Vortices incident upon an oscillating cylinder: Flow structure and loading

Interaction of an incident vortex street with an oscillating cylinder is ad dressed using high-image-density particle image velocimetry and simultaneou s force measurements. This approach reveals that the timing of the incident vortices relative to the cylinder motion controls the large-scale vortex f ormation 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 i ncident vortices dramatically shorten the formation length of vortices in t he near-wake and yield values of lift coefficient up to a factor of five la rger than that for an isolated cylinder subjected to controlled oscillation s in the absence of incident vortices. These alterations of the wake struct ure and loading occur in conjunction with globally locked-on patterns of in cident and shed vortices with respect to the cylinder oscillation. Differen t states of global lock-on are attainable for different values of timing of the incident vortices. (C) 1999 Academic Press.