Detailed characterisation, using PIV, of the flow around an aerofoil in a high-lift
configuration
AD Arnott~ (), G Schneider~, K-P Neitzke4, J Agocs~, A Schröder~, B Sammler~ & J Kompenhans~
~
DLR, Institute of Aerodynamics and Flow Technology,
Bunsenstrasse 10, 37073 Göttingen, Germany
4
Airbus Germany, Department EGXG,
Hünefeldstrasse 1-5, 28199 Bremen, Germany
Abstract
Within the framework of the EUROPIV-2 programme, experiments using particle image velocimetry
(PIV) were performed in the Low Speed Wind Tunnel (LSWT) of Airbus Bremen, Germany. The goal
was to expand and improve on work performed for EUROPIV-1 and investigate the flow 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° & 21 . 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 PIV is feasible in industrial-type wind tunnel tests, with high quality data showing the flow through
the slat/wing gap, the mixing and unsteady nature of the slat wake and slat gap flow with distance downstream and unsteady flow separation over the flap.
1 Introduction
Prior to the development of optical measurement techniques such as PIV or LDA, single- or multiplepressure-probe methods were the only means of investigating the velocity fields around wind tunnel models. However the probe methods suffer from introducing disturbances to the flow. In addition, a singlepoint technique (including LDA) suffers from the limitation of requiring much time. This is clearly a disadvantage for the investigation of high-lift systems, where there are separated regions plus wakes which
have unsteady boundaries. The ability to measure velocities in complete regions around different areas of
a model simultaneously is of huge benefit to the designers of high-lift systems, working as they do in an
increasingly competitive market that is driven by lower operating costs, higher performance, increased
safety margins and lower emissions to list some examples [5, 13, 14]. It is expected that the acquisition of
such test data from industrial wind tunnel tests will help greatly to improve the CFD codes that are used to
design such system.
1
PIV is suited very well to this, as it allows the measurement of fluid velocities over an area of a flow
field instantaneously [1, 7, 8, 10, 11, 15]. In addition, for all practical purposes, it can be considered nonintrusive. PIV allows a larger amount of flow information to be acquired in a shorter time than prior to
the invention of this technique, thus providing potential savings in wind tunnel time and costs.
PIV has already been applied to the study of the flow around a slat/wing combination as part of the
EUROPIV-1 programme [11], however for this case the model was not completely realistic of a high-lift
system as no deployed flap was present. In addition, the data were acquired from one area at a time, so it
was not possible to correlate the instantaneous images from one area with those from another. The work
undertaken in EUROPIV-2 builds on this experience by using the same model, this time with both a slat
and flap deployed, plus acquiring data from several regions around the model simultaneously. A prime
example of the benefits that could be gained using this approach is the ability to obtain flowfield data on
either side of e.g. the slat/wing gap and link the flow structures in the slat cavity with the development of
the high-velocity gap flow on the upper surface of the wing (an example is shown by Figures 15 & 16).
2 High-lift tests as part of the EUROPIV-2 programme
The EUROPIV-2 project acts as a framework to push the capabilities of PIV forwards. Through this (and
a parallel programme PIVNET-2) the transfer of such improvements, applications and results from universities and institutions to industry is aided. EUROPIV-2 is split into 5 themes, or work packages
(WP’s), the work reported here being carried out as part of WP3, which is devoted to demonstrating the
applicability of the technique, at large scales, in industrial wind tunnels. Testing in these situations, as
opposed to university laboratories, is generally characterised by large models and test facilities, plus intensive test programmes due to the short lengths of time that a model is in a wind tunnel because of cost
considerations.
Work Package 3 itself can be broken down into four sub-tasks.
One of these, Task 3.1: Two-
dimensional PIV tests in the low-speed wind tunnel of Airbus Bremen, Germany is the subject of this paper. Of prime interest in this task was the investigation of the flowfields through the gaps between slat &
wing and wing & flap. This meant a much more complicated set-up than for EUROPIV-1, involving the
generation of laser light sheets on both sides of the model, plus the physical fitting of four relatively large
cameras, sometimes with very little space between them, to view the areas desired. The use of a twodimensional, three-element model for this task was a compromise between a model which was not too
complex a shape to gain optical access for PIV, but for which the physics of the flow over it were still
complex, representative of high-lift systems and difficult for CFD codes to model.
