A JOINT PROGRAM TO IMPROVE PIV
PERFORMANCE FOR
INDUSTRY AND RESEARCH
EUROPIV 2
PROJECT N°:
Coordinator : Pr Stanislas
LML URA CNRS 1441
Bv Paul Langevin, Cité scientifique
F 59655 Villeneuve d'Ascq Cedex
GRD1-1999-10835
Tel: (33) 03 20 33 71 70
Fax: (33) 03 20 33 71 69
Email: stanislas@ec-lille.fr
http : www.univ-lille1.fr/europiv
Publishable Summary December 2003
LML URA CNRS 1441
EADS
DASSAULT AVIATION
CIRA
DLR
ISL
NLR
ONERA
ITAP
F
D
F
IT
D
F/D
NL
F
D
Delft University of Technology
Madrid University Carlos III
Oldenburg University
Rome University La Sapienza
CORIA UMR CNRS 6614
St Etienne University
Zaragoza University
DNW
NL
SP
D
IT
F
F
SP
NL
1. Introduction
Particle Image Velocimetry (PIV) is a unique, optical, non intrusive method to capture whole velocity fields in
flows in a very short time. It is suited especially to help the design of modern aircraft by improving the outputs
of industrial wind tunnel tests. PIV provides much more information in much less time than point measurement
techniques, giving a deeper insight in the flow physics, helping to understand complex flows and reducing
significantly wind tunnel costs. The EUROPIV 2 project was firstly aimed at improving the PIV technique in its
ability to resolve near wall flows, to assess turbulence and to capture vortices. It was also aimed at
demonstrating the usefulness of the method on three aeronautical problems of interest. It had finally the target of
preparing the future of the method by developing advanced research.
The work program of EUROPIV 2 was divided into 5 work packages (Table 1) and extended over 3 years
starting in April 2000. It was concluded by an open workshop which took place in Zaragoza on March 31st and
April 1st . The detailed results of the project were presented at this workshop, together with some significant
research going on outside the EUROPIV consortium. The proceedings of this workshop are published by
Springer Verlag. The aim of the present summary is to synthetise some significant results obtained by the
EUROPIV 2 consortium.
Table 1 : work program organisation
WP
WP 1
WP 2
WP 3
Title
Turbulence &
Assessment of vortical
structures
Full Scale Industrial Tests
DLR
DASA
Near wall flow
DELFT
Manager
Task
T 1.1
T 1.2
T 2.1
T 2.2
T 3.1
T 3.2
T3.3
T3.4
Title
Turbulence
Near wall
flows
Seeding
behaviour
Vorticity
estimation
DASA
Numerical
simulations
Modane
DNW LST
DELFT
DLR
DLR
DASA
DASSAULT
ONERA
Manager
CORIA
1
W/T
S2
NLR
Europiv publishable summary
December 2003
Table 1 : work program organisation (continued)
2.
WP
WP 4
WP 5
Title
Advanced PIV Developments
Management & Exploitation
Manager
ISL
LML
Task
T4.1
T4.2
T5.1
T5.2
Title
Advanced PIV techniques &
algorithms
Holographic
PIV
Management.
Explotation.
Manager
MADRID
ISL
LML
DLR
WP1 ASSESSMENT OF TURBULENCE AND NEAR WALL FLOWS
The main objectives of this work package were to assess the capability of PIV to measure turbulent flow and to
explore the capabilities of taking PIV measurements in near-wall turbulent flows.
1.E-02
36 x 36 interrogation windows
1.E-03
1.E-04
3
2
E11 (m /s )
As far as turbulent flows are concerned,
PIV is bringing a new insight by providing
the
instantaneous
spatial
velocity
distribution. As a recent measurement
technique, its dynamic range, accuracy and
spectral response have to be characterised
in detail and compared to more standard
tools such as hot wire anemometry.
Several teams of the EUROPIV 2
consortium did work on this problem,
using both real and synthetic images. LML
did work on real images and could provide
a model of the PIV spectrum [4] (see
Figure 1). CORIA did use synthetic images
generated from DNS data to characterise in
detail the improvement brought by
advanced PIV algorithms based on image
deformation [1] (see Figure 2).
1.E-05
HWA spectrum
PIV spectrum
Model
1.E-06
1.E-07
kmin
1.E-08
1
10
100
kc
1000
10000
k (rad/m)
Figure 1 : Comparison of turbulence spectrum obtained from
HWA and PIV. The PIV interrogation has been optimised using
the proposed model [4].
Figure 2 : Probability density function of the v velocity component, obtained by the analysis of synthetic
images of isotropic turbulence with three different PIV algorithms : conventional sub-pixel method (CPIV),
continuous window shifting (CWS), Multi-grid continuous window shifting technique with image distortion
(MDPIV). The results are compared to the DNS data. The peak locking effect is clearly visible in the first case.
2
December 2003
Beside the work on synthetic images, a
detailed experiment was performed on
grid turbulence by CIRA. This was done
in order to assess experimentally the
accuracy of the measurement of
turbulence statistics with PIV. Tests were
performed behind different grids, with
different measurement techniques (PIV,
LDA & HWA). The results obtained are
detailed in the Zaragoza proceedings
[39]. Figure 3 illustrates the effect of the
time separation between the two
exposures and of the number of samples
on the mean error on the turbulence
intensity.
mean error %
Europiv publishable summary
16
14
12
10
8
6
4
2
0
100
Dt=70E-6 s
Dt=90E-6 s
Dt=110E-6 s
Dt=45E-6 s - Re=6000
300
500
700
number of analized images
900
Figure 3 : Mean error on the turbulence intensity as a function of
the number of samples for different time separations.
