# 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
Figure 11: Micro-pressure wave amplitudes at measurement points under different opening ratios: (**a**) radiation radius \(r=20\) m; and (**b**) radiation radius \(r=50\) m.  
In Figure 11b, when the radiation radius (\(r\)) is 50 m, the trend of decreasing MPW amplitude with increasing opening ratio remains evident. However, the MPW amplitudes at the top and sides of the hood were relatively close, indicating that after propagating a certain distance, phenomena like wave reflection and superposition have already occurred. The MPW has fully developed into an ellipsoidal wavefront and is propagating towards a distant location, with pressure amplitudes nearly identical at different positions on the same ellipsoidal wavefront. Furthermore, when the radiation radius is 50 m, the dimensions of the hood without an opening at the exit are smaller than the MPW's radiation radius; Case 1 does not follow the previously described directional pattern of the MPW. Therefore, for precise control of the MPW amplitude within a specific radiation radius, it is essential to ensure that the total length of the tunnel exit hood exceeds the mentioned radiation radius. Based on the pressure amplitude of the MPW at various measurement points, correction coefficients \(k\) for the effective radiation solid angle were calculated. The correction coefficients \(k\) for different cases are shown in Tables 3 and 4. According to Equation (10), it is observed that the \(k\)-values are inversely proportional to the amplitude of the MPW. Hence, the lower the pressure amplitude of the MPW at a measurement point, the higher the calculated value of \(k\) obtained. When the radiation radius is 20 m, the minimum \(k\) for Case 1 is located at measurement point F1. For Cases 2-4, the minimum \(k\) are all located at measurement point T3. Moreover, for Case 2 and Case 3, the \(k\) at T3 are both lower than that of Case 4. This indicates that measurement points near the openings of the hoods are more susceptible to pressure fluctuations. When the radiation radius is 50 m, the values of \(k\) exhibited a symmetric distribution along the sides and top of the hood. At the F1 location, the \(k\)-values showed the most variation with changes in the hood structure's opening ratio, indicating that the F1 measurement point is more susceptible to pressure fluctuations under the condition of a 50 m radiation radius. When \(k\geq 1\), it indicates that the amplitude of the MPW at that point is equal to or smaller than that on an infinite plane. The values of \(k\) for the measurement points in front of the hood structure's exit plane are all greater than 1, indicating that setting an opening in the hood can effectively disperse the pressure relief area of the compression wave, expand the radiating area of the MPW in front of the exit plane, and thereby achieve an alleviating effect on the MPW. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Azimuth Angle (\({}^{\circ}\))**} & \multicolumn{4}{c}{**The Opening Ratio of Hood (\%)**} & \multirow{2}{*}{**Observation Plane**} \\ \cline{2-2} \cline{4-5}  & **0** & **30** & **60** & **90** \\ \hline
90 & 0.8 & 0.8 & 0.8 & 0.9 & Top \\
45 & 1.2 & 1.2 & 1.3 & 1.4 & (Plane _zoy_) \\
0 & 1.2 & 1.3 & 1.4 & 1.6 & (Plane _zoy_) \\ \hline
45 & 1.2 & 1.1 & 1.2 & 1.4 & Side \\
90 & 0.8 & 0.8 & 0.8 & 0.9 & (Plane _zox_) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Correction coefficients of effective radiation solid angle under different opening ratios (\(r=50\) m).