The Effect of Gap Length on Vortex Interactions and Performance of a Pitching Fin in the Near Wake of a Cylinder

Several studies have investigated improving the thrust performance of unsteady foils through interactions with vortex streets created by upstream bodies. Adjusting the phase of the interactions is shown to lead to variation in the performance of the downstream propulsor. However, two-way coupling between the propulsor and the upstream body, where the downstream propulsor also modifies the wake of the upstream body, has received little attention. Such a scenario provides insight into how a bio-inspired swimmer can influence its environment to enhance its performance. When a nominally two-dimensional panel pitching about its leading edge is placed in the near wake of a cylinder, it can substantially modify the cylinder shedding behavior, and the thrust performance is significantly modified from that of an isolated panel. In the present study, we systematically vary the gap distance between the cylinder and panel while measuring thrust and propulsive efficiency of the foil, and conduct a series of PIV measurements to quantify the resulting vortex wake and its interaction with the panel. Comparison is made between a D-shaped cylinder and a circular cylinder.