A detailed study of the assessment of lagrangian statistics with Particle tracking was performed by Rome
University (see Zaragoza proceedings [39]). The relation between results on real and synthetic images both in
a turbulent channel flow with both a classical Particle Tracking Velocimetry (PTV) algorithm and an advanced
PTV using Feature Tracking was studied.
Beside the experimental assessment, a Synthetic Image Generator was developed jointly by Delft University,
CORIA and Madrid University. This generator allows the production of realistic synthetic PIV images from a
known velocity field. It works both in the standard and stereoscopic PIV configurations. This appeared a very
useful tool to test the performances of the different algorithms.
Bias
écart moyenne-solution exacte
erreur rms
0.15
0.03
0.02
0.12
0.01
-1
-0.8
-0.6 -0.4
0
-0.2
0
-0.01
0.2
-0.02
0.4
0.6
0.8
1
cas_opt(w=0)
w=1 pix
w=2 pix
w=3 pix
w=4 pix
w=5 pix
cas_opt(w=0)
w=1 pix
w=2 pix
w=3 pix
w=4 pix
w=5 pix
0.09
0.06
0.03
0
-1
-0.03
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
u_exact(pixels)
u_exact(pixels)
Figure 4 : bias and rms error due to the out of plane component in PIV analysis (LML).
This SIG was used by CORIA to generate images
from DNS velocity fields (see Figure 2 and WP4).
It has also been used extensively by LML to test the
effect of various parameters in the bias and the rms
error of cross-correlation algorithms. The detailed
results were presented in Zaragoza (see the
proceedings [39]). Figure 4 illustrates the effect of
the out of plane component on the bias and the rms
errors and figure 5 shows the reduction of the peak
locking brought by sub-pixel shifting techniques.
0.6
HWA
Integer shift
Whittaker
0.5
0.4
0.3
0.2
0.1
0
-4
-3
-2
-1
0
1
2
3
4
Beside this characterisation of PIV algorithms,
u/u'
Delft and Madrid University have tried to define
some design rules in order to keep the PIV accuracy Figure 5 : reduction of peak locking effect by the use of a
within acceptable limits. For example, Madrid sub-pixel interpolation algorithm on PIV images from a
University has identified a new source of peak real turbulent boundary layer (LML).
locking related to the window size [2]. This team
has also worked on the data validation, interpolation and signal to noise ratio improvement [3]. Figures 6 gives
an example of the study by Madrid of different advanced algorithms and of their effect on the RMS error.
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Concerning near wall flows, LML has
performed a stereo PIV experiment
which allowed to put 10 PIV planes
parallel to the wall in the buffer layer
of the LML boundary layer wind
tunnel. The results of this experiment
are under analysis in order to
characterise quantitatively the turbulent
production mechanism in the near wall
region. Work was also performed by
DLR to minimise laser light diffusion
by the model surface in industrial wind
tunnels. This was successfully used in
the test campaign performed in EADS
Bremen wind tunnel in the frame of
WP3.1. Madrid University did finally
Figure 6 : Comparison by Madrid of the performances of different PIV show the ability of advanced LFCPIV
algorithms on synthetic images as a function of the spatial wavelength. algorithms to resolve accurately the
velocity gradients very near the wall
(see Figure 7), opening the possibility of assessing the wall friction from the linear part of the velocity profile.
Viscous sublayer (157 profiles average)
Distance to the wall [pixels]
120
100
80
60
40
20
0
0
1
2
3
4
5
6
7
8
Displacement [pixels/∆ t]
Figure 7 : Profile obtained by averaging the 157 profiles corresponding to the vectors of the thirteen lines closer
to the wall in an analysis with a grid distance of 8 pixels of a single image from the LML turbulent boundary
layer (Madrid).
To summarize, at the end of the project, the assessment of turbulence by PIV is better characterized. The
accuracy and spectral response of both standard and advanced PIV algorithms have been characterized. Design
rules for good PIV experiments in turbulent flows have been proposed. The development of the SIG has been of
great help to characterize both standard and stereoscopic PIV. Also, some improvements have been proposed for
the measurement near wall, both on the recording and processing sides. DLR has shown how to reduce wall
diffusion in an industrial experiment, Delft did the same for laboratory experiments in water. Madrid and
CORIA have worked on the analysis side, showing that advanced algorithms do bring improvements in the near
wall region and that specific procedures can be used to cope with arbitrary curved boundaries.
3.
WP2 ASSESSMENT OF VORTICAL STRUCTURES
The objective of this work package was to study problems associated with the application of PIV in vortical
flows, to develop and improve the seeding and evaluation methods, and to assess the results obtained for future
investigations of vortical flows.
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Europiv publishable summary
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As far as seeding is concerned, a very detailed study of seeding generators based on Laskin nozzles was
performed by DLR. This study led to a best practice guide, which was published in a scientific Journal [8,14].
Figure 8 shows a visualisation of the bubbly flow inside a transparent aerosol generator in optimal
configuration.
Figure 8 : Left: Laser light-sheet visualization of an air jet interacting with the liquid inside an aerosol
generator for tracer particle generation (p=1 bar). Center: detail picture of particle-filled air bubbles
immediately behind the nozzle exit. Right: detail picture of particle-filled air bubbles above the liquid surface
(DLR).
Theoretical and numerical studies were also performed by Madrid University on the behaviour of seeding
particles in vortical flows [9,10]. Figure 9 shows the result of a simulation performed by Madrid on an Oseen
vortex.
Figure 9. Azimuthal velocities (in m s−1) obtained applying PIV to synthetic images for (a) homogeneous
seeding and (b) vortex-induced inhomogeneous seeding (Madrid).
