# Froude number

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

It is sometimes called Reech-Froude number after Ferdinand Reech, who introduced it for testing ships and propellers in 1852. Also, a number of other French researchers used this number before Froude.

the Froude number is defined as

[itex]

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

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

[itex] {u\over \sqrt{g' h}}[itex]

where g' the reduced gravity (see Boussinesq approximation) and [itex]h[itex] 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.de:Froude-Zahl nl:Getal van Froude fi:Frouden luku

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