## 3 Numerical Simulation Results and Discussions  
### Flow Characteristic  
Figure 7 depicts the sketches of three-dimensional velocity profiles of jet flow at a three-lane tunnel exit. It can be observed that after the air is pushed out from the tunnel exit, due to the lateral fluctuation on the turbulence, the moving wind exchanges momentum with its surrounding stagnant air and drives the surrounding air molecules to move to produce an entrainment effect, thus continuously expanding the jet flow cross-section area. The three-dimensional wall jet can be categorized into the potential core region and decay region, depending on whether the maximum velocity \(u_{\text{max}}\) decreases. The decay of nondimensional \(u_{\text{max}}\) is shown by the solid line in Figure 8. It can be seen from Figures 7 and 8 that at the potential core region (where \(x\leq 60\) m), the jet flow fiercely entrains the surrounding air, thereby expanding the jet flow boundary and the flow rate altogether, while reducing the air velocity at the same time, but the \(u_{\text{max}}\) remains the same. At the decay region (where \(x>60\) m), with further development of the jet-flow entrainment and mixing effects, the \(u_{\text{max}}\) starts to reduce and the velocity profile also tends to flatten out. Figure 6: Impacts of outlet air velocity \(u_{\text{out}}\) on \(\varphi_{\text{c}}\). Figure 7: The sketches of three-dimensional velocity profiles of jet flow at tunnel exit. Figure 9 depicts the flow structure at the portals of twin tunnels with small spacing, of which Figure 9a-f show the flow structures under different staggered distance \(\Delta l\) when the lateral distance of the tunnel portals \(d=5\) m, inlet air velocity \(u_{\text{in}}=7.5\) m\(\cdot\)s\({}^{-1}\), and outlet air velocity \(u_{\text{out}}=1.5\) m\(\cdot\)s\({}^{-1}\). When \(\Delta l\leq 0\), the jet flow at the outlet deflects entirely under the influence of the negative-pressure zone at the inlet, as shown in Figure 9a-c. In the meantime, by referring to Figure 8a, it can be noticed that, different from the standard three-dimensional wall jet, the \(u_{\text{max}}\) at the moment begins to attenuate when \(x>0\) (the bigger \(\Delta l\) is, the quicker the attenuation will be); when \(\Delta l>0\), we can learn from Figure 9d,f and Figure 8b that the jet flow structure at the outlet when \(x\leq\Delta l\) and the law of \(u_{\text{max}}\) attenuation are both similar to those of the standard three-dimensional wall jet. However, when \(x>\Delta l\), the jet flow will deflect under the influence of the negative-pressure zone at the inlet, for which \(u_{\text{max}}\) will fall rapidly at a decay rate first increasing and then decreasing along with the increase of \(\Delta l\). Moreover, it can be perceived from the comparison between Figure 9g-h that the bigger \(u_{\text{out}}\) is, the bigger the inertia effect of the jet flow will be, and the less vulnerable it will be to the influence of the negative-pressure zone at the inlet and the fuller the jet flow will develop. In this way, the polluted air flowing into the adjacent air-inlet tunnel through the air-exhausting tunnel will be reduced. According to Figure 9g-i, when \(u_{\text{in}}\) increases, the bigger the influencing area of the negative-pressure zone is, the bigger the damage caused to the flow structure at the outlet will be, thus resulting in more polluted air flowing into the adjacent air-inlet tunnel through the air-exhausting tunnel. Finally, through comparing Figure 9i with Figure 9j, the bigger \(d\) is, the smaller the damage caused by the negative-pressure zone at the inlet to the flow structure at the outlet will be and the fuller the jet flow will develop; when \(d=20\) m, there is almost no polluted air flowing into the adjacent air-inlet tunnel through the air-exhausting tunnel.