As far as the post processing is concerned, LML and Madrid University have investigated the possibilities of
assessing the vortical structures present in industrial PIV images [6, 7, 11, 12]. A database was constructed
with images from DLR, NLR and LML containing large coherent vortices. Different vortical filters were tested
and discussed by both LML and Madrid. Figure 10, 11 and 12 give some samples of results. The problem of
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Europiv publishable summary
December 2003
the best-suited filter with respect to noise and accuracy has been addressed. A report was published giving
some guidelines.
1000
0.2
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
-0.02
900
800
700
y
600
500
400
300
200
100
0
0
500
x
1000
Figure 10 : PIV image in the wake of a model of aircraft and corresponding vorticity map deduced by LML
from the PIV analysis with a 4th order Richardson extrapolation scheme which is optimum to extract the main
vortex.
Figure 11 : PIV image in the wake of a model of
aircraft and corresponding vorticity map deduced
by Madrid from the PIV analysis with the LFC
PIV software. Small scale vorticity is detected.
Figure 12 : Vorticity map deduced by Madrid with the
LFCPIV software from a PIV record of the EADS Bremen
test campaign, showing the vortex shedding in the wake of
the leading edge slat.
Finally, NLR has investigated the differences in wake vortices assessment by PIV and 5 holes probes. The
experiment performed in WP3 was analysed in detail on this point of view and the results are reported in the
Zaragoza proceedings [39].
To conclude on this work package, the aerosol generators as utilized in wind tunnels have been improved in
order to be capable of generating particles with diameters smaller than 1 m which exhibit better flow following
properties in case of strong velocity gradients (vortices). During this work the understanding of how aerosol
generators for PIV really work has been considerably improved. In addition, a numerical simulation has been
carried out to study the velocity lag of tracer particles in case of given experimental conditions and seeding
particles. The simulated PIV recordings agree well with the real PIV recordings showing that the particle
following behavior in vortices has been correctly modeled.
Another objective of EUROPIV 2 was to provide PIV users with advanced estimators for the calculation of
important fluid mechanical quantities such as vorticity, together with some estimation of the accuracy . This
work has been performed carefully and in detail, leading to a number of publications at conferences and in
scientific journals. The findings are important for wake vortex related projects such as AWIATOR.
In order to provide the end-user of measurement techniques in wind tunnels or in industry with state-of-the art
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Europiv publishable summary
December 2003
knowledge about methods to measure velocity in vortical flows, the performance of stereoscopic PIV and fivehole probes has been assessed in a survey of steady and unsteady 3D flow fields of trailing vortices behind a
civil aircraft model. The results clearly showed the intrusive effect of the five-hole probes and the necessity to
utilize stereoscopic PIV in cases where vortical structures are fluctuating in space and time.
The technical and scientific output is of high quality. The results are presently used at the application of PIV to
study the wake vortices behind large transport aircraft in wind tunnels, water towing tanks and in catapult
facilities within international industrial projects.
4.
WP3 FULL SCALE INDUSTRIAL TESTS
The aim of this work package was three fold: - firstly, to demonstrate the maturity of the PIV method for
application in large industrial facilities; - secondly to provide useful data for three problems of present interest
for aircraft design : high lift configuration, wakes and transonic flows and - thirdly to investigate the potential
of detailed comparison and validation of CFD results with PIV data. For these purposes, three wind tunnel tests
were performed.
The first test was performed jointly by DLR and EADS in the EADS Bremen wind tunnel [16, 17, 38]. The
model was a 2D wing model, in high lift configuration with slat and flap, designed and manufactured by
ONERA. Figure 13 shows the model in the test section. An extensive test campaign, with a multiple camera
PIV set-up, allowed to obtain detailed velocity maps both in the slat and in the flap region simultaneously and
this for various angles of attack. Figures 14 and 15 show samples of mean velocity fields, for an angle of
attack of 12°, respectively around the leading and the trailing edges of the model.
U∞
Y / cm
X / cm
Figure 14 : Example of mean velocity field obtained
around the leading edge for set-up 2, = 12° (DLR).
Figure 13 shows the 2.1 m span model as it was
installed in the wind tunnel. For the PIV experiment,
four PCO cameras (1000 x 1300) were used simultaneously, together with two Nd-YAG laser systems.
Figures 14 and 15 give some samples of results from
two set-ups. In figure 14, the stagnation point on the
slat, the separation bubble between the slat and the main
body and the acceleration on the main body are clearly
visible. Figure 15 shows in more detail the mean flow
above the flap at = 19°. The wake of the main body is
clearly detectable and the efficiency of the slat gap is
also visible with the thin layer of high velocity adjacent
to the wall. The full database was processed in order to
allow detailed comparisons
with computations
performed by Dassault Aviation. These computations
were performed on the basis of the test parameters
investigated during the test campaign. Comparisons
were performed between the pressure distribution
obtained from a RANS solver and the experimental
7
cm
Figure 13 : High lift wing model in the
EADS Bremen wind tunnel.
U∞
cm
Figure 15 : Mean velocity field behind the trailing
edge of the main body, above the flap. Set-up 4,
= 19° (DLR).
Europiv publishable summary
December 2003
results. The agreement is fairly good as illustrated
by figure 16 which gives the pressure
distribution on the three components of the
wing for four different angles of attack.
Except at 6° (where it is 2°), a correction of
angle of attack of 4° is applied uniformly due
to the effect of the side walls.
