holographic piv for large scale...

Holographic PIV for Large Scale Facilities

H.Royer 1, N.Pérenne2, M.Stanislas2, J.C. Monnier3

1 ISL 5 rue du Général Cassagnou, BP 34 - 68301 St Louis, France2 Laboratoire de Mécanique de Lille UMR3701, Bd Paul Langevin, Cité Scienti-fique, 59655 Villeneuve d’Ascq cedex, France3 ONERA Centre de Lille, 5 Bd Paul Painlevé, 59045 Lille Cedex, France

Abstract

An holographic PIV experiment has been performed in a large scale wind tunnelin order to assess the interest of the method for such facility. The holographicmeasurements were compared to simulataneous stereoscopic PIV measurements.The results show that that hologaphic PIV has a potential of application aroundlarge facilities if a suitable recording medium is available. Presently, the photo-graphic material available is not sensitive enough and to heavy to process to per-mit useful experiments in such a situation.

1 Introduction

”Particle Image Velocimetry” (PIV) is a unique optical (non intrusive) methodwhich allows to capture instantaneously whole velocity fields in fluids. Its useful-ness has been demonstrated in a previous EUROPIV program [1]. The presentcontribution is part of the EUROPIV 2 program and deals with the extension ofPIV to holographic PIV. Digital PIV is able nowadays to provide 2D2C or 2D3Cdata in industrial facilities in a quite efficient way. As industrial flows are oftenquite complex and 3D, there is a need to extend in the future PIV to 3D3C meas-urements. To do so, two tracks are presently possible: scanning light sheet stereo-scopic PIV [2] and holographic PIV [3]. Due to technical limitations, the first ap-proach is presently limited to low velocity flows. It is thus of interest,to investigate the holographic approach to the extension of PIV to 3D3C. For thatpurpose, the application of a stereoscopic holographic set-up to a large wind tun-nel was invesigated. The aim of the present contribution is to present the main re-sults obtained.

The Europiv project [1] did show that holographic recording of micron parti-cles in a light sheet is possible, even at long distance, with the advantage of get-ting tiny particle images (~10 µm) as compared to standart recording throughlenses (~ 30 µm).

One of the main drawbacks put to evidence in this first Europiv program wasthat although holographic recording is 3D by nature, the accuracy on the velocity

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component normal to the hologram is much less than on the two componentswhich are parallel to the holographic plate [4]. This is why it was decided to in-vestigate a more sophisticated stereoscopic holographic set-up, allowing the sameaccuracy on the three velocity components, and to assess the problems linked tothe use of such a set-up around a large wind tunnel.

Light Sheet

Jet

H 1

H 2

Reference 1

Reference 2

Fig. 1. Holographic recording set-up based on two holograms and allowing the same accu-racy on the three velocity components.

Laser LensCL

P1

P2

M1

M'1

D1

L1

H1

M2

M'2

H2

L2

D2

Lighttrap

L1

L2

Fig. 2. Short distance recording set-up. The two holograms are set symmetrically on bothsides of the light sheet. They are illuminated by the light scattered by the particles (dottedline) and by the reference beam (continuous line). The first lens focusses the laser beam inthe region of interest. The cylindrical lens (CL) makes the light sheet. A telescope is set oneach reference beam (L1, L1; L2, L2) to make it parallel with a diameter of about 60 mm.

Fig. 1 gives a schematic principle of the arrangement selected. Two hologramsare set at ± 45° from the light sheet and record simultaneously the particle images.At reconstruction, the accurate velocity components measured parallel to eachhologram plane allow to reconstruct the full velocity vector with the same accu-

Holography and ESPI 335

racy. Moreover, if the angles are carefully set at ± 45°, the velocity componentare obtained by simple addition and subtraction of the components in the plane ofthe holograms.

For this study, the LML boundary layer wind tunnel, which is 1 m x 2 m incross section, was selected as representative of larger facilities. The first half ofthe project was used to prepare the experiment. Different holographic set-upswere tested. The laser, the synchronization, the seeding, the digital stereoscopicPIV measurements, the implementation of the holographic set-up around the windtunnel and the reconstruction set-up for post-processing of the holograms wereprepared. A test campaign was then performed in the large wind tunnel.

