Volumetric flow rate

In fluid dynamics and hydrometry, the volumetric flow rate, also volume flow rate and rate of fluid flow, is the volume of fluid which passes through a given surface per unit time (for example cubic meters per second [m³ s^-1] in SI units, or cubic feet per second [cu ft/s]). It is usually represented by the symbol Q. Volumetric flow rate should not be confused with volumetric flux, represented by the symbol q, with units of m³/(m² s), that is, m s^-1. The integration of a flux over an area gives the volumetric flow rate. Volumetric flow rate is also linked to viscosity.

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Given an area A, and a fluid flowing through it with uniform velocity v with an angle θ away from the perpendicular to A, the flow rate is:

In the special case where the flow is perpendicular to the area A, that is, θ = 0, the volumetric flow rate is:

If the velocity of the fluid through the area is non-uniform (or if the area is non-planar) then the rate of fluid flow can be calculated by means of a surface integral:

where dS is a differential surface described by:

with n the unit surface normal and dA the differential magnitude of the area.

If a surface S encloses a volume V, the divergence theorem states that the rate of fluid flow through the surface is the integral of the divergence of the velocity vector field v on that volume:

See also

• Air to cloth ratio
• Discharge (hydrology)
• Flowmeter
• Flux (transport definition)
• Mass flow rate
• Poiseuille's law
• Darcy's law
• Orifice plate