## 4 Numerical Simulation Results and Discussions
### Variation of the Environmental Impact Radius with Ambient Wind Velocity  
Let \(r\) be the horizontal distance from the center of the upper vent. Then, \(C_{\text{max}}\) is set as the maximum concentration of pollutants on the cylinder, with the center of the upper vent as the origin at the radius \(r\). \(C_{\text{max}}\) of the horizontal outlet when the discharge velocity \(V_{0}\) is 6 m/s and the ambient wind \(U\) is 0 m/s is presented in Figure 17.  
The concentration limits of various pollutants in various ambient air function zones are indicated in \"Ambient Air Quality Standards\" (GB3095-2012) issued by the Chinese government. Based on this, 0.2 mg/m\({}^{3}\), i.e., the NO\({}_{2}\) hourly average concentration limit of the class II ambient air function zone, was adopted in this study as the environmental standard. Meanwhile, the distance at which the concentration of pollutants discharged from the upper vent decreased to 0.2 mg/m\({}^{3}\) was deemed as the environmental impact radius \(r_{\text{criti}}\) of the upper vent. Since the value of \(r_{\text{criti}}\) has a certain impact on both the planning layout of buildings around the tunnel and the valuation of the surrounding land, it is necessary to study the \(r_{\text{criti}}\) value of NO\({}_{2}\) emission. The \(r_{\text{criti}}\) of the vertical outlet when the wind velocity \(V_{0}\) was 6 m/s and the ambient wind \(U\) was 3 m/s is shown in Figure 18.  
The distribution of \(C_{\text{max}}\) of the vertical and horizontal outlets along the \(r\) direction under various ambient wind velocities \(U\), when the discharge velocity of the vent \(V_{0}\) was 6 m/s, is shown in Figure 19. Figure 20 shows the environmental impact radius \(r_{\text{criti}}\) of pollutants discharged from the vertical and horizontal outlets under different ambient wind speeds \(U\) when the discharge velocity \(V_{0}\) was 3 m/s, 4 m/s, 5 m/s, and 6 m/s. Figure 17: \(C_{\text{max}}\) diagram. Figure 18: Diagram of \(r_{\text{criti}}\). As can be observed from Figure 19a, \(C_{\max}\) gradually attenuated with the rising of \(r\) under various \(U\) for the vertical outlet. In particular, \(C_{\max}\) near the upper vent decreased at a faster velocity, whereas \(C_{\max}\) far from the upper vent diminished slowly. Moreover, for \(U\leq 3\) m/s, the larger \(U\) was, the greater the shear force of the lateral airflow suffered by the polluted airflow, the greater the horizontal diffusion, and the more significant the tail swing effect obtained in combination with the flow structure shown in Figures 9 and 10. Hence, the slower the attenuation in \(C_{\max}\), the larger the \(r_{\rm{criti}}\) (as shown in Figure 20). When \(U=1\) m/s, for instance, \(r_{\rm{criti}}\) was 9 m; when \(U=3\) m/s, \(r_{\rm{criti}}\) was 14 m, up by 56%. The attenuation rate of \(C_{\max}\) under various \(U\) values started to converge when \(U\geq 3\)m/s. At the same time, \(r_{\rm{criti}}\) also tended to be invariant according to Figure 20.  
As can be seen from Figure 19b, \(C_{\max}\) first remained unchanged with the rising of \(r\) under various \(U\) for the horizontal outlet, and then began to decrease when far from the upper vent (\(r\) was roughly greater than 10 m). In addition, \(C_{\max}\) under various \(U\) tended to converge at the scope approaching the upper vent. By contrast, \(U\) had a significant impact on the attenuation rate of \(C_{\max}\) at the scope far from the upper vent (\(r\) was roughly greater than 20 m), that is, the greater the \(U\) value, the better the contribution to the turbulent mix of pollution gas and ambient air. Together with the flow structure shown in Figures 14 and 15, the shorter the discharge jet length of each outlet, the faster the decrease of \(C_{\max}\), resulting in the smaller \(r_{\rm{criti}}\) (as shown in Figure 20). To be specific, when \(U=1\) m/s, \(r_{\rm{criti}}\) was 47 m; when \(U=4\) m/s, \(r_{\rm{criti}}\) was 28 m, reduced by 40%.