experimental aerodynamicsexperimental aerodynamics · century by henri pitot) •!the pitot-static...

Experimental Aerodynamics

Lecture 2: Wind Tunnel Instrumentation

G. Dimitriadis and T. Andrianne

Experimental Aerodynamics Experimental Aerodynamics


Introduction

•! The ability to simulate a real flow in a wind tunnel is not useful in itself

•! The simulated flow must be observed for the experiment to have some use

•! The simplest type of observation is by just looking at the experiment

•! More precise observations can be obtained using several types of instrumentation


Scope of the lecture

•! Airspeed measurement –!Pitot tube –!Hot wire anemometry

•! Flow visualization –!Qualitative (wool, clay, oil, smoke, !) –!Quantitative (PIV, PSP, LDA,!)

•! Force measurement


Working section airspeed •! The most basic parameter in the wind

tunnel working section is the airspeed •! The airspeed can be measured using a

Pitot-static tube (developed in the 18th century by Henri Pitot)

•! The Pitot-static tube measures the total and static pressure in the wind tunnel working section

•! The difference between these two pressures is the dynamic pressure


Pitot-static Tube D

8 holes equally spaced on the periphery

8D 16D

3D

Total head connection

Static connection, p

Air velocity, V

pt = p +12!V 2

V =2 pt ! p( )

"


Pitot-static tube accuracy •! Pitot-static tubes can be accurate to 0.1%. •! Some corrections must be made for

Reynolds number and proximity to a wall. •! The total head measurement is accurate

as long as the angle between the tube and the flow is less than 3o.

•! The leading edge of the pitot affects the static pressure downstream. The static pressure holes must be placed sufficiently downstream to avoid this effect.


Speed setting •! In some cases it is not practical to use a pitot-static

tube in the working section. •! The model may interfere with the pitot

measurement or vice versa •! In such cases the pitot-static tube can be placed in

the settling chamber (ahead of the contraction cone) or just ahead of the working section.

•! The pitot-static tube placed in these positions can be calibrated to yield the correct working section airspeed using calibration runs (no model) and a second pitot-static tube in the working section.


Working section static temperature

•! The calculation of the test section airspeed from pitot-static tube data requires knowledge of the air density

•! The density can be obtained from the state equation if the static pressure and temperature are known.

•! The temperature can be measured using a wall-mounted thermometer or other temperature probe (static temperature is constant in a boundary layer)

V =2 pt ! p( )

"


Other uses of Pitot tubes

•! Static pressure measurement –!Long static tube

•! Flow direction measurement –!5-hole probe

•! Boundary layer measurement –!Total head rake

Hot wire

5-hole probe

Total head rake


Other uses of Pitot tubes

And of course ! Aircraft airspeed

Hot wire

5-hole probe

Total head rake Supersonic > corrections needed !



•! Airspeed measurement –! Pitot tube –!Hot wire anemometry




•! Blowing on hot food decreases its temperature

•! Same principle for HW •! Tungsten wire •! Different types:

–! CCA (current) –! CVA (voltage) –! CTA (temperature)

Hot wire anemometry

5-hole probe

Total head rake


Convective heat exchange Q = (Tw-T0) Aw h = A + B Un (n ~ 0.5)

where Tw = HW temperature T0 = Fluid temperature Aw = HW surface h = heat transfer coefficient (depends on fluid and wire characteristics

Hot wire anemometry

5-hole probe

Total head rake


CTA principle •!Servo amplifier keeps the bridge in balance (by controlling the current) •!Temperature is kept constant •!Bridge voltage (E) represents the heat transfer = direct measure of the fluid velocity

Hot wire anemometry

Total head rake

Thermal inertia of HW << High gain of the servo amplifier > Very fast response


Hot wire anemometry

5-hole probe

Total head rake

Source: Dantec Dynamics Posters

+ High frequency response > Study of boundary layers, turbulence of a flow field

- Intrusive, accurate but small spatial resolution


Hot wire anemometry Calibration

5-hole probe

Total head rake


Hot wire anemometry Calibration

5-hole probe

Total head rake Calibrated Airspeed (m/s)

Volta

ge (V

)

U = C0 + C1E + C2E2 + C3E3 + C4E4 + C5E5

ECORR = ETw !T0Tw !Tmeas

"

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0,5


Hot wire anemometry Different types of probes

5-hole probe

Total head rake

1 mm long and 5 "m in diameter

1D 2D 3D


•! It is often interesting to observe the simulated flow around the model in the wind tunnel.

•! Unfortunately air is colorless and transparent •! Several different methods exist:

Flow visualization

Qualitative methods Wool (cotton) tufts China clay Oil film Smoke

Quantitative methods Particle Image Velocimetry (PIV and Tr-PIV) Pressure Sensitive Paint (PSP), Laser Doppler Anemometry (LDA)!


