− f=

z0

f−1

Equation (6-3) Lens magnification factor continued.

6.2.5 The Effect of Aperture

The aperture influences the following three parameters:

• Amount of light collected: Larger aperture→ more light collected→ lower exposure

time possible → less shaky images.

• Depth of focus: Smaller aperture → increased depth of focus.

• Lens distortion effects: Larger aperture→ lower image quality (strongly dependent

of the lens quality).

In practice it is more common to express the influence of the aperture by using it to

normalise the focal length, resulting in the f-number shown in Equation (6-4).

f #=f

Daperture

Equation (6-4) f-number (normalised focal length)([Raf98], [Her02])

Increasing the aperture diameter will decrease the f-number. Although this is potentially

a bit confusing, introducing the normalisation does make sense as the parameters

influenced by changing the aperture are also inversely influenced by changing the focal

length.

That increasing the aperture (decreasing f#) will increase the amount of light collected is

106


quite straightforward to understand: The larger the opening the more light can get

through. The price however is an decrease in the depth of focus and to some degree in

image quality as will be explained in the next few paragraphs.

Using a lens to collect light will introduce errors which increase with increasing

distance the light ray collected by the lens has got from the optical axis. Closing the

aperture will limit the distance of the light rays from the optical axis and thus increase

the image quality. However such errors can be compensated bylens design and quality

and this should thus be less an issue when using a dedicated state of the art lens.

The depth of focus is the range normal to the object plane where the imaged object will

still be perceived as in focus. Strictly seen anything outside the object plane is out of

focus but the limited resolution of the human eye / CCD chip will make a slightly

unfocused object still be interpreted as sharp.

Figure 6.5 Effect of opening the aperture.(compare with [Raf98] Fig 2.25)

The influence the aperture has on the depth of focus is described in Figure 6.5. An

infinitesimally small point will ideally be projected as a point again, i.e. the rays of light

emitted from the point will intersect in the projected point. If the actual image

(projection) plane lies before or after the ideal projection plane however the rays of

107

point('out' of focus)object plane

Z0

z0

f f

image plane(CCD)

small ('closed')aperture

wide open aperture

ideal point projection

d poin

t

d poin

t

small ('focussed')projection

large ('unfocussed')projection


light won't intersect on the projection plane but be form a spot of finite extent. If this

finite spot is below the resolution of the CCD chip or human eye it won't be

distinguishable from an infinitesimally small point (perfect projection) and thus still

appear in focus. A spot size above the resolvable limit will in turn appear as blurred

and, in case of a very big spot, have such a low intensity (power per area – big area, low

power) that it won't even be noticed.

It is shown in Figure 6.5 that opening the aperture will causethe rays originating from a

point to have a very wide opening angle between the outmost rays when the aperture is

open. Contrary to a small aperture this will result in a very large and blurry looking

spot.

6.3 Properties of CCD Chips used for PIV

6.3.1 CCD Chips used for PIV

In order to record an image it is necessary to measure the spatial distribution of the light

energy impinging on the image plane in a certain period of time (exposure). In most

PIV applications this is done by using a CCD (charge coupled device) chip.

Figure 6.6 One CCD element (pixel).(from [LaV02b] Fig. 11.1; similar to [Raf98] Fig. 2.36)

Figure 6.6 will be used to to explain the basic principle of a CCD in very simple words

(for a more complete explanation consult [Raf98] Chapter 2.6.1). A photon (single

quantity of light) absorbed in the light sensitive region ofa single CCD element can

potentially release an electron (the likelihood of this event is called quantum efficiency

QE). The structure of the CCD chip allows such electrons to bestored as a charge,

108


proportional to the number of absorbed photons or light energy (intensity). This charge

can then be read and used as a measure for the light intensity.By reading the charge the

CCD potential will be reset and a new recording can begin.

Distributing such single CCD elements over an area as shown in Figure 6.7 results in

the CCD chip allowing to measure the discrete planar intensity distribution of light.

Figure 6.7 Arrangement of CCD elements in a PIV camera.(from [LaV02b] Fig. 11.2; compare [Raf98] Chapter 4.2.4 for greater detail)

A CCD chip is read row by row, which takes a considerable amount of time on the

order of 30ms. For PIV to work it must be possible to record to subsequent pictures on

the order ofµs between the two pictures. This can either be achieved by using more

than one CCD chip arranged in a complex system of prisms and shutters redirecting

light, with the main drawback being the prohibitive cost-factor. Luckily a much smarter

way has been found by first exposing the CCD elements and thentransferring their

charge onto separate storage areas located right next to each single element as indicated

in Figure 6.7. After a frame transfer time of less or equal 1µs the light sensitive area can

be exposed again while the stored charge is read out.

The timing associated with the readout of the CCD chip is shown schematically in

Figure 6.8 and will be addressed in further detail in Chapter7. More information can

also be found [Raf98] and PIV manuals such as [LaV02b] and [Dan00a].

109


Figure 6.8 Timing associated with CCD readout and PIV recording.(compare [LaV02b] Fig. 11.2 and [Raf98] Chapter 4.2.4)

6.3.2 CCD Damage (Overexposure)

Overexposing a single CCD element can lead to electrons overflowing onto

neighbouring elements typically filling entire columns as shown in Figure 6.9.

Figure 6.9 Pixel overflow fromreflections in multiphase flow.

Figure 6.10 Resulting permanent CCDdamage.

Figure 6.11 Strong reflections in dimples.above: from own test measurements on dimplesleft: from multiphase measurements at UNSW;

sample kindly provided by A. Litvak

110

time

even

t

readout image 1 readout image 2

time between pictures ∆t

frame transfer

frame transfer

exposure image 1 exposure image 2

laser 1 flash laser 2 flash


Such an overexposure of the CCD chip must not happen as it can lead to permanent

damage noticeable by single white pixels or entire white columns as shown in Figure

6.10. Overexposure is a widely known problem and it is not uncommon to see PIV

manufacturers warning their customers about the phenomenon on the first few pages of

their PIV manuals ([Dan00a], [LaV02b]). According to [LaV02b] the damage can also

happen when the camera is turned off and the laser light causes local heating of the

CCD chip.

Typical sources of overexposure are strong reflections from walls and other objects

such as bubbles in multiphase flow. A safe approach is to gradually increase the light

intensity as far as necessary but no further than to the pointwhere the highest pixel

intensity values get close to saturation (i.e. 1023 counts for the Redlake ES1.0 camera).

It is also recommended ([Dan00a], [LaV02b]) to always put the protection on cap the

camera lens while it is not used or the laser adjusted.

Preventing overexposure is sometimes a bit tricky and care must be taken. In Figure

6.11 for example the laser light intensity was increased during the experiment as an

unexplainable decrease of the recorded intensity was noted. It turned out that this was

due to the excessively applied seeding smoke which attenuated the laser light and once

it cleared resulted in an overexposed image. Luckily the camera still worked fine

afterwards without showing any damage but it could have well turned out otherwise.

6.4 'Safe' Approach to Getting a Picture

The following description is the authors approach to adjustthe laser power relative to

the camera in a safe way. It is a possible but neither a universal nor the only method

which could be applied.

Among the greatest dangers when trying to capture the laser light reflected from

particles, walls and other things is that one keeps increasing the intensity but just can't

get an image. If it turns out that the problem is not the laser intensity but that the picture

isn't displayed on the computer monitor it might be that one has just successfully

destroyed a CCD chip worth several thousand dollars.

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To avoid such a nightmare it is useful to make a systematic approach which ensures that

certain types of errors can be excluded:

• First of all the laser light sheet must be in the plane of interest with the beam waist

(thinnest part of the light sheet) being in the centre of where the picture should be.

→ Only align the light sheet with the protection cap on the camera lens!

• Then a calibration image must be taken (without the laser running).

→ Close the lens aperture (highest f#).

→ Place a calibration target in the region of interest (e.g. piece of paper with

calibration markings; Chapter 6.7).

→ Illuminate the calibration target with a normal lamp (not the laser).

→ Set the camera into video mode so that a live picture can be viewed.

→ Remove the protection cap and carefully increase the exposure time up to the

maximum possible in video mode @20-25 fps (~32ms). The reason for the long

exposure time is to ensure that the laser flash will be captured and not missed.

Reduce the illumination of the target if it already is too bright.

→ Open the lens aperture until you get a nice image.

→ Remove the target; if a wall is in the field of view no seeding is needed, otherwise

seed the area.

→ Make sure the seeding / wall is still visible from the illumination with the lamp.

→ Then turn on the laser at low powers (repeatedly flashing); increase the laser

power until the flash is visible next to the illumination from the normal lamp.

→ Once the flash is visible place the optical 532nm bandpass filter on the lens. Most

light from the normal lamp should now no longer be noticeable.

→ If necessary increase the laser light a bit more until it is clearly noticeable (don't

overexpose, check the pixel intensity values).

→ Now the camera can be switched from continuous live picture (video) mode to

triggered double frame exposure mode and the timing adjusted as described in

Appendix 13.

The above steps ensure that once synchronised correctly in triggered double frame

recording mode the laser light should be visible and the camera won't be overexposed as

long as the laser pulse energy is not changed. In case of having no picture this allows to

112


exclude the possibility of having a too low laser light intensity or a falsely adjusted lens

and instead narrow down errors to other parameters such as synchronisation or missing

seeding.

6.5 Dealing with Reflections

For a good quality PIV picture it is necessary to have a high scattering particle intensity

so that the contrast between particle and background is good. As the CCD chip must not

be overexposed the presence of strong reflections from walls and other objects will limit

the maximum laser power available which can result in a real problem when trying to

get useful images.

There are two main approaches which can be made: a) Reducing the reflected laser light

intensity or b) entirely filtering out the laser light.

Entirely filtering out the laser light can be done when usingfluorescent seeding

particles which absorb the laser light and emit photons at a higher wavelength. By

placing an optical cutoff filter in front of the lens the laser light will be filtered out and

reflections become less a problem. This approach is more common for liquid flows and

is especially useful with two-phase flow where reflectionsfrom bubbles would

otherwise inhibit PIV measurements (compare comments in [Dan00a] Chapter 4.7.7).

Reducing the reflected laser light intensity is a more difficult task. Possible approaches

include viewing from a different position, using matt blackpaint on the reflecting

surface, placing black cardboard behind reflecting acrylic and painting the reflecting

surfaces with fluorescent paint reflecting at a wavelengthfiltered out by the optical

bandpass filter ([LaV03c] and [Dan00a]).

