High-speed trains: Prediction of micro-pressure wave radiation from tunnel portals

Compression waves generated by trains entering a tunnel travel toward the opposite portal, where they are partly reflected back in the tunnel as expansion waves and partly radiated outside as micro-pressure waves. The intensity of the micro-pressure waves is related to the pressure gradient of the oncoming wave that, during its propagation within the tunnel, can either become steeper or smoother, according to the balance between viscous, thermal and nonlinear effects. Under the worst conditions, micro-pressure waves can become cause of concern producing noise and vibrations in the environment close to the tunnel exit.Starting from the observation that pressure waves propagation in tunnels is mainly a one-dimensional (1D) phenomenon, this work presents a method capable of predicting the pressure disturbances radiated from tunnel portals based on the classical linear acoustic formulation by Kirchhoff, in which the source data are obtained by the solution of the unsteady quasi-1D equations of gas dynamics.Comparisons with both full-scale and reduced-scale experiments about micro-pressure wave radiation are presented. The results show that the proposed, highly efficient method is able to capture the fundamental features of pressure waves propagation within tunnels as well as pressure radiation at their portals.