Abstract
Micro-pressure wave radiated from a tunnel exit is one of the important environmental problems in high-speed railways. The magnitude of the micro-pressure wave is approximately proportional to the pressure gradient of the compression wave arriving at the exit portal. Therefore, it is essential to investigate the distortion of the compression wave propagating in the tunnel in order to predict the magnitude of the micro-pressure wave. In this paper, we have investigated an optimum tunnel entrance hood by numerical simulation by which the pressure gradient of the compression wave arriving at the exit portal of a long slab track tunnel is minimized. The results of the simulation are as follows: (1) A tunnel entrance hood, which gives rise to minimum pressure gradient of the initial compression wave, does not necessarily correspond to the optimum tunnel-entrance hood for minimizing the micro-pressure wave at the exit portal when the tunnel is long. (2) The increase of the pressure gradient of the compression wave with the pressure gradient waveform ∂p/∂t having a trough between two peaks is smaller than the compression wave with flat ∂p/∂t waveform (without a trough) and the same value of the maximum ∂p/∂t. The results of this study will be useful for improving the efficiency of tunnel entrance hoods.
