VORTICES INCIDENT UPON AN OSCILLATING CYLINDER: FLOW STRUCTURE AND LOADING

Interaction of an incident vortex street with an oscillating cylinder is addressed using high-image-density particle image velocimetry and simultaneous force measurements. This approach reveals that the timing of the incident vortices relative to the cylinder motion controls the large-scale vortex formation in the near-wake, and thereby the phase shift between the loading on the cylinder and its motion. As a consequence, it is possible to change the sign of the fluid-dynamic work done by the fluid on the cylinder. The incident vortices dramatically shorten the formation length of vortices in the near-wake and yield values of lift coefficient up to a factor of five larger than that for an isolated cylinder subjected to controlled oscillations in the absence of incident vortices. These alterations of the wake structure and loading occur in conjunction with globally locked-on patterns of incident and shed vortices with respect to the cylinder oscillation. Different states of global lock-on are attainable for different values of timing of the incident vortices.