study of theoretical and experimental aspects of particle image velocimetry

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04.08.2010 Hermann-Fttinger-Institut fr Strmungsmechanik Page 1/12

Experimentelle Strmungsmechanik

Prof. Dr.-Ing. C.O. Paschereit

Hermann-Fttinger Institut

Study of Theoretical and Experimental Aspects ofParticle Image Velocimetry

by

Vineet Maheshwari

Email:[email protected]

Berlin, 04.08.2010

Technische Universitt Berlin

Institut fr Strmungsmechanik und Technische Akustik

- Hermann-Fttinger-Institut -

Fachgebiet Experimentelle Strmungsmechanik

Mller-Breslau-Str. 8

D-10623 Berlin
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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Study of Theoretical and Experimental Aspects of Particle Image Velocimetry


DECLARATION

I certify that the work described in this report has been done by me and I am solely responsible

for the preparation of this report.

(Vineet Maheshwari)


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ABSTRACT

During this internship, theoretical concepts involved in Particle Image Velocimetry (PIV) werestudied and then hands-on experience in this technique was gained by assisting in an ongoing

experiment. PIV is an experimental technique to visualize fluid flow and is used for visualization

of overall flow field in a region. It is not as precise as some of the other point-wise measurement

techniques like hot-wire anemometry or laser Doppler anemometry (LDA), but it is more suita-

ble to analyze flow over a larger region with considerably high precision. The experimental com-

ponent of the internship was part of the project on Active Flow Control of an Incompressible

Axisymmetric Jet using Flaps and Zero Mass-flux Excitation. The enhancement in mixing charac-

teristics of a jet with the ambient at different positions was investigated, under the introduction

of vortices by means of sinusoidal excitations. It was concluded that active control was more ef-

fective in the near field.


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CONTENTS

1. Principal of Particle Image Velocimetry......................................................................5

1.1 Tracer particles..............................................................................................5

1.2 Light source..5

1.3 Camera......6

1.4 Computer.....6

2. Experimental work........................................................................................7

2.1 Project title.........................................................................................................7

2.2 Brief project description.7

2.3 Experimental setup............................................................................7

2.4 Experimental procedure and data analysis........................................................8

2.5 Results and discussion.....................................................................9

2.6 Challenges faced........................................................................................11

2.7 Skills acquired.11

Acknowledgements..............................................................................12

References...12


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1. Principle of Particle Image Velocimetry

As the name implies, PIV measures the velocity at numerous points in a fluid flow by taking im-

ages of particles flowing with the fluid and analysing a sequence of these images. The experi-

mental set-up of a PIV system typically consists of several sub-systems. These include (i) a

source of tracer particles (seeding generator), (ii) a high intensity laser and related optical at-

tachments that convert laser beam into a thin sheet, (iii) one or two CCD camera(s) with selec-

tive colour filters and (iv) a data acquisition and analysis unit (computer).

1.1Tracer Particles (seeding)

They move along with flow and scatter the incident light towards the camera(s). In true sense,

visualization of flow is done by visualizing the motion of tracer particles. Being an indirect tech-

nique, PIV measures tracer particle velocities instead of fluid velocity. Therefore, fluid mechani-

cal properties of tracer particles have to be checked in order to avoid significant discrepancies

between fluid and particle motion. When the fluid in question is a gas, like in wind tunnels,

spheres of solid materials like polystyrene, aluminium or magnesium or smoke of liquids such as

oils are used as seeding. An atomiser is used as a seeding generator to create smoke out of oil

and pressurised air. In case of liquid flows, solid particles of larger diameter serve as tracer par-

ticles. The materials used polystyrene, aluminium or silver-coated glass spheres. Sometimes,

oxygen bubbles may also be used as tracers.

1.2Light source

Lasers are widely used in PIV, because of their ability to emit monochromatic light with highenergy density, which can easily be bundled into thin light sheets for illuminating and recording

the tracer particles without chromatic aberrations. Generally, Neodym-YAG (Nd:YAG) lasers are

used in PIV as they have a high amplification and good mechanical and thermal properties. A

typical PIV laser system essentially consists of a pair of lasers. These lasers shoot at small time

intervals (typically in micro-seconds). Two images are taken by the camera, each being illumi-

nated by a single laser. A common feature of most PIV laser systems is the presence of a quality

switch (Q-switch). It normally consists of a polariser and a Pockels cell. By including a Q-switch

inside the cavity, laser can be operated in a triggered mode. Quality of optical resonator

changes with Pockels cell voltage. Q-switch alters the resonance characteristics of the optical

cavity, allowing it resonate at the most energetic point of flashlamp cycle. This yields a very

powerful laser pulse, the so-called Giant pulse.