A more detailed comparison was performed
on the mean velocity fields around the model,
as provided by PIV and by the RANS
computations [39]. Figure 17 shows two
examples of this comparison, under the
leading edge slat and above the trailing edge
flap for an angle of attack of 12°. The global
agreement is quite good, even in the
separated regions. Discrepancies appear
above the suction side, in the region of
mixing of the slat wake and the main body
Figure 16 : Pressure distribution on the RA16SC1 wing in high boundary layer, where the RANS models are
lift configuration. Comparison between the computations with a known to be deficient, and also at higher
RANS solver and the experimental results (DASSAULT).
angle of attack where some 3D effect appear
in the experiments due to the lateral walls.
Figure 17 : Mean velocity modulus under the slat (left) and over the flap (right) for
between computations and PIV data (DASSAULT).
= 12°. Comparison
b - Instantaneous streamlines obtained from the Navier
a - Experimental instantaneous streamlines provided by
Stokes simulation.
PIV.
Figure 18 : Comparison of the instantaneous results provided by PIV with the unsteady computations performed
by DASSAULT near the trailing edge of the flap.
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Beside these steady computations, DASSAULT AVIATION did also perform unsteady RANS computation in
order to assess the ability of PIV to provide some hints on the unsteady aspects of the flow. The unsteady version
of the Aether code was used, which is derived from the steady version with a special attention to the time
resolution in order to have a more accurate time prediction (second order accurate in time). Figure 18 gives some
examples of instantaneous streamline patterns derived from individual PIV records near the trailing edge of the
flap, compared with a short time series from the computation. The experimental results did show that, although a
small separation appeared on the mean flow, some large unsteady separation was occurring from time to time in
this region. This phenomenon was qualitatively confirmed by the computation as clearly illustrated by figure 18.
In fact, both computation and experiment have revealed similar behaviour of the unsteady flow in three regions
where separation exists (under the slat, in the main body cove region and over the flap). By this way, a detailed
study of flow characteristics has been possible. This analysis allows to improve the understanding of physics of
the complex flow surrounding typical high lift configurations and therefore contributes to enhance our capability
to predict this kind of flow.
The second industrial demonstration was performed by
ONERA and DLR in the ONERA S2MA transonic wind
tunnel [15, 38]. The aim was to demonstrate the possibility of
making PIV measurement in a large transonic facility and, in
particular, to assess the specific seeding problems involved in
this type of facility. S2MA is a continuous pressurized
transonic and supersonic wind tunnel with a test section of 1.5
x 1.5 m2. The OAT15 model was provided by ONERA. It is a
half model of a swept transonic wing. The wingspan is 1.28 m
and the aerodynamic chord length at the position of the PIV
measurements was 0.354 m. Figure 19 shows a photograph of
the model installed in the test section. Figure 20 shows the
experimental set-up used for the PIV measurements. The
Figure 19 : Photograph of the model.
YAG laser was placed on the top of the wind tunnel, out of the
pressurised envelop. The laser beam was introduced into the
test section through the ceiling by means of three reflecting mirrors. It was then expanded into a light sheet by
means of an optical set-up made of a cylindrical and a spherical lenses. The sheet was perpendicular to the wing
and parallel to the main flow. The tracers used were DEHS droplets with a nominal size of one micron. They
were produced by four Laskin nozzle generators and were injected into the wind tunnel through a grid, at the
entrance of the settling chamber.
To record the images, a high resolution
CCD camera (1280×1024 pixels), was
set behind the large lateral window of the
Mirror
test section inside the pressure shell of
X95-2m rail
X95-0.5m rail X 100 cube
the wind tunnel. A special a constant
pressure enclosure was designed to
protect the camera from the pressure
X95-0.75m rail
variations that occur at wind tunnel start
and stop. The CCD sensor was activated
Ceiling of the test section
at a rate of 3 frames per second.
Measurements were performed on the
Wind
suction side of the wing, in a vertical
Light sheet
plane parallel to the free stream. This
Y
plane was set at 480 mm from the
X
O
Model
Test section axis
fuselage.
Figures 21 and 22 show an example of
the results obtained during this
Figure 20 : Experimental set-up for PIV measurements in the S2MA campaign. The mean velocity maps of
the longitudinal component at the 2
transonic wind tunnel.
different angles of attack are given.
Each map is obtained by averaging 80 instantaneous maps and contains 2900 velocity vectors. The results show
a strong influence of the angle of attack of the model. At = 3.7°, a shock wave can easily be located on the map
with a typical lambda shape caused by the shock-boundary layer interaction. At = 4.5°, the velocity close to the
model has increased; the shock wave has slightly moved downstream and is stronger, leading to a more violent
YAG Laser
Pressurised wall
Support
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Europiv publishable summary
December 2003
25
25
0
0
Y (mm)
Y (mm)
shock-boundary layer interaction: the lambda shock has moved upstream, and the area of the shock boundary
layer interaction is larger.
-25
-50
-25
-50
U (m/s): 300 310 320 330 340 350 360 370 380 390 400
125
150
175
Mach 0.82 - 3.7°
200
225
U (m/s): 300 310 320 330 340 350 360 370 380 390 400
250
125
X (mm)
150
175
Mach 0.82 - 4.5°
200
225
250
X (mm)
Figure 21 : Average velocity map at 3.7°.
Figure 22 : Average velocity map at 4.5°.
Significant difficulties were encountered during this experiment, such as facility vibrations, seeding problems,
light reflections on the model, and pressure and temperature variations. Last but not least, a fairly precise optical
set-up had to be installed on the roof of the wind tunnel: this was seen as a big challenge because of the wind
tunnel vibrations. However, all these problems were solved, and this campaign demonstrates that PIV
measurements are possible in large industrial wind tunnel facility, and more particularly in transonic flows.
This campaign was a success in European co-operation too; both DLR and ONERA PIV teams could easily work
together owing to the compatibility of their equipment.
