10.1063/5.0113647.1

Abstract

We computationally study the propulsive performance of a two-dimensional elliptic foil undergoing interlinked pitching-heaving motion. This motion is realized by pitching the foil about an axis on its centerline outside the foil and by varying the distance between the pitching point and the leading edge. A distance of 0 and −∞ corresponds to leading edge pitching and pure heaving. An in-house fluid-structure interaction solver based on the sharp interface immersed boundary method is employed to resolve the flow field around the foil. We conducted simulations for different cases of the location of the pitching axis and pitching frequency at a Reynolds number of 100. The thrust generation is explained by the dynamics of leading-edge and trailing-edge vortices. The wake corresponding to thrust is either reverse von Kármán or a deflected reverse von Kármán vortex street. Analysis revealed the existence of an optimal pitching point for maximum thrust or propulsive efficiency at a given reduced pitching frequency. The optimal regions of the thrust and propulsive efficiency are quantified as a function of reduced pitching frequency and the location of the pitching axis. The pitching point for the maximum thrust and efficiency is found to be different. We discuss the fluid-mechanical reasons for the variation of propulsive performance with the location of the pitching point and the pitching frequency and corroborate our reasoning with the wake signatures.