2
3 The experimental facilities
3.1 The Airbus Bremen low-speed wind tunnel
The Airbus Bremen low-speed wind tunnel (LSWT) is of open-circuit, Eiffel-type, with a closed working
section and a contraction ratio of 4.82:1. The tunnel is installed in a hall that provides the return circuit.
-1
-1
The operating speed range is from 6 ms to 75 ms . The working section is shown in Fig.1 (with the test
model installed) and is 4.45 m in length, with a cross section measuring 2.1 m
2.1 m. It can be seen that
the floor and ceiling are each fitted with a large turntable; these can rotate separately to each other, or be
coupled to rotate together. Along the side walls are fitted several glass windows, with an additional window set into the floor turntable. In addition, the working section is fitted with fillets along the streamwise
corners to reduce secondary flows and also provide internal lighting for the test section.
3.2 The model
The model used for the tests was the same RA16SC1 two-dimensional model as for EUROPIV-1 [11],
however this time with the flap deployed too. The whole model was manufactured by ONERA Lille,
France and has a span of 2.1 m, a maximum thickness of 0.08 m and with both slat and flap deployed, an
extended chord length of 0.691 m. Fig. 2 shows a section through the profile. The main wing is manufactured from aluminium, however the slat and flap are made from steel to minimise any aeroelastic deformations and for these tests, were set at droop angles of 30° and 40° respectively. To attach them to the
wing, two sets of streamlined tracks (of maximum thickness 5 mm) were designed specially. Each set
comprised four tracks for the slat and flap each. One set allowed the slat and flap to be attached in the
normal fashion for such devices as in Fig. 3(a) and were termed inner tracks. The other set were termed
outer tracks and were fitted to the upper surfaces of the wing, slat and flap to allow unrestricted optical
access into the slat and flap coves. The outer tracks for the slat are shown in Fig. 3(b).
The model was installed vertically in the wind tunnel as shown in Fig. 1. The leading edge of the wing
is towards the reader and thus the flow direction is into the page. At the roof, the model was mounted in a
bearing which allowed rotation and was covered with an aerodynamic fairing. The model thus penetrated
the level of the rectangular window in the tunnel floor turntable slightly as in Fig. 4, the model being
bolted to a support that had been designed to allow optical access to the upper surface of the wing. This
support was in turn attached firmly to a motorised turntable (Fig. 5) that drove the model to the test incidences desired. The hole left in the tunnel floor turntable was filled with wooden plates that were cut to
fit around the model section.
3
3.3 Measurement apparatus
A sketch showing an example arrangement of the experimental apparatus is shown in Fig. 6. On each side
of the working section, a light sheet was formed and directed into the wind tunnel through the tunnel side
windows. The cameras were mounted underneath the wind tunnel. Each camera was connected to its
own dedicated image acquisition PC and a fifth PC was used to synchronise the trigger signals to the cameras with the laser pulses via a purpose-built box of electronics [12] (hereafter known as the sequencer
box). The trigger signal to the frame grabber cards was carried by a single cable to a switch, after which
the signal was split to the PCs. The switch was used to ensure that the cameras received their timing signals simultaneously.
On a balcony on each side of the working section, a pair of Quantel Brilliant-B pulsed lasers was
mounted in a frame built from X-95 rails (Fig. 7). The lasers emitted green light at a wavelength of 532
nm, with a maximum energy of 320 mJ per pulse. Each frame also had its own set of optics to produce a
light sheet of the width and thickness desired. The light sheets were set up to enter the working section
horizontally through the windows in the side walls of the working section, at the height of the mid-span of
the model (Fig. 8). Great care was taken to ensure that the light sheets from both sides of the tunnel were
co-incident, i.e. at the same height and horizontal in both transverse and longitudinal directions. Otherwise, the light sheets would cross along one line in the working section and diverge from each other on
either side (similar to 2 LDA beams). Not only would this have meant that the PIV images on the two
sides of the model would not have been taken at the same spanwise location, but there would have been a
high risk of ghost images causing problems for the analysis also. During the actual measurements the lasers (and hence the camera triggering) were controlled from the tunnel control room.
