Forced vortex shedding and control in wakes behind a cylinder performing rotating oscillations

We study the wake of a cylinder performing rotary oscillations at moderate Reynolds number. We observe that the structure of the wake is strongly affected by the forcing parameters. The forced wake is characterize by a near wake 'lock-in' region where the vortices are shed at the forcing frequency and a far wake region where the vortices are reorganized to give the natural frequency of the unforced wake. We show that these modifications of the wake structure are correlated to the dynamic of the fluctuations downstream of the cylinder. We discuss these observations as the function of the forcing parameters which are the amplitude and the frequency of the oscillations and study the consequences of this modifications on the global drag apply on the cylinder. Then, we show the mechanism responsible of the fluctuations modification, and then of the modification of the drag coefficient. (authors)