초록
Lock-on characteristics of the flow around a circular cylinder performing a rotationally oscillation with a relatively high forcing frequency have been investigated experimentally using flow visualization and hot-wire measurements. Dominant parameters are Reynolds number (Re), amplitude of oscillation $(\theta_A)$, and frequency ratio $F_R=f_f\;/\;f_n$, where $f_f$ is the forcing frequency and if is the natural frequency of vortex shedding. Experiments were carried out under the conditions of $Re=4.14\times10^3,\;\pi/15\leq\theta_A\leq\pi/3$, and $F_R=1.0$. The effects of this active control technique on the lock-on flow regime of the cylinder wake were evaluated through wake velocity measurements and spectral analysis of hot-wire signals. The rotary oscillation modified the flow structure of near wake significantly. The lock-on phenomenon was found to occur in the range of frequency encompassing the natural vortex shedding frequency. In addition, when the amplitude of oscillation is less than a certain value, the lock-on phenomenon was occurred only at $F_R=1.0$. The lock-on range expanded and vortex formation length decreased as the amplitude of oscillation increases. The rotary oscillation generated small-scale vortex structure just near the cylinder surface.