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
where u is a representative speed, g the acceleration due to gravity, and h a representative length scale.
When used in the context of the Boussinesq approximation it is defined as
where g' the reduced gravity
(see Boussinesq approximation) and h 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.
