Strouhal number

Summary

The Strouhal number $\mathrm{St} = f L / U$ is a dimensionless number for unsteady flows. It compares the frequency of the flow with the rate at which the flow carries fluid past an object. It describes oscillating flows, above all the periodic shedding of vortices behind a bluff body. For a circular cylinder, vortex shedding happens at a Strouhal number close to $0.2$ over a wide range of flow speeds.