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25 years of PIV development for application in aeronautical test facilities

Jürgen Kompenhans and teamDepartment Experimental Methods

Institute of Aerodynamics and Flow TechnologyGerman Aerospace Center (DLR)

http://www.dlr.de/asE-mail: [email protected]

J. Kompenhans > 2Symposium 25 Years of PIV in aerodynamics, Göttingen, September 23 - 25, 2009

Contents

SymposiumObjectivesOrganization

Early PIV development at DLR for application in aeronautical test facilities (1984 - 1990)


Objectives of symposium

Commemorate the 25th anniversary of the first PIV measurements in a wind tunnel at DLR in Göttingen, performed in cooperation betweenUniversity of Oldenburg and DLRProvide an overview of the major milestones at the development of thegeneral PIV technique for application in aerodynamics and in EuropeProvide an overview of the PIV development in other areas and in othercountries, to a certain extentFind out whether experiences made at the development of PIV are of more general interest, e.g. as object of studies of experts in the field of history of technology (Falk Rieß and Andreas Junk, University of Oldenburg)Take this symposium as opportunity to provide an instrument to collectdocuments of the development of PIV on a broader basis than possiblevia a symposium and, in particular, such documents of the earlydevelopment of PIV, which are not easily (electronically) available today


Wiki-like web portal: The History of Particle Image Velocimetry

Information about its use: presentation of H. Frahnert today


Organization of symposium


Early PIV development at DLR for application in aeronautical test facilities (1984 - 1990)

First steps

PIV equipment

PIV results

Problems still existing in 1990

Progress 1990 - today


First steps: Environment in 1984

Communication & electronicsNo internet, no e-mail, no fax (use increasing)Mainly main frame computers and dedicated electronics formeasurement equipmentDesktop computers beginning to enter the market, but expensiveand < 1 MB memory

Optical techniques well advancedFor fluid mechanics:

Traditional techniques such as schlieren techniques and interferometryLaser light sheetLaser Doppler velocimetry (with dedicated electronics)Strong background in flow visualization in Göttingen (Ludwig Prandtl)


Separating Flow on a Knife-Edge

20 Hz frame ratealuminum flakes on water

Prandtl,Tietjens,Müller

1927-1933

© IWF, MPIDS


At DLR Göttingen:New quantitative methods needed for observation of unsteady flows forgiven applications within the objectives of our instituteSuch activities strongly supported by the new director of the instituteProf. Hans Hornung

Which new method to engage in?:Speckle methods known from measurement of discplacement of basilare membran (acoustics)Use of speckle methods to determine flow velocity in fluids, but doubtswhether technique will be transferable to aerodynamicsParticipation in VKI Lecture Series 1984-03 Digital Image Processing in Fluid Dynamics

Lecture by Roland Meynart and Louis LourencoLaser Speckle Velocimetry in fluid dynamics applicationsone application: jet flow in air

First steps: Activities in 1984


First steps: Activities in 1984DLR Göttingen and University of Oldenburg:

Klaus Hinsch, University of Oldenburg, was looking for a wind tunnel to explorethe potential of the speckle method to determine the degree of turbulenceOldenburg equipment was moved to Göttingen (incl. 10 J ruby laser)After some problems had been solved, recordings of good quality have beenobtained (lecture by Klaus Hinsch)

DLR Göttingen:Karl-Joachim Ebeling from the University of Göttingen suggested, that, ifcoherence is not required, we should buy a Nd:YAG laser instead of a ruby laser:

10 Hz repetition rate instead of 0.02 Hz Visible green light (Nd:YAG: λ = 532 nm) instead of dark red light (ruby: λ = 694 nm)Photographic material (KODAK Technical Pan 2415) of higher sensitivity for green lightFeasibility test on same set up with Nd:YAG laser borrowed from JK:

Much easier alignmentRecordings of tracer particles of same quality as with ruby laser

