THE ONSET AND DEVELOPMENT OF CIRCULAR-CYLINDER VORTEX WAKES IN UNIFORMLY ACCELERATING FLOWS

The influence of uniform flow acceleration on the stability and the characteristics of circular-cylinder wakes over a Reynolds-number range, 20 < R < 330, was investigated. Experiments were performed to examine the temporal evolution of the wake before, during, and after the onset of the wake instability. We have demonstrated in several ways that the wake is stabilized by flow acceleration: (i) the onset of the wake instability occurs at larger Reynolds numbers than in the steady flow case, (ii) the closed wake develops to states that would be unstable in a steady flow, and (iii) once vortex shedding does occur there is a reduction in instantaneous Strouhal number. We have also examined the temporal growth rate of the wake instability and find that it is directly proportional to the applied flow acceleration. Physical mechanisms are proposed to describe the experimental observations.