# Aerodynamic Characteristics When Trains Pass Each Other in High-Speed Railway Shield Tunnel
## 4 Results and Discussion
### Aerodynamic Effects at Different Intersection Positions  
The aerodynamic effects of the high-speed trains when they met in the shield tunnel were analyzed. The intersection points were selected as 100, 200, 300, 400, and 500 m from the tunnel entrance. The nose of the left train was 900, 700, 500, 300, and 100 m from the tunnel entrance, and the nose of the right train was 100 m away from the tunnel exit. The two trains traveled into the tunnel at a speed of 350 km/h. The pressure changes on the shield tunnel lining structure and the micro-pressure waves 20 and 50 m outside the tunnel exit under five working conditions were compared and calculated. Figure 13 shows the comparison of the pressure peaks in the tunnel at different intersections during the whole process of the train traveling in the tunnel. Figure 14 shows the pressure peaks of the lining structure of the shield tunnel under the two-stage conditions as the train traveled through (Figure 14a) and after completely exiting the tunnel (Figure 14b). Figure 13: Pressure decay rate of the point of measurement in the center of the tunnel after pulling out of the tunnel. Figure 14a shows that the pressure amplitude varied with the longitudinal position of the tunnel monitoring point. When the high-speed trains met in the middle of the tunnel, the pressure amplitude near the tunnel entrance was smaller, but it was larger in the middle of the tunnel. When the high-speed trains met in the center of the tunnel, maximum positive and negative peak pressures appeared. The peak pressure appeared at the central measurement point of the tunnel, and the measurement point of the peak pressure was symmetric. In these five intersection conditions, when the trains met 100 and 200 m from the tunnel entrance, the pressure peak generated was the lowest. The maximum positive peak pressure was much smaller than that when the trains met at the center of the tunnel. When the train intersected at 100, 400, and 500 m from the tunnel entrance, the positive and negative peak pressures at the intersection point were maximal. When trains met at 200 and 300 m, the maximum positive and negative peak pressures on the shield tunnel lining structure deviated from the intersection position. There may have been three reasons for the deviation [3]: (a) the initial compression wave from the head of the train entering the tunnel and the expansion wave formed by reflection at the tunnel exit, (b) the initial expansion wave generated when the rear of the train entered the tunnel and the compression wave formed by its reflection, and (c) the mutual pressure change caused by two trains meeting in the tunnel. Figure 14b shows that after the trains completely exited the tunnel, when the trains had intersected at distances of 200 and 500 m from the tunnel entrance, the maximum positive peak pressure was still greater than the positive peak pressure when the train was traveling in the tunnel. The overall value was less than the peak pressure when the train was traveling in the tunnel. This further verified the necessity of studying the two different stages of the train passing through the tunnel. Figure 15 shows the pressure curve of the micro-pressure wave outside the tunnel opening at different intersection positions. The micro-pressure waves generated when two trains intersected in the center of the tunnel were analyzed in Section 4.2. In this section, only the micro-pressure waves in the four working conditions are discussed. For the pressure curve at each measurement point, only the micro-pressure part that formed by the divergence of the compression wave pulse is analyzed. Figure 15 shows that a low-pressure wave formed 50 m from the tunnel exit caused by trains meeting at a distance of 200 m from the tunnel entrance, with a pressure of 23.6 Pa. On the whole, the micro-pressure wave at 20 m and 50 m outside the shield tunnel exit was less than 50 Pa and 20 Pa, respectively.