Froude number

Summary

The Froude number $\mathrm{Fr} = U / \sqrt{g L}$ is the dimensionless number that compares the inertia of a flow with gravity. It governs flows with a free surface, such as rivers, open channels and the waves made by ships. In shallow water it is the ratio of the flow speed to the speed of long surface waves, so $\mathrm{Fr} = 1$ separates two different regimes.