The performance of two self-adaptive flaps designed with a biomimetic approach is investigated. Each flap comprised a rigid plastic frame covered with a porous material (characterized by its solidity, σ) and is installed on the side of a square cylinder. In order to investigate the effect of the flap dynamics, the flaps are either positioned at a given angle (passive control) or hinged on their leading edges and left free to adapt to the flow changes (self-adaptive control). For the optimum position of 20° and σ=100%, a significant drag reduction of 30% is obtained over a large range of Reynolds numbers (Re 20,000 to 60,000). The investigation of the flow in the close wake of the model for both fixed and moving flaps reveals a modification of the flow topology and a possible change in the mode of vortex formation.

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