Vortex shedding from a ring in oscillatory flow

This paper completes a series of three on the effects of various kinds of upstream flow disturbance on the nature of vortex shedding behind a rectangular-sectioned ring. Results of an experimental study of the effects of free-stream velocity oscillations of various amplitudes and frequencies are presented. It is shown that lock-in phenomena can occur when the pulsation frequency is not too far from the natural shedding frequency (or half the latter). Despite the fact that in uniform flow the shedding process is much stronger and more coherent than is often the case for planar 2-D obstacles, the lock-in range at any given pulsation amplitude is at least as wide as for the equivalent planar body and the circumferential coherence of the shedding can be significantly weakened, even in the lock-in range.