In fluid dynamics, the Froude number (named after William Froude) is the reciprocal of the square root of the Richardson number.

It is defined as

<math>

u\over\sqrt{gh} <math> where <math>u<math> is a representative speed, g the acceleration due to gravity, and <math>h<math> a representative length scale.

When used in the context of the Boussinesq approximation it is defined as

<math> {u\over \sqrt{g' h}}<math>

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

The densimetric Froude number is usually preferred by modellers who wish to nondimensionalize a speed preference to the Richardson number which is more commonly encountered when considering stratified shear layers.

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

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