Characterization of the Flow Field around a Transonic Wing by PIV. A. Gilliot 1, J.C. Monnier 1, A. Arnott 2, J. Agocs 2 and C. Fatien 1 1 ONERA Lille,5 Bd Paul Painlevé, 59 045 Lille cedex France. 2 DLR Gottingen, Bunsenstrasse 10, 37073 Goettingen Germany. Abstract Two-component PIV measurements were realized in the S2 large scale transonic wind tunnel in Modane in April 2001 on a half model of a swept transonic wing provided by ONERA. The PIV measurements have been performed by ONERA and DLR. For the PIV campaign, the Mach number of the flow was 0.82. These PIV measurements were recorded above the suction side of the model for different values of the angle of incidence α. The PIV results show a strong influence of the incidence of the model. At α = 3.7, a shock wave can easily be located on the map with the typical lambda shape caused by shock boundary layer interaction. A comparison was made between PIV and pressure taps results previously obtained by ONERA. This campaign demonstrated that PIV measurements are possible in large industrial wind tunnel facilities and more particularly in transonic flow. It 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. 1 Introduction Particle Image Velocimetry (PIV) is a unique optical method to capture whole velocity fields in flows, in a very short time. Its use in large facilities has been demonstrated in the previous EUROPIV program [1]. It is especially suited to help designing modern aircraft by consequently improving the performance of industrial wind tunnels. PIV yields much more information in much less time, giving a deeper insight into the flow physics and saving large amounts of wind tunnel costs. Instantaneous velocity maps are obtained in a flow plane of interest. By postprocessing, it is possible to calculate average velocity maps, instantaneous or average vorticity maps, turbulence intensity maps and spatial correlation map. Hence, this measurement technique gives access to quantitative information which is very useful for flow characterization. It can both be used in aerodynamics
62 Session 1 and hydrodynamics, and the measurement range extends from very low speed to the supersonic regime [2]. For transonic flows, the PIV applications are very limited and generally they are only realised in research wind tunnels. The objective of this work is to demonstrate the possibility of using planar twocomponent (2D2C) PIV in a large industrial transonic facility to investigate problems of industrial interest around realistic aircraft geometries. The PIV measurements, in this work, have been realised by ONERA in cooperation with DLR around a OAT15 wing profile in the ONERA S2 MA large transonic wind tunnel in Modane [3]. The flow Mach number was chosen at 0.82. The various necessary equipment for these experiments (seeding device, laser, recording system,...) have been jointly provided by the ONERA and DLR teams. 2 Model and PIV set-up The S2MA large transonic wind tunnel of ONERA in Modane is a continuous pressurized transonic and supersonic wind tunnel. The OAT15 model provided by ONERA is a half model of a swept transonic wing. It has been manufactured at the beginning of 2000 as part of the ONERA internal Research Program «Active Flow Control» for buffeting studies. The wingspan is 1.28 m and the aerodynamic chord length at the position of the light sheet is 0.354 m. Figure 1 presents photograph of the model installed in the test section. The angle of attack of the model can very easily be changed during the measurements. Fig. 1. Photograph of the model Figure 2 shows the experimental set-up of the PIV measurements. The YAG laser was placed on the top of the wind tunnel, out of the pressurized envelope. The laser beam is introduced into the test section through the ceiling by means of three reflecting mirrors. It is then expanded into a light sheet by means of an optical set-up made of a cylindrical and a spherical lenses, with focal lengths respectively of 150 and 60 mm. The sheet is perpendicular to the wing.
