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The lasting high fuel cost has recently inspired resurgence in drag reduction research for vehicles. Active drag reduction of an Ahmed body with a slant angle of 25°, corresponding to the high-drag regime, has been experimentally investigated. Four individual actuations, produced by steady blowing and denoted as S1, S2, S3 and S4, are applied respectively at the upper and two side edges of the rear window, and along the upper and lower edges of the vertical base. While the individual actuations produce a drag reduction of up to 6-14%, their combination results in a drag reduction of 29%, higher than any previous drag reductions achieved experimentally and very close to the target (30%) set by automotive industries. Extensive flow measurements are performed, with and without control, using force balance, pressure scanner, hotwire, flow visualization and particle image velocimetry techniques. A marked change in the flow structure is captured in the wake of the body under control, including the flow separation bubbles, be over the rear window or behind the vertical base, and the C-pillar vortices, leading to significant pressure increases over the rear window and at the vertical base.

 


A conceptual model of flow structure around an Ahmed vehicle model in uncontrolled flow.

 

Conceptual model of the flow structure under the combined actuation of S1, S2, S3 and S4.