# Aerodynamic Analysis of the Opening Hood Structures at Exits of High-Speed Railway Tunnels
## 5 Results and Discussion
### Design Methodology for Entrance and Exit Hood Structures  
In previous studies on the design of entrance and exit hoods for tunnels, the typical approach has been to select optimization parameters based on the optimal conditions of entrance hoods [33; 34; 35]. Additionally, hoods are symmetrically positioned at the entrance and exit of the tunnel. However, the findings from the aforementioned research reveal that setting hoods with openings at the exit can effectively mitigate the impact of MPWs. Furthermore, the best mitigation results for MPWs are achieved when hoods are installed at both the entrance and exit. From the analysis results, it can be observed that past designs failed to consider the impact of exit hoods on mitigating MPWs, revealing flaws in their  
Figure 16: Micro-pressure wave amplitude at monitoring points affected by the entrance and exit hood: (**a**) radiation radius \(r=20\) m; and (**b**) radiation radius \(r=50\) m.  
methodology. Therefore, in the subsequent design of hoods, comprehensive consideration will be taken on the influence of entrance and exit hoods on the MPWs, as well as the surrounding topography of tunnel exits. Based on previous research findings, tunnel entrances utilize hoods to prolong the time required for compression waves to reach their peak, thus alleviating the impact of MPWs. However, the mitigation effect of entrance hoods is significantly influenced by factors such as cross-sectional area, opening area, opening location, and opening form. Under the combined influence of multiple factors, the relationship between pressure gradient and opening area is no longer purely linear. Additionally, existing research faces challenges in rapidly optimizing entrance hood parameters due to the influence of multiple variable factors [10]. Currently, artificial intelligence and deep learning algorithms have a unique advantage in addressing such complex nonlinear engineering problems [36; 37]. Scholars have put forward various algorithmic approaches for optimizing complex parameters in engineering [38; 39; 40]. Additionally, some scholars have proposed that [41; 42] using deep learning algorithms to solve fluid dynamics problems can enhance the computational efficiency of numerical simulations. However, there has been no systematic research on optimizing the parameters of high-speed railway tunnel entrance hoods using deep learning algorithms to date. In contrast, exit hoods release pressure in advance to reduce the pressure amplitude at the exit and expand the radiating area of the MPW, thereby increasing the effective radiation solid angle and consequently achieving the mitigation of MPWs. Furthermore, the key parameters influencing the amplitude and spatial distribution pattern of MPWs in exit hoods are the opening ratio and opening form, with relatively more singular controlling factors. Hence, for single-track tunnels, an approach involving asymmetric entrance and exit hoods can be employed in the design process. When designing entrance hood structures, the tunnel's internal pressure gradient should serve as a control criterion, and design cases with pressure gradient peaks that are relatively close to each other should be selected. For the exit hood, direct pressure release is achieved. Therefore, the total opening ratio should be increased while avoiding excessively large individual opening areas. Through the above design approaches, it is possible to effectively reduce the amplitude of MPWs in front of the exit plane and enhance the mitigation efficiency of the single-track tunnel hoods. For double-track tunnels, adopting the aforementioned asymmetric hood has certain limitations. When trains operate in reverse, the exit hood's opening ratio is relatively low, and this limits its effectiveness. On the other hand, having excessively large individual opening areas in the entrance hood can reduce the efficiency of alleviating the pressure gradient of the compression wave, thereby increasing the amplitude of the MPW.