Study on the adaptive relationship between train nose lengths and variable-section tunnel parameters : Aiming at mitigating micro-pressure waves

Environmental problems caused by micro-pressure waves (MPWs) are increasingly prominent with increase in train speed. Considering different models of trains, which may have differences in nose lengths (L_n), and certain extreme terrains where it is unsuitable to construct the buffer hood outside the tunnel, this study numerically investigated the adaptive relationship between Ln and variable-section tunnel parameters [i.e., the length (L₁) and area (S₁) of the entrance expansion section] for minimizing the MPW. The results show that, as L_n rises from 5 to 13 m, the formation of the MPW peak is continuously delayed, and the peak value of the MPW decreases; while the optimal L₁ remains at 20 m for different L_n. Additionally, the optimal S₁ scarcely changes with the increase in L_n as well, between 151 and 156 m², and its ratio to the section area of the main tunnel (70 m²) is approximately 2.2. The optimal parameters of the entrance expansion section play a universal role for trains with different L_n, which can alleviate the MPW amplitude with a value of 58% or higher. The findings can provide significant support for designing tunnels and tunnel hoods, focusing on alleviating acoustic noise.