Influence of the enlarged portal length on pressure waves in railway tunnels with cross-section expansion

The train/tunnel aerodynamic effects still remain a major concern during high-speed railway construction. At present, different sizes of tunnels with constant cross-section are generally adopted based on train speed and blockage ratio. However, for tunnels with constant cross-section, narrower cross-section leads to worse aero-dynamic effects while larger cross-section results in sharply increasing costs. Focusing on this problem, this paper proposes a new type of high-speed railway tunnel to mitigate the aerodynamic effects by enlarging the crosssection at both ends and reducing the cross-section in the middle portion of the tunnel. A three-dimensional, unsteady, compressible and RNG k-epsilon turbulence model is utilized to simulate the pressure waves induced by a high-speed train passing through such a tunnel. Three cases with different enlarged portal lengths are simulated. The pressure variations on the tunnel wall and train surface are compared with previous moving model tests to validate the numerical algorithm. The results show that the train-tunnel-entry induced pressure transients are greatly reduced compared with that of constant cross-section tunnel. In addition, the pressure gradient and micro-pressure wave (MPW) emitted from the tunnel exit are also considerably decreased. Due to the enlarged portal of the tunnel, pressure waves are repeatedly reflected, and a new compression wave is developed at the junction where the tunnel cross-section abruptly changes. As a result, the wave energy is dissipated, and its strength is reduced. However, it is revealed that the length of the enlarged portal has less influence on the maximum pressure gradient and the MPW.