Characterization and passive control of the wake behind a square-back Ahmed body at yaw

  • Author / Creator
    Booysen, Adriaan
  • The near wake behind the Ahmed body at zero-yaw has been extensively studied in the past, but comparatively little is known about the changes in the wake flow patterns during crossflow. Measurements of the near wake of a square-back Ahmed body were obtained at a Reynolds Number of ReH = 9.2104 for three yaw angles of  = 0°, 5°, and 10° using large-scale volumetric particle tracking velocimetry within a 350 mm  300 mm  180 mm volume that utilizes an innovative helium-filled soap bubble seeding system. The helium-filled bubble system integrated 48 nozzles into a streamlined, modular housing structure that was located before the wind tunnel contraction. It successfully maintained a seeding density of 0.02 particles per pixel at a 10 m/s freestream velocity which was critical to the measurement quality.
    With increasing yaw angle, the recirculating region on the leeward side grew in strength, shifting the separation bubble towards the windward side. The downwash motion from the top- edge and the spanwise motion from the leeward edge of rear-face also strengthened. Time- averaged vortex structures were identified within the mean flow showing a vortex ring that forms tangent to the separation bubble. At yaw, the vortex ring became skewed and four additional streamwise vortex structures developed. 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 the same. The majority of the energy within the yawed wakes was found on the windward side of the wake. 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, while the energy reduced to approximately 10% for the  = 5° and 10° wakes. The large drop in energy within the yawed wake first modes implied the suppression of the bi-stability phenomenon at these yaw angles.
    A supplementary study measured the aerodynamic loads acting on the same square-back Ahmed body geometry at a higher Reynolds number of ReH = 2.7105 across a yaw range of -12.5°    12.5°. The aerodynamics loads were analyzed to assess the effect of yaw. As yaw increased it was shown that the drag forces, side forces, roll moments, and yaw moments all increased. The latter three all had an approximately linear anti-symmetric distribution about zero-yaw. The increase in drag was attributed to an induced drag term which was found to be proportional to the side force. The force and moment components of the baseline case were also compared to those of four different geometry modifications, with the goal being drag reduction and reduced aerodynamic loading. The four geometries were a boat tail consisting of four inclined panels at the rear edges, a vertical splitter plate centered on the rear face, cylindrical protrusions on the side edges of the rear face, and a dual fan configuration with downward facing fans positioned at the top two corners. The boat tail was the most effective geometry modification. It resulted in a drag reduction of 26.8% and a side force reduction of 12.3%. The boat tail did however also correspond to the largest increase in yaw moment (41.7%).

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.