Detailed Characterisation, using PIV, of the Flow around an Airfoil in High-Lift Configuration.

Transcription

1 Detailed Characterisation, using PIV, of the Flow around an Airfoil in High-Lift Configuration. AD Arnott 1, G Schneider 1, K-P Neitzke 2, J Agocs 1, A Schröder 1, B Sammler 1 and J Kompenhans 1 1 DLR, Institute of Aerodynamics and Flow Technology, Bunsenstrasse 1, 3773 Göttingen, Germany 2 Airbus Germany, Department EGXG, Hünefeldstrasse 1-5, Bremen, Germany Abstract Within the framework of the European Union s EUROPIV-2 programme, experiments using Particle Image Velocimetry (PIV) were performed in the low-speed wind tunnel of Airbus Bremen, Germany. The goal was to investigate the flow over a two-dimensional, 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 wind tunnel. The PIV experiments were performed at the mid-span of the model, for incidences of 12, 17.5 and 19. Nearly 5 PIV images were obtained, 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 revealing the unsteady nature of the flow around high-lift configurations, the mixing that occurs across wakes, plus intermittent flow separation over the flap. 1 Introduction The aircraft industry worldwide places great importance still on high-quality wind tunnel data in order to validate its various CFD codes. However, prior to the development of optical measurement techniques, single- or multiple-pressure-probe methods were the only means of investigating the velocity fields around wind tunnel models. Unfortunately, probe methods suffer from introducing measurable disturbances to the flow. In addition, single-point techniques (including the optical one of Laser Doppler Anemometry) suffer from the limitation of requiring much time to measure a region step-by-step. This is clearly a disadvantage where there are regions of unsteady (including separated) flow. Such a flow is that around a deployed high-lift system, where flow separation, re-circulation, acceleration, wakes and mixing all occur. Therefore, in order to understand such flows, designers have a great need for data acquired from various

2 32 Session 1 regions around a model simultaneously,. The ability to accomplish this is of huge benefit to the designers, working as they do in an increasingly competitive market that is driven by lower operating costs and increased safety margins [3, 12, 13]. PIV would seem to be suited very well to this task, as it allows the measurement of the flow over an area instantaneously [1, 5, 6, 8, 9, 14]. In addition, for all practical purposes, it can be considered non-intrusive. However, to be of use to designers, it needs to be shown that PIV is a viable and valuable technique for industrial wind tunnel testing and not just small-scale testing in university laboratories. The former situations are usually quite different to the latter, being characterised by large models and test facilities, plus intensive test programmes. In this respect, the EUROPIV-2 project acts as a framework to push the capabilities of PIV forwards. The project is split into 5 work packages (WP s), the work reported here belonging to WP3. This is devoted to the application of the technique, at large scales, in industrial wind tunnels. 2 High-lift PIV tests PIV had been applied to a two-dimensional slat/wing combination already as part of the EUROPIV-1 programme [9]. The investigation was able to capture the flow in the leading edge region extremely well, however no flap was deployed and in addition, the data were acquired from one area at a time only. Another investigation published in the same year [7], looked specifically at the slat wake characteristics, again for a two-dimensional high-lift model. A third recent investigation [11] concentrated on the slat-cove region, however again this was concerned just with a single area. EUROPIV-2 pushes knowledge in this area forwards by the use of the same model as in EUROPIV-1, but with both slat and flap deployed. In addition, the data were acquired from four regions around the model simultaneously. Of prime interest was the investigation of the flowfields through the gaps between slat and wing and wing and flap. The use of a two-dimensional, three-element model 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. 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 is in a hall that provides the return circuit. The working section is shown in Fig. 1 and is 4.45 m in length, with a cross section measuring m 2. Along the side walls are fitted several glass windows, with