The cameras used were PCO Sensicam digital cameras, with CCD resolutions of 1280 × 1024 pixels.
These cameras incorporate cooling for the CCD array, so that the grey-scale sensitivity is 12-bit. The
lenses used were Zeiss 100 mm (f/2.8) or 180 mm (f/2.8), except for one case where a Tamron 300 mm
(f/2.8) lens was used to get greater resolution. However, it was clear before the model installation that
there would be insufficient room below the tunnel floor to mount the cameras and lenses vertically.
Therefore a system of flat rails was constructed on the red, motorised turntable and the cameras were
mounted on these, as in Fig. 9, along with high-quality mirrors so that the cameras could view the areas of
interest. Where it was not possible to mount a mirror at 45° exactly in order to view the area desired, the
corresponding camera was mounted in a Scheimpflug adapter. Due to the space limitations, it was not
possible mount motors to focus the lenses remotely, therefore focusing was manual.
The control and video signals for the cameras were transmitted through fibre optic cables to PCs in the
tunnel control room, as in Fig. 6. In order to help position the cameras, a portable PC was also placed under the tunnel. This PC had the same image acquisition hardware and software installed as that used to
acquire the actual PIV images on the PCs in the control room. By using a short length of fibre-optic cable
from the portable PC to a camera, it was possible to see what the camera was looking at whilst adjusting
its position from beneath the tunnel.
4
The seeding was provided by tiny particles of DEHS (Di-2-Ethylhexyl-Sebacat) oil. Two seeding generators [10] were placed on the floor of the wind tunnel hall, upstream of the tunnel intake. Thus the
complete hall surrounding the wind tunnel was seeded, i.e. the seeding was global. Each generator incorporated 5 nozzles, based on work performed for Work Package 2 in the EUROPIV-2 programme [6],
these being immersed in the DEHS. Compressed air was blown through the nozzles producing a mist of
particles in the space above the liquid oil. The size distribution of the particles is approximately bellshaped, with a modal diameter around 1 m.
4 Experimental conditions
4.1 Test and model configurations
-1
The tests were performed using a wind tunnel freestream speed of 54 ms . Three model incidences were
chosen for the PIV measurements, viz. 12°, 17.5° & 19°. The slat and flap positions stayed to all intents
and purposes the same during the tests, the only changes being which set of slat/flap tracks were used for
a particular camera arrangement. The change from one set to another produced measured changes in
model slat or flap position of (at most) 0.25 mm in X and 0.3 mm in Y [9]. The PIV tests were grouped
into four camera arrangements, these being shown in Fig. 10.
For set-ups 1–3, the inner slat and flap tracks were used as shown in Fig. 3(a). For set-up 4, the outer
slat track was fitted (Fig. 3, b), the inner flap track remaining as before. The reason for 2 window-a’s in
Fig. 10(b) is as follows: at
but not at
= 12°, the attachment line on the slat was in the field of view of the camera,
= 17.5° & 19°. Therefore, the mirror for window-a was shifted slightly downstream and the
experiments performed again at the two higher incidences.
As a result of experience gained in the EUROLIFT programme [2, 3], to remove spurious reflections
and blooming on the camera arrays due to the laser sheet striking the metal model surface, wide pieces of
matt-black, self-adhesive, foil were attached carefully to the model surface. This explains the black regions in Fig. 8. The foil itself is 0.13 mm thick and was not damaged during the tests by the energy in the
laser sheets. The effect of the foil in eliminating these problems can be seen in Fig. 11.
4.2 Experimental procedure
During each days’ measurements, the lasers were allowed to run in flashlamps only mode when no measurements were being performed. This was in order to keep them at a nearly constant working temperature,
this being beneficial to the quality of the laser sheet. For each new set of measurement windows, the following procedure was performed.
