ICIASF th International Congress on Instrumentation in Aerospace Simulation Facilities. DLR Göttingen, Germany

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1 ICIASF 3 2 th International Congress on Instrumentation in Aerospace Simulation Facilities DLR Göttingen, Germany August 25-29, 23

2 Application of Particle Image Velocimetry under Cryogenic Conditions H.Richard, W. Becker*, S. Loose, M. Thimm, J. Bosbach, M. Raffel Institut für Aerodynamik und Strömungstechnik Deutsches Forschungszentrum für Luft- und Raumfahrt (DLR) Bunsenstraße 1, D-3773 Göttingen, Germany * Kryo-Kanal Köln (KKK) Deutsch Niederländische Windkanäle (DNW) Linder Höhe, D Köln, Germany ABSTRACT Cryogenic investigations of the trailing vortices of large future transport aircrafts have been performed on an aircraft half-model with different wing tip devices. The model has been installed in the cryogenic wind tunnel (KKK) of the German- Dutch wind tunnels (DNW) in Cologne. During the tests, PIV flow field measurements have been performed in addition to conventional force, moment, and pressure distribution measurements. The measurements of the velocity field gave an insight into important aerodynamic effects as for example the dependends of vortex strength with changing angles of incidence as well as the differences in the spatial vorticity distribution in the wake. The PIV technique offers special advantages for the investigation of those wakes at very low temperatures, but also yields to special experimental difficulties, which are also described in this article. 1 INTRODUCTION With the increasing use of civil aircrafts, airport capacities and therefore start- and landing frequencies become more and more important. Therefore, massive research projects on wake vortices are driven by many aeronautical research organizations and by the aircraft manufactures. The subject of wake vortices is important for the efficiency of airports, since a reduction of wake vortex could lead to a reduction of the required distance between aircrafts and therefore increase the start and landing frequencies. The research activities are also driven by safety considerations, since several aircraft disasters have already been caused by wake vortex affection. Wake vortices in the fare field consist basically of two counterrotating vortices, generated at the end of the wings and the flaps during starting/landing in high lift configuration. The power and duration of these vortices are directly proportional to the weight of the aircraft and are a natural effect caused by the lift of any winged aircraft. They can last over several minutes having a very small diameter. Best conditions for the generation of strong and stable vortices have heavy and slow aircrafts with relative clean wing design. Full-scale wake vortices can be measured by several experiments using radar and laser-technologies. The aim of many activities now is the detailed investigation of the influence of flap, wing and wing tip designs at model scales in order to develop airplanes of more capacity that do not require larger distances between the airplanes. Since the wake vortices develop from a complex system of individual vortices, the study of the near wake directly behind the wing is frequently performed. For realistic investigations the Mach, as well as the Reynolds number has to be simulated accordingly. Therefore, cryogenic wind tunnels and non-intrusive optical flow measurement techniques like particle image velocimetry (PIV) are increasingly important for meaningful comparisons with the results of numerical investigations. 2 WIND TUNNEL FACILITY The cryogenic wind tunnel Cologne (KKK) is a continuously working low-speed tunnel with a closed tunnel circuit. The gas temperature in the tunnel circuit can be varied between 3 K and 1 K by injecting liquid nitrogen in order to achieve higher Reynolds numbers. The Reynolds number can then be increased by a factor of 5.5, while the drive power remains constant /3/$ IEEE 393

3 3 Re * K K ca p a b ility fo r 2D-m odels.1 x S x U Re=.2 1 K c a p a b ility fo r half-m odels 1 K 3 K 1 K c a p a b ility fo r IC E.3 Ma.4 Fig. 1 Re-Ma capability of DNW-KKK Due to the variation of the gas temperature, the influence of Mach number and Reynolds number on the aerodynamic coefficients of model measurements can be investigated separately. To enable cryogenic test operation, the KKK has the following subsystems: a closed test section including an access lock and a model conditioning room for handling the model while the tunnel circuit is held at low temperature a liquid nitrogen injection system to cool down the gas temperature an exhaust system to control the tunnel pressure LN 2 system Pump station LN 2 nozzles Fan Diffuser 1 Drive 3 MODEL GN 2 system Diffuser 2 Control room Test section Model access lock 5 m Exhaust system Screens Model-conditioning room Fig. 2 cross section of DNW-KKK Honey comb The PIV measurements were performed on a half model of a large commercial airplane as part of the M-DAW (Modeling and Design of Advanced Wing tip devices) research program. Three different wing tip devices were investigated: Küchemann wing tip, fence tip and winglet in take off configuration PIV TECHNIQUE In the past years the PIV method has gained continual acceptance as a valuable fluid mechanics research tool in a wide variety of applications. A summary of the theoretical and experimental aspects of the PIV technique is given by Adrian (1991) and Raffel et al. (1998). In this section the PIV subsystems specially developed for application in cryogenic flows will be described. First of all tracer particles, generated by an aerosol generator, have to be added to the flow. These particles have to be illuminated in a plane of the flow two times within a short time interval. The light scattered by the particles has to be recorded. The displacement of the particle images between the two light pulses has to be determined at the evaluation of the PIV recordings. 4.1 TRACER PARTICLES First applications of PIV have already shown that difficulties appear to provide high quality seeding in high speed air flows compared to applications in water flows or in low speed air flows (Höcker & Kompenhans 1991, Towers et al. 1991, Humphreys et al. 1993, and Molezzi & Dutton 1993). High speed air flows require small tracer particles in order to minimize the velocity lag. This problem is discussed in literature intensively (Hunter & Nichols 1985, Melling 1986, and Meyers 1991). However, by decreasing the particle size the light scattered by the particles will be reduced considerably. Thus, a modern high power pulse laser is necessary for recording of tracer particles with a diameter of one micron or less. A much larger volume has to be seeded for PIV than for LDA as the flow velocity is not only measured at a