6.6 Imaging Particles

When recording seeding particles two things are of interest: The intensity and the

recorded particle diameter in pixels. For the small size range of commonly used

particles both parameters follow more complex laws which will be not addressed in

great detail but described phenomenologically (refer to [Raf98] Chapter 2.4 for more

information).

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As a rule of thumb the light intensity scattered from particles will increase with the

particle diameter ([Käh04] Chapter 2.1). The intensity distribution around the particle

itself varies with the viewing angle in patterns similar to the one shown in Figure 6.12.

This variation of intensity with viewing angle can be observed by eye when watching

light scattered from particles in a light sheet.

Figure 6.12 Intensity distribution of laser light scattered from a 1µm particle.(from [Käh04] Fig. 2.2)

Getting an optimal imaged particle diameter is of importance to get accurate results as

was explained in Chapter 3 and optimally a particle will end up 2-3 pixels in size.

Due to the small size of the seeding particles the imaged diameter will no longer be

linearly related to the actual particle size according to the magnification factor M, but

be dependent of non-linear diffraction effects influencedby a number of parameters

(consult [Raf98] Chapter 2.4). This means that the projection of the particle will not

look like the particle itself but form a so-called Airy pattern as shown in Figure 6.13.

Figure 6.13 Airy pattern of light scattered from a particle.(from [Her02] Fig. 6-78)

Although the theory needed to calculate the effect of diffraction when imaging seeding

particles is quite complex, simplified approximations allowing to estimate the resulting

114


particle diameter on the image exist. Such approximations as shown in reference

[Raf98] Chapter 2.4 (and also partly in [Dan00a] Chapter 4.7.2.5) were used to

calculate the pixel size data shown in Figures 6.14 and 6.15,which suffice to make an

initial pixel diameter estimation for a broad number of cases.

The particle image diameter is mainly dependent of three parameters: The f-number f#,

the magnification factor M and the particle diameter dp.

It can be seen from both graphs that decreasing the magnification factor (increases the

field of view) results in a transition from an originally linear decrease proportional to

the magnification factor to an asymptotic decrease of the imaged diameter until it

reaches a limit like value whereafter further decreasing the magnification factor will

have no effect.

Most PIV cameras use a CCD chip sized close to 10mm (~9.1-9.2 mm for the Redlake

ES1.0 used in this report - [Red 02]). For common field of views between 50mm and

100mm (and larger) this will result in magnification factors ranged between 0.1 and 0.2.

The typical pixel size is around 10µm. As outlined in Chapter 3.5.5 it is desired that the

particle on the image is at least 1.5 pixels wide, with best results occurring between 2

and 3 pixels.

Air flows rarely tolerate having a particle size greater than 5µm and it can be seen from

Figure 6.15 that for common magnification factors increasing the particle size from

1µm to 5µm has got little effect on the particle image size (the intensity however will

increase quite substantially).

A far greater effect is obtained by increasing the f-number as shown in Figure 6.14 for a

1µm particle. Closing the aperture down to a resulting f-number of 16 will increase the

image diameter into the desired pixel range. As explained inChapter 6.2.5 the cost of

doing this will be a substantial loss of particle intensity which has to be compensated by

either increasing the laser power or the particle diameter.

115


Figure 6.14 Influence of the f-number on the image diameter for a 1mm particle.

Figure 6.15 Influence of the seeding particle size for an f# of 5.6 .(when changing the f# simply shift the curves along the y-axis to the same position as in Figure 6.14)

116

0.01 0.1 10

0.5

1

1.5

2

2.5

3Influence of f#

magnification factor M

imag

e di

amet

erin

10 µ

m u

nits

⇒ ≈

pix

els

f#=1

f#=2

f#=4

f#=5.6

f#=8

f#=11

f#=16

common magnification range desired imagediameter range

0.01 0.1 10

0.5

1

1.5

2

2.5

3Influence of particle size

magnification factor M

0.5 & 1 µm(very smalldifference)

5 µm

10 µm

20 µm

30 µm50 µm100 µm

imag

e di

amet

erin

10 µ

m u

nits

⇒ ≈

pix

els

(f#=5.6)

common magnification range

desi

red

imag

edi

amet

er r

ange

f# ↑

f# ↓


Another trick following the same logic as shown in Figure 6.5is to slightly unfocus the

image resulting in a blurring of particles, thus widening their image diameter ([LaV02a]

Chapter 4.7). The cost is again a decrease of intensity (the same amount of light is

distributed over a larger area) which has to be compensated by increasing the laser

power. Note that artificial blurring after the image has been recorded can increase and

broaden the correlation peak as well but the blurred single-pixel particles will still be

biased towards that particular pixel.

6.7 Calibrating the Image

The (digital) PIV evaluation will first evaluate the displacements in pixels which

without further scaling results in pixels per second. In order to to get a real world

velocity it is necessary to have a scaling function which relates pixels to coordinates.

Image distortion is caused by lens effects (barrel, cushion), viewing at an angle and

windows (viewing through a curved wall for example), with some examples shown in

Figure 6.16.

Figure 6.16 Possible image distortions.(from [LaV02a], Fig. 4.2)

In case of an undistorted image the calibration can be as simple as taking an image of a

ruler placed in the field of view. No image will be entirely free from distortion however

and as soon as the degree of distortion gets too high it is necessary take an image of

some sort of grid placed in the field of view.

By locating the points on the grid and associating them with distinct coordinates it is

possible to generate a mapping algorithm working as indicated in Figure 6.17 (more

information on some simple mapping algorithms can be found in [Raf98] Chapter

7.1.2). More powerful PIV software is capable of (semi-) automatically detecting the

117


points on a calibration target and associating them with coordinates with further

information available in the respective manuals ([LaV02a], [Dan00a]).

Figure 6.17 The process of dewarping a distorted image.(adapted from [LaV02a] Chapter 12.2 together with [Raf98] Fig. 7.2)

6.8 Viewing at an Angle – The Scheimplug Correction

Lens systems for photography are more of less exclusively designed for perpendicular

viewing. In situations where access is limited or the view would be obstructed it can

become necessary to place the camera at an angle to the regionof interest as is the case

when studying the flow inside the concave dimple cavity.

As cameras have got a finite aperture the depth of field (DOF)where the image will be

in focus is finite as well, so when viewing at an angle it can occur that the limited depth

of focus will make it impossible to cover the entire field of view as shown in Figure

6.18a.

To get the image back in focus again it is necessary to tilt theimage recording plane

(CCD chip) relative to the optical axis of the lens. Not only will the CCD chip be tilted

in case of ideal alignment, but as shown in Figure 6.18b it will also be off-centred

relative to the optical axis of the lens.

While it is possible to calculate the ideal position of lens and CCD chip ([Merk96],

[Dan00b]), the adjustment can also be done empirically as described in [LaV02a]. The

first step is to position the camera and roughly focus on the centre of the field of interest

while viewing a live picture. The next step is to tilt the CCD plane relative to the lens as

indicated in Figure 6.18b (the camera and lens must be mounted on a special

Scheimpflug adaptor to allow this). Tilting will require some refocussing and

repositioning of the camera and lens (note that the lens mustbe translated relative to the

118


CCD as well). Further opening the aperture will decrease thefield of view and increase

the sensitivity of this iterative procedure.

Figure 6.18 Scheimpflug correction when viewing at an angle α.(DOF: depth of focus, FOV: field of view)

While the Scheimpflug correction will give a sharp image it is important to know that

viewing at an angle will increase the perspective error introduced by particles moving in

the third velocity direction as described in Chapter 3.7.2.The Scheimpflug correction is

also commonly applied in stereoscopic PIV where two camerasare positioned at an

angle (the aforementioned perspective error can then be corrected).

Another effect of viewing at an angle is that the image will appear distorted whether it

is in focus or not, which must be taken into account as described in Chapter 6.7.

119

FO

V @

αDOF

part of image in focus @ angle α

a) No Correction: b) Scheimpflug Correction:

FO

V @

0°

part of image in focus @ 0°

α α'plane of focus'

3x intersection point

corrected CCDlocation


6.9 Using the Redlake Megaplus ES1.0 Camera

6.9.1 General Overview

The Redlake Megaplus ES1.0 Camera used in this project captures 10-bit pictures at a

resolution of 1 megapixels with a minimal time between the double frame exposure of

at least 0.2 µs.

A frame grabber controlled by a software called Epix is used to capture images from the

camera. When grabbing a picture the framegrabber takes whatever is available in the

camera output buffer and if the camera places no data in the output a time-out message

will occur after a certain period of time. It is important to understand that the camera

recording process works somewhat independently of the picture grabbing process. The

camera will read pictures from the CCD chip and make them available for the

framegrabber. The recording of the proceeding image will goon independently of

whether the framegrabber will read the image previously made available or not.

In the PIV double frame recording mode the camera is triggered via the trigger input

located on the back of the camera. It is possible to fine-tunethe synchronisation by

changing internal camera parameters as will be outlined in Chapter 7.

As most industrial CCD-cameras the Redlake PIV camera is also fitted with a c-mount

thread to connect to a lens. In order to connect a Nikon f-mount type lens an appropriate

adaptor is needed.

Details on using the frame grabber software to 'grab' an image from the camera are

given in Appendix 13 with further information on the camera available in the manual

[Red01].

6.9.2 Mounting on the ILA Scheimpflug Adaptor

As the only Scheimpflug mounts available during the projectwere designed for a PCO

camera a spacer plate allowing to mount the Redlake ES1.0 on the same Scheimpflug

adaptor was designed. Additionally another flange allowing to mount the Scheimpflug

device on a Dantec traverse system was made as well. Both drawings are available on

the web as Appendix 8.

120

Chapter 7 - Synchronising the Components

7. Synchronising the Components

7.1 Introduction

When recording double frame single exposure PIV pictures the time separation between

the two frames and laser flashes can get into the sub-microsecond range. For the

recording process to work the camera must switch from recording one frame to the next

one at exactly the right time or otherwise it can occur that both laser flashes will end up

on one single frame with the other frame not being exposed at all.

Each single event such as a flashlamp flash, laser flash (Q-switch), recording of frame

1, recording of frame 2 etc. is triggered by a trigger signal either generated internally in

the device itself or supplied externally. Different trigger events allow a different degree

of control: While most events are hardware specific and can't be user controlled at all

several can be related to another internal event by a software settable timing delay and

some can be controlled by applying an external user supplied TTL trigger signal.

