# Aerodynamic Analysis of the Opening Hood Structures at Exits of High-Speed Railway Tunnels
## 5 Results and Discussion
### Analysis of the Impact of Exit Hood Opening Ratio
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Simulation Case**} & \multicolumn{2}{c}{**P\({}_{\text{mpw}}\) (Pa)**} & \multicolumn{2}{c}{**Reduction Ratio (\%)**} \\ \cline{2-5}  & \(r\) = 20 & \(r\) = 50 & \(r\) = 20 & \(r\) = 50 \\ \hline Case 1 & 89.5 & 33.0 & — & — \\ Case 2 & 68.9 & 31.8 & 23.0 & 3.6 \\ Case 3 & 55.6 & 28.3 & 37.8 & 14.2 \\ Case 4 & 45.9 & 24.6 & 48.7 & 25.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of micro-pressure waves at monitoring points F1. Figure 8: Time-history curve of micro-pressure waves at F1 (\(r\) = 20 m). Figure 10 shows the generation, propagation, and evolution process of MPWs on the \(x=0\) m (tunnel centerline) plane. However, Figure 10a,c,e,g,i correspond to the condition without an opening in the hood (Case 1), while Figure 10b,d,f,h,j correspond to the condition with an opening at the top of the hood (Case 2). When t = 2.2 s, the compression wave reached the vicinity of the tunnel exit, and the exit of the hood starts generating a positive pressure zone and radiating MPWs outward. Currently, the pressure change is relatively small. In Case 2, the compression wave, in the presence of the opening, was released outward earlier, resulting in a lower pressure amplitude at the exit of the hood compared to Case 1. This situation led to a lower pressure gradient at the exit. Simultaneously, the radiation distance of the MPW is greater at the same moment compared to Case 1, as shown in Figure 10a,b. When t = 2.2-2.3 s, the pressure amplitude of the MPW outside the tunnel rapidly increased. In Case 1, the MPW approximately radiated outward from the exit of the hood in a semi-spherical shape. However, when obstructed by the topography near the mountain, the MPW gradually transformed into a 1/4 spherical shape and continued to propagate outward. In Case 2, both the opening and the exit of the hood released pressure outward. In these two areas, MPWs were generated and propagated outward, respectively. When the MPWs propagated to a certain radiation radius, the two waves superimposed and rapidly evolved into a 1/4 ellipsoidal shape, continuing to propagate further, as shown in Figure 10c-f. The results of this study are consistent with previous research findings [16], demonstrating that micro-pressure waves exhibit elliptical propagation characteristics. When t = 2.5 s, in Case 1, the MPW propagated outward and reached the vicinity of the mountain where reflection occurs. The reflected wave superimposed with the MPW radiated from the outlet and evolved into a 1/4 ellipsoidal shape, continuing its propagation, as shown in Figure 10g. When t = 2.6 s, the release of the compression wave was essentially complete, and the MPW maintained a certain front and continued to propagate further. As the propagation distance increased, the amplitude of the MPW weakened, and the fluctuations gradually dispersed in three-dimensional space, as shown in Figure 10i,j. In conclusion, when an opening hood was employed at the tunnel exit, it is possible to reduce the pressure amplitude and pressure gradient at the exit of the hood by releasing a portion of pressure in advance. Furthermore, the exit hood can disperse the release area of the compression wave and enlarge the radiating area of the MPW, thus effectively mitigating the impact of MPWs. Figure 9: Fitting curves between MPW amplitudes and the opening ratio of the hood. Based on the research findings, the installation of the exit hood can effectively reduce the amplitude of MPWs. However, there may be an increase in pressure amplitude in the vicinity of the hood's opening. Therefore, further research is essential for a better understanding of the distribution pattern of MPW amplitudes around the exit hood. To provide a more precise description of the impact of factors such as tunnel exit topography on the MPWs, [PERSON] [32] introduced the concept of the effective radiation solid angle and presented Equation (10).