## 2 Experimental Study  
### Tunnel Model  
The prototype tunnel is the cross-harbor tunnel from Guangzhou to Zhanjiang, China, which exhibits the unique V-shaped characteristics. The ventilation flow field in tunnel construction can be simplified as a viscous constant and incompressible fluid flow. Therefore, the Reynolds number is used as a similarity criterion, with a critical value of \(5\times 10^{4}\) based on experience [27; 28]. The wind speed in the model must be at least 2 m/s to ensure similarity between the model and the prototype (the critical wind speed is calculated by \(Re=uD/\
u\), where \(Re\) is the Reynolds number; \(u\) is the fluid flow rate, m/s; \(D\) is the equivalent diameter of the tunnel model, m; and \(\
u\) is the air kinematic viscosity, m\({}^{2}\)/s, which is taken as \(1.57\times 10^{-5}\) m\({}^{2}\)/s here). Based on the similarity theory [24; 29] and considering the convenience of practical operation and realistic conditions, the similarity ratio in this study was set at 1:25. A tunnel model with a length of 20 m was constructed. The physical drawings and specific dimensions can be seen in Figure 2 and Table 1, respectively. The model consisted of four parts, which were the main body of the tunnel model, the ventilation system, the pollutant-generating device, and the data-monitoring system. Later experiments were conducted on this ventilation platform. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Types**} & **Inner Diameter of** & **Inner Diameter of the Tunnel** & **Distance from the Duct Outlet** \\  & **(m)** & **(m)** & **(m)** \\ \hline Prototype tunnel & 12.6 & 2.0 & 25/50/75 \\ Small-scale model & 0.50 & 0.08 & 1/2/3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dimensions of prototype tunnel and subscale model (1:25). Figure 2: Experimental model: (**a**) the main body of the tunnel model, (**b**) the ventilation system, (**c**) the pollutant generation system, and (**d**) the data-monitoring system. As shown in Figures 2a and 3, the main body was made of 10 sections of 2 m-long galvanized iron pipe, closed at one end and open at the other. The closed end was used to mimic the working face in an actual construction tunnel and the main body was placed on several height-adjustable supports in order to change the slope of the model. As shown in Figures 2b and 4a, the ventilation system included a PVC air duct, a fan capable of a maximum capacity of 2326 m\({}^{3}\)/h, and a frequency modulator. The air duct was suspended at the top of the model. The fan frequency was adjusted to control the wind velocity at the duct outlet through a frequency modulator. As shown in Figure 2c, the pollutant generation device included a CO cylinder, a pressure-reducing valve, and a flow meter with a range of 0.1-1 L/min. The pollutant generation device was connected to the closed end of the model by a hose, and CO could be injected into the model. Likewise, the data-monitoring system is presented in Figure 2d, which acquired both wind speed and CO concentration within the model. Wind speed detection consisted of a hot-wire anemometer and fixed Pitot tubes (see Figure 4b,c). The hot-wire anemometer was used to calibrate the exit velocity of the air duct. The Pitot tubes were used as fixed-wind-speed test points in the model and were arranged according to the equal-area method, and the Pitot tubes were coupled to micro-differential pressure transmitters, which were connected to data acquisition modules to transfer the data to a PC. As indicated by the red arrow in Figure 2, starting at 1 m from the closed end of the model, a wind speed measurement section was set up at 2 m intervals along the horizontal direction of the model, for a total of 10 sections. The detection of CO concentration was accomplished by a pumped CO detector (see Figure 4d).