Lock-on Characteristics of Wake Behind a Rotationally Oscillating Circular Cylinder

Lock-on characteristics of flow around a circular cylinder oscillating rotationally with a relatively high forcing frequency have been investigated experimentally. Dominant governing parameters are Reynolds number (Re), angular amplitude of oscillation (theta(A)), and frequency ratio F-R = f(f)/f(n), where f(f) is a forcing frequency and f(n) is a natural frequency of vortex shedding. Experiments were carried out under the conditions of Re=4.14 X 10(3), pi/90 <=theta(A)<=pi/3, and F-R=1.0. The effect of this active flow control technique on the lock-on flow characteristics of the cylinder wake was evaluated with wake velocity measurements and spectral analysis of hot-wire signals. The rotational oscillation modifies the flow structure of near wake significantly. The lock-on phenomenon always occurs at F-R=1.0, regardless of the angular amplitude theta(A). In addition, when the angular amplitude is less than a certain value, the lock-on characteristics appear only at F-R =1.0. The range of lock-on phenomena expands and vortex formation length is decreased, as the angular amplitude increases. The rotational oscillation creates a small-scale vortex structure in the region just near the cylinder surface. At theta(A)=60 degrees, the drag coefficient was reduced about 43.7% at maximum.