3 Aeronautics 33 an additional window set into the floor turntable to provide optical access for the PIV system. 1 Y/cm X/cm Fig. 1. The working section of the LSWT (the flow direction is into the page) and the section profile of the RA16SC1 two-dimensional EUROPIV-2 model. 3.2 The model The model was the same RA16SC1 two-dimensional model as for EUROPIV-1 [9] and its section profile is shown also in Fig. 1. The model was manufactured by ONERA Lille, France and has a span of 2.1 m, a maximum thickness of.8 m and with both slat and flap deployed, an extended chord length of.691 m. The slat and flap were set at droop angles of 3 and 4 respectively. 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. The model passed through the tunnel floor turntable a short distance, it being attached to a motorised turntable underneath the tunnel. The gap remaining in the tunnel floor turntable was filled by 2 wooden plates cut specially to fit around the model profile. 3.3 Measurement apparatus Fig. 3 shows an example arrangement of the experimental apparatus. On each side of the working section, a light sheet was formed and directed into the wind tunnel through the tunnel side windows. To form the light sheets, two pairs of Quantel Brilliant-B pulsed lasers plus associated optics were mounted outside the working section, one on each balcony. The light sheets were set up to enter the working section horizontally, at the height of the mid-span of the model.

4 34 Session 1 Laser head & power unit Light sheet optics Sequencer Sequence control PC 4 PIV acquisition PCs U Camera positions Fibreoptic cables Light sheet through side window Fig. 2. Diagram of DLR s PIV equipment positioned around the LSWT. Fig. 3. The mounting of the cameras and mirrors under the tunnel floor. Fig. 4. The wooden panels in the tunnel floor, with viewing portholes inserted. The cameras used were PCO Sensicam digital cameras, with Zeiss and Tamron lenses of maximum aperture f/2.8 and various focal lengths. However, there was insufficient room below the tunnel floor to mount the cameras and lenses vertically. Therefore the cameras were mounted on a horizontal rail system, along with high-quality mirrors to fold the image path as in Fig. 3. For the majority of cases, the mirrors were at 45 ; however where not, the cameras were mounted in Scheimpflug adapters. So that the cameras could view the light sheet, portholes were cut in the wooden tunnel floor where necessary and thin glass plates fixed to cover them as in Fig. 4. Each camera was connected to its own dedicated image acquisition PC and a fifth PC was used to synchronise the cameras and laser

5 Aeronautics 35 pulses via a box of electronics referred to as the sequencer [1]. The trigger signal from the sequencer to the image acquisition PCs 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. Focussing of the lenses was manual. The seeding was produced by two DLR seeding generators [6], filled with DEHS (Di-2-Ethylhexyl-Sebacat) oil and the complete hall surrounding the wind tunnel was seeded, i.e. the seeding was global. The design of the generator nozzles was new however, based on work [4] performed earlier in the timescale of the EUROPIV-2 programme. 4 Experimental conditions The PIV tests were grouped into four camera arrangements, these being shown in Fig. 5. The tests were performed using a wind tunnel freestream speed of 54 ms -1, at three values of model incidence, viz. 12, 17.5 and 19. The slat and flap positions stayed the same during the tests c d e g a l (a) Setup (b) Setup g i h l k j (c) Setup (d) Setup-4 Fig. 5. The four groups of PIV experiments, showing the areas viewed by the cameras. For each new arrangement, following procedure was performed. Firstly, with the wooden panels in the tunnel floor removed, the calibration grids were fitted up