2 Preliminary Tests

The aim of these tests was to investigate different recording set-ups and to selectthe best suited one for the large scale experiment. Two set-ups were retained andused to record sample holograms in a laboratory environment. The first set-up,called “short distance” is illustrated in Fig. 2. As can be seen, the two hologramsare very near the flow region (about 25 cm in the present case) and the light dif-fracted by the particles is recorded directly on the plates. The advantage of thisset-up is the simplicity, the main complexity being the set-up to generate two ref-erence beams of equivalent characteristics.

Left image Right image

Fig. 3. Sample of reconstructed particle images obtained with the short distance set-up ofFig. 2. Only the central region is in focus because the light sheet is at 45° to each hologram.

The size of the light sheet was 0.5 mm x 50 mm. A Ruby laser was used for therecording, with a pulse energy of 2 x 75 mJ. The holographic plates were Agfa10E75. A sample of reconstructed particle images is given in Fig. 3 for each holo-gram. The reconstructed images are of good quality (the aberration spots have atypical size of 10 x10 x 40 µm) .

The second set-up tested is illustrated in Fig. 4. In this case the holograms arefar away from the flow and a relay lens is used to image the light sheet nearby thehologram. The main advantage of such a set-up is to allow measurement in the

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middle of a large wind tunnel, as the distance between the relay lens and the re-gion of interest is here of 1.5 m. The drawbacks are : - a loss of light, a more sen-sitive set-up and the need to reconstruct the image through the relay lens in orderto correct for the optical aberrations. The same ligth sheet, laser and hologramswere used as for the short distance set-up. Only the pulse energy was pushed to 2x 200 mJ.

Tests were performed on a free jet at 20 m/s with a time separation of 20 msbetween the two exposures. Fig. 4 presents the images reconstructed from bothholograms through the imaging lenses. The image quality is good and the particleimage pairs are clearly visible.

M

m2D1

L2

D2

L1

m1C2

C1

m'1

BS1

BS2

jetlight trap

RL1

RL2

H1

H2

CLm'2

Fig. 4. Long distance recording set-up. The cylindrical lens (CL) generates the light sheet.One set-up is folded in order to have the two holograms on the same side of the wind tun-nel. The relay lens RL1 and RL2, with a focal length of 0.75 m, do image the light sheetnearby the holograms.

Fig. 5. Reconstructed images from the right and left holograms of the set-up in Fig. 4.


2 Experimental Set-up

The preliminary test performed having shown that both set-ups were giving goodquality holograms, it was decided to retain the set-up at short distance for threemain reasons : - the flow region retained for the investigation was near the upperwall of the wind tunnel (imaging was not mandatory), - the recording optical set-up was simpler (which is important around a large facility), - the reconstructionset-up was simpler too as, without imaging lens at recording, the holographic im-age can be observed directly at reconstruction. Besides, it was necessary to set-upa stereoscopic PIV system for purpose of comparison. Preliminary tests showedthat the PIV camera could record directly through the holographic plates, withoutvisible loss of quality of the images.

2.1 Wind Tunnel

The experiments were performed in the wind tunnel shown in Fig. 6. The testsection is 2 m wide by 1 m high. Its full length is 21.6 m. The last 5 meters areoptically accessible from all sides. The free stream velocity can be set from 2 to10 m/s. An open loop configuration is obtained by opening two doors in the re-turn circuit of the wind tunnel. In this configuration, the flow seeding is notrecirculating.

For the holographic experiments, the tunnel was first operated in closed loop inorder to obtain a homogeneous seeding of the whole flow (using a PEG smokegenerator). The poor quality of the holograms recorded, due to the per-turbation of the reference beams by the seeding, motivated a change toward to theopen loop. It was then possible to seed only the region of interest (with micronolive oil dropplets), thereby reducing the noise recorded by the holograms.

