Forces and wake modes of an oscillating cylinder

This investigation considers the wake states of a cylinder subjected to forced oscillations at frequencies close to the Karman frequency. Two distinctly different wake states are observed. The emphasis is on the transition between these states, which is, characterized in terms of the lift force on the cylinder and the instantaneous patterns of vortex structures in tl-le near-wake. As the frequency of oscillation increases, there is simultaneously an abrupt jump in the lift force and a change in the mode of vortex shedding. The jump in the lift force involves a sharp increase in the magnitude of the lift coefficient and a phase shift of the order 180 degrees. The corresponding mode change involves an alteration in both the timing of the vortex initially shed from the cylinder and the overall pattern of vortices in the near-wake. Whilst previously these changes have been observed individually in separate forced vibration investigations, we show conclusive evidence that these two events are intrinsically linked. Moreover, for a narrow band of frequencies, a self-excited transition is possible, where the wake state changes while the cylinder oscillates at a constant frequency. (C) 2001 Academic Press.