The process of synchronisation is to control the different trigger events so that they

occur at the right time relative to each other.

7.2 Triggered Camera Control

7.2.1 Redlake ES1.0

Figure 7.1 shows the events occurring when recording doubleframes with the Redlake

ES1.0 camera.

The process of a double frame recording event is started by the positive (or negative)

flank of a user supplied trigger input signal at the back of the camera. It is important

that this input signal occurs at a fixed and known time spacing of at least 21µs but less

than 1019µs before the laser 1 flash. The beginning of the transition from the exposure

of image 1, which should capture the first laser flash, to theexposure of image 2 can be

set by the transfer pulse delay parameter (TPD). During the transition from exposure 1

to exposure 2 the image data is transferred to the masked storage area of the CCD chip

in the time called transfer pulse width (TPW) as explained inChapter 6.3. Then the next

121


image, which should capture laser flash 2, is exposed for thefixed time of 33ms needed

to make the image data available for the frame grabber. Once both images have been

obtained the next trigger signal can be sent to the camera.

Figure 7.1 Events and timing - Redlake ES1.0 camera.

It may first sound surprising that the transfer pulse width TPW is held variable instead

of just setting it to the lowest possible value, minimising the delay between the two

exposures. The author assumes that the reason for this is thesame as mentioned in the

PCO Camware manual ([PCO03a]), where it is stated that decreasing this time will

increase the effect of blooming, i.e. result in a noisier image.

As both TPD and TPW values can be set manually the camera offers quite a great range

for fine tuning the synchronisation. After some experimentation it was found that

Equation (7-2) offers a useful estimation of the TPD value but requires that the delay

between trigger signal and laser flash has been measured within reasonable accuracy.

An estimation for the TPW value is shown in Equation (7-3). Itis based on the

experimental observation that a time of 5µs between laser flash and transition to the

next image is ample for a long enough time between flashes∆t. Once∆t falls below 15

µs the events laser flash and transition to the next exposure are distributed

122

time

even

t

frame grabber readout image 1frame grabberreadout image 2

TPW

frame transfer

exposure image 1

33ms, fixed

laser 1flash

laser 2flash

20µs

external(user) trigger

TPD

exposure image 2

(fixed)

ttrig-flash time between pictures ∆t

time between trigger and laser 1 flash

TPD: 1, 2, ..., 999 µsTPW: 0.2, 0.3, ..., 5 µsSoftware settable by the user.


symmetrically, which requires adapting the TPW value. It can be that further fine tuning

is necessary after ∆t is reduced below 3µs.

t trig.flash−20µ s TPD 1019µ sEquation (7-1) – Transfer pulse delay range

TPD∈1,2, ...,999

TPD≈ t trig.flash−20µ s−TPW Equation (7-2) – Transfer pulse delay estimator

TPW= 5µ s for t≥15µ sTPW≈ t /3 for t15µ s

Equation (7-3)Transfer pulse width estimator

7.2.2 PCO Sensicam Timing for Comparison

The main difference between the PCO Sensicam as shown in Figure 7.2 and the

Redlake ES1.0 as shown in Figure 7.1 is that the PCO timing of exposure 1 is entirely

set by the duration of the external user trigger, which thus replaces the software settable

TPD of the Redlake.

Figure 7.2 Events and timing - PCO Sensicam camera.

In situations where a powerful external trigger device is available this can be seen as a

good thing as it allows great control not just over the initiation but also over the

duration of exposure 1. For setups not accommodating such a powerful external trigger,

as the case in this report where a simple function generator is used, being deprived of an

123

time

even

t

frame grabber readout image 1frame grabberreadout image 2

DTframe transfer

ttrig

exposure image 1

fixed, < 121.5ms

laser 1flash

laser 2flash

external(user) trigger exposure image 2

ttrig-flash time between pictures ∆t

time between trigger and laser 1 flash

DT: dead time 200ns or 1µs


additional timing feature such as TPD results in a substantial loss of (timing) resolution,

which is a major handicap and greatly limits the control and synchronisation

possibilities.

7.3 PIV Laser and Camera Synchronisation

7.3.1 Introduction

Now that the camera trigger scheme has been described ways togenerate the camera

input trigger signal in the right phase relative to the laser flashes are being discussed.

7.3.2 The Common Approach

The common approach usually found in commercial PIV systemsis to use a

synchronisation unit generating the trigger signals for the laser flashlamps, laser Q-

switches and camera. This way all PIV components are controlled from one central unit

which minimises the timing uncertainty propagation (jitter) introduced when one unit

starts triggering the next one.

7.3.3 Ekspla Laser

A first limiting feature of the Eskpla laser is that it doesn't allow triggering of

flashlamps and Q-switches from an external device other than its own control hardware.

There is an option however to apply an external synchronisation signal at 20±0.1 Hz

allowing to get the laser to operate in phase with other devices.

As no real synchronisation unit was available except for a function generator a

synchronisation output signal from the laser had to be used to trigger the camera. This

required studying the laser sync-out signals as described in the following Chapter 7.4.

7.4 Laser Sync-Out Signals

7.4.1 Introduction

The Redlake ES 1.0 cameras can be triggered by positive or negative TTL signals

having a signal level as shown in Figure 7.3. If possible the ability to trigger the camera

directly from the laser without having to use any further devices would be preferred.

Alternatively a function generator used by Stephen Hall ([Hall01a]) having a trigger

124


input which, triggered by the laser, could be used to initiate the generation of a special

trigger signal for the camera.

Figure 7.3 TTL trigger levels - input and output.(from [Kup03]: http://www.allaboutcircuits.com/vol_4/chpt_3/11.html)

An acceptable camera trigger input signal should have a correct TTL level (Figure 7.3)

and be consistent in time (i.e. constant delay relative to the first laser flash, low jitter).

7.4.2 Measurement Setup

For the first evaluations the signals from the laser were directly fed into an oscilloscope

(terminated with a 50Ω resistor) and compared with the laser flash picked up by a

Thorlabs DET210 fast photodiode, schematically shown in Figure 7.4.

Figure 7.4 Signal testing measurement setup.(some images are from [Eks97] and [Tek95])

Although not absolutely necessary the 50Ω terminator was used to damp out any high

125


frequency oscillations. The terminator was rarely used on the photodiode side though as

it would limit the width of its reaction to nanoseconds, which made it impossible for the

oscilloscope to resolve the flash when viewing over a several hundred microseconds

wide window.

To prevent damage to the fast photodiode from the high power laser it was attenuated

using a few sheets of white 80 gm/m2 printer paper.

7.4.3 Laser Back Panel Outputs

Three (four) BNC connecters located on the back panel were found to be of interest to

pick up a suitable trigger signal. These are the high voltagesync 1 and sync 2 outputs

(sync 2 was used in the previous work from [Iv02]) and the two dedicated sync out and

delayed sync out outputs (compare Figure 4.10).

The high voltage sync outputs are difficult to access as a t-piece would be necessary to

share the signal with the laser. It was decided to first focuson the other options and

after the front panel sync out appeared to be almost in phase with the laser 1 flash the

HV syncs were no longer regarded.

Both the sync out and the delayed sync out appeared to give thesame signal, although

the author surmises that the delayed sync out is related to laser 2 while the other is to

laser 1. From the measurements the back panel sync out signals were found to have the

following properties:

• When terminated with 50Ω a very nice 'rectangular' trigger shape is generated.

• The peak level when terminated with 50Ω is at 8 volts, which is outside the valid

TTL range (Figure 7.3) and can't directly be used.

• The signal is related to the flashlamp trigger and occurs about 300µs before the first

laser flash @100% energy.

→ Directly triggering the camera is possible within this timerange as discussed in

Chapter 7.2.

→ Changes in the energy % affect the timing, which is inconvenient if not

completely undesired.

• Regardless of the Q-switch frequency (1, 2, 4, 5, 10, 20 Hz) the frequency of the

126


back panel sync signal is at a constant 20Hz.

→ If the camera ignores the obsolete signals while being busy with reading the

image data to memory (what it didn't) this would not be a problem.

As apart from the level, which is addressed in the following Chapter 7.5, the signal

allows to directly trigger the camera without needing a further timing unit, it was

decided to further pursuit this option.

7.4.4 Laser Front Panel Sync Out

The trigger output on the laser front panel turned out to havea number of good

properties but one major disadvantage:

• The signal occurs 290ns before the laser 1 flash at the Q-switch and not the

flashlamp frequency.

→ 290ns is too short to be able to directly trigger the camera which requires the

signal to occur between 21µs and 1019µs before the the laser 1 flash.

• Terminated with 50Ω the trigger level is about 3.5V with a reasonably consistent

flank.

→ The TTL signal level is in the acceptable range (Figure 7.3).

7.5 Trigger Level Conversion, Back Panel Sync Out

7.5.1 Introduction

As it was desired to try and trigger the camera directly from the laser without a further

timing function, efforts were made to be able to use the back panel sync signal for this

purpose.

Due to the too high signal level of 8V a level converter generating a signal at a TTL

conform level had to be built. This was then used to directly trigger the camera and

applied in PIV measurements.

7.5.2 TTL Trigger Level Conversion

First it was intended to make a simple voltage divider using two resistors at the ratio

3:5. Thanks to the warning input of George Otvos however the author realised that the

TTL signal architecture does not allow such a trivial approach, which led to reviewing

127


literature on TTL logic ([Kup03]).

It is shown in Figure 7.5 that the level of a TTL input logic is low when it is closed (e.g.

bridged with a wire) and high when left floating (removing the wire). When placing a

resistor R over the TTL input connections a current will flowthrough the resistor and

result in a voltage U, which then defines the state of the input according to Figure 7.3.

Figure 7.5 TTL levels depending on the situation at the input.

As a consequence using a voltage divider made of resistors asshown in Figure 7.6 will

result in a trigger input level which is not clearly defined and for likely values of R2

would always be high regardless of the actual signal level.

Figure 7.6 TTL when using a voltagedivider.

Figure 7.7 TTL level conversion using atransistor (inverted logic!).

A simple solution found in reference [Kup03] (Vol. 4, Chapter 3, page 11) is shown in

Figure 7.7. If the input signal is high (e.g. 8V) the transistor will switch and TTL trigger

input situation will correspond to Figure 7.5a, giving a lowlevel. A low input level (i.e.

