# Aerodynamic Analysis of the Opening Hood Structures at Exits of High-Speed Railway Tunnels
## 6 Conclusions  
This study investigates the impact of the exit hoods on the amplitude and spatial distribution of MPWs in high-speed railway tunnels, and the following conclusions are made:1. The influence parameters of the exit opening hood are theoretically analyzed using the control volume method, and the results are validated through three-dimensional numerical simulation. The research findings indicate that the opening ratio is a key parameter influencing the mitigation effectiveness of the exit hood on the MPWs. The amplitude of MPWs shows an inverse proportionality relationship with the opening ratio. Setting up a hood with openings at the tunnel exit can reduce the pressure amplitude at the exit of the hood, increasing the effective radiation solid angle of the MPWs and consequently enhancing the mitigation effectiveness of the exit hood on the MPWs. 2. Different opening forms have a relatively minor impact on the pressure amplitude of MPWs in front of the hood exit plane. However, they do affect the spatial distribution of MPWs and the effective solid angle in the vicinity of the openings. Hence, when determining the opening form and placement of hoods, reliance solely on past single-point evaluation methods should be avoided. It is crucial to comprehensively consider the relative significance of various positions outside the tunnel, ensuring that the design of hoods maximally mitigates the impact on residents and critical facilities. 3. The installation of hoods at the entrance and exit of a tunnel significantly influences the mitigation of MPWs, with distinct mechanisms governing the alleviation effects of entrance and exit hoods. Compared to entrance hoods, exit hoods exhibit a more significant efficiency in mitigating the amplitude of MPWs within a certain range in front of the hood exit plane. When both the tunnel entrance and exit are equipped with opening hoods, the amplitude of MPWs is minimized, resulting in the highest efficiency in mitigating MPWs. 4. When conducting hood design, it is essential to consider the impact of entrance and exit hoods on the MPWs. For single-track tunnels, opening hoods can be asymmetrically placed at the tunnel entrance and exit. The entrance hood should be designed using cases that ensure the proximity of pressure gradient peaks. For the exit hood, it is important to increase the total opening ratio to enhance its efficiency in mitigating MPWs. 5. For the design of opening hoods in double-track tunnels, a symmetrical arrangement should be adopted at both the entrance and exit while simultaneously increasing the overall opening area. Meanwhile, the hoods should adopt a design with small, numerous openings, and the positioning of these openings should be carefully planned. This strategy aims to prevent any adverse effects on the mitigation of MPWs when trains operate in both directions. The tunnel exit hood structure has a certain alleviating effect on the MPWs. However, current research has only explored the alleviating mechanism of opening hoods on the MPWs at tunnel exits. Therefore, investigating the alleviating effects of different forms of exit hoods on the MPWs and assessing the impact of exit hoods on the pressure comfort of trains inside the tunnel will be the focus of future research. [PERSON] conceived and designed the analysis, collected and analyzed data, and wrote and revised the paper; [PERSON] conceived and designed the analysis, provided funding, and revised the paper; [PERSON] provided guidance on the numerical simulation software (Fluent 15.0); [PERSON] wrote and edited the paper and provided supervision; [PERSON] conducted verification and formal analysis. All authors have read and agreed to the published version of the manuscript. This research was funded by the by the Technological Key Research and Development Program of China Railway Corporation [P2021G053] and National Natural Science Foundation of China, grant numbers [51778539]. The authors are grateful to all the study participants.