Large-scale 3D-PTV measurement of Ahmed-body wake in crossflow

Abstract

The near wake of the Ahmed body at zero-yaw has been extensively studied in the literature, but comparatively little is known about the wake flow in crossflow condition. In this investigation, we characterize the near wake of a square-back Ahmed body at a Reynolds number of ReH = 9.2 × 104 for three yaw angles of β = 0°, 5°, and 10°. The measurements are carried out using 3D particle tracking velocimetry utilizing an innovative helium-filled soap bubble (HFSB) system for seeding a large domain of approximately 35 × 30 × 18 cm3. The results showed that increasing the yaw angle shifted the separation bubble towards the windward side and enlarged the recirculating region of the leeward side. The downwash motion from the top-edge and the spanwise motion from the leeward edge of rear-face also strengthened. A vortex ring formed in the periphery of the mean separation bubble. With increasing the yaw, the ring skewed toward the leeward edge and its windward potion became weaker. For the yaw cases, streamwise vortices appeared originating from the windward portion of the vortex ring. Spectral analysis showed that the overall energy of oscillations increased with yaw while the Strouhal numbers of the spanwise and wall-normal flow motions remained relatively constant. Proper orthogonal decomposition of the velocity field showed that the first spatial mode contained 32.8% of the total kinetic energy for the zero-yaw case. The energy of the first spatial mode reduced to approximately 10% for the β = 5° and 10° wakes due to the disappearance of the bi-stability mode. The energetic flow motions within the yawed wakes were primarily on the windward side of the body.