Abstract: A one-dimensional unsteady compressible nonisentropic flow model, incorporating a generalized Riemann variable characteristic line numerical simulation method and the rain flow counting method, is employed to explore the mechanisms and characteristics of pressure fluctuations at the center and inside the entrance and exit of a tunnel during the passage of a single train and the crossing of two trains at the same speed. The distribution patterns of the maximum positive and negative pressures, the maximum pressure peak-to-peak values at various measurement points inside the tunnel along its length, the influence of tunnel length on the maximum tunnel aerodynamic load, the impact of train speed on the magnitude, position, and frequency of amplitude, and the distribution characteristics of the critical tunnel length based on the maximum pressure peak-to-peak value inside the tunnel are obtained. The research results demonstrate the following: (1) The pressure at a given location in the tunnel measurement points remains constant for a certain time when a single train passes through and when two trains cross at a constant speed. After the train exits the tunnel, the pressure waveform repeats periodically, consistent with the propagation period of the compression or expansion wave inside the tunnel. (2) When a single train passes through a tunnel at three different speeds and tunnel lengths ranging from 0.5 to 10 km, the maximum positive pressure value occurs 200－600 m from the tunnel entrance. As speed increases, the position of the maximum negative pressure value and the pressure peak-to-peak value for certain tunnel lengths shift from the near-inlet side toward the near-exit side of the tunnel center. (3) At 400 km/h speed, the critical tunnel length based on different maximum pressure loads is 1.3 km for a single train passage and 1.4, 0.94, and 1.2 km for the central meeting of two trains at a constant speed. (4) The highest amplitude levels for single train passage and central meeting of two trains at 400 km/h can reach 4 800 to 5 200 Pa and 10 000 to 10 400 Pa, respectively.

Key words: high-speed railway tunnel, aerodynamic load, pressure attenuation, one-dimensional flow model with characteristic line method, rain flow counting method