volume flow rate
Volume flow rate measures how much fluid passes a point each second, by volume. It is what a tap's setting really controls, how fast a bath fills, or how much water a river carries. It answers the simple question: how quickly is fluid being delivered, counted in litres or cubic metres per second?
Precisely, the volume flow rate Q is the volume of fluid V that passes a chosen cross-section divided by the time t it takes: Q = V / t. Its SI unit is the cubic metre per second (m^3/s), though litres per second and litres per minute are common in daily life. There is a very useful second form: if the fluid moves at speed v through a pipe of cross-sectional area A, then Q = A × v, because in one second the fluid sweeps out a slug of length v and cross-section A. This is the quantity the continuity equation keeps constant along a pipe.
Volume flow rate is the everyday language of plumbing, irrigation, breathing, and blood circulation. One honest distinction to keep straight: volume flow rate is not the same as mass flow rate. Two streams can carry the same number of cubic metres per second yet very different masses if their densities differ, since mass flow rate is the density times the volume flow rate, rho × Q. For an incompressible liquid the two rise and fall together, but for a gas whose density changes they can part ways.
Water moves at 2 m/s through a pipe of area 0.005 m^2. The volume flow rate is Q = A v = 0.005 × 2 = 0.01 m^3/s, which is 10 litres every second, filling a 200-litre tank in about 20 seconds.
Volume flow rate is area times speed, the volume of fluid delivered each second.
Volume flow rate (m^3/s) is not the same as mass flow rate (kg/s). They differ by the fluid's density, and for a gas whose density changes they need not track together.