Froude number

The Froude number is the reciprocal of the square root of the Richardson number.

It is usually used in the context of the Boussinesq approximation and is defined as

where is a representative speed, g' the reduced gravity (see Boussinesq approximation), and a representative vertical lengthscale. Strictly, this is known as the densimetric Froude number.

It is usually used by modellers who wish to nondimensionalize a speed.

For example, the leading edge of a gravity current moves with a front Froude number of about unity.