0V) won't switch the transistor and the TTL trigger input will be left floating, resulting

in a high level as in Figure 7.5b.

The consequence of this inverting TTL level converter is that the trigger device must

also accept a negative trigger signal. This is the case with the Redlake ES1.0 PIV

camera but not with the function generator which is also available.

Consequently a level converter box containing a circuit corresponding to the one shown

128

ITTL

ITTL

ITTL

U=0V Trigger Input:low when closed

ITTL

=0A Trigger Input:high when left floating

ITTL

ITTL

ITTL

U=R·ITTL

Trigger Input:high, low orundefined depending on R

R

a) b) c)

TriggerSignal

ITTLI

TTL

ITTL

R1

R2 U=I

TTL·R

2

Trigger Input:never clearlylow

TriggerSignal

Trigger Input:High: if floatingLow: if transistor

is switched

R


in Figure 7.7 was built and applied.

7.5.3 Directly Triggering the Camera from the Back Panel Sync Out

Using the level converter circuit (Figure 7.7) set up as shown in Figure 7.8 resulted in

an inverted signal at the correct level successfully triggering the camera (Figure 7.9).

Figure 7.8 Setup when directly triggering the camera.

Figure 7.9 Plot of the signal before and after level conversion / inversion.

The big drawback turned out to be the 20Hz trigger signal frequency which is more than

the camera can handle (10Hz). While some pairs of images in a recording sequence

would to match each other, a substantial number of odd pictures would occur as well.

These odd pictures, which sometimes were double exposed andnever matched any

other pictures, tended to disturb the sequence of good pictures so much that a long and

tedious hand sorting would be necessary to sort out the pictures suited for PIV analysis,

which is undesirable. Consequently it was decided to focus on using the front panel

129


synchronisation signal, despite this needing another timing unit (function generator).

7.6 The Final Camera Synchronisation Method

7.6.1 The Trigger Signal Generation

Eventually the setup shown in Figure 7.10 was found to produce stable trigger signals at

a convenient time of 144µs (Figure 7.11) before the first laser flash.

The beginning of a trigger signal lies in the laser front panel sync out occurring 290ns

before the first double flash. As this is too short for the camera to react this signal is

used to trigger the function generator which generates a waveform having one single

rectangular trigger signal peak occurring 144µs before the next double flash (Figures

7.11 and 7.12).

Figure 7.10 Camera trigger signal generation.

Figure 7.11 Trigger signal recordings. Figure 7.12 Trigger signal delay.

130


The maximal waveform resolution of 16384 points or roughly 6µs for a signal duration

of 0.1s (10Hz frequency) made placing the trigger event at a suitable time before the

flash relatively simple. Note that this is only true for the Redlake ES1.0 camera. For a

camera like the PCO Sensicam where the entire exposure of thefirst image is controlled

by the duration of the trigger signal (Chapter 7.2) the resolution of 6µs is very poor,

particularly when trying to optimise delays for faster flows (>~5m/s depending on the

image scale).

Another option to adjust the occurrence of the trigger signal is by changing the

frequency of the function generator. This was done to adapt the first waveform

generated as reprogramming the waveform was found to be far more time consuming.

7.6.2 Setting up the Function Generator

As long as no settings within the laser are changed, most notably the EO-delays

accessible in the laser servicing regime ([Eks97] & Chapter4.5.13), all that has to be

done is to restore / recall the function generator settings (Figure 7.13).

Figure 7.13 Function generator settings.

The waveform was stored as function USR1. Instructions on how to reprogram the

waveform and on how to use the function generator are given in Appendix 9.

131

Chapter 8 - Seeding

8. Seeding

8.1 Introduction

Generation and distribution of appropriate light scattering particles in the flow is one of

the often forgotten and underestimated critical issues in PIV. Seeding particles have to

be introduced to the flow without significantly altering it, should faithfully follow the

flow path, have scattering properties allowing the generation of good images and must

not adversely affect the health of the people involved in dealing with them. While

achieving this is usually not a big deal in liquid flows, introducing suitable seeding

particles into air flows can cause serious difficulties anddelays in the progress of the

experiments.

A part of this project was to test, compare and discuss different types of seeding

particles. Next to the discussion of those results this chapter will also give a more

general overview of seeding particles as this is necessary to draw proper conclusions

from the observations and give recommendations.

Seeding is a complex issue and only a small part of the topic will be covered here.

Among the literature available references [Dan00a] (Chapter 4.5), [Raf98] (Chapter

2.1), [Mey01], [Mel97] and [Käh02] were the most extensively used.

8.2 Seeding Particle Requirements

8.2.1 Scattering Intensity and Resulting Image Size

In Chapter 6.6 it was discussed that the seeding particle size has got some influence on

the resulting size of the particle on the image. For most situations where the particle

size is less than 10µm the effect of the particle size on the image intensity of the

scattered light is far more important than its effect on the particle image size.

Next to the particle size the viewing angle (compare Figure 6.12) and particle index of

refraction further influence the resulting intensity. In reference [Mel97] the scattering

cross section Cs, being the ratio of the total scattered power to the incidentintensity,

was presented as a convenient measure for a particle's scattering capability.

132

Chapter 8 - Seeding

The most drastic change in the intensity of scattered light relevant for PIV occurs

around particle diameters of 1µm. As shown in Table 8-1 the jump in scattered intensity

from a 1µm to a 10µm particle is on the order of 103. Despite this apparent pitfall of

small diameters seeding airflows with 1µm sized particles is very common and feasible

in PIV applications ([Mel97], [Raf98]).

Particle diameter Typical scattering cross section Change of cross section

dp = 1µm Cs ≈10-12 m2 Cs ∞ (dp/λ)4

dp = 10µm Cs ≈10-9 m2 Cs ∞ (dp/λ)2

Table 8-1 Influence of the particle diameter on the scattered light intensity.(adapted from [Mel97] Table 1; λ is the wavelength i.e. 532nm for Nd-YAG)

For particles with diameters larger than 10µm the scattering intensity is roughly

proportional to the diameter squared and thus investigations in water flows where

particle sizes are on the order of 50µm can often make use of low power cw lasers (2-

5W continuous) for illumination ([Mel97] Table 4).

8.2.2 Ability to Follow the Flow

The size (diameter dp) and ratio of particle to flow density are the two most important

parameters characterising a particles ability to follow the flow.

Liquid flows have the huge advantage of a high density often closely matching that of

several seeding particle materials (such as polyamide power). This allows using larger

particle diameters sized around 20µm – 100µm without sacrificing the accuracy of a

particle's path. Air flows in contrast suffer from the low gas density which usually is on

the order of 102-103 lower than the seeding particle density. To compensate for this

density difference it is common for airflow investigationsto use rather small particles

sized on the order of 1µm-2µm, which in turn requires far higher light intensities.

A more quantitative estimation of a particle's deviance from the flow can be made by

solving the equation of motion for an idealised spherical particle. A good description

and discussion of this is given in references [Mey01] and [Mel97]. Though often not

relevant it is nevertheless important not to forget the effect of gravity on particles when

dealing with low velocity flows ([Raf98], Chapter 2.1.1).

133

Chapter 8 - Seeding

8.2.3 Particle Distribution

For optimal PIV results the seeding particles should be distributed homogeneously over

the entire viewing area and have an optimal seeding density resulting in a maximal

correlation peak. In this respect liquid flows again tend tobe much easier to handle than

air flows as will be discussed in the next few chapters.

8.2.4 Health, Pollution and Other Issues

Seeding particles must not have an adverse effect on the health of the people dealing

with them. This is especially a problem with air flows where it is likely that at least a

fraction of the seeding could be inhaled by the operators especially when working

around open loop windtunnels. Additionally most solid seeding particles as well as

many liquid ones will deposit on the walls and surfaces of objects exposed to them,

often resulting in a mess. In terms of deposits it is necessary not only to consider health

effects but also any other adverse effects the seeding material might have (e.g. corrosion

of parts, abrasive effects, volatility and flammability).Next to choosing suitable seeding

particles issues such as proper workspace ventilation and additional respiratory

protection should be considered as well.

Liquid flows in contrast usually are yet once more far simpler to handle as they mostly

are operated in closed loop configurations and the particles are bound in the water and

not in the air.

(Compare reference [Dan00a] for more details on these issues.)

8.3 Seeding Liquid Flows

Apart from open loop flows where the seeding must be suppliedcontinuously liquids

are usually quite simple to seed and it is often sufficient topre-mix some solid particles

with the fluid using simple methods such as stirring by hand ([Mel97] Chapter 5.1 and

[Raf98] Chapter 2.1.3). Lists of suitable particles can be found in references [Mel97] ,

[Raf98] and [Dan00a].

134

Chapter 8 - Seeding

8.4 Seeding Gas Flows

Gas flows can either be seeded by liquid droplets or solid particles. The difficulty is that

unlike with liquids, which can be seeded once and then contain the particles for a long

period of time, gas flows must be seeded more or less continuously whilst maintaining

the correct seeding density (number density), distribution and particle size. Another

requirement is that the flow should not be disturbed by the introduction of particles.

A fairly 'complete' overview of seeding generators is givenby Melling in [Mel97] with

further information available in [Raf98] and on the productwebsite of several

manufacturers. Following a brief listing of some possibilities to seed gas flows mainly

based on [Mel97] will be given.

Generating liquid particles:

• By condensation:

→ Foggers as known from the entertainment industry make use ofthis principle

based on evaporation of a liquid and condensation to fine droplets.

→ Difficult to regulate: As stated in [Mel97] such devices tend to be unsteady.

• By atomisation:

→ Droplets can be generated by sprays and other atomisers. Together with impactors

to remove larger droplets it is possible to generate narrow size distributions in the

range of 1µm.

Generating solid particles:

• By atomisation:

→ Is possible by atomising suspensions of particles in a volatile solvent.

• Directly from powders:

→ Can be done by in various ways depending on the power. Some particles already

disperse by shaking while others have to be generated in cyclones or introduced

through venturi nozzles as done in [Hall01a].

135

Chapter 8 - Seeding

8.5 Own Experiments

8.5.1 Introduction

The purpose of this subchapter is to give a qualitative review of the experience made in

using a number of different seeding particles in an experimental PIV setup as described

in Chapter 10. A PIV system from ILA was used for this comparison as the Ekspla

system was mainly tested with atomised vegetable oil.

