1. Flow equations

1.1 General equation

Flow equations express the conservation of mass. They are written for organs or parts of organs inside which the pressure can, to a first approximation, be considered uniform. The approximation is generally justified in the static regime, and remains justified in the variable regime at low frequencies (as long as the periods remain large compared with the propagation times of the pressure waves).

Simply write that the difference between the mass of fluid entering and the mass of fluid leaving during the unit of time is equal to the variation in the mass contained in the organ or portion of organ considered.

Let's refer respectively to