In fluid dynamics, the rate of fluid flow is the volume of fluid which passes through a given area per unit time. It is also called flux.
Given an area A, and a fluid flowing perpendicularly through it with uniform speed v, then the flux is
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If the rate of fluid flow is to be defined for a differential area which encloses a volume (see Gaussian surface), then it can be shown that the rate of fluid flow out of the volume is the divergence of the velocity vector field at a point inside that volume:
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