Influence of hood geometry on the compression wave generated by a high-speed train

A compression wave is generated when a high-speed train enters a tunnel. The wave propagates ahead of the train at the speed of sound. In a long tunnel nonlinear steepening of the wavefront produces the emission of a strong micro-pressure wave (mpw) from the distant tunnel exit. The mpw can produce structural damage to the tunnel and rattles in surrounding buildings. Nonlinear steepening can be countered by increasing the initial rise time of the compression wave by installing a tunnel entrance ‘hood’ consisting of a nominally uniform extension of the tunnel of larger cross-section. In this paper a theoretical examination is made of the influence on the wave of the rapid change in tunnel cross-section in the transition region between the tunnel and hood. It is shown that by optimally profiling the cross-sectional area changes across this region it is possible to minimize the amplitudes of second and third peaks of the compression wave pressure gradient. By this means the amplitudes of the secondary peaks in the micro-pressure wave are greatly reduced.