# 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  
The entrance hood primarily mitigated the MPWs at the tunnel exit by extending the initial rise time of the compression wave, thereby reducing the maximum pressure gradient. This effectively alleviates the impact of MPWs at the tunnel exit. Designing the entrance hood is relatively simple, and that is why most of the previous research has been focusing on optimizing it. However, the aforementioned study indicated that the exit hood also has a certain mitigating effect on the MPWs. Therefore, this section will comprehensively compare the mitigation effects of the entrance and exit hood structures on the MPWs. One study [32] investigated the optimization methods for multi-opening entrance hoods and obtained the following conclusions: the opening location and area of the hood are key optimization parameters influencing the amplitude of MPWs, and the relationship between the number of pressure gradient curve peaks \(M\) and the number of openings \(N\) is \(M=N+1\). When several peaks of the pressure gradient are close under the influence of the hood, it can be assumed that an optimally designed opening hood structure will have the highest level of performance (i.e., the smallest maximum value of MPW). In this section, we adopt the typical Japanese hoods from Ref. [32] as the research subject. By optimizing the opening position and opening area of the hood, we obtain the time-history curve of the compression wave at 50 m from the tunnel exit, as shown in Figure 14. After setting up the entry hood, the peak values of the first, second, and third waves of the pressure gradient are 12.2 kPa/s, 12.4 kPa/s, and 12.6 kPa/s, respectively. The three peak values of the pressure gradient can be approximately considered equal. Compared to the case without a hood, the reduction in the pressure gradient is the highest. Therefore, the arrangement of the entrance hood openings shown in Figure 15 is considered optimal. The hood shown in Figure 15 is placed at the tunnel entrance, exit, and entrance/exit, respectively. If hoods are only placed at the exit or entrance, equal-length hoods without openings will be installed \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Azimuth Angle (\({}^{\circ}\))**} & \multicolumn{4}{c}{**The Opening Form of the Tunnel Hood**} & \multirow{2}{*}{**Observation Plane**} \\ \cline{2-2} \cline{4-5}  & **without** & **Top** & **Side** & **Seam** \\ \hline
90 & 0.8 & 0.8 & 0.8 & 0.8 & Top \\
45 & 1.2 & 1.2 & 1.2 & 1.2 & (Plane _zoy_) \\
0 & 1.2 & 1.3 & 1.3 & 1.3 & (Plane _zox_) \\ \hline
45 & 1.2 & 1.1 & 1.2 & 1.2 & Side \\
90 & 0.8 & 0.8 & 0.8 & 0.8 & (Plane _zox_) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Correction coefficients of effective radiation solid angle under different opening forms (\(r=50\) m). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Azimuth Angle (\({}^{\circ}\))**} & \multicolumn{4}{c}{**The Opening Form of the Tunnel Hood**} & \multirow{2}{*}{**Observation Plane**} \\ \cline{2-2} \cline{4-5}  & **without** & **Top** & **Side** & **Seam** \\ \hline
135 & 1.0 & 0.6 & 0.8 & 0.7 & \\
90 & 1.3 & 1.1 & 1.2 & 1.2 & Top \\
45 & 1.1 & 1.5 & 1.6 & 1.6 & (Plane _zoy_) \\
0 & 1.0 & 1.6 & 1.7 & 1.7 & \\ \hline
45 & 1.1 & 1.5 & 1.5 & 1.5 & Side \\
90 & 1.2 & 1.1 & 1.1 & 1.1 & (Plane _zox_) \\
135 & 0.9 & 0.8 & 0.6 & 0.7 & (Plane _zox_) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Correction coefficients of effective radiation solid angle under different opening forms (\(r=20\) m). at the other portal of the tunnel. The parameters for setting up the entrance/exit hood structures are presented in Table 7.  
Figure 16 shows the influence of different entrance and exit hoods on the spatial distribution pattern of MPW amplitudes. When the radiation radius is 20 m, the implementation of entrance or exit hoods in the tunnel provided a certain degree of mitigation effect on the MPWs.