# Aerodynamic Analysis of the Opening Hood Structures at Exits of High-Speed Railway Tunnels
## 5 Results and Discussion
### Analysis of the MPW Mitigation Effects by Entrance and Exit Hood
However, various installation approaches of hoods have distinct impacts on the amplitude and spatial distribution of the MPWs. The greatest variation in the amplitude of the MPWs occurred when only an opening hood was set at the exit. The MPW amplitude was highest at measurement point S3, reaching 100.2 Pa. Compared to the case with only a single opening on the side of the hood (as described in Section 5.2), the MPW amplitude decreased by 13.7%. This indicates that increasing the number of openings in the exit hood can disperse the pressure relief area, effectively reducing the MPW amplitude near the openings of the hood structure. The difference in the distribution of MPW amplitudes is the smallest when only an opening hood structure is set at the entrance. This is because the entrance hood alleviates MPWs by reducing the pressure gradient of the compression wave without affecting the spatial distribution pattern of MPW amplitudes. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Hood Parameters** & **Entrance** & **Exit** & **Entrance and Exit** \\ \hline Opening ratio of entrance hood (\%) & 30 & — & 30 \\ Opening ratio of exit hood (\%) & — & 30 & 30 \\ Length of the hoods (m) & 21 & 21 & 21 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The setting of entrance and exit hood structure opening ratio. Figure 14: Time–history curves of compression waves under the influence of entrance hood: (**a**) initial compression wave; and (**b**) pressure gradient. Figure 15: Optimal conditions of entrance hood structure. If only an opening hood is set at the tunnel entrance or exit, the MPW amplitudes for the two cases are relatively close when the observation azimuth angle (\(\alpha,\beta\)) does not exceed \(45^{\circ}\). This indicates that both cases have similar effects on mitigating MPWs within a certain range in front of the exit plane of the hood. However, when an opening hood is placed at the tunnel entrance, it can effectively reduce the pressure gradient of the compression wave generated by the train entering the tunnel. Meanwhile, setting a hood at the tunnel exit can increase the effective radiation solid angle of MPWs. In contrast, setting a hood without opening with equal cross-sections at the tunnel entrance does not reduce the pressure gradient. Only the opening hood set at the tunnel exit can increase the effective solid angle by effectively releasing some pressure in advance, which helps to achieve the mitigation effect on the MPWs. Therefore, the installation of a hood at the exit provides a higher efficiency in mitigating the effects of MPWs compared to the installation at the entrance. When hoods with openings are installed at both the tunnel entrance and exit, the MPW amplitudes in front of the exit plane of the hood are the lowest among the three cases. This suggests that the installation of hoods with openings at both entrance and exit provides the highest efficiency in mitigating the effects of MPWs. At measurement point S3, the MPW amplitude under the case of entrance and exit hoods still experiences an increase. However, compared to the case with only an opening hood at the exit, the MPW amplitude decreased by 12.3%. When the radiation radius is 50 m, the spatial distribution pattern of MPW amplitudes is largely consistent with the previously mentioned results. However, the MPW amplitude for the case with an exit hood is slightly higher than the other two cases. In the case where only the exit hood is installed, the MPW amplitude at the F1 measurement point is 31.5 Pa. Compared to the conditions where only the entrance hood is installed and where both the entrance and exit hood are installed, the MPW amplitudes have increased by 11.1% and 8.4%, respectively. According to Equation (10), the maximum amplitude of the MPW is directly proportional to the maximum value of the pressure gradient. As a result, setting up entrance hoods in the tunnel can lead to effective mitigation of MPWs across varying radiation radii.