MULTI-WINDOW PIV FOR HIGH-LIFT MEASUREMENTS
Dr. A. Arnott✦ (), Dr. G. Schneider✦, Dr. K.-P. Neitzke4, Dipl.-Ing. J. Agocs✦,
Dr. B. Sammler✦, Dr. A. Schröder✦ & Dr. J. Kompenhans✦
✦
4
Dept. AS-EV, DLR, Bunsenstrasse 10, 37073 Göttingen, Germany
Dept. EGXG, Airbus Germany, Hünefeldstrasse 1-5, 28199 Bremen, Germany
ABSTRACT
Experiments using multi-window, particle image
velocimetry (PIV) were performed in the low-speed
wind tunnel of Airbus Bremen, Germany, over a twodimensional, slat/wing/flap model in a high-lift
configuration. For this, a system for acquiring PIV
data from multiple cameras simultaneously was
devised and installed underneath the LSWT. The
PIV experiments were performed at the mid-span of
the model, for incidences of 12°, 17.5° & 19°.
Nearly 5000 PIV images were obtained during the
tests, the images capturing the flowfields from the
slat wake, slat/wing gap, flow mixing over the main
wing, wing/flap-gap and wing wake. The analysis of
the results reveals that such a technique is a viable
and valuable tool for industrial wind tunnel tests,
with high quality data showing the unsteady nature of
the slat wake and slat gap flow with distance
downstream and unsteady flow separation over the
flap.
INTRODUCTION
designers require data from several regions around a
wind tunnel model simultaneously.
Such data have been obtained by DLR as part of the
EUROPIV-2 programme, around a simplified highlift configuration in the low-speed wind tunnel of
Airbus Bremen, Germany. In these experiments, data
from 4 regions was obtained simultaneously around
the model. For this, numerous problems had to be
overcome, examples of which include the extremely
small amount of space under the tunnel for the
cameras, reducing reflections from the surfaces and
the dynamic range of the flow. In addition, the tests
were performed under considerable time pressure.
These problems and the surmounting of them will be
discussed in the paper. By this it will be seen that
PIV is now maturing as a technique for application in
industrial wind tunnels and does not just belong to
the university laboratory.
THE WIND TUNNEL AND MODEL
The wind tunnel
To assist the aviation industry in designing efficient
high-lift devices, the improvement of CFD design
codes is of vital importance. This is an area, like any
other, where wind tunnel data is used to verify CFD
codes and assist in the design of slat/flap systems [3,
11]. In a flow region where the flow is unsteady,
even separated, such as coves and gaps left by the
deployment of high-lift surfaces, designers have a
great need for unsteady velocity data obtained over a
region at many instants in time, using a non-intrusive
technique.
The Airbus Bremen low-speed wind tunnel (LSWT)
is enclosed within a hall and is of the open-circuit
type with a closed working section. This is 4.45 m in
length, with a cross section measuring 2.1 m × 2.1 m.
and is shown in Fig.1, along with the model wing
which was mounted vertically. The leading edge of
the model is towards the reader and thus the flow
direction is into the page. Glass windows along both
side walls provided access for the laser light sheets
and additional windows were provided in the tunnel
floor for the cameras to view the laser sheets.
More traditional methods of measurement, such as
pressure-probe, or hot-wire rakes, suffer from
introducing measurable disturbances to the flow field.
In addition, laser Doppler anemometry, although
non-intrusive, is a point technique and requires much
time in order to make a flow field survey.
The model used for the tests was a two-dimensional,
three-component one, manufactured by ONERA,
Lille, using the RA16SC1 section [9]. It spanned the
complete height of the tunnel, passing a short
distance through the tunnel floor to be attached to a
motorised support turntable underneath the tunnel as
shown in Fig. 2. The gap remaining in the tunnel
floor turntable was filled by 2 wooden panels cut
specially to fit around the model profile. The
model’s sectional profile is shown in Fig. 8, with a
maximum thickness of 0.08 m and with both slat and
flap deployed, an extended chord length of 0.691 m.
The slat and flap were set at droop angles of 30° and
40° respectively.