Aeronautics 63 The tracers used were DEHS droplets, nominaly given for one micron in diameter. They were produced by four Laskin nozzle generators (Fig. 3) and were injected into the wind tunnel through a grid, upstream from the test section, at the entrance of the settling chamber (Fig. 4). YAG Laser Pressurised wall Support Mirror X95-0.5m rail X 100 cube X95-2m rail X95-0.75m rail Ceiling of the test section Light sheet Wind O Y X Model Test section axis Fig. 2. Experimental set-up. Fig. 3. Laskin nozzle generators. Fig. 4. Seeding grid. Measurements were realised on the suction side of the wing, in a vertical plane parallel to the free stream velocity vector. The lateral position of this plane is cho-
64 Session 1 sen according to the limited optical access of the test section. It is set at 480 mm from the fuselage. The intersection of the light sheet with the rotational axis of the model defines the origin of the co-ordinates system for the measurement plane, for which the x-axis is anti-parallel to the free stream velocity. In this system, the x and y co-ordinates of the leading edge are 21.1 mm and 53.4 mm respectively at 0 angle of attack. A high resolution CCD camera (1280 1024 pixels), stands behind the large window of the test section inside the pressure shell of the wind tunnel in a constant pressure enclosure designed to protect it from the pressure variations that occur at wind tunnel start and stop. It is equipped with a 50 mm f/2.8 lens and is connected through an optical fiber link to a micro computer on which the recorded frames are stored. The CCD sensor is synchronised and activated with a delay generator at a rate of 3 frames per second. 3 PIV Measurements The PIV campaign was realised by ONERA and DLR at the end of April 2001. Figures 5 and 6 show an example of the results obtained during this campaign. The mean velocity maps of the longitudinal component at the 2 different angles of attack are presented. Each map is calculated from the averaging of 80 instantaneous maps and contains 2900 velocity vectors. The results show a strong influence of the incidence of the model. At 3.7, a shock wave can easily be located on the map with the typical lambda shape caused by shock-boundary layer interaction. At a higher incidence of 4.5, the velocity close to the model has increased; the shock wave has slightly moved towards the trailing edge and is stronger, leading to a more violent shock-boundary layer interaction: the lambda shock moved upstream, and the area of the shock boundary layer interaction is becomes larger. 25 25 Y (mm) 0-25 Y (mm) 0-25 -50 U (m/s): 300 310 320 330 340 350 360 370 380 390 400 Mach 0.82-3.7-50 U (m/s): 300 310 320 330 340 350 360 370 380 390 400 Mach 0.82-4.5 125 150 175 200 225 250 X(mm) 125 150 175 200 225 250 X (mm) Fig.5. Average velocity map at 3.7. Fig. 6. Average velocity map at 4.5. The presence of the laser light reflection on the wing surface did not permit to obtain PIV measurements closer than 3 mm from the surface. At 3.7 angle of attack, a comparison was made between the PIV results and the pressure tap results which were available. At this incidence, the position of the shock wave measured
Aeronautics 65 with the PIV method is at 60% of the model chord (figure 7), the pressure taps method gives a position between 55% and 60% of the chord (shaded area in figure 8) [4]. Ma = 0.82 - incidence = 3.5 - Pi = 0.6 bar - U (m/s): 300 320 340 360 380 400 Fig. 7. PIV shock position : 60 %. kp KP 1,4 1,2 1 0,8 0,6 0,4 0,2 0 0 10 20 30 40 50 60 70 80 90 100 X/C Fig. 8. Shock position obtained with the pressure taps. 4 Conclusion In order to investigate problems of industrial interest around realistic aircraft geometries, 2D2C PIV measurements were made in an industrial transonic facility under typical conditions of industrial tests. 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 a transonic flow. 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.
66 Session 1 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). The authors would like to acknowledge the European Community which permitted to realise this work. A special thanks is expressed to all the people who contributed to the successful completion of the wind-tunnel test campaign. References [1] J.C. Monnier, G. Croisier,A. Gilliot, (2000) : Characterisation of the EUROPIV Test Experiment by PIV, Particle Image Velocimetry : Progress Towards Industrial Application, Editions Kluwer Academic Publishers. [2] A. Gilliot, :J.C. Monnier, C. Geiler, C. Fatien, (2001) : Vélocimétrie par PIV en grilles d aubes à R4.3 Modane, Rapport ONERA RT 13/01564 DDSS/ DAAP, Confidentiel Industrie. [3] J.C. Monnier, A. Gilliot, C. Fatien, Y. Agocs, A. Arnott (2003) : Transonic test in large industrial wind tunnel, Rapport ONERA RT 14/04009 DAAP. [4] Projet de Recherche Fédérateur : Contrôle actif d écoulements, Jean-Louis Gobert (ONERA) 1997-2001.