Oscillating foils produce thrust through the development of a jet-like average flow. It is found
that such jets are convectively unstable with a narrow range of frequencies of maximum
amplification, resulting in the formation of a staggered array of vortices with direction
opposite to that of the classical Karman street. A stable co-existence of the jet profile and the
large-scale patterns is ensured only at the frequency of maximum amplification, hence at this
frequency optimal efficiency is obtained, ie, maximum thrust per unit input energy. The …