Flow structure of wake behind a rotationally oscillating circular cylinder

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 (theta(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 x 10(3), 0 degrees <=theta(A)<= 60 degrees 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 theta(A), In addition, the drag coefficient was reduced when the frequency ratio FR was less than 1.0 (F-R < 1.0) while fixing the oscillation amplitude at theta(A) = 30 degrees. When the oscillation amplitude theta(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 theta(A) = 30 degrees. This type of active flow control method can be used effectively in aerodynamic applications while optimizing the forcing parameters. (c) 2006 Published by Elsevier Ltd.