Biomimetic bluff body drag reduction by self-adaptive porous flaps

The performances of an original passive control system based on a biomimetic approach are assessed by investigating the flow over a bluff body. This control device consists of a couple of flaps made from the combination of a rigid plastic skeleton coated with a porous fabric mimicking the shaft and the vane of the birdʼs feathers, respectively. The sides of a square cylinder have been fitted with this system so that each flap can freely rotate around its leading edge. This feature allows the movable flaps to self-adapt to the flow conditions. Comparing both the uncontrolled and the controlled flow, a significant drag reduction (≈22% on average) has been obtained over a broad range of Reynolds numbers. This improvement is related to the increase of the base pressure in the controlled case. The investigation of the mean flow reveals a noticeable modification of the flow topology at large scale in the vicinity of the controlled cylinder. Meanwhile, the study of the relative motion of both flaps highlights that their dynamics is sensitive to the Reynolds number. Furthermore, the analysis of the flow dynamics at large scale suggests a lock-in coupling between the flap motion and the vortex shedding.