flödesmängden

Flödesmängden, literally “flow quantity,” is a general term used in physics and engineering to describe the rate at which a quantity is transported by a flow. It is most often applied to fluids, but the concept also covers the flux of other properties such as mass, energy, or a tracer carried by the moving medium. In fluid dynamics, the most common forms are the volumetric flow rate and the mass flow rate.

Volumetric flow rate, Q, denotes the volume that passes through a cross-section per unit time. Its SI unit is cubic meters per second (m^3/s). The mass flow rate, ṁ, relates to Q by ṁ = ρ Q, where ρ is the fluid density; its SI unit is kilograms per second (kg/s). For a pipe with cross-sectional area A and average velocity v, Q = A v. The continuity equation states that for incompressible flow, Q remains constant along a duct.

A related concept is flux, the flow of a quantity across a surface. For a vector field F, the flux through a surface S is Φ = ∬_S F · n dA, where n is the surface normal. In the fluid context, F can be the mass flux ρ v.

Applications include water supply, ventilation, chemical processing, and hydrology. Flow meters measure Q or ṁ to monitor and control systems. See also volumetric flow rate, mass flow rate, flux, and the continuity equation.