Inertial, thermal and mach-number compressibility in railway tunnels

Three important types of flow compressibility in railway tunnel systems are highlighted and their influence is illustrated by means of specific examples. As far as possible, in each example, only one of the three types of compressibility is of strong importance. This has the benefit of clarifying the underlying roles of the different types and it also enables incisive comparisons to be made between two different 1D methods of numerical analysis that arc widely used for the simulation of such flows. One of these methods is especially suited to analyses of wave-like (lows, but does not necessarily satisfy overall conservation of mass, momentum and energy. The other satisfies overall conservation exactly in any particular tunnel reach and is inherently more appropriate than the first method for most types of flow, with the possible exception of the detailed analysis of very steep-fronted pressure waves.