## 3 Results and Discussion
### The Generation Mechanism of the Initial Compression Wave  
The complex pressure waves are generated as a high-speed train passes through a tunnel. Figure 9 depicts a schematic of pressure fluctuations at a measurement point within the tunnel. There are four primary processes in which the pressure changes at a measurement point within the tunnel. When the train's streamlined nose enters the tunnel, there is a rapid initial rise in pressure (\(\Delta p_{\text{N}}\)), commonly referred to as the initial compression wave. This is also the focus of this research. The second pressure rise (\(\Delta p_{\text{tr}}\)) is caused by the friction produced by the body part of the train entering the tunnel. The third process involves pressure decrease (\(\Delta p_{\text{T}}\)) caused by the expansion wave generated as the tail car enters the tunnel. When the head car passes through the measuring location, the pressure drops significantly (\(\Delta p_{\text{HP}}\)) [4].  
To illustrate the formation mechanism and process of the initial compression wave, Figure 10 depicts the time history of the initial compression wave and pressure gradient at roof measurement points located 5_Di_ and 7_Di_ away from the tunnel entrance. The moment when the train's nose reaches the tunnel entrance is defined as 0 s.  
Figure 10: The basic characteristics of the initial compression wave: (**a**) Trajectory diagram of the head car and its induced compression waves; (**b**) time history of initial compression wave and pressure gradient at roof measurement points located 5_Di_ and 7_Di_ away from the tunnel entrance.  
Figure 9: The pressure fluctuation signature at a fixed position in a tunnel [4].  
As shown in Figure 10, before the train enters the tunnel, the pressure at the measurement points inside the tunnel begins to rise. This is because the pressure disturbances generated by the train's movement in the open air propagate into the tunnel, causing the pressure to increase. The pressure rises due to the disturbances from the open air at the measurement points located at 5_Di_ and 7_Di_ are 708 Pa and 705 Pa, respectively. As the train's nose enters the tunnel, a sequence of compression waves is generated due to the variation in the annular cross-section of the train and the tunnel. The compression waves generated from the train's nose and shoulder propagate to the measurement points, denoted as points B and C, respectively. As a result, the AB segment of the pressure-time history curve is attributed to disturbances from the open air, while the BC segment is attributed to the train's nose entering the tunnel.  
Simultaneously, the distribution of the pressure gradient of the initial compression wave is closely related to the distribution of the nose-shaped cross-sectional area along the length of the train's nose. As the part of the streamlined nose with the maximum cross-sectional area change rate (denoted as the M section) enters the tunnel, the resulting compression wave causes the largest pressure gradient at the measurement point. Therefore, to mitigate the intensity of the micro-pressure wave, it is advisable to minimize abrupt changes in the cross-sectional area of the head shape.