The highly intense light beam coming out of the lasers need to be given proper shape and orien-

tation. It is done with the help of suitable optical attachments like mirrors and focussing lenses.

One important component of PIV laser optics is a cylindrical lens. It converts the final laser beam

into a thin sheet that illuminates the region of interest.


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1.3Camera

The camera used in PIV image recording is of CCD (charge coupled device) type. Since Nd:YAG

laser gives green light of wavelength 532 nm, selective colour filter is used that permits light of

only this wavelength to enter the camera, filtering out all other optical noises. The temperatureof CCD chip also needs to be controlled as over-heating may lead to electronic noises. So the

cameras are equipped with cooling fans. In case of 2D PIV, a single camera is used and it is

placed in such a way that object, lens and image planes are parallel to each other. In stereo PIV,

two cameras are used and they are placed at an angle with respect to the object plane. In order

to get a sharp image in this orientation, object, lens and image planes must meet at a point. This

is called Scheimpflug condition (Figure - 1).

Figure 1

1.4Data Acquisition

For acquisition of data (images), its processing and analysis, a computer with suitable software

is used. Many software are commercially available to process PIV data. This software (e.g. Vid

PIV) utilise image processing algorithms and correlation functions to calculate velocity vector

values at all interrogation points, giving a vector map for the whole region. Vector values are

stored and exported to other analysis software like MATLAB and Tecplot for further analysis and

interpretation. Standard PIV gives velocity information in a 2-D plane while stereo PIV gives

complete 3-D velocity field.


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2. Experimental Work

2.1 Project Title

The experimental work was part of the project on Active Control of an Incompressible Axisym-

metric Jet using Flaps and Zero Mass-flux Excitation.

2.2 Brief Project Description

An active flow control method of an axis-symmetric jet was investigated which, when activated,

generated streamwise vortices and thus enhanced mixing of the jet flow with the ambient. The

perimeter of the jet was equipped with six small flaps deflected away from the stream. Zero

mass-flux perturbations were being used to excite the flow. These excitations were introduced

in the flow through slots at the base of the flaps. Each of the flaps could be excited independent-

ly. In these investigations, the effect of an array of six individually controllable flaps on the glob-

al jet behavior was addressed. Each of the flaps could be excited in phase or with pre-fixed

phase shift. Effects of frequency and amplitude on the flow momentum, streamwise vorticity,

circulation and turbulence for a fixed flap deflection angle were part of the investigation. A ste-

reo-PIV setup was used to acquire complete flow field information. The emphasis was placed on

mapping the development of the trailing vortices in order to quantify the mixing achieved.

2.3 Experimental Setup

The wind tunnel used for the experiments was a low speed, circular cross-section, open circuit

tunnel with an open-air jet. The exit jet diameter was 90mm and it gave a maximum Reynolds

number of 90,000 based on jet diameter. The lip of the axisymmetric jet was equipped with six

small flaps deflected away from the stream at an angle of 30. The chord length of the flaps was15mm. The flaps incorporate a flow control slot (15x1.5mm) and each slot was connected to a

speaker via a flexible tube. A sine-wave was supplied to the speakers to produce the desired

frequency and amplitude through which zero mass flux excitation was introduced to the flow in

the axial direction. The control slot had been calibrated to get the desired amplitude of the exci-

tations. The velocity measurements were carried out using stereo-PIV in planes perpendicular to

the axis of the jet at axial locations ofx/D = 0.25, 0.5, 1.0, 2.0. Figures - 2 (a), (b) and (c) show

different views of the experimental setup.


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(a) (b)

(c)

Figure - 2

2.4 Experimental Procedure and Data Analysis

Complete flow field measurements were carried out using a stereo-PIV setup, at a Reynolds

number of 31000. The amplitude of the excitation was quantified by the non-dimensional para-

meter C, the momentum coefficient. It is defined as the ratio of the momentum added by the

control slot to the momentum of the main jet. The calibration of the excitation amplitude was

carried out using a hotwire, which was positioned directly in front of the slot, oriented parallel

to it. For each excitation frequency, the peak velocity of the jet was determined as a function of

the AC voltage of the excitation signal supplied to the speakers. The amplitude of the excitation

was varied in a range previously determined by earlier works. Within this range, a frequency

scan was carried out with reduced frequency F+ (dimensionless excitation frequency). The ste-

reo-PIV acquisition was phase-locked with the actuator signal and data at 16 different phases

were acquired. The software used for data acquisition and processing was VidPiv, developed by

Intelligent Laser Application, GmbH. Further processing and analysis of results were done with

previously developed scripts in MATLAB.


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2.5 Results and Discussion

Figure 3 shows the velocity field at x/D =1.0 in the absence of any control. The axial velocity

profile resembles an unaltered jet, showing that there is no effect of the presence of slots on the

jet.

