FISH WITHOUT FOOTPRINTS

Fish may propel themselves by successively generating and destroying tail tip eddies of alternating direction. Each eddy is produced by a sideways tail flip, and is destroyed by the return stroke during which a forward force F≈ 0·6KρhA3f3is generated, where ρ is the density of water, hthe height of the tail fin, Athe amplitude of maximum excursion from the centre line and f, the tail frequency. A dimensionless constant with a typical value K = 40 describes the entrainment of water into the tail tip vortex. The average forward power of this motion is P = 2·4KρhA4f3. The model accounts for both starts and steady swimming. © 1991 Academic Press Limited.