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, France
Laboratoire de Mécanique de Lille UMR3701, Bd Paul Langevin, Cité Scientifique, 59655 Villeneuve d’Ascq cedex, France
3
ONERA Centre de Lille, 5 Bd Paul Painlevé, 59045 Lille Cedex, France
2
Abstract
An holographic PIV experiment has been performed in a large scale wind tunnel
in order to assess the interest of the method for such facility. The holographic
measurements were compared to simulataneous stereoscopic PIV measurements.
The results show that that hologaphic PIV has a potential of application around
large facilities if a suitable recording medium is available. Presently, the photographic material available is not sensitive enough and to heavy to process to permit useful experiments in such a situation.
1 Introduction
”Particle Image Velocimetry” (PIV) is a unique optical (non intrusive) method
which allows to capture instantaneously whole velocity fields in fluids. Its usefulness has been demonstrated in a previous EUROPIV program [1]. The present
contribution is part of the EUROPIV 2 program and deals with the extension of
PIV to holographic PIV. Digital PIV is able nowadays to provide 2D2C or 2D3C
data in industrial facilities in a quite efficient way. As industrial flows are often
quite complex and 3D, there is a need to extend in the future PIV to 3D3C measurements. To do so, two tracks are presently possible: scanning light sheet stereoscopic PIV [2] and holographic PIV [3]. Due to technical limitations, the first approach 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 that
purpose, the application of a stereoscopic holographic set-up to a large wind tunnel was invesigated. The aim of the present contribution is to present the main results obtained.
The Europiv project [1] did show that holographic recording of micron particles in a light sheet is possible, even at long distance, with the advantage of getting tiny particle images (~10 µm) as compared to standart recording through
lenses (~ 30 µm).
One of the main drawbacks put to evidence in this first Europiv program was
that 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 components
which are parallel to the holographic plate [4]. This is why it was decided to investigate a more sophisticated stereoscopic holographic set-up, allowing the same
accuracy on the three velocity components, and to assess the problems linked to
the use of such a set-up around a large wind tunnel.
H1
Reference 1
Light Sheet
Jet
Reference 2
H2
Fig. 1. Holographic recording set-up based on two holograms and allowing the same accuracy on the three velocity components.
M'1
Laser
L1
D1
Lens
H2
M2
P2
L2
L1
CL
Light
trap
P1
M1
H1
D2
M'2
L2
Fig. 2. Short distance recording set-up. The two holograms are set symmetrically on both
sides of the light sheet. They are illuminated by the light scattered by the particles (dotted
line) and by the reference beam (continuous line). The first lens focusses the laser beam in
the region of interest. The cylindrical lens (CL) makes the light sheet. A telescope is set on
each 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 holograms
are set at ± 45° from the light sheet and record simultaneously the particle images.
At reconstruction, the accurate velocity components measured parallel to each
hologram 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 component
are obtained by simple addition and subtraction of the components in the plane of
the holograms.
For this study, the LML boundary layer wind tunnel, which is 1 m x 2 m in
cross section, was selected as representative of larger facilities. The first half of
the project was used to prepare the experiment. Different holographic set-ups
were tested. The laser, the synchronization, the seeding, the digital stereoscopic
PIV measurements, the implementation of the holographic set-up around the wind
tunnel and the reconstruction set-up for post-processing of the holograms were
prepared. 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 select
the best suited one for the large scale experiment. Two set-ups were retained and
used 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 holograms
are very near the flow region (about 25 cm in the present case) and the light diffracted by the particles is recorded directly on the plates. The advantage of this
set-up is the simplicity, the main complexity being the set-up to generate two reference beams of equivalent characteristics.
Left image
Right image
Fig. 3. Sample of reconstructed particle images obtained with the short distance set-up of
Fig. 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 the
recording, with a pulse energy of 2 x 75 mJ. The holographic plates were Agfa
10E75. A sample of reconstructed particle images is given in Fig. 3 for each hologram. The reconstructed images are of good quality (the aberration spots have a
typical size of 10 x10 x 40 µm) .
The second set-up tested is illustrated in Fig. 4. In this case the holograms are
far away from the flow and a relay lens is used to image the light sheet nearby the
hologram. 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 region of interest is here of 1.5 m. The drawbacks are : - a loss of light, a more sensitive set-up and the need to reconstruct the image through the relay lens in order
to correct for the optical aberrations. The same ligth sheet, laser and holograms
were used as for the short distance set-up. Only the pulse energy was pushed to 2
x 200 mJ.
