 The Strouhal number is an integral part of the fundamentals of fluid mechanics.The Strouhal number is often given as

where "f" is the frequency of vortex shedding, "L" is the characteristic length (for example, hydraulic diameter or the airfoil thickness) and "U" is the flow velocity.In certain cases, like heaving (plunging) flight, this characteristic length is the amplitude of oscillation.