Based on VKI experience and on experience gained at test in our wind tunnel, thedecision was made to go for ‚speckle methods‘The institute director Prof. Hans Hornung provided budget for the laser


First steps: Nd:YAG laser

DLR Göttingen and JK (laser manufacturer):Disadvantage of Nd:YAG laser (single oscillator)

max. and min. time separation of two illumination pulses generatedout of one oscillator during one illumination of the flash lamp(200 µs) is much less than with ruby laserDifficult to obtain two stable pulses with nearly same energyNot adequate for use in aerodynamics with request to be able to

measure in low speed boundary layer flows (∆t = 200 µs) as well in high speed transonic flows (∆t = 3 µs)

JK proposed to build a double oscillator Nd:YAG laser in October 1984(lecture by R. Kelnberger)JK HY200 Special system was delivered to DLR in summer 1985DLR PIV system (illumination, recording and evaluation) in operationsince 1986


PIV system

Seeding: Laskin nozzle, oil droplets ∅ = 1 µm, as for LDVIllumination: Nd:YAG laser, 2 x 70 mJ, λ = 532 nm, fr = 10 Hz,tpulse = 16 nsRecording: 35 mm photo camera, KODAK Technical Pan 2415, max 36 PIV recordings for one measurement, time consuming focussing processEvaluation: Young‘s fringes method, autocorrelation ⇔ optical/digital Fourier transformation(due to lack of computer power), up to 80 s per Interrogation Window, even with dedicated image processing computer (Kontron IPS, with 8 bitarray processor), 50 x 24 grid points = 26 hours per PIV recording

Seeding Focussing Evaluation


PIV system: Double oscillator Nd:YAG laser

Power supply

Delay electronics

Laser head

Optical set-upScheme Hardware

HY200 Special(still working today)


PIV results: jet flow

Jet flow, nozzle dia. = 15 mm, Uj = 180 m/s, ∆t = 3 µs, internal seeding

2D flow field, (ux – 0.8 Uj; uy)

No external seeding

No ambiguity removal

1986


PIV results: Grid turbulence

19872D flow field, 1.3 m downstream of grid, (u – U, v)

TuHW = 0.04 %

TuHW = 1.1 %

TuHW = 1.4 %

Low turbulence wind tunnel

PDFs of laminar and turbulent flow- Hot-wire- PIV


PIV results: Transonic flow with shock

High speed suck-down wind tunnelRun time 20 s

Homogeneous seeding at high flow velocitiesCondensationReflections close to wallStrong velocity gradients

2D flow field, (u – U, v) 1989


Problems still existing in 1990

Photographic recording: No online check of quality of recording possibleWet film processing: long time needed to assess quality of recording(few hours) and to obtain quantitative resultsPerspective error: only two components of velocity vector can bemeasuredEvaluation: Need to place both illuminations on the same frame (picture) due to smalltime delays required in aerodynamics resulted in necessity to useautocorrelation for the evaluation process⇔the problem of ambiguity (which image of a tracer particle is due to firstand which is due to second pulse)Powerful and homogeneous seeding for large and high speed wind tunnels


Progress: 1990 - today

These problems have been solved in the meantimeModern CCD sensors allow to place each illumination on a different frameUse of cross correlation algorithms removes ambiguitySophisticated peak finders and windows deformation techniques lead to better spatial resolution and increased number of vectors per PIV recordingStereoscopic PIV allows measurement of all three components of thevelocity vectorAfter 25 years of development and experiences gained at the mostdifferent applications PIV became a standard measurement technique in aerodynamics for application in large industrial test facilities at DLR projects, national and international projects and contracts, in particular to understand unsteady flow phenomena and to provide experimental datafor validation of numerical codes at DLR


The “Prandtl Movie” subjected to PIV Processing

Evaluation of Prandtl‘s moviewith speciallyadapted PIV software byChris Willert

2D – 2C(t)

Spatial jitterdue to movierecording notyet removed

vorticity

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