The unsteady flow past a rotationally oscillating circular cylinder is inve
stigated numerically for different values of oscillating amplitude and freq
uency at a Reynolds number of 200. A discrete vortex method, which was prev
iously presented and applied to the situation of flow past a rotating circu
lar 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 lo
ck-on range for vortex shedding is obtained. The influences of the oscillat
ing amplitude and frequency on the forces acting on the cylinder are determ
ined together with the flow pattern in the wake. For values of oscillating
frequency above lock-on frequency, it is found that drag coefficient is bel
ow that for the stationary cylinder in a quite wide range of forcing freque
ncy, 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 la
rge-scale anti-symmetrical configuration with a new frequency, which is sim
ilar in form to the Karman vortex street downstream of a stationary cylinde
r. The intensive vortex shedding and coalescence process behind the cylinde
r is studied. (C) 2001 Elsevier Science Ltd. All rights reserved.