Propagation characteristics of initial compression wave in cave and portal micro-pressure waves characteristics when 600 km·h^-1 maglev train entering tunnels

Based on a three-dimensional numerical simulation method, a one-dimensional compressible unsteady non-isentropic flow model and an improved generalized Riemann variable characteristic line method were developed. The initial compression wave propagation in the tunnel and the micro-pressure wave characteristics at the portal (default exit) of the tunnel when the tunnel entrance without and with an opening buffer structure were investigated. Analysis results show that compared to the nonbuffer structure at the tunnel entrance, the maximum pressure gradient of the initial compression wave generated by setting the opening buffer structure decreases by 67.56%. During the propagation of the initial compression waves in the tunnel, intensification first occurs, followed by attenuation. The critical lengths of the nonbuffer and opening buffer structures are 2 and 6 km, respectively, whereas the critical lengths of the tunnel satisfying the control standard of the micro-pressure waves are 33 and 34 km, respectively. Although the opening buffer structure can significantly reduce the maximum pressure gradient of the initial compression waves for a long tunnel, owing to the continuous intensification of the compression wave during propagation, the effect of the opening buffer structure on the mitigation of the micro-pressure waves is significantly weakened. Engineering measures (such as shafts) should be adopted to mitigate intensification. In addition, the effects of the buffer structure on the maximum pressure gradient of compression waves are different in the portals of different tunnel lengths. Therefore, the different types of buffer structure and length factors should be combined to determine the corresponding optimal tunnel length matching relationship. 1 tab, 24 figs, 33 refs.