In dealing with the flow of fluids, and the reactions set up on moving surfaces, water and steam turbine engineers have to deal with definite masses of water or steam flowing along channels whose boundary walls are fixed.

Fig. 1 (reproduced from Stodola's Steam Turbines, 3rd edition) shows the nature of the assumption made for the so–called impulse turbine, assumptions which hold to an equal approximation for water turbines, showing nozzle and moving vanes

Reaction turbines present a more complicated state of affairs, which offers a less direct analogy with aeroplanes and airscrews, and which therefore need not be considered here.

In the figure (1) the mass of working fluid delivered per second is the cross section of the nozzle or channels between vanes, at any point, multiplied by the velocity of the fluid relatively to the walls of the channel at that point.