Tests were performed on a free jet at 20 m/s with a time separation of 20 ms
between the two exposures. Fig. 4 presents the images reconstructed from both
holograms through the imaging lenses. The image quality is good and the particle
image pairs are clearly visible.
m'2
m'1
BS1
M
light trap
CL
jet
BS2
D1
RL1
L2
L1
m2
D2
C2
m1
RL2
H1
C1
H2
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 tunnel. The relay lens RL1 and RL2, with a focal length of 0.75 m, do image the light sheet
nearby the holograms.
Fig. 5. Reconstructed images from the right and left holograms of the set-up in Fig. 4.
Holography and ESPI 337
2 Experimental Set-up
The preliminary test performed having shown that both set-ups were giving good
quality holograms, it was decided to retain the set-up at short distance for three
main reasons : - the flow region retained for the investigation was near the upper
wall of the wind tunnel (imaging was not mandatory), - the recording optical setup was simpler (which is important around a large facility), - the reconstruction
set-up was simpler too as, without imaging lens at recording, the holographic image can be observed directly at reconstruction. Besides, it was necessary to set-up
a stereoscopic PIV system for purpose of comparison. Preliminary tests showed
that the PIV camera could record directly through the holographic plates, without
visible loss of quality of the images.
2.1 Wind Tunnel
The experiments were performed in the wind tunnel shown in Fig. 6. The test
section is 2 m wide by 1 m high. Its full length is 21.6 m. The last 5 meters are
optically accessible from all sides. The free stream velocity can be set from 2 to
10 m/s. An open loop configuration is obtained by opening two doors in the return circuit of the wind tunnel. In this configuration, the flow seeding is not
recirculating.
For the holographic experiments, the tunnel was first operated in closed loop in
order to obtain a homogeneous seeding of the whole flow (using a PEG smoke
generator). The poor quality of the holograms recorded, due to the perturbation of the reference beams by the seeding, motivated a change toward to the
open loop. It was then possible to seed only the region of interest (with micron
olive 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 independent Nd:Yag laser cavities, each of them producing more than 300 mJ per
pulse at 12.5 Hz. The pulse duration is 5 ns. This system can be modified to a configuration with two amplified cavities (Fig. 7). In this case, the energy delivered is
around 600 mJ per pulse. In this configuration, the two lasers can be seeded, increasing the coherence length from a few cm to more than 2 m. The seeding system, however, includes an opto-electronic feed-back loop which requires a sufficient 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 the
holographic 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 holographic
purposes. On the synchronization point of view, the laser triggering system was
bypassed by providing the flash lamps and Pockels cells signals directly to each
laser from the stereo PIV system.
Frequency
Oscillator on carbon rods
Amplifiers
Seeder
doublers
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 tunnel
experiment. A scheme is given in Fig. 8. P is a prismatic glass plate (4% reflection) which produces the two reference beams. L1 + L1 (resp. L2 + L2) form a
collimator which enlarges the reference beam to 60 mm. D1 and D2 are densities
which attenuate the references by a factor of about 25. M are mirrors folding the
beam path. The huge attenuation of the references (about 2.105) accounts for the
weak diffusivity of the particles.
Thanks to the preliminary studies, the installation of the holographic system at
LML was quite straightforward. The difficulties came with the first recordings.
The sensitivity evaluations corresponding to the change of wavelength were correct (from ruby laser at 694 nm to YAG laser at 532 nm), but the specific installation (wind tunnel + laser) generated a very large amount of spurious light. Three
Holography and ESPI 339
factors were identified. First, the upper and lower windows (made of standard
glass) 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, even
the 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 light
pulse among a train of 100 pulses contributing to the thermal stability of the laser
cavity. Of course, the 99 extraneous pulses had to be eliminated and a specific
shutter system was designed for this purpose.
The third cause is related to the tunnel configuration. When operated in closed
loop the whole tunnel was filled with particles. Thus, although the useful part was
only about 10 cm high, the effective length of the light sheet was equal to the
height of the tunnel (1 meter). The importance of this point is increased by the fact
that the light diffused by the lower (useless) part of the light sheet is much brighter
due to a smaller angle of diffusion. Thus it was necessary to seed the tunnel only
in a small region surrounding the useful zone.
