# Aerodynamic Analysis of the Opening Hood Structures at Exits of High-Speed Railway Tunnels
## 1 Introduction  
High-speed railway transportation offers distinct advantages in terms of energy conservation and carbon emission reduction. Against the backdrop of increasingly severe environmental and energy challenges, many countries are actively promoting the development of high-speed railways. With the continuous increase in train operating speeds, the issue of train-tunnel aerodynamic effects becomes more prominent, especially the problem of exceeding the micro-pressure wave limit. Based on field-measured data, [PERSON] [1] indicated that the pressure amplitude of MPWs at tunnel exits is directly proportional to the cube of the train's entry speed into the tunnel. Furthermore, as the train speed increases,the amplitude of MPWs will also experience a significant rise. MPWs not only pose a severe threat to the internal structure of the tunnel but also affect the lives of the surrounding residents [2]. Additionally, the pressure fluctuations generated by high-speed flowing air also impact the stability of trains during operation [3,4]. Therefore, addressing the issue of MPWs holds significant importance for ensuring the safety and efficient operation of high-speed railways. Previous studies have indicated that installing hoods at the tunnel entrance area to decrease the pressure gradient of the compression waves can effectively control the micro-pressure waves [5,6]. Numerous scholars have extensively investigated the alleviating effects of tunnel entrance hoods on the MPWs. [PERSON] et al. [7,8] conducted a detailed analysis of the aerodynamic effects of compression waves generated by trains passing through various hoods. They established a relationship between the wavefront shape and the hood structure. Additionally, the study utilized acoustic theory to predict compression waves within the tunnel, and the reliability of these theoretical calculations was confirmed through experimental methods. [PERSON] et al. [9] conducted a study on the variations in pressure gradient after implementing single and double-seam opening hoods at tunnel entrances. Through model experiments and on-site testing, they confirmed the pressure gradient alleviation effect of seam hoods on compression waves. These research findings provide a fresh perspective on optimizing hood design. [PERSON] et al. [10] systematically investigated the mechanism by which hood structure openings influence the pressure gradient of compression waves. They thoroughly considered factors such as opening position and area under varying train speed conditions and their impact on the pressure gradient of compression waves. [PERSON] et al. [11,12] employed a two-dimensional axisymmetric numerical simulation model to simulate the propagation of compression waves. They achieved this by introducing a pressure inlet boundary at the upstream tunnel boundary. Consequently, they summarized the effects of tunnel exit hoods on the propagation and reflection characteristics of MPWs. Through on-site testing and numerical simulation, [PERSON] et al. [13] discovered that the installation of snowshed structures with openings between tunnel clusters has a mitigating effect on the MPWs. [PERSON] et al. [14] conducted a study on the aerodynamic effects of dividing-type hoods at tunnel exits. They substantiated the effective alleviation of MPWs by this structure through model experiments. Utilizing acoustic theory, [PERSON] [15] derived a predictive formula for MPWs that takes into account the topographical factors. They also outlined the impact of surrounding topography on the MPWs at tunnel exits. [PERSON] et al. [16] used a two-dimensional axisymmetric simulation model to simulate the generation and propagation process of MPWs. They investigated the mitigation effects of MPWs concerning the geometric shapes of various tunnel exits and summarized the relationship between tunnel exit topography and the effective solid angle coefficient.