EFFICIENT FOIL PROPULSION THROUGH VORTEX CONTROL

We investigate the problem of a heaving and pitching hydrofoil in an i nflow that consists of a uniform velocity field and a staggered array of vortices. The foil can exploit the energy in such a Karman vortex s treet for efficient propulsion of animals or submarines. Through a two -dimensional inviscid analysis, me find that the phase between foil mo tion and the arrival of inflow vortices is a critical parameter. Every thing else being equal, the highest efficiency is seen when this phase is such that the foil moves in close proximity to the oncoming vortic es. Different modes of vortex interaction in the wake results from a v ariation in this phase, and we show that the how downstream of the foi l is related to the foil input power.