Fig. 6. LML boundary layer wind tunnel.

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2.2 Laser system

The laser system was specifically built by BMI. It basically consists of four inde-pendent Nd:Yag laser cavities, each of them producing more than 300 mJ perpulse at 12.5 Hz. The pulse duration is 5 ns. This system can be modified to a con-figuration with two amplified cavities (Fig. 7). In this case, the energy delivered isaround 600 mJ per pulse. In this configuration, the two lasers can be seeded, in-creasing the coherence length from a few cm to more than 2 m. The seeding sys-tem, however, includes an opto-electronic feed-back loop which requires a suffi-cient number of pulses before reaching stabilization. Preliminary tests did show(i) that the energy per pulse (close to 600 mJ) should be high enough, although theholographic plates used are more sensitive to the red than to green light, and (ii)that the coherence length of the seeded cavities was long enough for holographicpurposes. On the synchronization point of view, the laser triggering system wasbypassed by providing the flash lamps and Pockels cells signals directly to eachlaser from the stereo PIV system.

Fig. 7. LML Laser system in twin amplified seeded cavities configuration.

2.3 Holographic recording set-up

As was explained above, the short distance set-up was retained for the wind tunnelexperiment. A scheme is given in Fig. 8. P is a prismatic glass plate (4% reflec-tion) which produces the two reference beams. L

1 + L1 (resp. L

2 + L2) form a

collimator which enlarges the reference beam to 60 mm. D1 and D2 are densitieswhich attenuate the references by a factor of about 25. M are mirrors folding thebeam path. The huge attenuation of the references (about 2.105) accounts for theweak diffusivity of the particles.

Thanks to the preliminary studies, the installation of the holographic system atLML was quite straightforward. The difficulties came with the first recordings.The sensitivity evaluations corresponding to the change of wavelength were cor-rect (from ruby laser at 694 nm to YAG laser at 532 nm), but the specific installa-tion (wind tunnel + laser) generated a very large amount of spurious light. Three

Seeder AmplifiersOscillator on carbon rodsFrequencydoublers


factors were identified. First, the upper and lower windows (made of standardglass) reflect the light sheet and the unavoidable dusts diffuse it in all directions ;as the light sheet was about 105 times more intense than the reference beams, eventhe second-order reflections were very efficient.

The interposition of masks finally reduced this factor to an acceptable amount.Second, the YAG laser being repetitive, it was necessary to extract the useful lightpulse among a train of 100 pulses contributing to the thermal stability of the lasercavity. Of course, the 99 extraneous pulses had to be eliminated and a specificshutter system was designed for this purpose.

The third cause is related to the tunnel configuration. When operated in closedloop the whole tunnel was filled with particles. Thus, although the useful part wasonly about 10 cm high, the effective length of the light sheet was equal to theheight of the tunnel (1 meter). The importance of this point is increased by the factthat the light diffused by the lower (useless) part of the light sheet is much brighterdue to a smaller angle of diffusion. Thus it was necessary to seed the tunnel onlyin a small region surrounding the useful zone.

Laser

Lens

CL

P1

P2

M1

M'1

D1

L1

H1

M2M'2

H2

L2

D2

Lighttrap

L1

L2

window

window

1 m

Y

X

Z

area of interest

Fig. 8. Holographic optical set-up used for the wind tunnel experiments.

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An important feature of this experiment is that the holographic recordings wereachieved simultaneously with more classical recording of the particle images ontwo CCD cameras located behind the holograms (Fig. 9). This allows a directcomparison of the images of the light sheet as well as of the corresponding veloc-ity maps.