Four seeding generators dividable into three main groups shown in Figure 8.1 have been

compared. Samples of the PIV test images have been made available in a zip file on the

web (Appendix 10).

Figure 8.1 Evaluated seeding generators.

8.5.2 Fogger ('Smoke')

Two different brands of fog machines were tested: A relatively cheap one from Dick

Smith Electronics (M6000) run with Dick Smith fog juice (N6000) and an expensive

high quality Concept Colt 4 Turbo run with Concept smoke fluid type A.

While the data available on the resulting particle size is rather vague the particle size

section on the Concept website (www.concept-smoke.co.uk)claimed that concept

smoke generators would produce particles sized less than 0.6 microns with the peak

being within 0.2-0.3 microns, compared to the 1-5 micron range obtained with

conventional foggers.

Due to the claimed small particle size of the concept fogger it would have been

expected to give worse results but apparently the opposite turned out to be the case.

Generating useful PIV images with the Concept fogger seemedgenerally easier than

136

foggers solid particles(hollow spheres)

shaker and sieve

atomizer(vegetable oil)

pressurized air

tank with laskin nozzle and impactor plate

Chapter 8 - Seeding

with the Dick Smith fogger. The reason for this is that while non of the two devices

allowed obtaining pictures of nicely looking discrete particle peaks the Concept fogger

resulted in moving patterns of varying intensity which apparently were correlateable

between the two exposures. In contrast the Dick Smith fog machine resulted in a usually

very dense fog, giving what seemed to be random intensity distributions which were

widely inconsistent between two exposures apart from 'gaps' (zones with little smoke)

making apparent that the exposures do belong together. DickSmith fog obtained

towards the end of the intermittent output however appearedmore clustered and in part

similar to the Concept fog, which again was useful to evaluate.

The general impression of both devices was that they are difficult to control in output

quantity and density. Instead of producing nice discrete particle images both fogs

resulted in a more hazy seeming image ('speckling') not overly suited for PIV

evaluation. In part the small cross section of the test wind-tunnel could have contributed

to this but attempts in measuring with fog of reduced densityfailed just as well as the

resulting image was still hazy but less intense.

8.5.3 Hollow Glass Spheres

Q-Cell brand type 5020 hollow glass spheres assumed to be thesame as used by S. Hall

([Hall01a] Table 6.1) with a 5µm mean diameter and a particle density of 600 kg/m3

were used. As the small windtunnel only needed lower quantities a sieve attached to a

shaker was used to distribute the particles, allowing fairly good control. If necessary

larger quantities could be distributed using a venturi typeseeder as in Fig. 5.9 of

[Hall01a].

In terms of evaluation the produced images contained nice discrete particle peaks

spanning up to six pixels which although almost to big (Chapter 3.7.6) led to good

correlation results without ever having to optimise any seeding parameters. On the other

hand it was found that the particles had already absorbed some moisture and thus tended

to stick together giving super-particles not following theflow that faithfully any more.

The particles also were likely to make a mess and stuck on everything they got in

contact with. Finally the dust clouds produced by the particles – if not contained –

required wearing a face mask.

137

Chapter 8 - Seeding

8.5.4 Vegetable Oil

Using a Laskin type atomiser (described in Chapter 8.6) to produce vegetable oil

droplets made seeding the flow a lot easier than the smoke machines. Due to the

construction of the atomiser it was possible to adapt the seeding production rate and the

steady operation of the device allowed a much easier handling than the somewhat

unwieldy intermittently operating fogging machines. The droplets gave relatively nice

intensity distributions which were a lot better than what was obtained with the smoke

machines. Due to the small diameter in the range of 1µm the intensity of scattered light

was relatively low compared to the hollow glass spheres. Furthermore the resulting

particle image diameter tended to be too small which would have to be improved by

unfocussing or closing the aperture (Chapter 6.6). The oil turned out not to be overly

messy but just as with the smoke machines working in a well ventilated room seemed to

be advantageous.

8.5.5 Comparison Table

Foggers Hollow SpheresAtomised

Vegetable Oil

intensity of scatteredlight

fair but not discrete excellent relatively low

particle image sizepoor

(small particles)good

fair, room forimprovement

relative frequency(image density)

poor(very dense)

good good

production rateover time

poor(intermittent)

good, but constantsupply not that easy

good, very stableand constant

controlpoor (Dick Smith)

fair (Concept)fair-good depending

on apparatusgood, but within

limits

pollutionfair

(ventilation)mess potentialif not contained

fair(ventilation)

healthgood ventilation

requiredrespiratoryprotection

good ventilationrequired

Table 8-2 Comparison of the three seeding generators tested.

138

Chapter 8 - Seeding

8.6 The Laskin Based Seeding Generator at UNSW

8.6.1 Introduction

The seeding generator based on Laskin nozzles was found to beone of the most useful

devices for seeding airflows in situations where it is desired to avoid using solid

particles such as hollow glass spheres.

As using such a seeding generator raises a number of issues, most notably the potential

health hazard it is the intention of this chapter to discuss some of the important aspects.

8.6.2 Working Principle

The seeding generator used in this project was based on a model from PivTec

(www.pivtec.com) and consisted of five Laskin nozzles which could be turned on and

off independently.

An actual Laskin nozzle as shown schematically in Figure 8.2consists of a closed pipe

with holes of 1mm diameter at the side located right below a socalled feedhole-ring. In

operation pressurised air is guided through the nozzle which is submerged in liquid

forming tiny droplets captured in air bubbles and transported to the surface.

Figure 8.2 A single Laskin nozzle.(from [Raf98] Fig. 2.10)

A schematic of the utilised seeding generator based on five such Laskin nozzles is

shown in Figure 8.3. Pressurised air is supplied via a pressure regulating valve and then

is fed into a Laskin nozzle located in the centre of a closed container. As the container

is sealed a pressure will build up inside it and create an airflow dragging the smaller

particles outside.

139

Chapter 8 - Seeding

According to the PivTec (www.pivtec.com) specifications and according to the

observations made by Kähler et al. [Käh02] the typical expected mean particle size will

be around 1µm. The particles can be guided to a suitable location via an outlet hose. By

own experimentation it was found that this outlet hose should have a big enough cross

section (~1 inch diameter) and not be too long in order to get agood mass flow and to

prevent that only the very small particles will get through.It is possible to control the

mass flow by regulating the pressure and by opening and closing the valves leading to

the additional nozzles. When operating all nozzles it was found that some non-atomised

liquid would be expelled too.

Figure 8.3 Schematic of the utilised seeding generator based on Laskin nozzles.

The seeding generator in its original configuration must not be used with corrosive

liquids and especially not with water! Before being able to use the generator it first had

to be cleaned from rust as it was filled with water. This is due to the Laskin nozzle pipes

made of steel and not of brass or plastics like most other parts. A solution would be to

replace the steel pipes with ones made of brass or stainless steel. According to the

observations of [Käh02] it would suffice to just drill some 1mm diameter holes in the

end region of the closed pipes and omit the feedhole-ring.

Recommended reading on Laskin nozzle based atomisers includes references [Käh02],

[Mel97] and [Raf98] (Chapter 2.1.3).

140

pressurisedair supply

pressure regulationvalve

5 Laskinnozzles

open / closevalve for 4 nozzles

impactorplate

seedingliquid

outlet hose

Chapter 8 - Seeding

8.6.3 Health Issues

As the seeding particles would be generated in an open loop wind tunnel placed in a

ventilated room, investigations on any adverse health effects the atomised substances

could have on the people working in the vicinity of the windtunnel were made. It was

found that the American ESTA (www.esta.org) organisation provides a lot of useful

information on the health effects and safety of atomised fogging fluids and that the

American OSHA agency (www.osha.gov) provides further useful information,

especially on the health effects of atomised vegetable oil.

The typical way to deal with the presence of the substances isto introduce exposure

limits (usually over 8-hours). Often a differentiation between the total dust and

respirable fraction exposure limit is given of which the latter only covers the particles

smaller than ten microns ([HSE97]) and should be used with foggers.

Glycol Recommended Limit

Tri- & Di- ethylene glycol 10 mg/m3

Mono- & Di- propylene glycol 10 mg/m3

Glycerin 10 mg/m3

Vegetable Oil 5 mg/m3

Table 8-3 Exposure limits (8hr average).(composed using [Coh97], [HSE97] and [OSHA04])

An overview of the commonly recommended exposure limits is given in Table 8-3.

According to [Coh97] it is believed that these concentrations won't compromise "the

intended aesthetic goal" in theatrical productions, from which the assumption can be

drawn that the same will also be true for PIV applications if good ventilation is ensured.

A really serious analysis however would still require concentration measurements in the

exposed environments, which is possible by renting appropriate aerosol monitoring

devices.

Among other issues not directly addressed here are the potential of burns from fogging

machines and the dangers involved in handling pressurised air used with many types of

atomisers. In another note many users might have noticed that the artificial fog tends to

dry out lungs and eyes which is due to the hygroscopic property of glycols.

141

Chapter 8 - Seeding

8.6.4 Seeding Liquids

Next to health effects criteria for liquids used in atomiserbased seeding generators are

the resulting particle size, the evaporation time (particle stability) and the general ability

to result in a droplet.

Seeding Material Comments

Water Would be safe for humans but needs an evaporation inhibitor([Dan00a]; health issues?); Will corrode common steel.

Fogging Fluid Advantage of existing MSDSs; Expensive to use; Corrosivenessmust be tested.

Glycerine Recommended in [Dan00a], Cheaper than fogging fluids.Fogging fluid component; Corrosiveness must be tested.

Vegetable Oil,Corn Oil

Cheap; No corrosive problems.

Olive Oil Slightly more expensive than vegetable oil; Said to withstandhigher temperatures than vegetable oil ([Dan03]).

DEHS Di-Ethyl-Hexyl-Sebakat; Excellent for producing stable aerosols;Vaporises without residues; Said to be non-toxic (?) - [ILA03c].

Silicon Oil Said to be very satisfactory in [Dan03]; Health issues?Table 8-4 Different seeding materials for use in Laskin based seeding generators.

Table 8-4 lists a number of liquid seeding materials which could be considered for use

in a Laskin based seeding generator as in Figure 8.3.

From a safety point of view water would appear to be an excellent seeding material but

it can only be used in containers which won't corrode in contact with water and more

importantly it requires an evaporation inhibitor to be added ([Dan00a] Chapter 4.5.2.2).

