Perforated tunnel exit regions and micro-pressure waves: geometrical influence

The effectiveness of long, perforated exit regions in reducing the radiation of micro-pressure waves (MPWs) from railway tunnels is assessed. Such disturbances always occur, but their amplitudes are usually small. For the particular case of high speed trains, they can reach levels that would cause annoyance in the absence of suitable counter-measures. This risk is especially large in the case of long tunnels. The general behaviour of wave reflection/transmission/radiation at a perforated exit region has been explored in previous papers that have (i) quantified the dependence on characteristics of the incident wavefront reaching the exit region from further upstream in the tunnel and (ii) validated the numerical methodology in a searching manner). Some notable differences have been found in comparison with criteria that have long been known for unperforated exit regions. In particular, the resulting MPW amplitudes depend upon the amplitudes of incident wavefronts as well as upon their steepnesses. The present paper summarises these outcomes and uses the methodology to explore issues of importance in practical design, namely the dependence of the effectiveness of perforated exit regions on their length and cross-sectional area. Once again, differences are found from behaviour of unperforated regions.