PIV measurements of the wake behind a rotationally oscillating circular cylinder

The near-wake behind a circular cylinder undergoing rotational oscillatory motion with a relatively high forcing frequency has been investigated experimentally. Experiments were carried out varying the ratio of the forcing frequency f_f to the natural vortex shedding frequency f_n in the range of 0.0 (stationary) to 1.6 at an oscillation amplitude of θ_A=30° and Reynolds number of Re=4.14×10³. Depending on the frequency ratio (F_R=f_f /f_n), the near-wake flow could be divided into three regimes—non-lock-on (F_R=0.4), transition (F_R=0.8, 1.6) and lock-on (F_R=1.0) regimes—with markedly different flow structures. When the frequency ratio was less than 1.0 (F_R⩽1.0), 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. The entrainment of ambient fluid seemed to play an important role in controlling the near-wake flow and shear-layer instability. In addition, strong vortex motion was observed throughout the near-wake region. The flow characteristics changed markedly beyond the lock-on flow regime (F_R=1.0) due to the high frequency forcing. At F_R=1.6, the high frequency forcing decreased the size of the large-scale vortices by suppressing the lateral extent of the wake. In addition, the interactions between the vortices shed from both sides of the cylinder were not so strong at this forcing frequency. As a consequence, the flow entrainment and momentum transfer into the wake center region were reduced. The turbulent kinetic energy was large in the region near the edge of the recirculation region, where the vortices shed from both sides of the cylinder cross the wake centerline for all frequency ratios except for the case of F_R=1.6. The temporally resolved quantitative flow information extracted in the present work is useful for understanding the effects of open-loop active flow control on the near-wake flow structure.