6 36 Session 1 to the wing at the light sheet height. This aided the positioning of the cameras and their mirrors. If necessary a new porthole was cut in the wooden plates and a glass plate bonded to the underside as in Fig. 4. The wooden plates were then replaced and fixed prior to the acquisition of calibration images. For the actual PIV acquisition, the tunnel hall was blacked out and fine adjustments to the focii of the cameras were carried out on the seeding particles themselves before data acquisition commenced. In addition, for the cameras that had a mirror at other than 45, it was necessary to make some adjustment to the Scheimpflug adaptor(s). For each incidence, a sequence of 34 image pairs was acquired from each camera with an inter-pair interval of ⅓ s and saved to hard disc. This was then repeated twice to give 12 image pairs for each window. In total, including calibration images, nearly 5 PIV images were acquired. This amounted to slightly more than 25 Gb of raw images. 5 Data reduction and discussion of results For the analysis, it was necessary firstly to de-warp all images [2] to remove perspective distortions. The analysis then proceeded using a pixel interrogation window, with a 16 pixel step. Each interrogation window was allowed to search within a larger area to obtain its 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. The origin of the co-ordinate system is the leading edge of the main wing itself as shown in Fig. 1. In all cases, the accuracy in the (X, Y) co-ordinates of the vectors is ±.5 mm. Unfortunately, space limitations permit the presentation of results from selected areas only: a more complete discussion is provided in the EUROPIV-2 technical report. In particular, at the scale of the figures, the reproduction of the overview in Setup-1 would give little insight into the flow. The use of colour in the version on the CD that accompanies the book makes it easier to discern the features of the flow. In all figures, the tail of the vector is at the centre of the interrogation window position. 5.1 The leading edge Figs. 6 and 7 show the averaged velocity vector maps in the slat/wing-gap region for 12 and 19 respectively. Although the use of lack of colour in the book version makes many features (such as the velocity defect across the slat wake) impossible to discern, the development of the shear layer from the separation at the lower, trailing-edge of the slat can still be seen. At 12, the path of the averaged shear layer lies a few cm downstream of its position at 19. In addition, the recirculation zones in the slat cove and the flow acceleration through the slat/wing-gap are prominent. At 12 incidence, the maximum flow velocity just downstream of the gap is between 14 and 145 ms -1. At 19, this increases to between 16 and 165 ms -1.

7 Aeronautics Vel [cm/s] Fig. 6. Averaged velocity vector maps on both sides of the slat/wing-gap at α = 12 (taken from Setup-2). Every 2 nd vector has been omitted for clarity Vel [cm/s] Fig. 7. Averaged velocity vector maps on both sides of the slat/wing-gap at α = 19 (taken from Setup-2). Every 2 nd vector has been omitted for clarity.

8 38 Session 1 However, on their own, the averaged images do not show the full picture. The flow in the slat cove is extremely unsteady: the path of the shear layer wandering in space with time. Fig. 8 shows a more detailed view in this area at 12. The turbulent nature of the shear layer from the lower, trailing-edge of the slat can be seen from the variations in the directions of the vectors. In addition, small structures in the re-circulation zone, plus reattachment on the downstream side of the slat, have been captured. From a series of instantaneous vector maps, the PIV technique allows such instabilities in shear layers and wakes over time to be recorded. 5.2 The trailing edge Moving downstream to look at the trailing edge of the main wing and wing/flapgap, there are also considerable differences in flow behaviour with increasing incidence. Fig. 9 shows the flow pattern in this region at α = 12, averaged from 99 images, in which the acceleration of the flow through the wing/flap-gap can be seen clearly. The development and mixing of the slat wake was also captured in window-g, although it is not clear in the black and white book version. With the use of colour on the accompanying CD to show levels of velocity magnitude, it is possible to see the features in the slat and wing wakes quite easily. It is possible in the monochrome version however, to make out the basic form of the wake from the main wing in window-i. Vel [cm/s] Fig. 8. Instantaneous PIV map in the slat cove (window-l) at α = 12 (from Setup-2). Every 2 nd row of vectors has been omitted for clarity.