Indeed recording the CCD images through the holograms allows to perform(and record) a holographic restitution quite easily : once the experiment isfinished, 2 holograms and the 2 corresponding CCD image pairs are available; thelatter can be saved to disk and the holograms developed in a dark room ; then put-ting the holograms back in front of the cameras and illuminating them with onlythe reference beam, allow to observe the double-exposed particle images, whichcan be acquired as if the cameras were looking at real particles. The direct and re-constructed CCD images can then be compared, and also processed by standardPIV algorithms (either auto or cross-correlation) ; again the resulting velocityfields can be compared very easily. However, such a set-up would not be possiblein the general case of a full 3D holographic measurement (not limited to a 2D lightsheet) and the holographic restitution set-up of

ONERA has also been used to demonstrate the possibility of afterward recon-struction.

2.4 Stereoscopic PIV set-up

2.4.1 Recording set-up

In PIV, by combining the information from 2 cameras, it is possible to obtain thethird velocity component, resulting in 2D3C velocity fields. This double PIV con-figuration, called stereo PIV, is now a mature technique (see for instance the re-view by Prasad [1]). For the present study a commercially available system wasused, namely the FlowMaster package from LaVision.

H1H2

Light trap

window

CCD 1CCD 2

Y

X

Z

H

L

Fig. 9. Simultaneous recording of holograms and on PIV CCD cameras on top of thewind tunnel.


The light sheet set-up was common to the holographic set-up. The implementa-tion of the recording set-up is shown in Fig. 9. The camera field of view was ini-tially positioned very close to the upper boundary of the flow. However, it ap-peared that the holographic restitution was more efficient for the particles thatwere located deeper in the wind tunnel ; this fact was evidenced by visual in-spection during the ‘in situ’ restitution stage and is probably due to the smaller an-gle of diffusion (and thus the increased amount of light coming from theparticles) as the viewing direction gets more vertical.

Thus the approximate vertical distance from the optical center of camera lensesto the middle of the field of view was H = 60 cm (compared to L = 40 cm hori-zontally).

Initially the light sheet was wide enough to use the full field of view of thecameras (approximately 4 cm wide by 9.5 cm high, see below) but later the extentof the light sheet in the Y direction was reduced to about 2 cm in order to get ahigher object beam intensity for the holograms.

2.4.2 Acquisition and processing of digital images

The LaVision hardware and software used consist of - two PCO SensiCam cam-eras and their interface boards for PCI bus. SensiCam cameras are able to acquire1280x1024 image pairs with interframing time as small as 200 ns, in the limit of 8images/s. They are Peltier cooled and provide a 12 bits dynamic range. - An ISAbus board which generates the trigger signals for the laser and cameras : this iscalled the Programmable Timing Unit (PTU). - The DaVis software provides agraphical interface for (i) controlling the data acquisition (PTU and cameraboards) and (ii) processing the images (including stereoPIV calibration).

2.4.3 Scheimpflug mounts, optics and calibration grid

In order to use the angular configuration shown in Fig. 9, one needs to tilt theoptical axis of the camera lens with respect to the CCD plane in order to fulfill theScheimpflug condition. A special Scheimpflug mount was developed for thatpurpose. The camera on the mount is shown in Fig. 10. Nikon lenses of focallength f = 100 mm were used in these experiments with M = f/(H2+L2)1/2 = 0.14.

Since the aperture was limited to f# = 22 during the experiments, the diffractionspot size was ddiff = 2.4 f# (M+1) λYag = 32 µm i.e. more than four pixels of a PCOSensiCam. Such particle images are big in terms of ideal PIV analysis, but owingto the large amount of light needed by the holographic plates, this small aper-ture was necessary.

In order to determine accurately the geometrical properties of the stereoPIV set-up, the DaVis system relies on a set of 4 grid images: two calibration images (onefor each camera) must be taken first, then a known displacement is applied to thereference grid in the direction perpendicular to the light sheet, and two other cali-bration images are taken. This displacement was accomplished using a combina-tion of 2 three-axes translation stages, which also allowed to check the parallelismbetween the calibration grid and the light sheet. The cameras being located on op-

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posite sides of the light sheet in the present set-up, the calibration procedure re-quired a transparent grid, upon which some crosses were regularly spaced (at 3.4mm intervals. The magnification in the center of the field of view was around 78µm/pixel along X and 40 µm/pixel along Y.

