Control of vortex shedding on a circular cylinder using self-adaptive hairy-flaps

Experiments on separation control using flexible self-adaptive hairy-flaps are presented herein. The wake-flow behind a circular cylinder is investigated without and with flexible hairy-flaps at the aft-part of the cylinder. Flow dynamics and hair motion were measured by particle image velocimetry and image processing in a range of Reynolds number . The experiments and POD analysis show, that the hairy-flaps alter the natural vortex separation cycle in such a way that the vortices do not shed in a zig-zag like arrangement as in the classical von Kármán vortex street but in line in a row with the cylinder wake axis. Thus, the wake-deficit is largely reduced. Furthermore, flow fluctuations are considerably reduced about 42% in streamwise and 35% in transversal direction compared to the reference case without hairy-flaps, too. The condition for this mode change is the lock-in of the vortex shedding with a traveling wave running through the flexible hair bundles in transversal direction at the aft-part of the cylinder. As a consequence, the vortex shedding frequency is increased, the length of the separation bubble is decreased and drag force is decreased, too. The lock-in appears as a jump-like change of the shedding frequency and a jump in the Strouhal–Reynolds number diagram. However, when the characteristic length for the normalized frequency is chosen as the length of the separation bubble instead of the cylinder diameter, the Str–Re dependence is regular again. This hints on the relevance of the resonator model as proposed by Sigurdson and Roshko (1988) [16] on vortex shedding mechanism when boundary conditions are changed such as in our case, where the hairy-flap bundle imposes a flexible wall with visco-elastic coupling in transversal direction.