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 vis ualizing water flows around it at Reynolds numbers of 260-2460. It was observed that the frequency of vortex shedding from the oscillating c ylinder tends to be equal to n times the cylinder's oscillation freque ncy (n = 0.5, 1, 2, 3,...), even if the oscillation frequency is small er than the natural Karman vortex frequency. This tendency can be cons idered a sort of ''lock-in'' phenomenon, and its features vary with bo th oscillation amplitude and Reynolds number. It was also known that f lows 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 Karm an vortex street appears instead of the twin vortex street. When the v ortex shedding is out of a lock-in state, the flow pattern of the wake becomes very complex. Although the instantaneous vortex shedding freq uency varies with the change of relative velocity in one cycle of the cylinder oscillation, the rate of variation of the vortex shedding fre quency is greater than that of the relative flow velocity. Even if the oscillation frequency is as small as about 10% of the Karman vortex f requency, the vortex shedding shows a tendency to deviate from the sta te of so-called quasi-steady vortex shedding.