Modern digital PIV is suited to the study of unsteady
flow fields extremely well [5, 6, 8, 12]. Several
studies have been performed already [7, 9, 10],
however the data have tended to be obtained from
one area of the flow at a time. Obviously, to improve
the modelling of the development of wakes and their
mixing, or interactions between slat and flap further,
LIKELY PROBLEMS USING PIV IN
INDUSTRIAL WIND TUNNEL TESTS
Early on in the planning for the test programme it
became clear that various problems would have to be
surmounted. Chief amongst these was the limited
space underneath the tunnel, there being a vertical
gap between tunnel floor and model support of only
c.320 mm. This meant that it was impossible to
mount the cameras vertically. Neither was it possible
to mount the cameras above the roof of the working
section, due to a turntable mechanism being situated
there.
A second problem was the fact that the model
surfaces were bare aluminium and steel. It was
anticipated that reflections from the model surface
would cause considerable “blooming/flare” on the
camera arrays, rendering it impossible to obtain
velocity information in those regions.
A third problem involved the acquisition and analysis
of the data. Previously, where data had been
acquired from only one flow region at a time, the
time interval between laser pulses could be optimised
for each region. However here, measurements were
planned to be taken in areas such as the slat leading
edge, slat cove, wing/slat-gap and wing trailing edge,
all simultaneously. As such the dynamic range of the
flow was anticipated to be from 0 m/s to c.170 m/s.
Optimising the pulse separation for the slower
regions would cause problems for the correlation of
the seeding particles in the high-velocity regions.
The surmounting of these problems is described
where relevant in the paper.
THE EXPERIMENTAL APPARATUS
Fig. 3 shows a sketch of the experimental apparatus.
On the balcony outside each side of the working
section, a pair of Quantel Brilliant-B pulsed lasers,
plus associated optics to form the two light sheets,
were mounted. The light sheets were set up to enter
the working section horizontally through the tunnel
side windows, at the height of the mid-span of the
model.
Digital PCO Sensicam cameras were used, with
various Zeiss and Tamron lenses of fixed focal
lengths ranging from 100 mm to 300 mm, all of
which having maximum apertures of f/2.8. However,
as mentioned, the cameras themselves were too long
to be mounted vertically underneath the tunnel floor,
even before adding any lenses. To overcome this, a
system of flat rails was devised to mount the cameras
on, with high-quality mirrors in special holders to
fold the image path as in Fig. 4. This was tested in
DLR Göttingen and Airbus Bremen before entry into
the tunnel. So that the cameras could view the light
sheet, portholes were cut in the wooden floor panels
where necessary and thin glass plates fixed to cover
them as in Fig. 5.
A switch was incorporated in the cable transmitting
the trigger signal from the sequencer to the image
acquisition PCs. The switch was used to ensure that
the cameras received their timing signals
simultaneously.
The seeding was produced by two seeding
generators, of a DLR design [8], filled with DEHS
(Di-2-Ethylhexyl-Sebacat) oil and the hall
surrounding the wind tunnel was seeded completely.
The design of the generator nozzles was based on
work [4] performed earlier in the timescale of the
EUROPIV-2 programme.
EXPERIMENTAL CONDITIONS
All tests were performed at a freestream speed of
54 ms-1, for three model incidences, viz. 12°, 17.5° &
19°. The slat and flap positions stayed the same
during the tests however.
For certain regions around the profile, the amount of
light reflected and scattered from the shiny metal
surface was considerable. This led to areas where no
particle information could be acquired as shown in
Fig. 6. Fortunately however, experience from the
EUROLIFT PIV tests [1] was valuable in overcoming
this: matt black, self-adhesive plastic foil was used to
cover the wing surface. This had the effect of
eliminating the areas of flare, leaving a thin stripe
where the laser sheet hit the wing as in Fig. 7.
The PIV tests were grouped into four camera
arrangements, these being shown in Fig. 8. Where
necessary, a wooden floor panel was removed, a new
porthole cut into it and a glass plate bonded to the
underside as in Fig. 5. The panel was then replaced
and fixed prior to the acquisition of calibration
images.
