concept

Strouhal number

A dimensionless number describing the rhythm of a beating fin or wing: frequency times stroke amplitude, divided by forward speed. It says how far apart each beat leaves its vortex in the wake. The strange thing is that dolphins, tuna, sharks and penguins — lineages with nothing to do with one another — all swim most efficiently inside the same narrow band, 0.20 to 0.40. Outside it the vortices stop meshing, the wake turns chaotic, and the animal burns more energy to cover the same distance.