Without the evaporation inhibitor water would evaporate, resulting in too small droplets

and poor fog properties (for PIV). Furthermore the evaporation inhibitor would raise

new health issues.

As fogging fluids are widely used in the entertainment industry and numerous MSDSs

exist it was considered to directly use them in the seeding generator. For this purpose a

corrosion experiment shown in Appendix 11 was undertaken and it appeared that the Le

Maitre brand general fogging fluid would corrode the Laskinnozzle pipes so this option

142

Chapter 8 - Seeding

was never tested.

Apart from using a fogging fluid a much cheaper option would be to use one of their

components as listed in Table 8-3. It appears that glycerineis mentioned in reference

[Dan00a] as a useful fogging fluid and it allows higher fog concentration levels than

vegetable oil (Table 8-3). Before it can be used it would either have to pass a similar

corrosion test as in Appendix 11 or the Laskin nozzle pipes inthe seeding generator

would have to be replaced with corrosion resistant ones.

Another very cheap option is the vegetable oil which as used in this report gave

satisfactory results. Olive oil as a slightly more expensive option is said to withstand

somewhat higher temperatures ([Dan03]). Generally most oils similar to vegetable oil

and probably also the glycols / glycerine are said to producedroplets in the size range

of 1µm ([Mel97], [Käh02], [Raf98]).

Among the more "specialised" seeding liquids are DEHS, which is well known for its

excellent properties as an aerosol, and silicon oil, of which the toxicological properties

would still have to be ascertained.

143

Chapter 9 - PIV Evaluation using VidPIV

9. PIV Evaluation using VidPIV

9.1 Introduction

Once the PIV images have been recorded they have to be evaluated using a software

capable of handling the methods and algorithms described inChapter 3. As two

software dongles are available for the VidPIV software usedwith the school's ILA PIV

system it was decided to use the second dongle with the EksplaPIV system set up in

this project.

Most details on handling the program won't be addressed hereas they can be found in

the manual [ILA03a]. Therefore first a brief overview on using VidPIV as well as on

how to import the images will be given. Additionally a Matlabscript written to

overcome unnecessary intensity downsampling due to the '8-bit image import only' limit

of VidPIV will be presented.

9.2 A Brief Overview of the Steps Involved in using VidPIV

9.2.1 General

VidPIV is based on modules which themselves can have submodules fulfilling different

purposes with a very simple example given in Figure 9.1. For the Ekspla system the

first step would be to import image pairs and a calibration image. Based on the

calibration image it then is necessary to define the image regions to exclude from the

analysis (annotations) and to define a scale (mapping) by positioning markers on pixels

and assigning corresponding real world coordinate values.As final step before cross

correlation the time between the images has to be set in the timing module.

It is possible to use several cross correlation modules witheach allowing to define a

different interrogation window size and spacing. After cross-correlation a number of

error-vector filters and interpolation functions as well as adaptive cross-correlation

modules for further refinement are available.

Once the vector field has been processed as desired, it is possible to calculate further

quantities such as average velocities and vorticity. Eventually the images and data can

144


be exported for further processing and presentation.

Figure 9.1 Simple example workflow in VidPIV.

9.2.2 Importing Image Pairs

Figure 9.2 Importing image pairs.

To import image pairs an "Importation (Dual)" module has to be created at the

beginning of the evaluation as in Figure 9.1. Bitmap pairs can then be imported by

145


right-clicking on the icon. Although called bitmap pairs VidPIV can import many file

formats such as bmp, tif and jpg but apparently only as 8bit images with other bit depths

such as 10bit and 16bit tiff always resulting in black pictures (Chapter 9.3).

In the image import interface shown in Figure 9.2 it is first necessary to select the

images to import ordered according to their exposure (firstexposure first). If the first

image in the selection doesn't belong to the second, the cross correlation analysis will

fail. The selected images are then imported by activating both base and cross selection

options, clicking sequence and import.

9.3 Preparing n-bit Images for the 8-bit Importer

9.3.1 Introduction

Although not explicitly stated in the manual it turned out that VidPIV is only able to

import 8-bit images. Most image processing software will downsample the intensity

levels proportionally when saving 10-bit images as 8-bit images, which reduces the

information contained in the image by a factor of 4. From Chapter 3.7.8 it can be

implied that this will not have a great influence on the result as long as the intensity

range between a bright particle and the dark background is reasonably high. Due to

insufficient laser power or due to disturbing reflections from walls it is not unusual

however that the intensity range between particle and background is low, often lower

than the 256 levels of 8-bit images.

In Figure 9.3b it is illustrated what happens when proportionally downsampling a 10-bit

to an 8-bit image. While its profile still looks roughly the same the resolution of the

particle intensity peaks has been reduced by factor 4. A muchmore elegant approach

shown in Figure 9.3c is to extract the intensity range between the camera black noise

level and the highest particle intensity level and resample this as an 8-bit image.

146


Figure 9.3 Converting 8-bit to 10bit images.

9.3.2 The Matlab Program

As no pre-written software allowing to extract a certain particle intensity range was

available, a Matlab script given in Appendix 12 was written for this purpose.

The example syntax "LevelConv('e:\tmp\',100,300)" wouldextract the image intensity

data between the bit levels 100 and 300 of all tif images contained in the folder 'e:\tmp'

into an 8-bit image marked by the prefix '8bit'.

Two program versions exist, with V0.11 proportionally distributing the selected range

along 8-bit, i.e. 100 in the example being 0 and 300 being 255,linearly interpolated in-

between. V0.20 in contrast will only proportionally downsample if the selected range

exceeds 8-bits, i.e. 100 → 0 and 300 → 200 in case of the example.

147

0 32 64 96 1280

64

128

192

256b) 8-bit downsampled

Pixel

Inte

nsi

ty

0 32 64 96 1280

64

128

192

256c) 8-bit range extracted

Pixel

Inte

nsity

0 32 64 96 1280

256

512

768

1024a) 10-bit image

Pixel

Inte

nsity

10b

it ra

nge

8bi

t ra

nge

proportional downsampling to 8-bit → loss of information

useful range with particle information

extracting and re-sampling the useful range

Chapter 10 - The Test Rig

10. The Test Rig

10.1 Introduction

At the beginning of the project it was intended to verify the the PIV system based on the

Ekspla laser by comparing experimental results obtained ona certain experiment with

experimental results obtained on the same experiment usingthe school's ILA PIV

system. As one part of the project was to compare the image quality of hollow glass

spheres to fog it was decided to design an experiment where this could be done with

both PIV systems. This required the test rig to be portable and in addition allow testing

the hollow spheres without making a mess as this would not be acceptable in most

testing environments.

Based on these requirements a brainstorming resulting in the two main initial concepts

shown in figure Figure 10.1 was done. As concept (b) resembled a vacuum cleaner,

which is a fairly simple solution, the decision to pursuit a vacuum cleaner driven wind

tunnel device was made.

Figure 10.1 Initial test rig concepts.

10.2 The Test-Windtunnel

The final result is shown in Figures 10.2 and 10.3. For simplicity the inlet and the

connection to the vacuum cleaner were made of cardboard heldtogether by gaffa tape.

Glued acrylic plates formed the main part of the wind tunnel with the test section

having a removable lid. To straighten the flow honeycombs made out of drinking straws

glued together with superglue were used.

148

drive

test-section

test-section drivefilter

a) closed loop b) open loop


To determine the necessary cross section of the wind tunnel the volume flow of the

vacuum cleaner was estimated using a handheld propellor anemometer. Based on this

data a cross section resulting in velocities around 10 m/s was chosen.

Figure 10.2 The windtunnel schematically.Figure 10.3 Windtunnel and

Vacuum Cleaner.

10.3 The Windtunnel in Operation

The windtunnel used in the configurations shown in Figures 10.4 and 10.5 turned out to

work fairly well with the only problem being hollow glass sphere deposits on the wind

tunnel walls and on the vacuum cleaner bag, resulting in a velocity drop from ~10m/s to

less than 3m/s.

149


Figure 10.4 The windtunnel used with the ILA PIV system.

Figure 10.5 The windtunnel used with the Ekspla PIV system.

150

Chapter 11 - System Usage Summary

11. System Usage Summary

11.1 Introduction

The aim of this chapter is to list the main steps when using theEkspla PIV system by

referring to other parts of the report and the appendices.

11.2 Steps when using the System

1. Prepare the test rig and the seeding generator.

→ The seeding should appear in the region of interest.

2. Set up the laser and light sheet.

→ Take safety precautions (Chapter 4.3).

→ Read the laser operation instructions (Chapter 4.4).

→ Align and mount the lightsheet in the usage direction (Chapter 5.4.3, Appendix 7).

3. Align the the camera on the target region.

→ Place an illuminated target in the test plane.

→ Adjust the camera on the target and focus the lens (live image → Appendix 13).

→ Take and store a calibration image.

4. Synchronise laser and camera.

→ Make all connections (laser front panel sync out→ function generator front panel

trigger in → CH1 out → camera back panel trigger in – Chapter 7.6).

→ Program the function generator (Chapter 7.6.2, Appendix 9).

→ Determine the TPD and TPW settings (Chapter 7.2 and Equations 7-2 & 7-3; For

∆t between flashes ≥15µs TPD = 119µs, TPW = 5µs.

→ Set the TPD and TPW settings via the Epix software (Appendix 13).

→ Set the laser intensity – approach as described in Chapter 6.4 and consult

Appendix 13. Don't overexpose the CCD chip!

→ Record single image pairs:

⇒ Make sure each image contains one laser flash only. Otherwise adjust TPD and

TPW.

151

Chapter 11 - System Usage Summary

⇒ Adjust the laser power to obtain similar image intensities between the two

images.

5. Record PIV images.

→ Image sequence recording (Appendix 13).

→ Optimise the image (particle intensity, size and density).

→ Optimise the timing between images (displacement around 3-5 pixels).

→ Export the recorded images as 16bit tifs (Appendix 13).

6. Analyse the PIV images.

→ Prepare the images for VidPIV import as 8-bit images (Chapter 9.3).

→ Analyse the images in VidPIV (Chapter 9).

152

Conclusion

12. Conclusion

Setting up the PIV system would not have been possible without a proper understanding

of the theoretical and practical basics involved and thus these aspects formed a major

part of this report.

The laser, constituting the core component of PIV illumination, was set up by applying

basic knowledge of pulsed lasers and input from the manufacturer(Chapters 4.2 & 4.5).