Fig. 10. Camera on Scheimpflug mount.

2.5 Synchronization

The laser system used at LML was synchronized using the PTU includedin the FlowMaster, which delivered 5 trigger signals :

• 2 flash-lamp signals and 2 Pockels triggers (one for each laser pulse). Theflashlamps were run continuously (with a 12 Hz frequency) to stabilize the ther-mal state of the laser system. The Pockels signals were only generated (that is, thelaser fired) for those cycles when images were actually taken from the cameras.

• 2 camera triggers : one for each PCO SensiCam. They were activated at 8 Hzto take into account the maximum framing rate of the camera.

• 1 signal to open an external shutter in front of the laser source.This shutter is needed due to the fact that several laser pulses are necessary be-

fore the coherence length of the seeded laser is guaranteed. The light generated bythese preliminary pulses, must not get to the holographic plates which should rec-ord one and only one double pulse. A Compur shutter was in front of the lasersource. This shutter was driven by the FlowMaster PTU.


2.6 Holographic reconstruction set-up

For the exploitation of the holograms recorded, the holographic reconstructionbench of ONERA [6] was modified and a specific software was developed. Ascheme is showed in Fig. 11. The light source is an Argon-Ion laser. The outputpower is about 200 mW at 514.5 nm (TEM00, linearly polarized). This wave-length is very close to the one used at recording (532 nm). In this condition, themagnification is nearly unchanged. First, the laser beam passes through a Galileantype expander. The magnification is about 60. It allows to generate a uniform ref-erence beam for the reconstruction of holographic images. The hologram ismounted on a rotating support which allows to adjust the angle between the refer-ence beam and the holographic plate. This support is fixed on a translation stageallowing to move the hologram in three orthogonal directions. This last mechani-cal system consists of precision translation stages. Each of them is equipped with astepper motor. The reconstructed particle image field is recorded with a 512x512pixel CCD-camera. By moving the hologram parallel to the image-plane of thecamera, it is possible to digitize a large area of the reconstructed particle imagefield. A software developed specially for this study, allows to drive simultane-ously the laser, the camera and the displacements of the holograms. Two photo-graphs of the reconstruction holographic bench are shown in Fig. 12.

Argon Laser

Beam-Expander

Holographic image

CCD Camera

Hologram

Reference beam Reconstruction beam

Fig. 11. Scheme of the holographic reconstruction set-up.

Fig. 12. Photographs of the reconstruction set-up.

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3 Results

The purpose of the experiments reported here was not to study thoroughly the dy-namics of a given flow; the stress was put on recording techniques and the effortswere devoted to enhancing the implementation of the chosen holographic set-up,rather than varying the flow parameters. The results presented in this section arebased on a couple of records performed in the same experimental conditions; theyare representative of the quality of measurement that was achieved during the ex-perimental program. The field of view is located in the vicinity of the (horizontal)upper window and the free stream velocity was set to approximately 5 m/s.

3.1 Image quality

The raw data for PIV are of course particle images ; Fig. 13a shows an example of« direct » images (obtained during the course of the experiment) as delivered bycamera #2 (see Fig. 4) ; the flow is directed « upward » and the wind tunnel win-dow is located at the left of the picture. As already mentioned, one can observe inFig. 13 that (i) the light sheet extent has been reduced (in order to increase the il-lumination intensity), (ii) the particle images are relatively large (small aperturedue to the amount of light) and (iii) the light sheet is inclined (to get rid of the wallreflections more efficiently).

a) b)

Fig. 13. PIV image recorded by camera #2. Image a) is the direct image recorded simulta-neously with the hologram (single exposure). b) is the image recorded after in situ holo-graphic reconstruction (double exposure).

Fig. 13b shows the corresponding reconstructed holographic image, providedby the same camera and upon reference beam illumination of the developed holo-graphic plate ; these images were typically acquired with an aperture of f# = 5.6.The image is double-exposed as the holographic plate (the PIV analysis will thenrely on auto-correlation computations). The possibility to compare direct and re-


constructed images of the same particles (e.g. Fig. 13a and 13b) is indeed a uniquefeature of the experiments reported here.

