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Swimming and flying animals propel themselves by generating forces against a fluid medium or, in other words, by transferring momentum to the fluid in time. Momentum transfer is always restricted to the propulsor and its vicinity. The induced velocity differences (shear) in the surrounding fluid cause rotation in the water which can be visualized and quantified using vorticity. At low Reynold’s number (Re), the elevated vorticity regions stay attached to the propulsor and decay and die almost instantly after the propulsion stops. At higher Re, the accelerated masses of water separate from the propulsor and form entities. At intermediate Re, intermediate phenomena are observed. Since vortices require a beginning and an end, the associated vortices show up as line vortices (almost straight) or horseshoe vortices (curved) both with a beginning and an end at a fixed plane or at the surface, or as rings. In a free fluid, rings appear to be the only way for a vortex to exist. These fluid mechanical principles can clearly be recognized in animal-generated wakes, although the interpretation at different Reynold’s numbers is sometimes counter-intuitive. Here, the manner in which the flow phenomena scale with Re in animal-generated wakes is illustrated with flow analysis examples from larval and adult copepods, barnacles, fish larvae, frogs, adult fish and birds.

Vortices Rule The Wake: Structure And Reynolds ScalingOf Animal-generated Wakes