This required analysing and troubleshooting its operationby using information obtained

from high voltage pulse measurements, infrared visualisation and laser beam energy

measurements. Having the two lasers set up properly then allowed to combine (overlap)

the two beams and install the KTP crystal. In particular the alignment of the frequency

doubling KTP crystal, which converts infrared to green laser light at an efficiency

depending on the angle of incidence, turned out to be tricky and required several

attempts into reading and rethinking the sparse information available.

To create a light sheet (LS) a lens and laser beam redirectionsystem was designed,

requiring knowledge in geometrical- and laser optics. The more crucial aspects of

devising the lens system were to ensure that the lens materials would withstand the high

power laser beams, to avoid back reflections into the laser and to have a versatile

design. To the author's great delight the resulting light sheet worked just as predicted by

the calculations and fulfilled its purpose. Although quiteversatile already, the sheet

optics still has room for optimisation, most notably in increasing the light sheet angle by

utilising a stronger cylindrical lens, requiring the manufacture of a special lens holder.

Other improvements which could be made are to design and employ a better sheet

rotation module (Figure 5.10) and to allow translating the lens system in the plane

perpendicular to its optical axis, thereby increasing the alignment possibilities.

After proper analysis of potential synchronisation signals and camera parameters,

setting up and synchronising the camera with the laser became a fairly straightforward

step. In theory the system would even allow delay times between laser flashes of less

than a microsecond but it is expected that doing so will require further fine tuning of the

153

Conclusion

currently applied synchronisation parameters.

In operation the laser control application written in Labview appeared as remarkably

useful, not just due to its comparably intuitive interface but also due to the ability to

remotely fine tune the laser pulse energy from the same computer where the resulting

camera image is monitored. Problematic on the other hand was applying proper seeding.

Although a Laskin nozzle based seeding generator was found to be able to meet the

requirements of health issues, convenience and controllability, optimising the

introduction of seeding particles into an airflow is expected to remain one of the central

challenges in many PIV experiments.

Apart from controlling the hardware, theoretical knowledge on the output of the PIV

evaluation method depending on its input was found to be essential for obtaining

accurate and justifiable results.

It was shown that in order to obtain accurate PIV results a good image quality with

particles sized 2-3 pixels at a high intensity and suited density of occurrence (~10

particles per interrogation area) is necessary. Increasing the particle size can be done by

closing the aperture or unfocussing and thereby blurring the picture (Chapter 6.6). Both

methods result in a loss of intensity and require increasingthe laser pulse energy. Next

to the maximum laser power a major limit in the obtainable particle intensity are

reflections from walls which have to be dealt with, for example by using fluorescent

paints and black backgrounds.

In terms of PIV evaluation it was discussed that apart from just doing a simple cross

correlation analysis modern PIV programs, including the used VidPIV from ILA, allow

further refinements such as adaptive cross correlation which lead to an increase of the

resulting accuracy and resolution (vector density).

With respect to flow around dimples, one of the main reasons for establishing this PIV

system, it was reviewed from literature that the interesting part of the flow is highly

three dimensional. The established PIV system allows measuring two components only

and the not measured third component will result in an added uncertainty and the

154

Conclusion

necessity to decrease the time between exposures in order toprevent the particles to

move out of the light sheet between two illuminations. This perspective error will get

even worse when having to study the flow inside the concave region of a dimple where

viewing at an angle may be required. While the two component system should still

suffice for initial analyses and to gather experience, eventually a stereo PIV system

employing two cameras will be necessary for obtaining more accurate and complete

results.

Determining a PIV measurement uncertainty for a certain situation will not be simple if

great detail is desired as this requires numerous analyses from examining the optical

setup to performing monte carlo simulations. An easier approach is to estimate the order

of the resulting uncertainties from the phenomena presented in Chapter 3.7 by using the

graphs, tables and equations included in this report. Doingthis will also reveal potential

improvements for the applied PIV setup and thus definitely is worthwhile when the

ability to justify the obtained results is an essential requirement.

In closing it can be summarised that a working two velocity component digital PIV

system has been established and successfully run. It is not perfect but it fulfils the major

requirements of a modern tool in experimental fluid mechanics.

155

References

References

General Literature

[Coh97] Raymond, G. E.:Recommended Exposure Guidelines for Glycol Fogging

Agents. The Cohen Group & Entertainment Services & Technology

Association, 1997.

[Dub02] Beitz W., Grote K.-H.: Dubbel Interaktiv. Taschenbuch für den

Maschinenbau Version 2.0.Springer, 2002. (Electronic Media - in

German).

[Fox94] Fox R.W., McDonald A.T.: Introduction to Fluid Mechanics.New York:

Wiley, 1994.

[Hall01a] Hall S.D.: An investigation of the turbulent backward facing step flow

with the addition of a charged particle phase and electrostatic forces.

Ph.D. Thesis, University of New South Wales, School of Mechanical and

Manufacturing Engineering, Sydney, Australia, 2001.

[Hall01b] Hall S.D., Fletcher C.A.J., Behnia M., Morrison G. (2001):The design

and optimisation of a digital, cross-correlation PIV system, a sub-system

approach. Report. Centre for Advanced Numerical Computation in

Engineering and Science, Australian Technology Park, Sydney, CANCES

No. 7, pg. 1-39.

[Her02] Hering E., Martin R., Stohrer M.: Physik für Ingenieure. 8. Auflage.

Springer, 2002 (in German).

[HSE97] HSE Consulting and Sampling, Inc.:Literature Review for Glycerol and

Glycols. For Entertainment Services & Technology Association, pdf

(HSE.pdf) dated 1997-09-02.

156

General Literature

[Inc02] Incropera F.P., DeWitt D.P.: Fundamentals of Heat and Mass Transfer.

New York: Wiley, 2002 (Fifth Edition).

[Iv02] Leong I. (Ivan Tan Eng Leong): The Development Of Particle Image

Velocimetry (PIV) In Aerodynamics Measurements.Bachelor of

Engineering Thesis, University of New South Wales, School of

Mechanical and Manufacturing Engineering, Sydney, Australia, 2002.

[Kr99] Kreith F., Boehm R.F., et. al.: Mechanical Engineering Handbook –

Chapter 4: Heat and Mass Transfer”. Boca Raton: CRC Press LLC, 1999.

[Kup03] Kuphaldt T. R.: All About Circuits. www.allaboutcircuits.com, 2003.

(Copyright).

[Leo04a] Leonardi E., Timchenko V.: Project Background Information Handout

(Part E – Project Description; Project Title: Heat TransferEnhancement

Techniques for Air Conditioning and Refrigeration Equipment). University

of New South Wales, School of Mechanical and Manufacturing

Engineering, Sydney, Australia, 2004.

[Leo04b] Leonardi E.: Non-Dimensional Groups. (MECH9620 Computational

Fluid Dynamics Lecture Handout).University of New South Wales,

School of Mechanical and Manufacturing Engineering, Sydney, Australia,

2004.

[Lit03] Litvak A.: Laser Safety Guidelines.University of New South Wales,

School of Mechanical and Manufacturing Engineering, Sydney, Australia,

April 2003.

[OSHA04] OSHA: Occupational Safety and Health Guideline for Vegetable OilMist.

From www.osha.gov, 2004-11-04.

157

General Literature

[Rand04] Randall, R. B.: UNSW MECH9310 Advanced Vibration Analysis. Course

Text and Handouts.University of New South Wales, School of

Mechanical and Manufacturing Engineering, Sydney, Australia, 2004.

[Tek03] Tektronix Inc.: AFG310 and AFG320 Arbitrary Function generator.

User Manual. 071-0175-50, Year not specified.

Obtained from www.tektronix.com, 071017550.pdf, created 2003-03-17.

[Tek95] Tektronix Inc.: TDS 340A, TDS 360 & TDS 380 Digital Real-Time

Oscilloscopes. User Manual. 070-9459-04, 1995.

Obtained from www.tektronix.com, 070945904.pdf, created 1998-02-24.

Publications on Dimples

[Bunk03] Bunker R.S., Donnellan K.F.: Heat Transfer and Friction Factors for

Flows Inside Circular Tubes With Concavity Surfaces.ASME Journal of

Turbomachinery, Vol. 125, pp. 665-672, 2003.

[Burg03] Burgess N.K., Oliveira M.M., Ligrani P.M.: Nusselt Number Behaviour

on Deep Dimpled Surfaces Within a Channel.ASME Journal of Heat

Transfer, Vol. 125, pp. 11-18, 2003.

[Chen01] Chen J., Müller-Steinhagen H., Duffy G.G.: Heat Transfer

Enhancement in Dimples Tubes.Applied Thermal Engineering, Vol. 21,

pp.535-547, 2001.

[Grif03] Griffith T.S., Al-Hadhrami L., Han J.-C.: Heat Transfer in Rotating

Rectangular Cooling Channels (AR=4) With Dimples.ASME Journal of

Turbomachinery, Vol. 125, pp. 555-563, 2003.

158

Publications on Dimples

[Isa03] Isaev A.A., Leont'ev A.I.: Numerical Simulation of Vortex Enhancement

of Heat Transfer under Conditions of Turbulent Flow Past a Spherical

Dimple on the Wall of a Narrow Channel.High Temperature, Vol. 41, No.

5, pp. 665-679, 2003.

[Khal01] Khalatov A., Kozlov A., Agachev R., Syred N., Shchukin A.:Effect of

Surface Curvature on Heat Transfer and Hydrodynamics Within a Single

Hemispherical Dimple.ASME Journal of Turbomachinery, Vol. 123, pp.

609-613, 2003.

[Lig01] Ligrani P.M., Mahmood G.I., Harrison J.L., Clayton C.M., Ne lson

D.L.: Flow Structure and Local Nusselt Number Variations in a Channel

with Dimples and Protrusions on Opposite Walls.International Journal of

Heat and Mass Transfer, Vol. 44, pp. 4413-4425, 2001.

[Mah01] Mahmood G.I., Ligrani P.M., Hill M.L., Nelson D.L., Moon H,- K.,

Glezer B.: Local Heat Transfer and Flow Structure on and Above a

Dimpled Surface in a Channel.ASME Journal of Turbomachinery, Vol.

123, pp. 115-123, 2001.

[Vic02a] Vicente P.G., García A., Viedma A.: Experimental Study of Mixed

Convection and Pressure Drop in Helically Dimpled Tubes forLaminar

and Transition Flow.International Journal of Heat and Mass Transfer,

Vol. 45, pp. 5091-5105, 2002.