4 single point but in a whole plane. Moreover, at PIV there should be no gaps in the seeding, as this would lead to data dropout in the instantaneous velocity field. It is known from simulations of the PIV evaluation process, that typically 15 particles per interrogation volume are essential for a high quality evaluation (Keane & Adrian 199). Thus, a powerful aerosol generator is required for large aerodynamic facilities. For our experiments we have utilized several Laskin nozzles in parallel. This proved to be sufficient to seed the wind tunnel in a short duration of time (appr. 5 min). The use of a specially developed ring impactor ensured, that this large amount of particles could be used, without any pollution of the insulating material of the cryogenic tunnel by any larger droplets. Finally we could attain a high density of tracer particles of the order of up to 1 particles/mm 3 (this corresponds to 3 particles per interrogation volume) during the experiments. The aerodynamic diameter of the olive oil particles was about 1 m. In contrast to prior PIV applications, the measurements described here were additionally complicated by the fact that the air, that is generating the droplets within the Laskin nozzles and the oil droplets themself, are cooled down to cryogenic conditions within a very short time. Therefore, tracer particles had to be generated with extremely dry air. The seeded air passed an injection system that contained of a specially developed nozzle inside the wind tunnel where transonic flow conditions were obtained. Fig. 3a Seeding devices (particle generator and impactor) attached to the particle injection system of KKK Fig. 3b PIV recording (wing tip device in the background) Figure 3b shows a PIV recording obtained at 17 Celsius and a Mach number of.2. The recording illustrates the high homogeneity and visibility of tracer particles. 4.2 ILLUMINATION SYSTEM A single oscillator Nd:YAG pulse laser from INNOLAS was used for illumination. This new design enables to generate light pulses from beams with an excellent spatial intensity profile, which is identical for both light pulses and therefore perfectly overlapping for pulse separation times between 1 and 2 micro seconds. The output energy was 2 12 mj during these measurements. This energy was sufficient to illuminate a sheet with a height of 2 cm, when recording the light scattered by the 1 m particles on a highly sensitive CCD cameras. The time delay t between the two laser pulses was set between 7 s and 2 s for these experiments. The optical access to the test section was complicated by the fact that the test chamber of the wind tunnel is on a temperature level below -17 C during the tunnel run. Therefore, the laser was located outside the tunnel, whereas the light sheet optic was mounted into the wall of the test chamber. The light was directed into the tunnel through a small orifice in the wall. The optical path between laser and observation area had a length of more than 1.5m. Figure 4 shows the light sheet optic installed inside the wall of the wind tunnel, a part of its airconditioned box can be also seen. 395

5 could be traversed remotely in two directions.15 m behind the light sheet plane. Schlieren and condensation close to the glass window in front of the camera have been avoided by an electrically heated foil attached to the window. INSOLATION and HEATING FOILS motors camera 1376 x 14 px resolution 12 bit dynamic range lens INSOLATION and HEATING FOILS belt Fig. 5a camera and focussing system Fig. 4 Thermal insulated light sheet optic The size of the observation area was approximately 8 x 11 mm 2. The light sheet thickness was adjusted to 1.5 mm. The sheet was oriented normal to the main flow direction.15 meter behind the wing tip device. 4.3 RECORDING An almost immediate availability of the PIV data is possible by using video based approaches to PIV recording. New CCD architectures allow high resolution PIV measurement in a single exposure/double frame mode suited for crosscorrelation evaluation. In order to be able to install this type of camera under cryogenic conditions, some special developments, which are not commercially available had to be made. One major problem is associated with the limited amount of light scattered by the small tracer particles. Typically, the depth of focus at recording is of the order of 1-5 mm in our experiments, which means that the intensity of the light as scattered by out-offocus particles is not high enough to expose the CCD sufficiently. Focusing The strong temperature changes and the fact that the test section can not be assessed during and after the cool down period, required a high quality and reliable focusing device, in order to ensure focusing for different test condition. For this purpose our camera was equipped with a new and very small device for fast focusing. A CCD camera (PCO SensiCam) with 14 x 132 px spatial resolution was mounted in a electrically heated and thermally insulated box. The position of the CCD camera 396 Fig. 5b Model and camera traverse system Figure 5 shows the wing tip of the model and the camera inside the traversing system of the wind tunnel. 4.4 EVALUATION AND POST PROCESSING Each PIV recording was interrogated using the PivView2.1 software in small sub areas (2 x 2 pixel interrogation areas with a step size yielding an overlap of 5% along the x and y-axis). The local displacement vector for the images of the tracer particles has been determined for each interrogation area by means of a multiple path scheme, based on image deformation algorithms. This approach significantly reduces measurement noise due to the elimination of the in-plane loss of pairs. The field of view used during these experiments was 84 x 112 mm 2 which yields to interrogation areas of 1.6 x 1.6 mm 2,.8 mm spacing between each vector and approximately 14 vectors per field. 5 RESULTS FROM PIV MEASUREMENTS