9 g Aeronautics 39 Near (X, Y) = (57, -7) cm in Fig.9 it also appears that a flow separation might be occurring over the trailing edge of the flap at 12. Looking more closely at the instantaneous vector maps, it was found that separation and a region of flow recirculation indeed occurred, the phenomena being intermittent however. As the model incidence was increased from 12, this separation occurred less frequently until at 19 it was almost absent. For this reason, the averaged vector map at 12 shows the existence of a recirculation zone, but at higher incidences, the averaged flow pattern gives the impression that the higher velocity flow through the wing/flap-gap remains attached, which is false. However, at the two higher incidences, the separation did not occur frequently enough to affect the averaging process. This illustrates the danger of using averaged data alone and shows how important and useful the technique of PIV is for fluid measurement. 5 Window-g Window-i Vel [cm/s] Window-k Fig. 9. Averaged velocity vector maps at the trailing edge at α = 12 (taken from Setup-3). Every 2 nd vector has been omitted for clarity. The two instantaneous vector maps in Figs. 1 and 11 illustrate the intermittency of the separation. They show the vector fields in window-i, with a time interval of.66 s, i.e. Fig. 11 was acquired.66 s after Fig. 1. The change from Fig. 1 to Fig. 11 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 approximately 5 ms -1 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.

10 4 Session 1 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. -5 Vel [cm/s] Fig. 1. Instantaneous PIV map above the flap (window-i) at α = 12 (from Setup-3). Every 2 nd row of vectors has been omitted for clarity. 6 Conclusions Multiple-window PIV has been applied to the wind tunnel testing of a slat/wing/flap model, in an industrial environment, with very good results. Therefore PIV is feasible for high-lift testing in industrial wind tunnels. This technique to capture the flowfield in different regions around a model simultaneously is deemed very beneficial for the comparison of experimental results with numerical codes. 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 and is therefore considered very successful.

11 Aeronautics 41-5 Vel [cm/s] Fig. 11. Instantaneous PIV map above the flap (window-i) at α = 12 (from Setup-3). The image was acquired.66 s after Fig 1. Again every 2 nd row of vectors has been omitted for clarity. 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 23), 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: GRD ). 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] Adrian RJ (1991) Particle-imaging techniques for experimental fluid mechanics. Ann Rev Fluid Mech. 23:

12 42 Session 1 [2] Ehrenfried K (21) Processing calibration-grid images using the Hough transformation. In: Proc 4 th Intl Symp on PIV, September, Göttingen, Germany: paper 142. [3] Hansen H (1998) Überblick über das Technologieprogramm Hochauftriebskonzepte (HAK). DGLR Jahrestagung, Bremen, Germany. [4] Kähler CJ, Sammler B, Kompenhans J (21) Generation and control of particle sizes for optical velocity measurement techniques in fluid mechanics. In: Proc 4 th Intl Symp on PIV, September, Göttingen, Germany: paper [5] Kompenhans J, Dieterle L, Raffel M, Monnier J-C, Gilliot A, De Gregorio F, Pengel K (21) Particle Image Velocimetry: status of development and examples of application in industrial test facilities. In: Proc 3rd ONERA-DLR Aerospace Symposium, 2 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 (2) Particle Image Velocimetry in Aerodynamics: Technology and Applications in Wind Tunnels. J Visualisation 2: [7] Paschal K, Jenkins L, Yao C (2) Unsteady slat-wake characteristics of a 2-D high-lift configuration. AIAA paper [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 (2) Particle Image Velocimetry - Progress towards Industrial Application (eds). Kluwer. [1] Stasicki B, Ehrenfried K, Dieterle L, Ludwikowski K, Raffel M (21) Advanced synchronisation techniques for complex flow field investigations by means of PIV. In: Proc 4 th Intl Symp on PIV, September, Göttingen, Germany: paper [11] Takeda K, Ashcroft GB, Zhang X (21) Unsteady aerodynamics of slatcove flow high-lift device. AIAA paper [12] Thibert J J (1993) The GARTEUR high-lift research programme, high-lift system aerodynamics. AGARD CP-515: 16/1 21. [13] Thibert J J, Reneaux J, Moens F, Preist J (1995) ONERA activities on highlift devices for transport aircraft. In: CEAS forum on high-lift and separation control, University of Bath, Bath, UK. [14] Willert C, Raffel M, Kompenhans J, Stasicki B, Kähler C (1996) Recent applications of particle image velocimetry in aerodynamic research. J Flow Meas Instrum 7: