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June 19 (Mon) – 21 (Wed), 2017 / Grand Hotel, Pusan, Korea. Analysis of turbulent flow around a 3D NACA0015 wing using StereoPIV and TomoPIV. Gwenaël ...
The 12th International Symposium on Particle Image Velocimetery June 19 (Mon) – 21 (Wed), 2017 / Grand Hotel, Pusan, Korea

ISPIV2017

Analysis of turbulent flow around a 3D NACA0015 wing using StereoPIV and TomoPIV 1

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Gwenaël Acher , Lionel Thomas , Benoit Tremblais and Laurent David 1

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Institut PPRIME, UPR3346, CNRS – Université de Poitiers – ISAE-ENSMA, France [email protected] 2

Université de Poitiers, Laboratoire XLIM, Axe ASALI/SRI, CNRS 7252, France

ABSTRACT The development of three-dimensional velocity measurement techniques has made tremendous advancements over the past ten years, enabling the understanding of a wide range of highly three dimensional flows. The review made by Scarano [1] presents the working principles of tomographic Particle Image Velocimetry (TomoPIV) and several experiments where this technique can benefit the understanding of the flow organization. The assessment of major tomographic algorithms has been performed by Thomas et al [2] on synthetic data and experimental data of a pulsed jet in cross flow. Among the flows which can take advantage of this newly developed technique is the wake of a finite span airfoil placed at high incidence. Indeed, the generation of the tip vortex and its interaction with the wake are important phenomenon especially when considering dynamic stall. The use of Tomographic PIV and stereo PIV is therefore essential for the understanding of such complex three dimensional flow organization. The turbulent flow around a NACA0015 of chord 80mm and span 140mm is investigated in the present study. The rigid airfoil is molded with an Araldite resin reinforced by a silica filler, around a 8mm diameter stainless steel rod placed at a quarter of chord of the airfoil. The wing tip is formed by a semicircle joining the suction side and pressure side. It is placed at 30 degrees of angle of attack, in a water tunnel with a 230mm square test section at a velocity of 1.25m/s, providing a chord based Reynolds number of Rec=100 000. In order to analyze the three dimensional flow around the airfoil, time-resolved stereoscopic particle image velocimetry (TR-SPIV) and time-resolved tomographic particle image velocimetry (TR-TomoPIV) are performed in 3 different planes/volumes along the wingspan. The planes of SPIV and the center of the volumes of TomoPIV are positioned at a half span, three quarters of span and at the tip of the airfoil. The SPIV is performed with two high speed cameras Photron Fastcam SA1.1 recording at 2 kHz. The first camera is positioned in the axis of the airfoil providing a view of the flow without any shadow created by the wing and the second one is placed at the same altitude at 30 degrees downstream. The setup has been designed aiming at two goals: the first one is to allow the visualization of the suction side and the wake, which are the most valuable regions, in SPIV. A symmetrical setup would have created hidden zones both in pressure and suction side. The second objective reached by this setup is that classical PIV is still available with camera 1 in the upstream regions where the flow is mainly two dimensional. When performing the TomoPIV, the goals remain the same: properly resolve the wake region without shadow on the suction side and be able to perform standard PIV upstream where a shadow region exists for some cameras. Thus, to allow volume reconstruction, two Photron Fastcam SA-Z cameras are added to the setup. The SA-Z cameras are positioned like the SA1-1 cameras in SPIV and the remaining two SA1-1 cameras are placed on the same side, looking downward, one upstream and the other downstream of the profile. A picture of the setup is presented in figure 1 to illustrate the position of the cameras and the configuration of study. Every camera visualizing the scene with an angle is mounted with a Scheimpflug adaptor to remain focused in the whole measurement zone. The images are 1MegaPixels and the field is about 200mm x 200mm for SPIV and 200mm x 200mm x 12mm for TomoPIV. For both SPIV and TomoPIV, sets of ten seconds are recorded and processed using the

FTEE algorithm which is based on trajectory evaluation (Jeon et al [3]). The flow is analyzed with both SPIV and TomoPIV, the results of the two techniques are compared, and advantages, drawbacks and complementarity of the techniques are discussed. The mean horizontal velocity obtained by SPIV at half-span and at the tip of the wing 1)is displayed in figure 2.

Figure 1 : Picture of the TomoPIV setup

Figure 2 : Mean horizontal velocity in the half span plane and the tip plane

ACKNOWLEDGEMENT The grant of Gwenaël Acher’s PhD thesis is supported by the Direction Générale de L'Armement.

REFERENCES [1] Scarano F “Tomographic PIV principles and practice” Measurement Science and Technology (2012) [2] Thomas L, Tremblais B, David L “Optimization of the volume reconstruction for classical Tomo-PIV algorithms (MART, BIMART and SMART): synthetic and experimental studies” Measurement Science and Technology (2014) [3] Jeon YJ, Chatellier L, David L “Fluid trajectory evaluation based on an ensemble-averaged cross-correlation in time-resolved PIV” Experiments in fluids (2014)