Firstly, with the wooden panels in the tunnel floor removed, the calibration grids were fitted up to the
wing at the mid-span. The cameras and their mirrors were positioned to view the regions for the test using
5
the portable PC mentioned earlier. This being done, holes were cut in the wooden plates where necessary
and thin glass plates bonded to their undersides to produce viewing portholes for the cameras as in Fig.
3(a). The wooden plates were then replaced and fixed. Each camera’s video output was connected back
to its acquisition PC, the lenses focused and calibration images taken. After the calibration images had
been acquired, the lenses’ apertures were opened to maximum for the PIV experiments and the working
section cleared.
The tunnel was made ready to run, the hall blacked out and the tunnel then run at a speed of 10 m/s.
The seeding was introduced into the hall and the lasers operated on high energy, with 1/3 s time interval
between one pair of pulses and the next. Fine adjustments to the focus of the cameras were carried out on
the seeding particles themselves before data acquisition commenced. In addition, for the cameras which
had a mirror at other than 45°, it was necessary to make adjustments to the Scheimpflug condition, in order to obtain sharp particles across the whole width of the image(s). For these tasks, one person with
protective eyewear and two-way radio went beneath the tunnel to the cameras and subject to instructions
from people watching the monitors in the control room, adjusted the focus and Scheimpflug conditions as
required. On completion of this, the tunnel was accelerated to 54 m/s and the experiments commenced.
For each incidence, the trigger synchronisation switch was opened and the image acquisition programmes on the four PCs set to acquire 34 image pairs. The trigger switch was then closed and acquisition commenced. Once all PCs had acquired the images to their RAM, they were stored to disk. For each
case, this process was repeated twice, thereby giving 102 image pairs for each window. The model incidence was then changed. On completion of the incidence polar, the tunnel was shut down, the lasers returned to flashlamps only mode and where necessary, the cameras moved to their positions for the next
set-up.
5 Data reduction
In total, including calibration images, nearly 5000 PIV images were acquired, amounting to a little over
25 Gb of raw images. All the PIV images have now been analysed to give instantaneous and averaged
velocity vector maps. For the analysis, the origin of the co-ordinate system is the leading edge of the
main wing itself as shown in Fig. 2. In all cases, the accuracy in the (X, Y) co-ordinates of the vectors is
0.5 mm.
Firstly it was necessary to de-warp all images [4] to remove perspective distortions. All images were
then rotated and flipped (mirrored) so that the grid lines were parallel to the edges of the images and the
direction of the freestream flow was from left to right in the images. To replicate the wing contour and
avoid regions of obviously false vectors due to shadows etc., masks were created by hand and used in the
analysis.
The analysis was performed using 32 32 pixel interrogation windows, with a 16 pixel step. Each interrogation window was allowed to search within a larger area to obtain the most plausible correlation, as in
6
some cases, the particles had travelled further than the downstream edge of the interrogation window.
This resulted from the time interval between laser-pulse being a compromise when performing PIV measurements in regions were the dynamic range of the flow is extremely large, e.g. around the slat/wing gap
-1
where the velocity magnitude ranged from 0 to c.170 ms . Outliers were removed by setting upper and
lower boundaries for the velocity components in the X- & Y-directions. 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.
The analysed data have been made available to the Dassault company in France for comparison with
Navier-Stokes computations.
6 Discussion of results
The results are presented, firstly as an overview around the model, then as close ups of the leading and
trailing edges. Although the physical scales in some of the figures can make them look rather like contour
maps, they are actually vector maps. In all figures, the tail of the vector is at the centre of the interrogation window position and the legend shows the velocity magnitude.
6.1 Overall flow picture
Figs. 12 and 13 show the averaged velocity vector maps for 12° & 19° respectively. Especially clear is
the separation line from the lower, trailing-edge corner of the slat and its development into a shear layer in
the slat cove. The most obvious difference in flow behaviour in the slat cove with incidence is the line
taken by this shear layer as it wends its way in the positive Y direction to attach on the downstream side
of the slat. In addition, the flow acceleration through the slat/wing gap is particularly prominent. At 12°
-1
incidence, the maximum flow velocity just downstream of the gap is between 14,000 and 14,500 cms .
-1
At 19°, this increases to between 16,000 and 16,500 cms .
The development of the slat wake, gap flow and main wing boundary layer can be seen in windows-e &
-g, these features being more prominent at 19° due to the colour scales used.
6.2 The leading edge
This set-up gave a more detailed view of the flow in the slat/wing gap region. Fig. 14 shows the averaged
velocity vector maps at 12°. In addition, set-up 2 allowed the attachment line on the leading edge of the
slat to be measured. The wake of the slat is defined clearly here and the wing boundary layer can be seen
developing and thickening also.
Figures 15 & 16 show more detailed views in the area of the slat cove and downstream of the slat/winggap respectively: instantaneous vector maps of the flow in windows-l & -c at 12° are shown. The shear
layer and re-circulation zone close to the downstream side of the slat can be followed. The meandering
7
nature of this shear layer and small structures in the re-circulation zone are depicted rather well by Fig. 15.
In addition the slat wake can be discerned at the right-hand side of Fig. 16. From the series of instantaneous vector maps, the technique allows instabilities over time to be recorded.
6.3 The trailing edge
Moving downstream to look at the trailing edge of the main wing and wing/flap gap, there are also considerable differences in flow behaviour with increasing incidence. Figs. 17 & 18 show this region at
=
12° & 19° respectively. Unfortunately there are a few regions of drop outs at the higher incidence, near
the bottom of window-k. The much higher density of vectors in window-j is due to the use of a 180 mm
objective, whereas for the other regions, a 100 mm objective was used.
In window-g, the contour levels show that as the incidence changes from 12° to 19°, although the
-1
higher velocity regions (8,500–9,000 cms ) of the outer, potential flow extend further downstream, the
velocity deficit in the slat wake becomes greater. However the most obvious changes are over the flap in
windows-i and –j. At 12°, it seems that a separation occurs over the flap at about 50% of its chord length,
whereas this is not evident at 19°. Fig. 18 shows a close-up of this region at
= 12°, where the deccel-
eration of the flow over the flap downstream of the wing/flap-gap can be seen more clearly (n.b. every 2
nd
vector has not been plotted here for clarity).
Set-up 4 gave information over a greater area around the trailing edge of the flap than set-up 3 and the
averaged velocity vectors are shown in Figs. 20 & 21 for
= 12° & 19° respectively. It is clear from
Fig. 20 that a time-averaged, re-circulatory flow exists over the rear half of the flap. However at the
higher of the 2 incidences shown, it seems that the relatively high-velocity flow through the wing/flap-gap
remains attached to the flap surface.
What actually happens however can only be discovered by looking at the instantaneous vector maps.
By this, it was found that separation and flow re-circulation over the flap indeed occurred, however the
phenomenon was intermittent. At
= 12°, the separation occurred quite frequently, but less so as the in-
cidence was increased to 17.5°, becoming almost absent at 19°. For this reason, the averaged vector map
at 12° shows the existence of a re-circulation zone, but at higher incidences, the separation did not occur
frequently enough to affect the averaging process, giving the impression that the higher velocity flow
through the wing/flap-gap remains attached, which is false. This illustrates the danger of using averaged
data alone and shows how important and useful a technique PIV is for fluid measurement.
The two instantaneous vector maps in Figs. 22 & 23 are for
= 12° and illustrate the intermittency of
the separation. They show the vector fields in window-i, with a time interval of ⅔ s, i.e. Fig. 23 was acquired ⅔ s after Fig. 22. The change from Fig. 22 to Fig. 23 is immediately obvious: in the former the
high velocity flow through the wing/flap-gap remains attached in the field of view, whereas in the latter a
strong separation has occurred. In the separated region, reversed flow with a maximum magnitude of ap-
8
-1
proximately 50 ms occurs and in addition, several vortical sub-structures can be seen. Other recordings
of the separation which have been acquired are less dramatic than this, but the phenomenon still occurs
nevertheless.