Light
trap
H1
H2
window
Y
Z
area of interest
X
1m
window
CL
L1
L2
P2
P1
M2
M'1
M'2
L2
D1
D2
L1
M1
Lens
Laser
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 were
achieved simultaneously with more classical recording of the particle images on
two CCD cameras located behind the holograms (Fig. 9). This allows a direct
comparison of the images of the light sheet as well as of the corresponding velocity maps.
Indeed recording the CCD images through the holograms allows to perform
(and record) a holographic restitution quite easily : once the experiment is
finished, 2 holograms and the 2 corresponding CCD image pairs are available; the
latter can be saved to disk and the holograms developed in a dark room ; then putting the holograms back in front of the cameras and illuminating them with only
the reference beam, allow to observe the double-exposed particle images, which
can be acquired as if the cameras were looking at real particles. The direct and reconstructed CCD images can then be compared, and also processed by standard
PIV algorithms (either auto or cross-correlation) ; again the resulting velocity
fields can be compared very easily. However, such a set-up would not be possible
in the general case of a full 3D holographic measurement (not limited to a 2D light
sheet) and the holographic restitution set-up of
ONERA has also been used to demonstrate the possibility of afterward reconstruction.
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 the
third velocity component, resulting in 2D3C velocity fields. This double PIV configuration, called stereo PIV, is now a mature technique (see for instance the review by Prasad [1]). For the present study a commercially available system was
used, namely the FlowMaster package from LaVision.
Light trap
CCD 1
CCD 2
L
H
H2
H1
window
Y
Z
X
Fig. 9. Simultaneous recording of holograms and on PIV CCD cameras on top of the
wind tunnel.
Holography and ESPI 341
The light sheet set-up was common to the holographic set-up. The implementation of the recording set-up is shown in Fig. 9. The camera field of view was initially positioned very close to the upper boundary of the flow. However, it appeared that the holographic restitution was more efficient for the particles that
were located deeper in the wind tunnel ; this fact was evidenced by visual inspection during the ‘in situ’ restitution stage and is probably due to the smaller angle of diffusion (and thus the increased amount of light coming from the
particles) as the viewing direction gets more vertical.
Thus the approximate vertical distance from the optical center of camera lenses
to the middle of the field of view was H = 60 cm (compared to L = 40 cm horizontally).
Initially the light sheet was wide enough to use the full field of view of the
cameras (approximately 4 cm wide by 9.5 cm high, see below) but later the extent
of the light sheet in the Y direction was reduced to about 2 cm in order to get a
higher 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 cameras and their interface boards for PCI bus. SensiCam cameras are able to acquire
1280x1024 image pairs with interframing time as small as 200 ns, in the limit of 8
images/s. They are Peltier cooled and provide a 12 bits dynamic range. - An ISA
bus board which generates the trigger signals for the laser and cameras : this is
called the Programmable Timing Unit (PTU). - The DaVis software provides a
graphical interface for (i) controlling the data acquisition (PTU and camera
boards) 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 the
optical axis of the camera lens with respect to the CCD plane in order to fulfill the
Scheimpflug condition. A special Scheimpflug mount was developed for that
purpose. The camera on the mount is shown in Fig. 10. Nikon lenses of focal
length 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 diffraction
spot size was ddiff = 2.4 f# (M+1) λYag = 32 µm i.e. more than four pixels of a PCO
SensiCam. Such particle images are big in terms of ideal PIV analysis, but owing
to the large amount of light needed by the holographic plates, this small aperture was necessary.
In order to determine accurately the geometrical properties of the stereoPIV setup, the DaVis system relies on a set of 4 grid images: two calibration images (one
for each camera) must be taken first, then a known displacement is applied to the
reference grid in the direction perpendicular to the light sheet, and two other calibration images are taken. This displacement was accomplished using a combination of 2 three-axes translation stages, which also allowed to check the parallelism
between 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 required a transparent grid, upon which some crosses were regularly spaced (at 3.4
mm intervals. The magnification in the center of the field of view was around 78
µm/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 included
in the FlowMaster, which delivered 5 trigger signals :
• 2 flash-lamp signals and 2 Pockels triggers (one for each laser pulse). The
flashlamps were run continuously (with a 12 Hz frequency) to stabilize the thermal state of the laser system. The Pockels signals were only generated (that is, the
laser 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 Hz
to 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 before the coherence length of the seeded laser is guaranteed. The light generated by
these preliminary pulses, must not get to the holographic plates which should record one and only one double pulse. A Compur shutter was in front of the laser
source. This shutter was driven by the FlowMaster PTU.
