Feedback control for reducing the pressure drag of bluff bodies terminated by a backward-facing step

Four flows over a backward-facing step are considered. These exhibit a range of flow physics, and are of relevance to the flow over the rear of “squareback” road vehicles. Computational flow simulations are used as a test-bed to devise a linear feedback control strategy which achieves a mean base pressure recovery (equivalent to reducing pressure drag) for all four flows. The strategy is based on the premise that reducing pressure drag fluctuations improves mean pressure recovery. Thus the feedback control objective is to attenuate base pressure force fluctuations. The response of each of the flows to actuation is characterised via harmonic forcing system identification. Feedback control is found to successfully achieve a mean pressure recovery for all four of the flows, and is particularly effective for 2-D geometries (with either laminar or turbulent separation). The approach uses only body-mounted sensing and actuation and could be applied experimentally.