6 14.2 different angles of attack: respectively (a):2.9 and (b): 1.2. On figure 7 (a) the vortex, as well as the shear layer of the winglet, are clearly visible. The vortex core diameter is around 4mm. For a higher angle of attack the vortex core diameter is much larger, as it can be seen in figure 7 (b) and is approximately 1mm in diameter. The result shown below are preliminary results obtained during the test with a model equipped with winglet tip. Figure 6 shows averaged vorticity and velocity fields (only one vector over four is displayed for clarity) measured a 169K with a free stream velocity of 53.1 m/s and a geometric angle of attack of 1.3. This result highlights the advantage of the traversing system which allows scanning a large field of view at a high spatial resolution. Thus after merging the individual observation areas, the total area is 25 x 2 mm2 and the resulting field is composed by around 4 vectors. Two vortices can be clearly seen: The upper vortex is generated by the tip of the winglet and the lower one by the gradient in lift distribution due to the crank of the airfoil geometry. Vorticity [1/s] Vorticity [1/s] (a) (b).2 Fig. 7 Averaged vorticity and velocity field (only one vector over four is displayed) around a winglet tip device. Free stream velocity: 41.2 m/s, temperature: 1K, Mach:.2, geometric angle of attack: (a): 2.9, (b): Vorticity [1/s] As mentioned before, these results are preliminary and further investigations have to be made in order to extract all the vortices characteristics properly CONCLUSIONS Fig. 6 Averaged vorticity and velocity field (only one vector over four is displayed) around a winglet tip device. Free stream velocity: 53.1 m/s, temperature: 169K, Mach:.2, geometric angle of attack: 1.3 Force and moment measurements, pressure measurements by tappings, and PIV recordings have been performed in order to investigate the aerodynamic effects in the wake of different wing tip devices attached to an aircraft half model. The PIV measurement of the flow velocity fields behind the trailing edge and the subsequent evaluation and Figure 7 shows the winglet tip vortex measured at 1k with a free stream velocity of 41.2 m/s for 2 397

7 analysis of pressure and velocity data yield detailed information on the features of such flow fields. In spite of difficult recording conditions the velocity data obtained is well suited for future comparison with data obtained by numerical simulations. Limits of imaging have been presented based on practical experiences. Valuable flow field information can be obtained within short measuring time, though imaging small tracer particles through density gradients is problematic and the seeding of the cryogenic flow required new developments. ACKNOWLEDGEMENT The authors would like to thank the M-DAW partners to have given us the chance to perform this test and especially Mr. D. Meißner and Mr. E. Elsholz for their help. The M-DAW project ( Modeling and Design of Advanced Wing tip devices ) is partly funded through the European Commission within the 5 th Frame work programme. The project is coordinated by Airbus-UK. REFERENCES Adrian R J 1991: Particle-Imaging techniques for Experimental Fluid Mechanics. Ann. Rev. Fluid Mech 23, pp Höcker R, Kompenhans J 1991: Application of particle image velocimetry to transonic flows. Appl. of Laser Techniques to Fluid Mechanics, ed. R.J. Adrian et al., Springer Verlag, Berlin, Heidelberg, Tokio, pp Humphreys W M, Bartram S M, Blackshire J L 1993: A Survey of Particle Image velocimetry Applications in Langley Aerospace Facilities. Proc. 31st Aerospace Sciences Meeting, Reno, AIAA paper 93/41. Hunter W W, Nichols C E (eds.) 1985: Wind Tunnel Seeding Systems for Laser Velocimeters. NASA Conference Publication 2393, NASA Langley. Keane R D, Adrian R J 199: Optimization of particle image velocimeters. Part I: Double pulsed systems. Meas. Sci. Technol. 1, pp Meyers J F 1991: Generation of Particles and Seeding. Von Kárman Institute for Fluid Dynamics, Lecture Series , Brussels. Melling A 1986: Seeding Gas Flows for Laser Anemometry. Conf. on Advanced Instrumentation for Aero Engine Components, AGARD-CP 399, paper 8. Molezzi M J, Dutton J C 1993: Application of particle image velocimetry in high-speed separated flows. AIAA Journal, Vol. 31, No. 3, pp Raffel, M.; Willert, C.; Kompenhans, J., 1998: "Particle Image Velocimetry, A Practical Guide, Springer-Verlag. Towers C E, Bryanston-Cross P J, Judge T R 1991: Application of particle image velocimetry to largescale transonic wind tunnels. Optics & Laser Technology Vol. 23, No 5, pp