The last experiment of this work package was performed by NLR and DNW in the DNW-LST wind tunnel [39].
The aim of this experiment was to compare the data provided by stereo PIV and 5-hole probe rake on the far
wake of aircraft. This was the first test in Europe of a stereo PIV system in a large industrial facility.
The DNW-LST low speed wind tunnel has a
cross section of 3.0 x 2.25 m2 and a test
section length of 8.75 m with excellent visual
accessibility through removable transparent
side panels. This low speed wind tunnel is
known to have a remarkably low free stream
turbulence of only 0.025%. The maximum
wind speed is 80 m/s. An F29-1-2 generic
civil aircraft full-span model was used with
fuselage and wings only. The model was
equipped with inboard and outboard flaps and
slats. Figure 23 shows a sketch of wing
geometry and the flap and slat system. The
measurements concentrated on the vortex
emanating from a 35o deflected inboard flap.
Two model configurations were tested:
- No slats mounted, inboard flap deflection
Figure 23 : Sketch of the wing with available high lift devices.
35-10/0 , outer flap deflection 5 , = 7o.
- Inner slat deflection 10 , outer slat deflection 20 , inner flap deflection 35-10/0 ,
outer flap deflection 15 , α = 18o.
Figure 24 shows a photograph of the model in the wind tunnel, together with the 5 holes probe rake usually used
in such a facility to characterise the wake of the model. For conventional measurements, NLR designed and built
a rake equipped with 18 miniature 5-hole probes with spherical heads (Ø 2.5 mm, probe pitch of 15 mm). The
rake sting was mounted on a streamlined horizontal strut (Ø 70 mm) and fixed to a Y-Z traversing mechanism
(see figure 24). The rake was continuously traversed in the spanwise Y-direction (traversing speed 5 mm/s),
while taking measurements every second. With two intermediate traverses a measurement grid of ∆Y∆Z = 5 5
mm2 was obtained.
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For PIV, a double cavity 2 380 mJ 10 Hz Quantel YAGlaser was used. The laser light sheet was inserted into the
test section from above. Two PCO SensiCam PIV cameras
of 1280
1024 pixel resolution were used for recording
(Figure 25). The sampling rate of the velocity was 6 Hz.
For each PIV camera a Scheimpflug adapter (from DLR)
was used to compensate for left-right variations in depth of
field. For cross correlation of the images the PIVview
software package, developed by DLR, was applied. The
stereoscopic reconstruction was realised by a DLR and
DNW developed software.
This experiment shows that both completely different
Figure 24 : Model and 5-hole probe with attach- measurement techniques S-PIV and 5-hole probe give
comparable results for the steady flow field. Getting into
ment rod in DNW-LST.
details, there was evidence that S-PIV provides more
accurate and smoother results than 5-hole probe, which show rake-related artefacts. Applying S-PIV it was
feasible to measure the intrusive effects of the 5-hole probe: clear evidence was found for the introduction of a
vertical offset in the flow field introduced by the downstream horizontal strut of the 5-hole probe system.
Concerning the unsteady characteristics of the flow field, interesting dynamic features of the wake vortex were
found, such as a direction of preference for the vortex core vibration and a double peak in the flow lag of the
vortex core perpendicular to the vortex core vibration. This is clearly illustrated by figure 26 which compares
the out of plane velocity component distribution obtained just by spatial averaging the PIV data (as is done with
the 5-hole probes) and by taking the vortex center of each instantaneous map as origin before the averaging
procedure.
Free flow = 60 m/s
1975 mm
1025 mm
PIV camera
f = 85 mm
PIV camera
f = 100 mm
F29-1-2
2
18
0m
m
1500 mm
MRP
8
24
1500 mm
XB
0m
m
tunnel
wall
5 hole probe
Figure 25 : Example of wind tunnel set-up showing the model, the PIV and the 5-hole probe set-up.
As a final remark, although not being as accurate as S-PIV, the 5-hole probe has the advantages that the set-up
and also the processing is performed more quickly, while a large flow field can be measured in a single run.
Contrary, S-PIV is limited to smaller flow fields. So both measurement techniques can be considered
complementary in usage: first, one measures the complete field by 5-hole probe rake, to find the most interesting
areas, after which those areas of highest interest can be investigated in more detail and more accurately by
(stereoscopic) PIV.
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Europiv publishable summary
December 2003
Figure 26 : Surface plot of the out-of-plane velocity component for configuration 5, measured by S-PIV, timeaveraged (left) and core position aligned (right).
As a conclusion, this work package did successfully demonstrate the robustness and reliability of the PIV
technique as a tool to investigate the flow field around complex models in all types of industrial wind tunnels.
As a consequence, PIV is used now in many important European aeronautical programs. It can be said that
EUROPIV 2 has given confidence to the end users in the reliability and the interest of the method. This was one
of the most important goals of the project and it can be considered as fully satisfied.
5. WP4 ADVANCED PIV DEVELOPMENTS
The purpose of this work package was to look at some advanced developments such as new algorithms and
stereoscopic PIV in order to improve the performances of the method. A second objective was to look at some
optical extensions of PIV such as ESPI and holography.
The development of advanced PIV algorithms has been pursued mainly by CORIA [38] and Madrid [19, 21,
22, 27, 28, 30, 34, 36]. CORIA has developed an advanced algorithm based on interrogation window
deformation and has started to validate it with synthetic images generated from DNS (figure 27). For that
purpose, the EUROPIV Synthetic Image Generator was used extensively. Madrid did also take into account a
compensation of the particle pattern deformation in the LFCPIV software. Test have been performed on
synthetic images in a vortex core, as illustrated by figure 28.
