## 4 Numerical Simulation Results and Discussions
### Characteristics of the Air Flow in the Horizontal Pollution Discharge Pattern  
Figure 12 displays the flow structure and velocity vector with discharge velocity of \(V_{0}=6\) m/s without considering the ambient wind when pollutants were discharged horizontally at the upper vents. Without ambient wind, the upper side of the turbulent jet discharged from the horizontal outlet was not limited, and the jet boundary expanded continuously under transverse pulsation. The lower side of the jet was close to the ground (distance from the ground, 0.8 m). Due to the Coanda effect [29], the jet could not entrain air, the static pressure of the gas was low, and the static pressure of the upper air flow of the jet was high. The difference in upper and lower pressure led to jet bending and a gradual close of the jet axis to the wall surface, forming a wall boundary layer.  
Figure 13 presents the attenuation of the dimensionless axial velocity \(V_{\text{max}}/V_{0}\) of the jet discharged from each outlet without considering the ambient wind. Near the horizontal outlet, \(V_{\text{max}}/V_{0}\) decayed more slowly than that of a free jet, but faster than that of a three-dimensional wall jet. However, with the increase in the distance from the horizontal outlet, the attenuation rate of \(V_{\text{max}}/V_{0}\) was gradually consistent with that of a three-dimensional wall jet.  
Figure 12: (**a**) Flow structure and (**b**) velocity vector of the horizontal outlet when \(V_{0}=6\) m/s without considering the ambient wind.  
Figures 14 and 15 show the flow structure and NO\({}_{2}\) concentration distribution in ambient wind direction corresponding to positive \(x\)-direction and discharge velocity \(V_{0}\) = 6 m/s for ambient wind speeds \(U\) = 1 m/s, 2 m/s, 3 m/s, and 4 m/s. Figure 16 presents the attenuation of the dimensionless axial velocity \(V_{\max}/V_{0}\) of the jet discharged from each outlet by considering the ambient wind.  
Figure 14: Velocity profiles of the horizontal outlet when \(V_{0}\) = 6 m/s without considering the ambient wind: (**a**) \(U\) = 1 m/s; (**b**) \(U\) = 2 m/s; (**c**) \(U\) = 3 m/s; (**d**) \(U\) = 4 m/s.  
Figure 13: \(V_{\max}/V_{0}\) attenuation of the horizontal discharge jet without considering the ambient wind.  
Figure 16: Axial velocity attenuation of the horizontal discharge jet under different \(U\): (**a**) horizontal outlet 1; (**b**) horizontal outlet 2; (**c**) horizontal outlet 3; (**d**) horizontal outlet 4.  
Figure 15: NO\({}_{2}\) distributions of the horizontal outlet when \(V_{0}=6\) m/s without considering the ambient wind: (**a**) \(U=1\) m/s; (**b**) \(U=2\) m/s; (**c**) \(U=3\) m/s; (**d**) \(U=4\) m/s.  
velocity of the jet at each outlet decayed faster, and the jet length was shorter, as shown in Figure 16.  
Therefore, the phenomenon of pollutants discharging horizontally at the upper vent can be regarded as semi-confined buoyant jet diffusion under the momentum of the ambient wind. Moreover, the movement of pollutants in the horizontal direction was comprehensively affected by diffusion induced by concentration difference, jet momentum, and convection caused by the ambient wind.