The time-resolved flow field in the wake of a rectangular bluff body in ground proximity is examined through wind tunnel experiments. In addition to an extensive assessment of the baseline wake dynamics, the study also investigates the impact of passive (i.e., base flaps) and active (i.e., fluidic oscillators) flow control on the wake dynamics. The velocity field downstream of the model is acquired with a stereoscopic high-speed particle image velocimetry system at several streamwise and crosswise sections. Coherent wake structures are determined by conditional averaging, spectral analysis, and spectral proper orthogonal decomposition. The baseline flow field is dominated by a wake bi-stability that is characterized by a random shift between two stable wake states. The bi-stability is governed by the model’s aspect ratio and occurs in the vertical direction, because the model height is 1.35 times larger than its width. Higher frequency modes with less energy content as determined in the appropriate literature are identified and visualized. A coupling between these modes and the bi-stability is discussed. Flow control has a significant impact on the wake dynamics. When passive flow control is applied, the bi-stability of the wake is still present for a flap angle of \(20^\circ\). The higher frequency modes are still detectable but weakened. The turbulence intensity is significantly reduced when the flow attaches to the base flaps and the bi-stability is inhibited. When active flow control is applied, the higher baseline frequencies are suppressed in addition to the absence of the bi-stability. Solely the dominant mode at a Strouhal number of about 0.08 remains present for all flow control configurations. This mode is attributed to an alternating shear layer oscillation.
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The authors would like to thank Moritz Sieber for supporting the data processing with the spectral proper orthogonal decomposition. This work is part of the research project ”Investigation of the Unsteady Wake behind a Generic Tractor-Trailer with Different Boundary Conditions” (PA 920/26-1). The authors would like to thank the German Research Foundation (DFG) for its financial support.
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Schmidt, H.-., Woszidlo, R., Nayeri, C.N. et al. The effect of flow control on the wake dynamics of a rectangular bluff body in ground proximity. Exp Fluids 59, 107 (2018). https://doi.org/10.1007/s00348-018-2560-x