It would seem likely that the occurrence, or not, of the separation is dependant on the behaviour of the
flow through the wing/flap-gap and therefore on the nature of the flow in the flap cove which could influence the gap flow. This is an area where the instantaneous images will be looked at in more detail.
7 Conclusions
Multiple-window PIV has been applied to the wind tunnel testing of a slat/wing/flap model in a high-lift
configuration, in an industrial environment, with very good results. Therefore PIV is feasible for industrial wind tunnel tests.
The use of four cameras to capture the flowfield in different regions around the model simultaneously
has many benefits for the comparison of experimental results with numerical codes. It is believed that this
technique can help to develop improvements in the latter.
Such tests are complex however and (sometimes considerable) testing is required with regard to camera
viewpoints etc. prior to the entry into the wind tunnel.
In particular, the interactions between areas of unsteady flow (e.g. slat or flap coves) and the flow
slightly downstream of these areas can now be investigated in more detail.
At an incidence of 12°, an unsteady separation was evident over the flap. This occurred less as incidence was increased, becoming almost absent at 19°.
The campaign was able to fulfil its intended task for WP 3.1 and is therefore considered very successful.
Acknowledgements
The work has been funded as part of the EUROPIV-2 programme (EU project G4RD-CT-2000-00190). In
addition, the authors would like to thank especially (in alphabetic order) Dipl.-Ing. Peter May, Dipl.-Ing.
Klaus Muthreich and Dipl.-Ing. Susanne Wyrembek of Deutsche Airbus, Bremen, for their assistance,
humour and hospitality during the experimental programme.
References
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Mech. 23: 261–304.
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[2] Arnott AD, 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.
[3] Dieterle L, Kompenhans J, Schneider G (2001) PIV adaption and pre-testing for low Re tests in the
LSWT Bremen. EUROLIFT technical report TR1.3.1-4, partner report for EU programme GRD11999-10015.
[4] Ehrenfried K (2001) Processing calibration-grid images using the Hough transformation. In: Proc 4
th
Intl Symp on PIV, 17–19 September, Göttingen, Germany: paper 1042.
[5] Hansen H (1998) Überblick über das Technologieprogramm Hochauftriebskonzepte (HAK). DGLR
Jahrestagung, Bremen, Germany.
[6] Kähler CJ, Sammler B, Kompenhans J (2001) Generation and control of particle sizes for optical
th
velocity measurement techniques in fluid mechanics. In: Proc 4 Intl Symp on PIV, 17–19 September, Göttingen, Germany: paper 1117.
[7] 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.
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[9] Monnier J-C (2003) Private communication by e-mail.
[10] Raffel M, Willert C, Kompenhans J (1998) Particle image velocimetry - a practical guide. Springer,
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[11] Stanislas M, Kompenhans J, Westerweel J (2000) Particle Image Velocimetry - Progress towards Industrial Application (eds). Kluwer.
[12] Stasicki B, Ehrenfried K, Dieterle L, Ludwikowski K, Raffel M (2001) Advanced synchronisation
th
techniques for complex flow field investigations by means of PIV. In: Proc 4 Intl Symp on PIV,
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[13] Thibert J J (1993) The GARTEUR high-lift research programme, high-lift system aerodynamics.
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Figures
Fig. 1. The model in the LSWT; the flow direction Fig. 2. The section profile of the RA16SC1 model.
is into the page.
U
Inner tracks
U
Outer slat tracks
b) the outer slat tracks.
(a) the inner slat and flap tracks.
Fig. 3. The slat and flap tracks, plus viewing portholes cut in the wooden panels fitted into the tunnel
floor for optical access to the plane of the light sheet.
model
model support
model support
Fig. 4. The model support underneath the LSWT.
Fig. 5. Motorised turntable underneath the LSWT.