Holography and ESPI 343
2.6 Holographic reconstruction set-up
For the exploitation of the holograms recorded, the holographic reconstruction
bench of ONERA [6] was modified and a specific software was developed. A
scheme is showed in Fig. 11. The light source is an Argon-Ion laser. The output
power is about 200 mW at 514.5 nm (TEM00, linearly polarized). This wavelength is very close to the one used at recording (532 nm). In this condition, the
magnification is nearly unchanged. First, the laser beam passes through a Galilean
type expander. The magnification is about 60. It allows to generate a uniform reference beam for the reconstruction of holographic images. The hologram is
mounted on a rotating support which allows to adjust the angle between the reference beam and the holographic plate. This support is fixed on a translation stage
allowing to move the hologram in three orthogonal directions. This last mechanical system consists of precision translation stages. Each of them is equipped with a
stepper motor. The reconstructed particle image field is recorded with a 512x512
pixel CCD-camera. By moving the hologram parallel to the image-plane of the
camera, it is possible to digitize a large area of the reconstructed particle image
field. A software developed specially for this study, allows to drive simultaneously the laser, the camera and the displacements of the holograms. Two photographs of the reconstruction holographic bench are shown in Fig. 12.
Holographic image
Hologram
Argon Laser
Beam-Expander
CCD Camera
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 dynamics of a given flow; the stress was put on recording techniques and the efforts
were devoted to enhancing the implementation of the chosen holographic set-up,
rather than varying the flow parameters. The results presented in this section are
based on a couple of records performed in the same experimental conditions; they
are representative of the quality of measurement that was achieved during the experimental 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 by
camera #2 (see Fig. 4) ; the flow is directed « upward » and the wind tunnel window is located at the left of the picture. As already mentioned, one can observe in
Fig. 13 that (i) the light sheet extent has been reduced (in order to increase the illumination intensity), (ii) the particle images are relatively large (small aperture
due to the amount of light) and (iii) the light sheet is inclined (to get rid of the wall
reflections more efficiently).
a)
b)
Fig. 13. PIV image recorded by camera #2. Image a) is the direct image recorded simultaneously with the hologram (single exposure). b) is the image recorded after in situ holographic reconstruction (double exposure).
Fig. 13b shows the corresponding reconstructed holographic image, provided
by the same camera and upon reference beam illumination of the developed holographic 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 then
rely on auto-correlation computations). The possibility to compare direct and re-
Holography and ESPI 345
constructed images of the same particles (e.g. Fig. 13a and 13b) is indeed a unique
feature of the experiments reported here.
Fig. 13b is also interesting in that it exhibits a rightward increase of image intensity, that is downward in the laboratory frame of reference since the camera
considered here is #2. As already mentioned, this can be partly explained by the
smaller angle of diffusion corresponding to those particles which are located at
greater X (see Fig. 3) and that is why the field of view was moved downward in
the course of the experiments. Fig. 13 b is representative of the best image quality
which could be obtained in the above described configuration. In fact, as will be
seen later, the image quality of hologram H2 was of poorer quality, due to spurious light diffused by one optical component which was partly burned during the
course of the experiment. Harmfully this was discovered when dismounting the
set-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 holographic prints of a common reference target. The coordinates of the two end points
of this target were obtained with an accuracy of ±0.1 mm. As already explained in
the 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 reconstruction was performed using the DaVis software based on the Soloff algorithm.
The velocity scaling of the pixel displacements is provided by (i) mapping functions determined at calibration stage and (ii) input of the time separation between
the two laser pulses (80 µs in the experiments reported here).
First of all, the flow area obtained from the holographic reconstruction is much
smaller than the stereoPIV one. This is due to the poor quality of hologram H2, as
indicated above. In the reconstructed area, the overall magnitude of the velocities
is 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 magnitudes, for the in-plane components (vector plot) as well as for the out-of-plane
component (contour plot).
As a conclusion, one can say that the agreement between both methods is
qualitatively satisfying, although owing to the experimental difficulties encountered, 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 one
set of these holographic plates was reconstructed at ONERA. As the two exposures were recorded on the same holographic plate, the holograms were analyzed
using the auto-correlation method. In this condition, the direction of particles motion cannot be determined directly from the holograms. This sign, which is the
same all over the field, was derived from a priori knowledge of the flow. To improve 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 other
according to the mean displacement vector.