Conventional PIV (32x32)
PIV with distortion (8x8 pixels)
Figure 27 : Comparison by CORIA of the results obtained with a conventional algorithm and an advanced
algorithm with window deformation on synthetic images generated from a DNS with the EUROPIV Synthetic
Image Generator.
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Europiv publishable summary
December 2003
Signal peak − Average value
S/N= Highest noise peak − Average value
a
b
Figure 28 : S/N ratio before and after compensation of the particle pattern deformation in a vortex core. (Code
for grey levels : S/N >2 - White; S/N from 2 to 1 - Grey; S/N < 1 - Black) (a) Only discrete offset of
interrogation windows. (b) Compensation of the particle pattern deformation (Madrid).
Tests were also performed by Madrid on images coming from the industrial experiments from WP 3. Figure 29
shows an example of the improvement brought by an advanced processing method such as LFCPIV on the
assessment of the small scale vorticity in the wake of the slat of the EADS Bremen experiment on a wing in
high lift configuration.
Figure 29. Comparison of application of conventional PIV and LFC-PIV to a real image from an industrial
facility. Dark grey is solid objects; light grey means places where reflections and shadows suppress all data. a)
Vorticity plot obtained from conventional PIV data. b) Vorticity plot obtained from LFC-PIV (Madrid).
As far as stereo PIV is concerned, a model has been introduced by CORIA and Delft in the EUROPIV SIG
[39]. This model allows the simulation of all stereoscopic configurations. Figure 30 illustrates one of those
configurations. Figure 31 gives samples of synthetic images together with the reconstructed velocity field for a
simulation of a pipe flow. Such a model was used by Delft, in conjunction with a laminar pipe flow
experiment, to study the accuracy of stereo PIV [39]. It is a useful tool to help the preparation of an
experiment by optimising the stereo PIV parameters. It is distributed freely with the Zaragoza proceedings and
through the PIVNET 2 thematic network. Delft University did also look at the accurate determination of the
position of the light sheet with respect to the calibration target in a real PIV experiment [39]. Tests were
performed in a laminar pipe flow by scanning the calibration target in depth accurately. Figure 32 shows the
experimental set-up used and figure 33 illustrates the results obtained. This problem of stereoscopic calibration
was also addressed by LML, who did compare different stereoscopic reconstruction algorithms on a calibration
experiment [39], and by St Etienne University [25, 38], who also did study a specific telecentric stereoscopic
set-up [18, 20, 23, 26]. At the end of the project, St Etienne did look at a method allowing the calibration to be
performed without knowing a priori the position of the target [39].
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Europiv publishable summary
December 2003
Stereo PIV images
Figure 30 : Schematic with all the
conventions used by SIG to recreate a
stereoscopic angular PIV system with
cameras in opposite views and laser sheet
perpendicular to the main flow direction
(Delft).
Reconstructed velocity field
Figure 32 : Specific calibration system Figure 31 : Example of synthetic stereoscopic PIV images as
designed to study the accuracy of generated by the EUROPIV Synthetic Image Generator developed
positioning of the calibration target with by Delft, CORIA and Madrid.
respect to the light sheet (Delft).
As far as optical extensions of PIV are concerned,
Zaragoza University and ITAP did a thorough
study of the ESPI (Electronic Speckle Pattern
Interferometry) method as a substitute to
stereoscopy in difficult optical conditions [29, 33,
37, 38]. A successful joint experiment was
performed by Zaragoza University and LML in the
LML Boundary Layer wind tunnel [29, 38]. The
aim of the experiment was to validate the ability of
ESPI to measure the out of plane component by
comparing the results with a standard stereoscopic
PIV set-up. Figure 34 shows the experimental setup in the wind tunnel. Four PCO cameras were
used, together with a specific four YAG Lasers
system. This allowed the simultaneous recording
of a stereoscopic PIV velocity field in the best
stereo configuration (90° between the two camera
axis) and a joint PIV/ESPI recording giving also
the three velocity components in the field of view.
Figure 35 gives an example of comparison of both
records.
Figure 33 : Accurate determination of the position in
depth of the light sheet (Delft).
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Europiv publishable summary
December 2003
Figure 34 : Joint stereo PIV and ESPI/PIV
set-up. Cameras 1 & 2 record the stereo
PIV images. Camera 3 records a standard
PIV image. Camera 4 records the ESPI
Speckle pattern. A reference beam is
needed for Interferometry (Zaragoza/LML).
a)
b)
Figure 35 : 3-C velocity measurements from the experiments at the
LML wind tunnel; the vector fields represent the in-plane
components, while the colour contour maps represent the out-ofplane component. a) Data from stereo PIV; b) Data from
ESPI&PIV (Zaragoza/LML).
The holographic approach to PIV in a large scale facility was studied by ISL. Two holographic set-ups were
realised and successfully characterised in the laboratory [39]. A joint full scale experiment was performed with
LML and ONERA in the LML wind tunnel [39] to compare the holographic and standard stereoscopic
approaches. The main conclusion was that, in the present state of the art, the stereo PIV approach using CCD
cameras is much more flexible and effective around a large facility.
Air-foil
object
Measurement
volume
Reference beam
generation
Holographic
plate
Object
illumination
Dual beam Nd:YAG laser
Figure 36 : Experimental set-up used by Oldenburg and DLR to record LiF holograms in the DLR wind tunnel.
