ABSTRACT
Previous chapters of this book have been concerned almost exclusively with steady flow – that is, flow in which the velocity, pressure, density and so on at a particular point do not change with time. Admittedly, flow is rarely steady in the strictest sense of the term: in turbulent flow, for example, countless small variations of velocity are superimposed on the main velocity. But if the values of velocity and of other quantities at any particular point are averaged over a period of time and the resulting mean values are unchanging, then steadiness of flow may be assumed.