Vz: -0.5 -0.4 -0.3 -0.2 -0.1
0
1 m/s
0.1 0.2 0.3 0.4 0.5
10
y
0
-10
10
20
Vz: -0.5 -0.4 -0.3 -0.2 -0.1
30
0
x
40
50
60
1 m/s
0.1 0.2 0.3 0.4 0.5
10
y
0
-10
10
20
30
x
40
50
60
Fig. 14. comparison of the 2D3C velocity fields directly recorded by CCD (top) and reconstructed from the holographic images (bottom). The in-plane components (VX, VY) are
represented 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 used
here 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 reconstructed particle image field (see Fig. 11), the particle images are correctly focused
in a central band of CCD. Outside this band, they are blurred. Therefore, it is necessary to move the holograms in the object-plane of the CCD camera to scan the
whole holographic image as illustrated by Fig. 15.
Holography and ESPI 347
a)
b)
Fig. 15. PIV images obtained by scanning the holograms on the holographic reconstruction
bench. a) hologram H1, b) hologram H2.
Vz: -0.5 -0.4 -0.3 -0.2 -0.1 0
1 m/s
0.1 0.2 0.3 0.4 0.5
y
10
0
-10
-60
-50
-40
-30
-20
-10
x
0
10
20
30
40
50
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 the
light sheet), it is easy to reconstruct the three components of the velocity field
from the two velocity maps provided by the analysis of the images in Fig. 15. The
result 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 the
contour plot. The size of these velocity fields is about 115mmx15mm, which is
much wider than the field recorded by the CCD camera.
In fact, the light sheet was recorded by the holograms down to the lower wall of
the wind tunnel (1m), but only the upper boundary layer was seeded. Because of
the spurious reflections mentioned above in the recording set-up of the hologram
H1, the quality of its reconstructed particle images field is lower than the one of
H2. This difference of quality affects the velocity map. Nevertheless, the velocity
magnitude and direction is in good agreement with the preceding results. This is
emphasized in Fig. 17 which shows a zoom in Fig. 16 scaled with the plots shown
in 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 suffer
significantly from the poorer quality of hologram H1.
1 m/s
Vz: -0.5 -0.4 -0.3 -0.2 -0.1 0
0.1 0.2 0.3 0.4 0.5
y
10
0
-10
10
20
30
x
40
50
60
Fig. 17. Zoom in the right part of Fig. 16 to be compared to the stereo PIV and in-situ reconstruction results of Fig. 14.
4 Conclusion
An experiment was conducted to investigate the possibilities of applying holographic PIV in a large scale facility. Preliminary laboratory experiments did show
that good quality images could be recorded with two different set-ups : a long
distance and a short distance set-up. The short distance set-up was retained for the
large 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 reflections and diffusion.
- The localization of these spurious light sources was very tedious and not fully
successful, due mainly to the size of the facility. The use of the long distance
set-up would have allowed to solve this problem more easily.
- The sensitivity of the holographic plates was a real problem, although Agfa
10E75 were used and the laser energy was 600 mJ/pulse.
- The in situ reconstruction of the holograms during the tests was mandatory in
order to obtain usable holograms.
- A priory check of the ability of each optical component to sustain the high
light power is also mandatory, as these components are not easy to access
during the tests.
- The fact that the holograms have to be chemically processed makes the experiment much more tedious. This was particularly evidenced here as a stereo
PIV set-up was operating simultaneously. It took one day to set-up and was
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providing the velocity maps in nearly real time, while the processing of the
holograms took about 1 hour before a possible reconstruction.
Nevertheless, two pairs of exploitable holograms were recorded and one of those
was reconstructed and processed a posteriori. The results show that the velocity
field 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 application
around large facilities if a suitable recording medium is available. Presently, the
photographic material available is not sensitive enough and to heavy to process to
permit 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 velocimetry
Meas. Sci. Technol. 13 No 7 (July 2002) R61-R72
[4] L. Lourenco 1988 : Some comments on particle image displacement velocimetry, Von
Karman Institute for Fluid Dynamics, Lecture Series 1988-06.
[5] A.K. Prasad 2000 : Stereoscopic particle image velocimetry. Experiments in
Fluids 29, 103-116.
[6] C. Geiler, M. Stanislas, H. Royer, T. Fournel 1992 : Automatic assessment of
aerosol holograms for granulometry and velocimetry.
Sixth international symposium on flow visualization, Yokohama, October 1992.