For the actual data acquisition, the tunnel hall was
blacked out completely. At each incidence, 34 image
pairs were acquired from each camera, with at a time
interval of 1/3 s from one pair to the next and saved
to hard disc. This acquisition process was then
repeated twice for each camera before changing
incidence. In total, nearly 5000 PIV images were
acquired (including calibration images).
This
amounts to just over 25 Gb of raw data.
DATA ANALYSIS AND DISCUSSION
Firstly it was necessary to de-warp all images [2] to
remove any perspective distortion. From these
images, masks were created for areas, such as where
the wing profile was in the picture, to save
computational time. Next, the problem concerning
the dynamic range of the flow was tackled. In certain
regions, the seeding particles were moving so fast
that by the time the second laser pulse of a pair
occurred, they were further downstream than the
preferred size of the interrogation window (32×32
pixels). Therefore, the analysis proceeded using this
size of window with a 16 pixel step, however each
window was allowed to search within a larger area
around it (e.g. 64×64 or 128×128) to obtain the most
plausible correlation. Any holes left by single (or
pairs) of outliers were filled using an interpolation
pass requiring a minimum of five neighbouring
vectors in order to be filled.
All images have now been analysed and archived as
vector maps in the TECPLOT data format. Only
selected images are presented here, due to space
considerations. The (X, Y) = (0, 0) origin of the coordinate system is the leading edge of the main wing
itself as shown in Fig. 2. In all cases, the accuracy in
the (X, Y) positions of the vectors is ±0.5 mm and the
tail of the vector is at the centre of the interrogation
window.
Figs. 9 & 10 show the mean flow pattern in the
slat/wing-gap region for 12° & 19° respectively, each
averaged from 102 vector maps. Although the use of
black and white limits the number of features that can
be made out, it is possible to discern the slat wake
slightly. More obvious are the differences in the
development of the shear layer from the separation
from the lower, trailing-edge of the slat and the size
of the re-circulation zone. For 12°, the mean path of
the shear layer lies as much as 2 cm further
downstream than at 19°.
However the path of this shear layer wanders in space
with time.
Consequently, the size of the recirculation region and flow structures within, vary
too.
Figs. 11 & 12 show examples of the
instantaneous flow in this region for 12° & 19°
respectively. In either one of the two Figs., the
vector maps on the 2 sides of the slat/wing-gap were
acquired simultaneously. In both regions, small
structures in the re-circulation zone, plus
reattachment on the downstream side of the slat, have
been captured. The images show the unsteady nature
of the flow in this region and from the data set, it
would be reasonable to make estimates of the rms
variation in space of the shear layer from the mean,
for the benefit of designers seeking to validate their
numerical CFD codes.
In addition, the flow acceleration through the
slat/wing-gap is prominent. The velocities through
the gap in the two regions are consistent (the use of
colour would confirm this). At 12° incidence, the
maximum flow velocity just downstream of the gap
is between 140 and 145 ms-1. This increased by
approximately 14% as the incidence was increased to
19°.
Moving downstream to look at the trailing edge of
the main wing and wing/flap-gap, considerable
variations in flow behaviour were found. Figs. 13 &
14 show the instantaneous vector maps from the 4
camera positions of Fig. 8(c), at two instants in time,
for α = 12°. The Figs. show very interesting flow
behaviour at the trailing edge of the model and
illustrate the usefulness of PIV as a measurement
technique. The 4 vector maps in either Fig. 13 or 14
were acquired simultaneously, those in Fig. 14 show
the flow 2/3 s later than in Fig. 13. Again the lack of
colour makes it difficult to distinguish certain
features, although the development of the wake from
the main wing into region i is clear in both Figs. To
make the plots clearer, for regions g, i & k, every 2nd
vector only has been plotted; for region j, every 4th
vector only has been plotted.
The most interesting behaviour at the trailing edge
concerns the wing/flap-gap flow and flap boundary
layer in region i. In Fig. 13 the high velocity flow
through the wing/slat-gap can be distinguished and
remains attached to the flap over most of region i. In
region j, there appears to be a small separation at the
trailing edge of the flap, with low-velocity,
circulatory structures being evident. Two-thirds of a
second later, the flap boundary layer has separated
violently and a relatively strong, coherent backflow is
evident, in some cases of up to 50 m/s. Examining
the complete data set, this separation was
intermittent, although occurring often enough at 12°
for the averaged vector maps to show the existence of
the separated region.
