When a high-speed train enters a tunnel it induces a nearly plain pressure wave running through the tunnel.
Two effects occur: on the one hand the wave is reflected at the open ending, on the others hand the tunnel
boom can occur at the far end. This is an explosion-like pressure-wave leaving the tunnel.
It can damage the tunnel and surrounding structures. The effect is comparable to the sonic boom produced
by an airplane and the noise is therefore disturbing for local residents. The amplitude of this pressure wave
can be reduced by decreasing the speed of the train. But, since high-speed trains should keep their velocity
also in tunnels structural modifications (tunnel portal) are necessary to decrease the pressure-gradient of
the wave front which is proportional to the amplitude of the sonic boom.
To investigate the effectiveness of structural modifications experiments can be performed using DLR's
Tunnel Simulation Facility (TSG). In such experiments mainly the pressure-time dependence at the tunnel
wall is measured.
Fig. 1: Tunnel entry schematic: processes and measured pressure-time dependence
Fig. 2: Tunnel entry (tunnel portal) with ICE3 model on TSG