Alleviation of micro-pressure waves radiated from tunnel hoods

Abstract

When a train enters a tunnel, an impulsive pressure wave, called a micro-pressure wave (MPW), radiates from the tunnel exit. The MPW causes infrasound and audible noise near the tunnel portals; therefore, it should be reduced for environmental reasons. A typical countermeasure is the installation of a hood at the tunnel entrance portal. In a previous study, we proposed a new type of hood that combined two hoods with constant and different cross-sectional areas (a two-step hood). The two-step hood was more effective than the existing hoods. However, because the cross-sectional area of the portal of the two-step hood was larger than that of the existing hoods, the MPW radiated by a train entering the opposite tunnel portal increased in a double-track tunnel. This study developed a new concept for a tunnel hood that reduces the radiated MPW (performance at the MPW radiation stage) while maintaining the performance in terms of reducing the maximum pressure gradient of the compression wave (CW) generated by a train entering a tunnel (performance at the CW generation stage). The desired specifications for satisfying both performances were determined through calculations based on acoustic theory, and their performances were investigated through model experiments using a train model launcher. As the results, the desired specifications were lh3/lw>0.6, lh1≈lh2>lwMt/1-Mt, σh1=1.4, and σh2=2.5, where lh1, lh2, and lh3 are lengths of first hood, second hood, and added lateral wall, respectively, lw is wavefront thickness, Mt is the train’s Mach number, and σh1 and σh2 are the cross-sectional area ratio of the first and second hoods to tunnel, respectively. The developed hood is expected to provide more effective performance in mitigating MPW than existing hoods.