  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 in 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 bird's feathers, respectively. The sides of a square cylinder have been
fitted with this system so as to enable the flaps to freely rotate around their
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 (up to 22%) has been obtained over a broad range of
Reynolds number. The investigation of the mean flow reveals a noticeable
modification of the flow topology at large scale in the vicinity of the
controlled cylinder accounting for the increase of the pressure base in
comparison with the natural flow. Meanwhile, the study of the relative motion
of both flaps points out that their dynamics is sensitive to the Reynolds
number. Furthermore, the comparative study of the flow dynamics at large scale
suggest a lock-in coupling of the flap motion and the vortex shedding.
