# Aerodynamic Analysis of the Opening Hood Structures at Exits of High-Speed Railway Tunnels
## 5 Results and Discussion
### Analysis of the Effects of Different Exit Hood Opening Form  
The above research indicates that when an opening is introduced into the hood at the tunnel exit, it results in an increase in pressure amplitude in the vicinity of the hood's opening area. To investigate the influence of different opening forms of exit hoods on the spatial distribution pattern of MPWs, three distinct opening forms are selected for comparative analysis: top opening, side opening and seam opening, as shown in Figure 12. The dimensions of the hoods are as follows: the cross-sectional area of the hood is the same as that of the tunnel, and the total length is 21 m for all cases, with an opening ratio of 30%. In the subsequent analysis, Case 1 is taken as a reference case for comparison. The spatial distribution of the MPW amplitudes is shown in Figure 13. When the MPW has a radiation radius of 20 m, the spatial distribution pattern in the condition without an opening (Case 1) is consistent with the previous conclusion. The MPW distribution patterns in front of the exit plane (\(\alpha\),\(\beta<90^{\circ}\)) for the top-opening hood, side-opening hood, and seam-opening hood exhibited similar spatial trends. Additionally, the MPW amplitudes in these three conditions are relatively close, and they increased with the increase in observation angle. At the F1 measurement point, the MPW amplitude for the seam-opening hood is 66.6 Pa, representing a reduction of 25.6% compared to the condition without an opening. At measurement points behind the exit plane (\(\alpha\),\(\beta>90^{\circ}\)), the MPW amplitudes for these three opening cases exhibited distinct spatial distribution patterns. Among them, the variation in MPW amplitudes is most pronounced at points S3 and T3. The top-opening hood exhibited the highest MPW amplitude at measurement point T3, with a value of 116.1 Pa, while the lowest MPW amplitude was observed at measurement point S3, with a  
Figure 12: Hood structures with different opening forms: (**a**) top opening hood; (**b**) side opening hood; and (**c**) seam opening hood. value of 97.3 Pa. For the side-opening hood, the MPW amplitude reached its maximum at measurement point S3, with a value of 116.1 Pa, while the minimum amplitude was recorded at measurement point T3, with a value of 93.9 Pa. In the case of the seam-opening hood, the amplitude of the MPW was relatively similar at measurement points T3 and S3, with values of 106.3 Pa and 105.4 Pa, respectively. This phenomenon happens because of the compressional wave releasing a portion of its pressure as it passes through the opening, which is installed in the hood structure. The varied opening forms led to pressure release occurring at different positions, and subsequently influenced the spatial distribution patterns of the micro-pressure waves in the vicinity of the hoods. When the radiation radius is 50 m, the four different opening cases of the hoods exhibited similar spatial distribution patterns of MPWs, and the MPW amplitudes were relatively close. This is consistent with the conclusion drawn in Section 5.1 of this paper. From the results shown in Figure 13, it can be concluded that when the opening form of the exit hood changes, there is almost no impact on the MPW in front of the exit plane. However, it only has a significant influence on the amplitude of the MPW in the vicinity of the opening. Combining the results from Figure 11, it is once again confirmed that the opening ratio is a crucial parameter affecting the mitigation effectiveness of the hood structure against MPWs. Effective radiation solid angles for different forms of hoods are shown in Tables 5 and 6. The correction factor \(k\) for effective radiation solid angles near the hoods falls within the range of \(0.6<k<1.7\), which closely aligns with the results by [PERSON] [30]. When the radiation radius is 20 m, different opening forms of the hoods yield \(k>1\) in front of the exit plane. After the openings were installed, compared to Case 1, the correction factor \(k\) increased slightly.