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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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