VORTEX SHEDDING FROM AN OSCILLATING CIRCULAR-CYLINDER IN A UNIFORM-FLOW

In this study, the vortex shedding from a circular cylinder in forced oscillation in the direction of a uniform flow was investigated by visualizing water flows around it at Reynolds numbers of 260-2460. It was observed that the frequency of vortex shedding from the oscillating cylinder tends to be equal to n times the cylinder's oscillation frequency (n = 0.5, 1, 2, 3,...), even if the oscillation frequency is smaller than the natural Karman vortex frequency. This tendency can be considered a sort of ''lock-in'' phenomenon, and its features vary with both oscillation amplitude and Reynolds number. It was also known that flows of vortex shedding wakes in the lock-in states can be classified into three typical patterns depending on the lock-in frequency ratio n (n = 0.5, 1, 2, 3,...). At the lock-in states of n = 2, 3,..., a set of vortices are shed with shorter and longer periods during one cycle of cylinder oscillation. At the lock-in state of n = 1, a twin vortex street is formed; twin vortices are also formed at n = 0.5, but a Karman vortex street appears instead of the twin vortex street. When the vortex shedding is out of a lock-in state, the flow pattern of the wake becomes very complex. Although the instantaneous vortex shedding frequency varies with the change of relative velocity in one cycle of the cylinder oscillation, the rate of variation of the vortex shedding frequency is greater than that of the relative flow velocity. Even if the oscillation frequency is as small as about 10% of the Karman vortex frequency, the vortex shedding shows a tendency to deviate from the state of so-called quasi-steady vortex shedding.