[Vic02b] Vicente P.G., García A., Viedma A.:Heat Transfer and Pressure Drop

for Low Reynolds Turbulent Flow in Helically Dimpled Tubes.

International Journal of Heat and Mass Transfer, Vol. 45, pp. 543-553,

2002.

159

Literature on PIV, Laser and Optics


[Bol99] Bolinder Jonas: On the accuracy of a digital particle image velocimetry

system. Technical Report, Lund Institute of Technology, SWEDEN, 1999.

[CAS99] CASIX, Inc.: Principles of Nonlinear Optical Crystals.

http://www.u-oplaz.com/crystals/crystals01.htm, Revision: Apr, 24 1999.

[Dan00a] Dantec Dynamics:FlowMap Particle Image Velocimetry Instrumentation

– Installation & User’s guide.Skovlunde, Denmark: Dantec Measurement

Technology A/S. Publication no.: 9040U3624. Fifth edition. August 2000.

[Dan00b] Dantec Dynamics: FlowMap 3D-PIV System – Installation & User’s

guide. Skovlunde, Denmark: Dantec Measurement Technology A/S.

Publication no.: 9040U4112. Fourth edition. August 2000.

[Dan02] Dantec Dynamics: 80X70 High Power Nd:YAG light-sheet series

(Publication No.: pi400007). pi400007.pdf, obtained from

www.dantecdynamics.com, pdf created 2002-02-15.

[Dan03] Dantec Dynamics:Seeding Generator for LDA and PIV (Publication No.:

pi260307). PI260307-liquid-seeding-generator.pdf, obtained from

www.dantecdynamics.com, pdf created 2003-08-27.

[Ed05] Edmund Optics: The Importance of Cleaning Optics. Technical support

article, published on www.edmundoptics.com, January 2005.

[Eks97] Ekspla Experimental Lasers Ltd.: Pulsed Nd:YAG Laser NL301-2G:

Technical Description & User's Manual. Vilnius, 1997.

[Forl00] Forliti D. J., Strykowski P. J., Debatin K.: Bias and Precision Errors of

Digital Particle Image Velocimetry.Experiments in Fluids, Vol. 28, pp.

436-447, 2000.

160


[Hart00] Hart D. P.: PIV Error Correction.Experiments in Fluids, Vol. 29, pp. 13-

22, 2000.

[Hua97] Huang H., Dabiri D., Gharib M.: On Errors of Digital Particle Image

Velocimetry.Measurement Science and Technology, Vol. 8, pp. 1427-

1440, 1997.

[ILA03a] ILA - Intelligent Laser Applications GmbH: Introduction to VidPIV 4.5

[User Manual]. Jülich, June 2003.

[ILA03b] ILA - Intelligent Laser Applications GmbH: LS Mini Light Sheet

Optics. [Datasheet]. ILA_data_lightsheet2003.pdf, obtained from

www.ila.de, pdf created 2003-08-11.

[ILA03c] ILA - Intelligent Laser Applications GmbH: ILA Datasheet DI(2-

ETHYLHEXYL) SEBACATE.ILA_manual_DEHS.pdf obtained from

www.ila.de, pdf created 2003-05-09.

[Käh02] Kähler C.J., Sammler B., Kompenhans J.:Generation and Control of

Tracer Particles for Optical Flow Investigations in Air.Experiments in

Fluids, Vol. 33, pp. 736-742, 2002.

[Käh04] Kähler C.J.: The significance of coherent flow structures for the turbulent

mixing in wall-bounded flows.Dissertation, Georg-August-Universität,

Göttingen, 2004.

[LaV02a] LaVision GmbH: DaVis FlowMaster Software Manual for DaVis 6.2.

Göttingen: LaVision GmbH, August 2002.

[LaV02b] LaVision GmbH: DaVis FlowMaster Hardware Manual for DaVis 6.2.

Göttingen: LaVision GmbH, August 2002.

[LaV03a] LaVision GmbH: DaVis FlowMaster Dual Plane Manual for DaVis 6.2.

Göttingen: LaVision GmbH, February 2003.

161


[LaV03b] LaVision GmbH: Pro-Package Manual for DaVis 6.2.Göttingen:

LaVision GmbH, February 2003.

[LaV03c] LaVision GmbH: DaVis FlowMaster - Getting Started Manual for DaVis

6.2. Göttingen: LaVision GmbH, March 2003.

[LaV03d] LaVision GmbH: Light Sheet Optic.Specification Sheet as obtained from

Lastek (www.lastek.com.au): DS Sheetoptic.pdf, pdf created 2003-02-20.

[Mel97] Melling A.: Tracer Particles and Seeding for Particle Image Velocimetry.

Measurement Science and Technology, Vol. 8, pp. 1406-1416, 1997.

[MelG02] Melles Griot: Optics Guide.

http://www.mellesgriot.com/products/optics/toc.htm, Melles Griot, 2002.

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Estimate Depth-of-Field and Sharpness in the Photographic Image.

Published by the author, Internet Edition, 2002 / (1990),

http://www.trenholm.org/hmmerk/download.html

[Merk96] Merklinger H.M.: Scheimpflug’s Patent.Photo Techniques, Nov/Dec

1996.

[Mey01] Meyers J.F.: Generation of Particles and Seeding.Von Karman Institute

for Fluid Dynamics, Lecture Series 1991-08, 1991.

[Oph03] Ophir Optronics: Introduction and Tutorial. Excerpt of the Full Catalog,

pp. 4-13, introduction_and_Tutorial.pdf, obtained from

www.ophiropt.com, pdf created 2003-01-25.

[PCO03a] PCO: CamWare. Operating Instructions. MA_CWOPIE_0305.pdf,

obtained from www.pco.de, pdf created 2003-05-09.

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obtained from www.pco.de, pdf created 2003-05-12.

162


[Raf98] Raffel M., Willert C., Kompenhans J.: Particle Image Velocimetry: A

Practical Guide. Berlin, New York: Springer, 1998.

[Red 02] Redlake MASD, Inc.: MegaPlus Model ES 1.0 Series Datasheet.

ES1-0_data.pdf, from www.redlake.com, pdf created 2002-02-08.

[Red01] Redlake MASD, Inc.: The MegaPlus Model ES 1.0 Series Cameras

User’s Manual. Manual No. 91000064-004, Revision A August 29, 2001.

[Sch88] Schott: Resistance of Optical Glasses to Short Laser Pulses.Technical

Information No. 21, Mainz, Germany: Schott Glaswerke, 1988.

[Schä99] Schärli M.: Optik: Radiometrie / geom. Optik.Lecture notes, University

of Applied Sciences Aargau, Northwestern Switzerland, 1999/2000. (In

German).

[Wer02] Wereley S. , Gui L.: ME 595W Fundamentals of Particle Image

Velocimetry Spring 2002. Purdue University 2002. As found on

http://widget.ecn.purdue.edu/~me595w/

[West93] Westerweel J.: Digital Particle Image Velocimetry – Theory and

Application –.PhD Thesis, Delft, the Netherlands: Delft University Press,

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Measurement Science and Technology, Vol. 8, pp. 1379-1392, 1997.

163

Homepages Used for General (Product) Information


www.concept-smoke.co.uk Concept Smoke Systems (fogger).

www.dantecdynamics.com Dantec Dynamics A/S – Manufacturer of flow

measurement systems such as PIV.

www.ekspla.com EKSPLA – Laser manufacturer.

www.esta.org Entertainment Services & Technology Association.

Organisation with excellent information on

entertainment safety, especially foggers.

www.glendale-laser.com Bacou-Dalloz Group, section on laser eyeware.

www.ila.de Intelligent Laser Applications GmbH – Manufacturer of

flow measurement systems such as PIV.

www.lavision.de LaVision GmbH – Specialised on imaging systems such

as PIV.

www.mellesgriot.com Melles Griot – Manufacturer of optical components and

other instruments associated with photonics.

www.mt-berlin.com Molecular Technology (MolTech) GmbH – Used for

information on non-linear crystals.

www.new-wave.com New Wave Research – Manufacturer of PIV lasers.

www.ni.com National Instruments; Producer of the Labview

Software.

www.nikon.com /

www.nikon.ch /

www.europe-nikon.com/

Nikon – Photography (Top Quality Camera Lenses).

164


www.ophiropt.com Ophir Optronics Inc. - Manufacturer of Laser Power and

Energy Measurement Equipment.

www.osha.gov The U.S. Occupational Safety & Health Administration.

Used for information on atomised oil health hazards.

www.oxfordlasers.com Oxford Lasers Imaging Division – Specialised on

imaging systems such as PIV.

www.pco.de PCO: Manufacturer of scientific cameras.

www.photonics.com General photonics product news. Mainlyused the

dictionary.

www.pivtec.com PivTec GmbH. Manufacturer / reseller of useful

components for PIV (seeding generator).

www.redlake.com Redlake MASD, Inc. - Digital imaging systems;

manufacturer of the PIV camera used in this report.

www.schott.com SCHOTT AG – Manufacturer of high quality glass used

for lenses.

www.tektronix.com Manufacturer of measurement equipment(oscilloscopes,

function generators, ...).

www.thorlabs.com Thorlabs, Inc. – Manufacturer of opticalcomponents and

other instruments associated with photonics.

www.u-oplaz.com U-Oplaz Technologies Inc. – Used for information on

non-linear crystals.

165

Appendices

Appendices

All appendices are available seperately on http://www.yaroc.ch/unsw/.

Appendix 1: Pulsed Laser Specifications.

Appendix 2: Changing the Flashlamps.

Appendix 3: Labview Laser Control Documentation.

Appendix 4: Light Sheet Optics Calculation (Mathcad Sheet).

Appendix 5: Thin Lens Combination Calculator (Mathcad Sheet).

Appendix 6: Light Sheet Configurations.

Appendix 7: Light Sheet Mount and Mirror Alignment.

Appendix 8: Adaptor plates to mount the ES1.0 camera on the Scheimpflug

adaptor and the Scheimpflug adaptor on the Dantec traverse

(drawings).

Appendix 9: Programming and Setting the Function Generator.

Appendix 10: PIV Test Images.

Appendix 11: Corrosion Properties of Fogging Fluids.

Appendix 12: Matlab Image Intensity Level Conversion.

Appendix 13: Using the Redlake ES1.0 Camera.

166

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