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Europiv publishable summary
December 2003
A second holographic approach, the Light in Flight holography, was studied by Oldenburg University [22, 25,
31, 32, 38]. Again the method was first developed and characterised at the lab level. Then, a large scale
experiment was performed jointly by Oldenburg University and DLR in a DLR subsonic blow down wind
tunnel. The flow under study was the wake of an airfoil. Figure 36 shows the optical set-up used for these
experiments. A specific reconstruction set-up (not shown) was developed to process the holograms
automatically. Several sets of holograms were recorded during this test campaign. These holograms were
difficult to analyse as the concentration of bright particle images was fairly low (due to the large cross-section of
the illuminated area). Figure 37 gives an example of the results obtained with the best set recorded. The
conclusions were comparable to those obtained from the ISL/LML experiment. Holography is not mature
enough for routine application in large scale facilities, due mainly to the lack of a suitable recording medium.
The potential of the method is high, nice results can be obtained at the laboratory level and a lot of developments
are still needed at this stage before the method can be transposed to industrial facilities for routine applications
such as those performed nowadays by standard PIV.
20
Z / mm
15
10
5
0
5
10
Y/m
m
15
0
20
10
Z
0
X / mm
X
Y
Figure 37 : Evaluated wind tunnel flow, 16.640 vectors have been obtained by three-dimensional grey-value
correlation. The plane-like distribution of the vectors is a result of a relatively large separation between adjacent
image slices, from which 128 enter in each correlation (Oldenburg).
As a conclusion, a lot of high level research work has been performed in this work package by the EUROPIV 2
consortium. This has been done on many aspects of the method and validated by a good deal of scientific
publications. Improvements have been brought mainly to the stereoscopic recording and to the advanced
algorithms. The interest of ESPI has been demonstrated. Also, the possibilities of holographic PIV have been
assessed. Most of these results are presented in the Zaragoza proceedings. Some of them have opened new routes
of research for the future.
6.
CONCLUSION
The main objective of the EUROPIV and EUROPIV 2 projects was to speed up the transfer of the PIV technique
to the European industrial wind tunnel for Aeronautics.
At the beginning of EUROPIV in 1995, CCD PIV was at an early stage of development and the method was
mainly applied in research facilities. At the end of EUROPIV 2, less than 10 years later, the method is routinely
16
Europiv publishable summary
December 2003
applied in most of the big facilities in Europe. The EUROPIV consortium has made several successful
demonstrations in various European wind tunnel (AIRBUS Bremen wind tunnel, ONERA Modane S2 transonic
wind tunnel, DNW-LST wind tunnel with stereo PIV), which have given confidence to both the users and the
wind tunnel operators. If one compares to the Laser Doppler Velocimetry technique, this is a great success.
DNW and the EREA have now PIV teams which are operational, cooperating and able to set-up state of the art
PIV experiments in any European wind tunnel.
The method has been , is and will be used in many important European projects such as APIAN, AEROMEMS I
& II, C-Wake, S-Wake, TILTAERO, EUROLIFT, HELIFLOW, AWIATOR, DESIDER.. Besides, the PIV data
recorded by the EUROPIV consortium (notably on the high lift configuration in the Bremen wind tunnel) has
been found of valuable scientific interest and are used for comparison with advanced CFD approaches. The
advanced PIV measurement technique has shown its usefulness to get a detailed view to the flow field around a
complex high lift system. The detailed knowledge of this flow field is today necessary to understand high lift
configurations and to improve them.
The PIVnet European network, which originates from the EUROPIV consortium has been and is still very active
in transferring the method toward industrial applications, in the field of Aeronautics , but also to a broader
spectrum (Naval, Household appliances, Car Industry…).
The second objective of this project was to support the European community in PIV, which is very active, in its
research to develop the method. Several of the leading teams in Europe are in the consortium and it was of
interest to develop a synergy between them. This again has worked quite well. A friendly and fruitful
cooperation was developed, which was extended through the PIVnet network and which will last long after the
project. From this cooperation, some significant results have been obtained :
-
a standard Synthetic Image Generator (SIG) has been developed and extensively used by the consortium. It
will be made available with the proceedings of the Zaragoza workshop and advertised through the PIVnet2
thematic network to push it as an international standard.
The performances and accuracy of the standard PIV techniques have been better assessed and improved by
the individual and cooperative work of several teams of the consortium. The SIG has helped significantly to
these research.
The PIV seeding of wind tunnels has been improved and characterized quantitatively, so that a best practice
guide has been realized and published in an international journal and is provided in the Zaragoza
proceedings.
The assessment of vortices from PIV velocity maps has been thoroughly studied by LML, Madrid and
NLR, both on the theoretical and practical points of view, providing clear guidelines for extracting the right
intensity of large vortical structures such as those encountered in wake flows and in turbulence.
Significant contributions have been given on advanced PIV methods, including holographic approaches.
Both the recording set-ups and the advanced processing software have been studied, leading to some
significant improvements : standard stereoscopic systems have been characterized and a specific Sheimpflug
mount has been designed and manufactured with a SME. Telecentric lenses approaches have been
characterized. Advanced PIV software based on interrogation window filtering (LFCPIV) or image
deformation have been developed and extensively characterized. Original holographic PIV set-ups have
been designed and tested allowing to investigate the advantages and drawbacks of this approach.
Finally, the most significant results have been presented in a workshop in Zaragoza in Spring 2003, which has
gathered most of the European specialists of the method and has led to about 30 original contributions. The
proceedings of this workshop, which was of good scientific level, are published by Springer Verlag [39] in order
to make the acquired knowledge available to the scientific community.
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[20] T. Fournel, C. Fournier, C. Vincent, Single-lens stereoscopic arrangement with overlapping of the views. 8th
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Göttingen, 17-19, Sept 2001.
