Numerical simulation of flow past a rotationally oscillating cylinder

The unsteady flow past a rotationally oscillating circular cylinder is investigated numerically for different values of oscillating amplitude and frequency at a Reynolds number of 200. A discrete vortex method, which was previously presented and applied to the situation of flow past a rotating circular cylinder, is developed to simulate this type of flow. The approach used to understand the vortex formation process is to trace the motion of fluid particles. Some basic behaviors of vortex shedding are revealed and the lock-on range for vortex shedding is obtained. The influences of the oscillating amplitude and frequency on the forces acting on the cylinder are determined together with the flow pattern in the wake. For values of oscillating frequency above lock-on frequency, it is found that drag coefficient is below that for the stationary cylinder in a quite wide range of forcing frequency, and the efficacy of drag reduction becomes increasingly remarkable as the amplitude increases. Furthermore, it is observed that vortices shed at forcing frequency from the cylinder interact in the wake and result in a large-scale anti-symmetrical configuration with a new frequency, which is similar in form to the Karman vortex street downstream of a stationary cylinder. The intensive vortex shedding and coalescence process behind the cylinder is studied. (C) 2001 Elsevier Science Ltd. All rights reserved.