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 (θA  ), and frequency ratio FR=ff/fnFR=ff/fn, where ff is the forcing frequency and fn   is the natural frequency of vortex shedding. Experiments were carried out under conditions of Re=4.14×103Re=4.14×103, 0°⩽θA⩽60° and 0.0⩽FR⩽2.0. Rotational oscillation of the cylinder significantly modified the flow structure in the near-wake. Depending on the frequency ratio FR  , 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 FR=1.0FR=1.0 and the frequency range of the lock-on regime expanded with increasing oscillation amplitude θA. In addition, the drag coefficient was reduced when the frequency ratio FR   was less than 1.0 (FR<1.0FR<1.0) while fixing the oscillation amplitude at θA=30°θA=30°. When the oscillation amplitude θA   was used as a control parameter at a fixed frequency ratio FR=1.0FR=1.0 (lock-on regime), the drag reduction effect was observed at all oscillation amplitudes except for the case of θA=30°θA=30°. This type of active flow control method can be used effectively in aerodynamic applications while optimizing the forcing parameters.