# Froude Number

## An introduction to and a definition of the Froude Number

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**The Froude Number** is a dimensionless parameter measuring of the ratio of the inertia force on a element of fluid to the weight of the fluid element - the inertial force divided by gravitational force.

The Froude Number can be expressed as

Fr = v / (g l)^{1/2}(1)

where

Fr = Froude number

v = velocity

g = gravity

l = characteristic length

The Froude Number is relevant in fluid dynamic problems where the weight of the fluid is an important force.

In general this is the situation for free surfaces like cold windows and hot radiators. It is used in momentum transfer in general and open channel flow and wave and surface behavior calculations in particular.

The Froude Number is also important in analyzing flow in spillways, weirs, channel flows, rivers and ship design.

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