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stereoscopic PIV Meas. Sci. Technol. 12 (2001) 1371-1381
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multigrid particle image velocimetry methods: application of a dedicated weighting function and symmetric
direct correlation. Meas. Sci. Technol. 13 (2002) 963–974
[29] A. Lecuona, Advanced PIV algorithms : Why and when advanced PIV algorithms? PIVNET2/ERCOFTAC
SIG32 workshop, Lisbon, 2002.
[30] J. Loberta, P. Arroyo, N. Pérenne, M. Stanislas, Joint PIV & ESPI compared to standard stereo PIV : some
results from a turbulent boundary layer. 11th Int. Symposium Application of Laser Techniques to Fluid
Mechanics, Lisbon, July 2002
[31] A. Lecuona, J. Nogueira, P.A. Rodrıguez, D. Santana, Accuracy and time performance of different schemes
of the local field correction PIV technique. Experiments in Fluids 33 (2002) 743–751.
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[34] J. Lobera, N. Andrés and M.P. Arroyo, Digital image plane holography as a three-dimensional flow
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Applications. 5th International Symposium on PIV, 2003, PIV’03 Paper 3002.
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Korea, September 22-24, 2003 PIV’03 Paper 3250.
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[37] A Lecuona, J Nogueira, P A Rodríguez and A Acosta, PIV Evaluation Algorithms for Industrial
Applications. To appear in MST.
[38] J. Lobera, N. Andrés and M.P. Arroyo, Digital Speckle Pattern Interferometry (DSPI) as a holographic
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[39] Proceedings of the EUROPIV 2 Workshop on Particle Image Velocimetry. Zaragoza (Spain), March the
31st – April the 1st 2003. Editors : M. Stanislas, J. Westerweel, J. Kompenhans. Springer Verlag.
Contact points
Pr. M. Stanislas
Laboratoire de Mécanique de Lille URA 1441
Bd Paul Langevin, Cité Scientifique
59655 Villeneuve d’Ascq cedex, France
Phone: 33 (0)3 20 33 71 70
Fax: 33 (0)3 20 33 71 69
Email: stanislas@ec-lille.fr
Dr. K.P. Neitzke
Abteilung EGXG
AIRBUS Deutschland GmbH
Hünefeldsr.1-5
28199 Bremen, Germany
Phone: 49 421 538 4704
Fax: 49 421 538 5034
Email: Klaus-Peter.Nietzke@airbus.com
Mr. Tran Dac N.
Dassault Aviation SA
78 quai Marcel Dassault
92552 St Cloud, France
Phone: 33 (0)1 47 11 35 09
Fax: 33 (0)1 47 11 45 35
Email: dac.tran@dassault-aviation.fr
Mr. F. De Gregorio
CIRA
Via Maiorise
81043 Capua, Italy
Phone: 39 0823 62 3721
Fax: 39 0823 96 9272
Email: f.degregorio@cira.i
Dr. J. Kompenhans
DLR
Bunsenstrasse 10
37073 Göttingen, Germany
Phone: 49 551 709 22 52
Fax: 49 551 709 28 30
Email: juergen.kompenhans@dlr.de
Mr. H.P.J Veerman
NLR (EI)
Anthony Fokkerweg2
1059 CM Amsterdam, PO Box 90502
1006 BM Amsterdam, The Netherlands
Phone: 31 20 511 36 77
Fax: 31 20 511 3210
Email: veerman@nlr.nl
Dr. J-C. Monnier
ONERA Centre de Lille
5 Bd Paul Painlevé
59045 Lille Cedex, France
Phone: 33 (0)3 20 49 69 43
Fax: 33 (0)3 20 49 69 53
Email: monnier@imf-lille.fr
Pr. J.Westerweel
Delft University of Technology
Laboratory for Aero & Hydrodynamics
Leeghwaterstraat 21,
2628 CA Delft, the Netherlands
Phone: 31 15-278-6887
Fax: 31 15-278-2947
Email: J.Westerweel@wbmt.tudelft.nl
Pr. A. Lecuona
Univ. Carlos III
Escuela Politécnica Superior
Calle Butarque 15
28911 Leganés Madrid, Spain
Phone: 34 (9)1 624 94 75
Fax: 34 (9)1 624 94 30
Email: lecuona@ing.uc3m.es
Pr. K. D. Hinsch
Carl v. Ossietzky University Oldenburg
Physics Department/ Applied Optics
26111 Oldenburg, Germany
Phone: 49 441 798 3510
Fax: 49 441 798 3576
Email: klaus.hinsch@uni-oldenburg.de
Pr. Cenedese
Dept. of Idraulica, transporti e strade,
“La Sapienza” University, Via Eudossiana 18
00184 Roma, Italy
Phone: 39 06 44 58 50 33
Fax: 39 06 44 58 094
Email: antonio.cenedese@uniroma1.it
Dr. B. Lecordier
UMR CNRS 6614 CORIA
Site Universitaire du Madrillet
76801 St Etienne du Rouvray Cedex
Phone: 33 (0)2 32 95 36 81
Fax: 33 (0)2 32 91 04 85
Email: bertrand.lecordier@coria.fr
Dr. T. Fournel
Laboratoire TSI
Université Jean Monnet
23 rue du Dr P. Michelon
42023 St Etienne Cedex 2, France
Phone: 33 (0)4 77 48 51 77
Fax: 33 (0)4 77 48 51 20
Email: fournel@univ-st-etienne.fr
Pr. Pilar Arroyo
Universidad de Zaragoza
Departamento de Fisica Aplicada C/Pedro Cerbuna 12
Facultad de Ciencias, 50009 Zaragoza, Spain
Phone: 34 9 76 76 24 41
Fax: 34 9 76 76 1233
Email: arroyo@posta.unizar.es
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