On cylinders undergoing one- and two-degree of freedom forced vibrations in a steady flow

Formation of vortices in the near wake of circular cylinders is discussed. Two different cases are compared: starting flow around an initially stationary cylinder and flow around an oscillating cylinder in a steady freestream. The effects of formation time on the morphology of the vortices are shown, as well as some consequences thereof.; For starting flows, the critical formation time defines the point where the wake transitions from the initially symmetrical state to the intermediate asymmetrical state. The asymmetrical state breaks down into the periodic shedding state normally associated with cylinder flows. It appears that the wake reaches a critical level of vorticity annihilation at the critical time. This triggers an exponential growth of asymmetry in the near wake. Evidence of this process can also be seen in the early time force data.; For oscillating flows, the critical time defines the transition from vortex to vortex-and-tail morphology. First, phase averaged vorticity fields are presented showing the changes in the wake with forcing frequency and streamwise motion. These changes are related to the formation time, and related to similar effects seen in other flows. In addition, prolonged formation is related to the observed switch in the phase of the vortex shedding. The effects of streamwise motion are also shown, including the increased phase coherence of the shedding via coordination of the shedding process and the ability to adjust formation time via streamwise acceleration. The latter was used to demonstrate a plausible explanation for the vortex pair formation process observed by some researchers by showing how the formation process affects the number of vortices generated per cycle.