11
Control
PC
Laser head &
power unit
4 PIV
acquisition
PCs
Sequencer
Fibreoptic
cables
Light sheet
optics
U
Camera
positions
Light sheet
through side
window
Fig. 6. Diagram of DLR’s PIV equipment positioned around the LSWT.
Fig. 7. One set of lasers and optics on a balcony.
Fig. 8. Checking the alignment of the light sheets.
Fig. 9. The flat rail system for the cameras and mirrors underneath the tunnel floor.
12
cc
a
d
e
g
l
(a) set-up 1
(b) set-up 2
gg
i
h
k
l
j
(c) set-up 3
(c) set-up 4
Fig. 10. The four groups of PIV experiments, showing the areas viewed by the cameras.
pixel blooming
wing surface
surface
wing
(a) no foil applied.
(b) with the black, self-adhesive foil.
Fig. 11. The problem of spurious reflections from a model’s surface and their elimination using mattblack, self-adhesive foil.
13
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Fig. 12. Averaged velocity vector maps for set-up 1 at
= 12°. The apparent region of drop-outs (hole)
in window-l is not actually there. The velocity vectors in that region are so small, that at this scale it
looks like a hole exists..
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Fig. 13. Averaged velocity vector maps for set-up 1 at
= 19°. Again the velocity vectors in window-l
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2000
1000
0
10
c
Y [cm]
5
0
-5
l
-10
a
-15
-15
-10
-5
0
5
10
15
20
X [cm]
Fig. 144. Averaged velocity vector maps for set-up 2 at
= 12°.
1
Vel [cm/s]
11000
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
0
-1
-2
Y [cm]
-3
-4
-5
-6
-7
-8
-9
-7
-6
-5
-4
-3
-2
-1
0
1
2
X [cm]
Fig. 15. Instantaneous PIV map in the slat cove (posl) at
simultaneously with Figure 16.
15
= 12° (from set-up 2). This map was acquired
7
Vel [cm/s]
15000
14000
13000
12000
11000
10000
9000
8000
7000
6000
5000
4000
6
5
Y [cm]
4
3
2
1
0
-1
-2
-4
-3
-2
-1
0
1
2
3
4
5
6
7
X [cm]
Fig. 16. Instantaneous PIV map downstream of the slat/wing-gap (posc) at
= 12° (from set-up 2, al-
though every 2nd row of vectors has been omitted for clarity). This vector maps was acquired simultaneously with Fig. 15, although different colour scales are used to highlight different features.
g
i
k
j
16
Fig. 17. Averaged velocity vector maps for set-up 3 at
= 12°.
g
i
k
j
Fig. 18. Averaged velocity vector maps for set-up 3 at
= 19°.
Vel [cm/s]
12000
11000
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
g
5
Y [cm]
i
0
-5
k
40
45
50
55
60
X [cm]
Fig. 19. Close-up of the averaged velocity vector maps at the trailing edge at
up 3). Every 2nd vector has been omitted for clarity.
17
= 12° (taken from set-
-4
-5
-6
Y [cm]
-7
-8
Vel [cm/s]
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
-9
-10
-11
-12
-13
56
57
58
59
60
61
62
63
64
65
66
67
X [cm]
Fig. 20. Averaged vector map for window-j at 12° (set-up 4).
-4
-5
-6
Y [cm]
-7
-8
Vel [cm/s]
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
-9
-10
-11
-12
-13
56
57
58
59
60
61
62
63
X [cm]
Fig. 21. Averaged vector map for window-j at 19° (set-up 4).
18
64
65
66
67
Y [cm]
0
-5
Vel [cm/s]
11000
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
50
55
60
X [cm]
Fig. 22. Instantaneous PIV map above the flap (window-i) at
nd
= 12° (set-up 3). Every 2 row of vectors
has been omitted for clarity.
Y [cm]
0
-5
Vel [cm/s]
11000
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
50
55
60
X [cm]
Fig. 23. Instantaneous PIV map above the flap (window-i) at
= 12° (from set-up 3). The image was
acquired ⅔ s after Fig 22. Again every 2 row of vectors has been omitted for clarity.
nd
19