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.
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