The effect of a perturbation on the flow over a bluff cylinder

Measurements are presented of the frequency of vortex shedding from a circular cylinder, a D section cylinder and a flat plate for a flow that has a single frequency perturbation superimposed on its mean velocity component. It was found that over a range of velocities the vortex shedding locks into the frequency of the perturbation such that the shedding frequency is half the perturbation frequency. The range of velocities for which lock-in occurs increases as the perturbation amplitude is increased. With lock-in, the mean base pressure was found to decrease and measurements of the vortex shedding from the circular cylinder show that the correlation of vortex shedding increases. The changes associated with lock-in were found to be greatest for the circular cylinder.