Fig. 13b is also interesting in that it exhibits a rightward increase of image in-tensity, that is downward in the laboratory frame of reference since the cameraconsidered here is #2. As already mentioned, this can be partly explained by thesmaller angle of diffusion corresponding to those particles which are located atgreater X (see Fig. 3) and that is why the field of view was moved downward inthe course of the experiments. Fig. 13 b is representative of the best image qualitywhich could be obtained in the above described configuration. In fact, as will beseen later, the image quality of hologram H2 was of poorer quality, due to spuri-ous light diffused by one optical component which was partly burned during thecourse of the experiment. Harmfully this was discovered when dismounting theset-up at the end of the test campaign.

3.2 StereoPIV results

For the purpose of comparison, care was taken to acquire CCD images and holo-graphic prints of a common reference target. The coordinates of the two end pointsof this target were obtained with an accuracy of ±0.1 mm. As already explained inthe previous section, the full extent of the field of view will not be exploited here ;Fig. 14 shows the area located at 5 mm < X < 60 mm. The stereoscopic recon-struction was performed using the DaVis software based on the Soloff algorithm.The velocity scaling of the pixel displacements is provided by (i) mapping func-tions determined at calibration stage and (ii) input of the time separation betweenthe two laser pulses (80 µs in the experiments reported here).

First of all, the flow area obtained from the holographic reconstruction is muchsmaller than the stereoPIV one. This is due to the poor quality of hologram H2, asindicated above. In the reconstructed area, the overall magnitude of the velocitiesis consistent with the free stream velocity of 5 m/s, as measured by a Pitot tube.Furthermore, both plots of Fig. 14 exhibit the same flow directions and magni-tudes, for the in-plane components (vector plot) as well as for the out-of-planecomponent (contour plot).

As a conclusion, one can say that the agreement between both methods isqualitatively satisfying, although owing to the experimental difficulties encoun-tered, a more accurate (statistical) error analysis is useless.

3.3 Results from the holographic restitution set-up at ONERA

During the LML tests, two good pairs of PIV holograms were recorded. Only oneset of these holographic plates was reconstructed at ONERA. As the two expo-sures were recorded on the same holographic plate, the holograms were analyzedusing the auto-correlation method. In this condition, the direction of particles mo-tion cannot be determined directly from the holograms. This sign, which is thesame all over the field, was derived from a priori knowledge of the flow. To im-prove the measurement accuracy, an integer shifting technique was applied. The

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position of the two interrogation windows was offset with respect to each otheraccording to the mean displacement vector.

x

y

10 20 30 40 50 60-10

0

10

Vz: -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.51 m/s

x

y

10 20 30 40 50 60-10

0

10

Vz: -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.51 m/s

Fig. 14. comparison of the 2D3C velocity fields directly recorded by CCD (top) and recon-structed from the holographic images (bottom). The in-plane components (VX, VY) arerepresented as vectors and the out-of-plane component VZ is shown by the contour plot.The x and y axes correspond to the reference frame defined in Fig. 3 and 4 ; the unit usedhere is mm. A speed of 4.5 m/s in the y direction was globally removed from both fields.

As the object-plane of the CCD camera is not superimposed with the recon-structed particle image field (see Fig. 11), the particle images are correctly focusedin a central band of CCD. Outside this band, they are blurred. Therefore, it is nec-essary to move the holograms in the object-plane of the CCD camera to scan thewhole holographic image as illustrated by Fig. 15.


a) b)Fig. 15. PIV images obtained by scanning the holograms on the holographic reconstructionbench. a) hologram H1, b) hologram H2.

x

y

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50-10

0

10

Vz: -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.51 m/s

Fig. 16. 3C velocity map reconstructed from the analysis of holograms H1 and H2.

