 Applying conservation of mass to this control volume, the mass flow rate (the mass of fluid flowing per unit time) is given by

where "v"1 is the speed in the front of the rotor, "v"2 is the speed downstream of the rotor, "v" is the speed at the fluid power device, "ρ" is the fluid density, the area of the turbine is given by "S", and formula_2 and formula_3 are the areas of the fluid before and after reaching the turbine.So the density times the area and speed should be equal in each of the three regions