Extraction of Flow Energy by Fish and Birds in a Wavy Stream

Abstract

This paper makes an attempt to explore the possibility for birds and aquatic animals to extract intrinsic flow energy from a wavy (or even turbulent) stream for enhancing their locomotion through the fluid medium. Sea gulls and pelicans have been observed to skim ocean waves over a long distance without making noticeable flapping of their wings as in ordinary flight. In a study of migrating salmon, Osborne (1960) found that an increased flow rate in a swollen river did not slow down the salmon, while en route to spawn, by nearly as much a margin as would be predicted by the law of resistance in proportion to the square of their velocity relative to the flowing water. A few possible reasons have been proposed, including the prospect that the flow energy associated with the waves and eddies in a stream can be extracted and used to generate thrust for locomotion.