We have been concerned to date with steady flow of water in pipes but it is inevitable that there will be applications where the rate of flow changes. The behaviour of the water is by no means simple and, in the right circumstances, can be destructive. Transient pressures of 40 bar can be obtained easily. Most people will meet the effects of such changes as “water hammer” but it is a wider field and it is of interest to mechanical engineers. We shall find that when the rate of flow changes slowly the water behaves in a relatively simple way but if a steady flow is interrupted by, say, the complete and sudden closure of a valve the result is much more complicated. In between we have the behaviour of the water when a valve is closed rapidly but not suddenly.
I suppose that it would be possible to construct a computer programme to model the behaviour of water in any pipe, under any head, for any initial condition of flow and any given method of closure of any valve but I do not think that many engineers would be very interested in using it. What is needed is a general understanding of the main characteristics of the behaviour of the water with unsteady flow and an awareness of when the effects of these transients are going to become important.
We shall have to build the physics for this system and we shall find that it would become much easier if the effects of friction in the pipe are ignored. Fortunately we already know from our work on pipes and bearings that water is one of the most mobile liquids that we encounter in engineering and the losses caused by viscosity are amongst the lowest that we have to deal with. It is probably good enough for our purposes to limit our interest to water and then to try to account for the fundamental behaviour of the water in a pipe. This can then be transferred to other applications.