Figure 3 Axial velocity field atx/D =1.0 (Singh et al, AIAA Paper 2010-4417)

Figures 4 (a), (b) and (c) show development of flow over three axial locationsx/D = 0.5, 1.0 and

2.0 respectively at F+= 0.2 and C= 0.45. It can be seen that the introduction of excitation caus-

es the flow near the flaps to get distorted and move towards the flaps at six flap locations, one

of them marked as P in the first figure. As we move downstream, the distortions begin to ap-

pear in regions between the flaps as well. This indicates the presence of a symmetrical system of

stream-wise vortices. On going away further, these regions of distortion begin to merge rapidly.

Figure 4 Axial velocity field,x/D = 0.5, 1.0 and 2.0, F

+

= 0.2 and C= 0.45 (Singh et al, AIAA Pa-per 2010-4417)


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Figure 5 Axial velocity field,x/D = 1.0, F+= 0.2 and C= 0.15, 0.45 and 0.75 (Singh et al, AIAA

Paper 2010-4417)

Figure 5 shows the effect of change in amplitude of excitation. Cases (a), (b) and (c) represent

C = 0.15, 0.45 and 0.75 respectively at F+ = 0.2 andx/D = 1.0. It can be easily seen that increase

in amplitude of excitation increases the deflection of flow towards the flaps but this effect satu-

rates at still higher amplitudes.

Figure 6 Streamwise vorticity, C = 0.3, F+ = 0.1, (a) = 0

o, (b) = 90

o, (c) = 180

oand (d) =

270o

(Singh et al, AIAA Paper 2010-4417)


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Figure 6 shows strength of stream-wise vortices at C = 0.3, F+ = 0.1 at four phase angles. At 0

o,

two pairs of counter-rotating vortices can be observed near each flap. At 90o, a circular system

of vortices is formed in the shear layer, with the pairs close to the flaps travelling into the ambi-

ent air. At 180, the vortices that were previously observed have moved out further and have al-

ready dissipated to a large extent. At 270o, strong pairs could be seen near the flaps.

2.6 Conclusions

Active flow control using zero mass-flux excitation is applied to study mixing characteristics in a

circular jet equipped with six finite span flaps along its perimeter. The above discussions result in

following main conclusions:

Zero mass-flux excitation parallel to the flow is effective in attaching the flow to the flaps andalso in generating streamwise vortices.

The location as well as the strength of these vortices strongly depends on the excitation fre-quency.

The effect of increasing the excitation amplitude saturates at higher amplitudes.

2.7 Challenges Faced

The laser-sheet optics is an essential aspect of a PIV measurement. Proper illumination of thedesired region requires a thin and focused light sheet. The position of laser-sheet with respect to

the calibration target is also very important in order to get accurate and precise data. Adjusting

the laser-sheet was a tedious and time-consuming job that required a lot of patience.

The amount of seeding is a big factor that decides the quality of results. Insufficient seeding

gives inaccurate results, while excessive seeding also deteriorates the quality of results as thesize of seeding particles becomes too big for the available pixel size. So maintaining the opti-

mum amount of seeding in the test area was a tough job and tested the experience of my men-

tors.

Taking sharp calibration pictures is very important to achieve good accuracy. For that, it must be

ensured that Scheimpflug condition is met and the focus is properly adjusted. The task of adjust-

ing the camera to get optimum calibration pictures was tough and challenging.

2.8 Skills Acquired

Working on the above experimental setup gave me practical experience in the following areas:

I learned all the aspects of the experimental technique of PIV, viz. selection of components, set-

ting up of experimental setup, calibration, taking measurements, processing of data and data

analysis.

Creating a three dimensional model of the setup in the software SolidWorks gave me ampleamount of experience with Computer Aided Design.


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Acknowledgements

I thank Prof. Dr.-Ing. Christian Oliver Paschereit for this opportunity to learn at Hermann-Fttinger Institute (HFI/ISTA), Technical University Berlin and gain this invaluable and useful ex-

perience. I also gratefully acknowledge the support and guidance of Dr.-Ing. Christian Navid

Nayeri, Mr. Yogesh Singh and Mr. Hanns Mller-Vahl in completing this internship.

References

1. Particle Image Velocimetry A Practical Guide by M. Raffel, C. Willhert and J. Kompen-

hans.

2. Singh, Y., Mueller-Vahl, H., Greenblatt, D., Nayeri, C.N., Paschereit, C.O., Active Control of

an Incompressible Axisymmetric Jet using Flaps and Zero Mass-flux Excitation, AIAA Paper

2010-4417, 2010.

3. Wikipedia.org

study of theoretical and experimental aspects of particle image velocimetry

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