 where

This relationship leaves Strouhal dimensionless, although a dimensionless approximation is often used for "C"3, resulting in units of pulses/volume (same as K-factor).In swimming or flying animals, Strouhal number is defined as 

where,

In animal flight or swimming, propulsive efficiency is high over a narrow range of Strouhal constants, generally peaking in the 0.2 < St < 0.4 range.This range is used in the swimming of dolphins, sharks, and bony fish, and in the cruising flight of birds, bats and insects.