Effect of rotational oscillation upon fluid forces about a circular cylinder

The present work aims at a computational study of the effect of various parameters that influence fluid-force behavior when a circular cylinder is subjected to rotational oscillations. A numerical simulation is conducted using the unsteady form of Reynolds-averaged Navier-Stokes equations combined with the k-ε model of turbulence. The study is carried out to examine the influence of various flow parameters, such as oscillation non-dimensional frequencies (0.1-2), rotational non-dimensional amplitudes (0.25-3) and Reynolds numbers (2000-30,000). Special attention is focused on the resonance condition at lower frequency and the subsequent drag reduction at higher frequency. It is found that the peak value of fluid forces on the circular cylinder increases abruptly when the forcing frequency is closer to the vortex shedding frequency. This is followed by a drag reduction at higher forcing frequencies, which becomes very strong at a non-dimensional oscillation frequency around 1 and at rotational amplitudes larger than 1. The former phenomenon is found to intensify with an increase in Reynolds numbers, but the latter almost preserves its strength in the range of Reynolds numbers considered here.