Due to the simple arrangement of the two holograms (they are at 45° to thelight sheet), it is easy to reconstruct the three components of the velocity fieldfrom the two velocity maps provided by the analysis of the images in Fig. 15. Theresult of this reconstruction is shown in the Fig. 16. The in-plane components (VX,VY) are represented as vectors and the out-of-plane component VZ is shown by thecontour plot. The size of these velocity fields is about 115mmx15mm, which ismuch wider than the field recorded by the CCD camera.

In fact, the light sheet was recorded by the holograms down to the lower wall ofthe wind tunnel (1m), but only the upper boundary layer was seeded. Because ofthe spurious reflections mentioned above in the recording set-up of the hologramH1, the quality of its reconstructed particle images field is lower than the one ofH2. This difference of quality affects the velocity map. Nevertheless, the velocitymagnitude and direction is in good agreement with the preceding results. This isemphasized in Fig. 17 which shows a zoom in Fig. 16 scaled with the plots shownin Fig. 14. The comparison shows that the overall shape of the velocity field is

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comparable, but it also clearly illustrates the fact that the holographic results suffersignificantly from the poorer quality of hologram H1.

x

y

10 20 30 40 50 60-10

0

10

Vz: -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.51 m/s

Fig. 17. Zoom in the right part of Fig. 16 to be compared to the stereo PIV and in-situ re-construction results of Fig. 14.

4 Conclusion

An experiment was conducted to investigate the possibilities of applying holo-graphic PIV in a large scale facility. Preliminary laboratory experiments did showthat good quality images could be recorded with two different set-ups : a longdistance and a short distance set-up. The short distance set-up was retained for thelarge scale tests for the sake of simplicity of the optical arrangement.

From these large scale experiments, the following conclusion can be drawn :- The fact of moving the holographic set-up from the laboratory to the wind

tunnel has increased significantly the spurious light due to all kinds of reflec-tions and diffusion.

- The localization of these spurious light sources was very tedious and not fullysuccessful, due mainly to the size of the facility. The use of the long distanceset-up would have allowed to solve this problem more easily.

- The sensitivity of the holographic plates was a real problem, although Agfa10E75 were used and the laser energy was 600 mJ/pulse.

- The in situ reconstruction of the holograms during the tests was mandatory inorder to obtain usable holograms.

- A priory check of the ability of each optical component to sustain the highlight power is also mandatory, as these components are not easy to accessduring the tests.

- The fact that the holograms have to be chemically processed makes the ex-periment much more tedious. This was particularly evidenced here as a stereoPIV set-up was operating simultaneously. It took one day to set-up and was


providing the velocity maps in nearly real time, while the processing of theholograms took about 1 hour before a possible reconstruction.

Nevertheless, two pairs of exploitable holograms were recorded and one of thosewas reconstructed and processed a posteriori. The results show that the velocityfield could be recovered in a wider field than with the direct stereo CCD set-up.But the accuracy of the measurements could not be assessed.The present global conclusion is that hologaphic PIV has a potential of applicationaround large facilities if a suitable recording medium is available. Presently, thephotographic material available is not sensitive enough and to heavy to process topermit useful experiments in such a situation.

References

[1] “ Particle image Velocimetry : Toward Industrial Application”M. Stanislas, J. Kompenhans, J. Westerweel Editors.Kluwer Academic Press, March 2000.[2] C. Brücker 1996 : 3-D scanning Particle Image Velocimetry (3-D SPIV)VKI LS 1996-03 on Particle Image Velocimetry.[3] K. D. Hinsch 2002 : Holographic particle image velocimetryMeas. Sci. Technol. 13 No 7 (July 2002) R61-R72[4] L. Lourenco 1988 : Some comments on particle image displacement velocimetry, VonKarman Institute for Fluid Dynamics, Lecture Series 1988-06.[5] A.K. Prasad 2000 : Stereoscopic particle image velocimetry. Experiments inFluids 29, 103-116.[6] C. Geiler, M. Stanislas, H. Royer, T. Fournel 1992 : Automatic assessment ofaerosol holograms for granulometry and velocimetry.Sixth international symposium on flow visualization, Yokohama, October 1992.

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