Abstract: The continuous improvement of train operation speed is likely to lead to sonic booms in long tunnels of high-speed railways, negatively impacting the acoustic environment and traffic order at tunnel entrances. However, the mechanism of sonic booms and related aerodynamic index characteristics remain unclear. Based on a real vehicle test conducted in the Wan′an tunnel of the Beijing-Hong Kong high-speed railway, the authors analyze the mechanism and causes of sonic booms, the frequency domain characteristics of the phenomenon, the pressure gradient, and the behavior of micro-pressure waves. A three-dimensional refined numerical simulation supplements the real vehicle test conditions, examining the aerodynamic mitigation effects of various measures for the test tunnel. The results reveal the following: (1) The compression wave intensifies under the nonlinear effect during its propagation in the ballastless track tunnel, leading to the occurrence of sonic booms. (2) When sonic booms occur, the low-frequency band of 0－20 Hz is the main component of the sonic boom noise both outside and inside the tunnel. (3) When the train speed is below 300 km/h, the compression wave intensifies weakly within the tunnel, and no sonic boom noise is detected at the tunnel entrance. However, at a train speed of 300 km/h, the compression wave intensifies significantly in the tunnel, and the amplitude of the micro-pressure wave at the entrance increases nearly 5.5 times compared to that at 250 km/h, resulting in detectable sonic boom noise at the entrance. (4) For the test tunnel, the mitigation effects of three types of buffer structures on micro-pressure waves are ranked as follows: adding dynamic openings at both the inlet and outlet simultaneously < adding dynamic openings at the outlet < opening three inclined shafts.

Key words: high-speed railway tunnel, aerodynamic effect, field test, pressure wave, sonic boom