# Dictionary:Filtration rate

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The velocity V of fluid movement, equal to the volume of fluid moved through a cross section of area A per unit of time, given by

${\displaystyle V={\frac {q}{A}}=-{\frac {k}{\mu }}\nabla P}$,

where ${\displaystyle \nabla P}$ is the pressure gradient, k the permeability, and ${\displaystyle \mu }$ the viscosity. The apparent filtration rate in a porous medium is ${\displaystyle V={\frac {q}{\phi }}A}$, where ${\displaystyle \phi }$ is the porosity.