Method of characteristics in quasi-steady compressible flows

Differences are highlighted between applications of the method of characteristics (MOC) to compressible and incompressible flows. Attention is focussed on regions of steady or nearly-steady flow that commonly occur locally within general unsteady flows. It is shown that, except at very small Mach numbers, fixed-grid MOC methods can give strongly distorted predictions of axial variations of pressure and velocity. Furthermore, integration time steps chosen for maximum accuracy in one location are likely to be less suitable in other locations. Whilst greatest attention is paid to flows with moderate subsonic Mach numbers, a second purpose of the paper is to show why classical applications of fixed-grid MOC to liquid transients in pipes are inherently incapable of modelling steady flows exactly.