The occurrence of the
separation is felt to be linked to the behaviour flow
from the flap cove (region k) as it flows through the
wing/flap-gap, as the vectors in this region for Figs.
13 & 14 do exhibit different behaviour. This will be
looked at in more detail in the future.
As the incidence was increased, the separation
became more intermittent, until at 19° it was almost
absent. The averaged velocity vector maps at
incidences of 17.5° & 19° show no existence of any
separation. However this illustrates extremely well
the danger of using averaged data alone, when such
intermittent flow behaviour occurs. In addition the
benefit and usefulness of a multi-window PIV
technique for the measurement of fluid flow are
confirmed.
CONCLUSIONS
Multiple-window
PIV,
using
4
cameras
simultaneously, has been applied to the testing of a
high-lift wing configuration, in an industrial wind
tunnel.
Due to the results obtained, the technique is deemed
to be successful and a valuable tool for wind tunnel
testing.
Such an approach is deemed beneficial for the
comparison of experimental results with numerical
codes.
In particular, the interactions between areas of
unsteady flow can now be investigated in more detail.
Here, an intermittent separation was measured over
the flap component of the model.
ACKNOWLEDGEMENTS
This work has been performed under the EUROPIV 2
project: EUROPIV 2 (A Joint Program to Improve
PIV Performance for Industry and Research) is a
collaboration between LML URA CNRS 1441,
Dassault Aviation, DASA, ITAP, CIRA, DLR, ISL,
NLR, ONERA, DNW and the universities of Delft,
Madrid, Oldenburg, Rome, Rouen (CORIA URA
CNRS 230), St Etienne (TSI URA CNRS 842) and
Zaragoza. The project is managed by LML URA
CNRS 1441 and is funded by the CEC under the IMT
initiative (contract no: GRD1-1999-10835).
In addition, the authors would like to thank (in
alphabetic order) Dipl.-Ing. Peter May, Dipl.-Ing.
Klaus Muthreich and Dipl.-Ing. Susanne Wyrembek
of Airbus Bremen, Germany, for their hospitality and
collaboration during the experiments.
REFERENCES
1] Arnott, A.D., Kompenhans, J., Schneider, G.,
Agocs, J., Sammler, B., Schröder, A., 2003,
“PIV experiments in the low-speed wind tunnel
of Airbus Bremen, Germany”. EUROLIFT
technical report TR1.3.1-8, partner report for EU
programme GRD1-1999-10015.
2] Ehrenfried, K., 2001, “Processing calibrationgrid images using the Hough transformation”,
Proc. 4th Intl. Symp. on PIV, 17–19 Sep.,
Göttingen, Germany, paper 1042.
3] Hansen, H., 1998, “Überblick über das
Technologieprogramm Hochauftriebskonzepte
HAK”, DGLR Jahrestagung, Bremen, Germany.
4] Kähler, C., J., Sammler, B., Kompenhans, J.,
2001, “Generation and control of particle sizes
for optical velocity measurement techniques in
fluid mechanics”, Proc. 4th Intl. Symp. on PIV,
17–19 Sep., Göttingen, Germany, paper 1117.
5] Kompenhans, J., Dieterle, L., Raffel, M.,
Monnier, J.-C., Gilliot, A., De Gregorio, F.,
Pengel, K., 2001, “Particle Image Velocimetry:
status of development and examples of
application in industrial test facilities”, Proc. 3rd
ONERA-DLR Aerospace Symp., 20–22 June,
Paris, France.
6] Kompenhans, J., Raffel, M., Dieterle, L.,
Dewhirst, T., Vollmers, H., Ehrenfried, K.,
Willert, C., Pengel, K., Kähler, C., Schröder, A.,
Ronneberger, O., 2000 “Particle Image
Velocimetry in Aerodynamics: Technology and
Applications in Wind Tunnels”, J. Visualisation,
Vol. 2, pp 229–244.
7] Paschal, K., Jenkins, L., Yao, C., 2000,
“Unsteady slat-wake characteristics of a 2-D
high-lift configuration”, AIAA paper 2000-0139.
