## 4 Conclusions  
In this paper, the ventilation safety in the construction of a large-slope V-shaped tunnel was investigated through a series of small-scale experiments for the first time, and the coupled effects of ventilation parameters and structural characteristics on construction tunnels, which have seldom been considered together, were taken into account. The effects of four variables, namely digging distance (\(L_{d}\)), the distance from the outlet of air duct to the closed end (\(L_{0}\)), the slope of the uphill section (\(\theta\)), and air supply speed (\(u_{0}\)), on the ventilation flow field and pollutant transport patterns in a V-shaped tunnel were investigated. The primary conclusions from this work are summarized as follows:  
1. The order of importance of the effect on the magnitude of the air return velocity is: \(u_{0}>L_{d}>\theta\approx L_{0}\). Where increases in \(u_{0}\) and \(L_{d}\) cause \(u_{\rm return}\) to increase and decrease, respectively. \(\theta\) and \(L_{0}\) have no significant effect on \(u_{\rm return}\) to a limited extent. The \(u_{\rm return}\) as a function of \(u_{0}\) is derived. 2. Pollutant transport characteristics of V-shaped tunnels and flat tunnels were compared. The peak of CO concentration along the longitudinal direction of the model does not decay unidirectionally as in the case of flat tunnels but shows fluctuating decay. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{\(u_{0}\)} & \multicolumn{6}{c}{**Stage I**} \\ \cline{2-7}  & \(f_{\bf 1}(t)\) & \(f_{\bf 2}(t)\) & \(f_{\bf 3}(t)\) & \(f_{\bf 4}(t)\) & \(k_{\bf 1}\) & \(R^{\bf 2}\) \\ \hline
4 m/s & \(0.011296t/T_{0}-\) & \(-0.85433t/T_{0}+\) & \(-10.0323(t/T_{0})^{2}+\) & \(30.48233(t/T_{0})^{2}-\) & \(-0.03046\) & \(0.892\) \\
6 m/s & \(0.01009t/T_{0}-\) & \(-0.13118t/T_{0}+\) & \(-2.95656(t/T_{0})^{2}+6.12402t\) & \(16.52183(t/T_{0})^{2}-\) & \(-0.0015\) & \(0.924\) \\
8 m/s & \(0.01396\) & \(0.21549\) & \(-3.31801\) & \(38.29427t/T_{0}+\) & \(16.88465\) & \(-0.0015\) & \(0.924\) \\
8 m/s & \(0.08281t/T_{0}-\) & \(-0.6216t/T_{0}+\) & \(-5.59428(t/T_{0})^{2}+\) & \(13.52062(t/T_{0})^{2}-\) & \(-0.0498\) & \(0.884\) \\
0.11163 & \(0.89907\) & \(12.78034t-7.3066\) & \(33.28801t/T_{0}+16.15787\) & & & \\
10 m/s & \(0.04982t/T_{0}-\) & \(-0.30228t/T_{0}+\) & \(-4.02468(t/T_{0})^{2}+9.31002t\) & \(9.88393(t/T_{0})^{2}-\) & \(-0.00224\) & \(0.935\) \\
0.07595 & \(0.53842\) & \(-5.36407\) & \(26.43139t/T_{0}+13.14085\) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: CO transport function for stage I of the diffusion zone. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{\(u_{0}\)} & \multicolumn{6}{c}{**Stage II**} \\ \cline{2-7}  & \(f_{\bf 5}(t)\) & \(f_{\bf 6}(t)\) & \(f_{\bf 7}(t)\) & \(k_{\bf 2}\) & \(R^{\bf 2}\) \\ \hline
4 m/s & \(0.14269t/T_{0}-\) & \(-1.64291(t/T_{0})^{2}+3.02047t/T_{0}-\) & \(6.99498(t/T_{0})^{2}-20.7725t/T_{0}+\) & \(-0.02483\) & \(0.909\) \\
0.19804 & \(0.26269\) & \(6.82536\) & \(-0.02483\) & \(0.909\) \\
6 m/s & \(0.22029t/T_{0}-\) & \(-1.13271(t/T_{0})^{2}+0.073205t/T_{0}+\) & \(6.79006(t/T_{0})^{2}-18.2798t/T_{0}+\) & \(-0.03139\) & \(0.933\) \\
0.29843 & \(1.67585\) & \(5.34493\) & \(-0.03139\) & \(0.933\) \\
8 m/s & \(0.06615t/T_{0}-\) & \(-0.31577(t/T_{0})^{2}+0.17934t/T_{0}+\) & \(2.3919(t/T_{0})^{2}-9.71992t/T_{0}+\) & \(-0.03853\) & \(0.950\) \\
0.09061 & \(0.808\) & \(3.34618\) & \(-0.03853\) & \(0.950\) \\
10 m/s & \(0.02331t/T_{0}-\) & \(-0.41196(t/T_{0})^{2}+1.18699\) & \(t/T_{0}-\) & \(0.91471(t/T_{0})^{2}-7.17993t/T_{0}+\) & \(-0.05098\) & \(0.950\) \\
0.02105 & \(0.60127\) & \(3.69721\) & \(-0.05098\) & \(0.950\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: CO transport function for stage II of the diffusion zone. Unconventionally high pollutant concentration occurs in the horizontal section of the model, making the V-shaped tunnel have critical ventilation areas other than the working area. 3.