Flow structure of wake behind a rotationally oscillating circular cylinder

Abstract

Flow around a circular cylinder oscillating rotationally with a relatively high forcing frequency has been investigated experimentally. The dominant parameters affecting this experiment are the Reynolds number (Re), oscillation amplitude ( θ A ), and frequency ratio F R = f f / f n , where f f is the forcing frequency and f n is the natural frequency of vortex shedding . Experiments were carried out under conditions of Re = 4.14 × 1 0 3 , 0°⩽ θ A ⩽60° and 0.0⩽ F R ⩽2.0. Rotational oscillation of the cylinder significantly modified the flow structure in the near-wake. Depending on the frequency ratio F R , the cylinder wake showed five different flow regimes, each with a distinct wake structure. The vortex formation length and the vortex shedding frequency were greatly changed before and after the lock-on regime where vortices shed at the same frequency as the forcing frequency. The lock-on phenomenon always occurred at F R = 1.0 and the frequency range of the lock-on regime expanded with increasing oscillation amplitude θ A . In addition, the drag coefficient was reduced when the frequency ratio F R was less than 1.0 ( F R < 1.0 ) while fixing the oscillation amplitude at θ A = 30 ° . When the oscillation amplitude θ A was used as a control parameter at a fixed frequency ratio F R = 1.0 (lock-on regime), the drag reduction effect was observed at all oscillation amplitudes except for the case of θ A = 30 ° . This type of active flow control method can be used effectively in aerodynamic applications while optimizing the forcing parameters.