OPTIMAL THRUST DEVELOPMENT IN OSCILLATING FOILS WITH APPLICATION TO FISH PROPULSION

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, i.e., maximum thrust per unit input energy. The nondimensional frequency of maximum amplification (Strouhal number) is in the range of 0·25 to 0·35. Experiments confirms this results, while the analysis of a large number of data from observations on fish and cetaceans confirm that optimal fish propulsion is achieved within this range of Strouhal number. © 1993 by Academic Press, Inc.