Temporal evolution of wake behind a rotationally oscillating circular cylinder

Abstract

The temporal evolution of the near-wake behind a circular cylinder undergoing rotational oscillatory motion with a relatively high forcing frequency has been investigated experimentally using a dynamic particle image velocimetry velocity field measurement technique. Experiments were carried out varying the frequency ratio FR (=ff∕fn), the ratio of the forcing frequency ff to the natural vortex shedding frequency fn, in the range of 0.0 (stationary) to 1.0 at an oscillation amplitude of θA=30° and Reynolds number of Re=4.14×103. Depending on the forcing condition (FR), the near-wake flow showed markedly different flow regimes: stationary (FR=0.0), non-lock-on (FR=0.4) and lock-on (FR=1.0) regimes. However, at all the frequency ratios tested, the power spectral density distributions showed a single dominant peak at the same vortex shedding frequency. As the forcing frequency was increased, the rotational oscillatory motion of the cylinder decreased the length of the vortex formation region and enhanced the mutual interaction between large-scale vortices across the wake centerline. In particular, the entrainment of ambient fluid seemed to play an important role in controlling the near-wake flow and shear-layer instability. The shear-layer instability showed different mechanisms compared with the case of small oscillation amplitude. A strong vortex motion appeared in the near-wake region. In addition, the rotational oscillatory motion of the cylinder changed the local mean flow structure significantly; thereby mean velocity profiles for each frequency ratio varied markedly in the vortex formation region. The configuration of vorticity contours shrunk toward the cylinder as the frequency ratio was increased. The temporally resolved flow information extracted in the present work is useful for understanding the evolution of vortex structure and analyzing the effects of open-loop active flow control technique on the near-wake flow structure.