Computational and experimental study on flow around a rotationally oscillating circular cylinder in a uniform flow

The effect of rotational oscillation on flow field and fluid forces of a circular cylinder in a uniform flow is studied by three-dimensional computation by large eddy simulation and flow measurement by particle image velocimetry at Reynolds number 2000. The experimental characteristics of the flow indicate the enhancement of vortex shedding at natural frequency, the suppression of vortex shedding at high frequency and the recovery that follows. The above are accurately predicted by numerical simulations except for the slightly smaller estimate of wake suppression effect at high frequency. On the other hand, the fluid force characteristics on the cylinder are studied by numerical simulation, which indicates the drag enhancement and reduction corresponding to the variations of flow field at low and high frequencies, respectively. The drag reduction at high frequency is magnified with an increase in oscillation amplitude, but it can be saturated at higher amplitude. The mechanism of drag reduction is caused by the combined effect of forcing frequency and oscillation amplitude due to the modification of wake flow at high frequency and the delay in flow separation, while the fluctuating lift force slightly increases at a high frequency caused by the small-scale vortex shedding in synchronous with the rotational oscillation.