Oil flow visualization •! Lubricating oil is applied in small

dabs on several locations on the model.

•! When the wind is turned on, the oil will follow the local streamlines.

•! Oil flows will indicate the boundary of flow separation, since the oil cannot cross the separation boundary.

•! Thinning down the oil can also indicate the laminar-turbulent flow transition boundary.

•! Often, color pigments are added to the oil in order to aid visualization. Sometimes the paint is fluorescent and can be lit by a UV light.


Smoke Flow Visualization

•! Producing smoke is not easy. Burning stuff is not recommended.

•! The most popular smoke production methods are: –!Fog generators, such as those used in

night clubs. These are good for thick smoke streaks

–!Smoke wire


Smoke Wire

Source: Sharul Sham Dol et al. at 4th WSEAS International Conference on Fluid Mechanics and Aerodynamics

Smoke wires only work in low speed and low turbulence conditions! Airspeed < 10m/s


Smoke Wire

Source: Sharul Sham Dol et al. at 4th WSEAS International Conference on Fluid Mechanics and Aerodynamics


Smoke flow visualization examples

Stalled wing

Car

Circular cylinder


Smoke visualization videos •! Flow around Delta wing and ducted fan at low speed •! Flow unsteadiness due to:

–! High angle of attack –! Wind tunnel turbulence –! Smoke generator quality


Particle Image Velocimetry •! PIV works on the principle of lighting up small

particles in the airflow using a laser light. •! A laser surface is created in the working

section and a camera is focused on this surface.

•! Particles are injected in the flow. These are light, small and invisible. They can be tiny was droplets

•! When the particles pass under the laser they are lit up and can be photographed by the camera

•! PIV can be also performed in 3D (Stereo PIV)


PIV principle

2D PIV 3D (Stereo) PIV


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Tr-PIV principle

Experimental Aerodynamics 77#

Image 1

Image 2

Sub-Image 1

Sub-Image 2

Cross-Correlation Area of search

Tr-PIV – image analysis


PIV Examples

Guerney flap Circular Cylinder

Helicopter rotor


ULg Tr-PIV example Rectangular cylinder undergoing pitch flutter oscillations

Tr-PIV (slow motion)


Advantages and disadvantages •! Tufts:

–! Advantages: easy to install, cheap –! Disadvantages: can affect other measurements, need to choose the right length/rigidity

of fabric, may need strong light and high speed camera, discrete surface-only flow visualization

•! Smoke: –! Advantages: flow visualization of entire sections of the flowfield, beautiful pictures and

movies –! Disadvantages: More suitable for low Reynolds number flows, turbulence mixes the

smoke, can be corrosive and pollutant. •! Clay/Oil:

–! Advantages: easy to apply, cheap, can take pictures after wind has been turned off. –! Disadvantages: surface-only flow visualization, need a lot of experience to mix and apply

correctly, oil requires cleaning. •! PIV:

–! Advantages: flow visualization of entire sections of the flowfield, high speed, can extract quantitative data

–! Disadvantages: expensive instrumentation, many parameters to set, requires a lot of experience, visualization window is small at high frame rates.


Aerodynamic load measurements

•! One of the most important functions of wind tunnels is to provide estimates of the aerodynamic loads acting on bodies moving through air.

•! The first such measurement were obtained using actual balances.

•! Since then, aerodynamic load measurement devices are called aerodynamic balances.


Wind tunnel balances

•! There are several types of wind tunnel balance: –! Internal balances: They are placed inside the

model. –!External balances: They are placed around or

under the working section. –!Strain gauge balances: Most modern balances

feature strain gauges –!Rotary balances: for propellers and other

rotating models


Internal balance

•! They usually come in the form of stingers •! They can measure six loads (three forces

and three moments) •! They must be attached near the model s

centre of gravity


External balance

•! External balances come in several degrees of complexity and size.

•! The complexity depends on the number of loads that the balance must measure (maximum of 6).

•! The most advanced type of external balance currently in use is the pyramidal balance.


External balance design


VKI, 6-comp balance for L-2A


1-component balance

•! The ULg wind tunnel has a very simple 1-component balance mounted under the turntable.

•! It measures lift only using a load cell.

•! Drag and side force can be measured using a strain gauged support on which the model can be attached


Horizontal axis balance

•! The ULg wind tunnel also features a horizontal axis balance.

•! The balance features four vertical force sensors, two horizontal force sensors and a moment sensor.

•! It can measure all 6 aerodynamic loads but the model must be mounted on the horizontal axis.


ULg horizontal axis balance

Force sensor

Torque sensor

Horizontal axis balance


Rotating balances •! For helicopter rotors, propellers, wind

turbine rotors etc

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