# 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  
For this section, four different hoods were selected to investigate the influence of the opening ratio of the exit hood on the MPW. In the subsequent analysis, the hood without an opening (Case 1) was chosen as the control group, with specific parameters shown in Table 1. The specific parameter settings for the exit hood are shown in Figure 7. During the simulation process, the fixed circumferential arc length (\(D\)) of the opening is 4 m. The distance between the opening and the exit of the hood (\(A\)) is 7 m, while the distance between the opening and the tunnel exit (C) is 6.5 m. By changing the length of the opening (\(X\)), the total opening ratio of the hood (the opening area divided by the cross-sectional area of the hood) can be adjusted. The total length of the hood is \(L\). Furthermore, all the hoods in this paper have a total length (\(L\)) greater than 20 m. Based on the conclusions in Section 4.2 of this paper, subsequent analysis could disregard the impact of variations in the total length of the hoods on the amplitude of the MPWs. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Hood Parameters** & **Case 1** & **Case 2** & **Case 3** & **Case 4** \\ \hline \(X\) (m) & — & 7.5 & 15 & 22.5 \\ Total length of the hood (m) & 21 & 21 & 28.5 & 36 \\ Opening ratio of the hood (\%) & — & 30 & 60 & 90 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Numerical simulation parameters of tunnel hood. Figure 7: Exit hood structure opening layout diagram. As indicated by existing research findings [16; 30; 31], micro-pressure waves manifest directionality when radiating outward from the tunnel exit. The MPW amplitudes recorded at monitor points directly in front of the exit axis (\(\alpha\),\(\beta\) = 0) are noticeably higher than those at other points. To investigate the influence of the exit hood on the MPWs in the area directly in front of the exit axis, the time-history curve of MPWs is shown in Figure 8 for the case when the radiation radius is \(r\) = 20 m. With the installation of the exit hood, the waveform of the MPWs at point F1 remained unchanged. However, the peak values of the MPWs at this point decreased as the opening ratio of the hood increased. Table 2 shows a comparison of the MPW amplitudes at point F1 under different circumstances. With a radiation radius of \(r\) = 20 m, Case 1 exhibited the highest MPW amplitude at point F1, with a value of 89.5 Pa. However, upon the installation of the exit hood with an opening, the minimum MPW amplitude was reduced to 45.9 Pa. Compared to Case 1, setting an opening significantly reduces the MPW amplitude at point F1. The reduction can reach a maximum of 48.7%. Hence, setting an opening in the exit hood leads to a higher efficiency in mitigating MPWs. The fitting curve of MPW amplitude concerning the opening ratio of the hood is shown in Figure 9. At point F1, the amplitude of the MPW decreased as the opening ratio of the hood structure increased. A negative linear function was employed to fit the relationship between the MPW and the opening ratio of the exit hood under different radiation radii. The fitting correlation coefficients R\({}^{2}\) were all greater than 0.98. At point F1, the maximum amplitude of MPWs occurs when there is no opening in the hood. However, when the opening ratio reached 90%, the minimum amplitude of MPWs was observed. This trend holds under various radiation radii. Furthermore, both the amplitude and attenuation ratio of MPWs decreased as the radiation distance increased, which is consistent with previous research findings [5]. To further investigate the impact mechanism of exit hoods on the MPWs, this study will analyze the pressure distribution characteristics of MPWs under different exit hoods.