Simple and accurate analysis of MPWs: Near-field and far-field

A simple and efficient method of simulating wave propagation in radially-symmetric environments is used to extend the practical scope of assessing micro-pressure wave (MPW) radiation from railway tunnel portals. The method utilises a recently developed solution technique based on a one-dimensional Method of Characteristics (MOC) for spherical wave propagation. Close similarities exist with corresponding MOC methods used to simulate uniaxial wave propagation inside tunnels and this enables a simple, continuous coupling to be made between the internal and external flow domains. Integral coupling has numerical advantages, but its primary benefit is physical realism, enabling account to be taken of behaviour that is neglected in many simpler analyses. The coupled analysis is used to illustrate strong differences between outcomes resulting from the arrival of low-frequency and high frequency wavefronts at a tunnel portal. The most useful purpose of the new coupled methodology is expected to lie in the efficiency with which multiple simulations can be undertaken. This is because it is far less resource-intensive than 3-D methods of allowing for coupling. Another, less general benefit, is a demonstrated ability to use MOC to simulate train-entry, wave-propagation and MPW radiation in a single unified manner, albeit only in a strictly limited range of cases that will rarely exist in older tunnels.