8] Raffel, M., Willert, C., Kompenhans, J., 1998,
“Particle image velocimetry - a practical guide”,
Springer, Berlin/Heidelberg/New York.
9] Stanislas, M., Kompenhans, J., Westerweel, J.,
2000, “Particle Image Velocimetry - Progress
towards Industrial Application”, (eds), Kluwer.
10] Takeda, K., Ashcroft, G. B., Zhang, X., 2001,
“Unsteady aerodynamics of slat-cove flow in a
high-lift device”, AIAA paper 2001-0706.
11] Thibert, J., J., 1993, “The GARTEUR high-lift
research
programme,
high-lift
system
aerodynamics”. AGARD CP-515, 16/1–21.
12] Willert, C., Raffel, M., Kompenhans, J., Stasicki,
B., Kähler, C., 1996, “Recent applications of
particle image velocimetry in aerodynamic
research”, J. Flow Meas. Instrum., Vol 7, pp
247–256.
FIGURES
U∞
Fig. 1. The EUROPIV-2 model in the LSWT, Bremen
(the flow direction is into the page).
Fig. 2. Underfloor mounting of the model
Control
PC
4 PIV PCs
Laser
Switch
Optics
Sequencer
Fibre-optic
cables
Light sheet
U∞
Camera positions
Light sheet
Laser
Fig. 3. Schematic of the arrangement of PIV equipment around the LSWT, Bremen.
Mirror
Camera
Model
support
U∞
Fig. 4. CCD Camera, X-95 rail and mirror
system underneath the LSWT
Fig. 5. Portholes cut into the wooden plates in the tunnel
floor for the cameras to look through.
Regions of flare
on CCD array
U∞
U∞
No flare, just the line of the
laser sheet on the model
Fig. 7. PIV image from a metal wing covered with
black, self-adhesive plastic foil.
30
30
20
20
10
10
Y [cm]
Y [cm]
Fig. 6. PIV image from an uncoated metal wing
0
0
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X [cm]
(c) set-up 3
50
60
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(b) set-up 2
30
Y [cm]
Y [cm]
(a) set-up 1
20
30
X [cm]
40
50
60
70
-30
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-10
0
10
20
30
X [cm]
(d) set-up 4
Fig. 8. The 4 experimental set-ups of the cameras: the boxes show the fields of view.
Y [cm]
5
Vel [cm/s]
18000
17000
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0
0
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-5
0
5
X [cm]
Fig. 9. Averaged vector maps of the slat/wing-gap region at 12° (every 2nd row is not plotted for clarity).
Y [cm]
5
Vel [cm/s]
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X [cm]
Fig. 10. Averaged vector maps of the slat/wing-gap region at 19° (every 2nd row is not plotted for clarity).
7
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Vel [cm/s]
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X [cm]
Fig. 11. Instantaneous vector maps of the slat/wing-gap region at 12° (every 2nd row is not plotted for clarity).
Both vector maps were acquired simultaneously.
7
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5
4
3
2
1
Vel [cm/s]
17000
16000
15000
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7
X [cm]
Fig. 12. Instantaneous vector maps of the slat/wing-gap region at 19° (every 2nd row is not plotted for clarity).
Both vector maps were acquired simultaneously.
10
Vel [cm/s]
12000
11000
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
g
5
i
Y [cm]
0
-5
k
-10
-15
35
j
40
45
50
55
60
X [cm]
Fig. 13. Instantaneous vector maps of the wing/flap-gap region at 12°, all four maps were acquired simultaneously.
For clarity, in regions g, i & k, every 2nd vector only is plotted, for region j, every 4th vector only is plotted.
65
10
Vel [cm/s]
12000
11000
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9000
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7000
6000
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g
5
i
Y [cm]
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-5
k
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35
j
40
45
50
55
60
65
X [cm]
Fig. 14. Instantaneous vector maps of the wing/flap-gap region at 12°, 2/3 s later than Fig.13. Again, all 4 vector maps were acquired simultaneously.
For clarity, in regions g, i & k, every 2nd vector only is plotted, for region j, every 4th vector only is plotted.