## 3 Scale Model Test  
The mathematical model was verified by the experimental results obtained with a 1/20 scale ventilation model tunnel with one upper vent from Zhejiang University. The schematic diagram of the model is shown in Figure 4. The length of the tunnel was 22 m,  
Figure 4: Schematic diagram of the boundary conditions: (**a**) external air area for the horizontal upper vent; (**b**) upper vent in the horizontal discharge pattern; (**c**) external air area for the vertical upper vent; (**d**) upper vent in the vertical discharge pattern. and the section size was 0.6775 m \(\times\) 0.35 m. There are two types of upper vents, namely, a horizontal vent discharging pollutants and a vertical vent discharging pollutants, which can be replaced and spliced according to the test requirements. The discharge area of the vertical outlet was 0.2 m\(\times\)0.2 m, and the area of the horizontal outlet was 0.1 m \(\times\) 0.1 m (four sides). In the model tunnel, the airflow entered into a region of quadratic resistance law [28] when the tunnel wind speed was greater than or equal to 2.5 m/s. In environmental impact analysis, NO\({}_{2}\) is often used as a primary indicator. However, because of the toxicity of NO\({}_{2}\), in this experiment we replaced NO\({}_{2}\) with CO\({}_{2}\), which is less harmful, cheaper, easy to obtain, and easy to measure. The pollutant-releasing device of the model test included a CO\({}_{2}\) cylinder, a CO\({}_{2}\) two-stage pressure-reducing valve (better pressure-stabilizing effect), and a vortex flow meter (DN200, CERTEON, China, Shanghai), as shown in Figure 5a. A stepless frequency conversion pipe fan was used to simulate axial flow fans (Figure 5b). A high-pressure variable-frequency fan was connected to the nozzle through a conductive pipe to simulate the jet fan (Figure 5c). The frequency converter was controlled by PLC 200 to start, stop, and regulate the speed of these model fans. The maximum fan volume in this model is 800 m\({}^{3}\)/h, and the maximum jet speed is 50 m/s. There are 2 jet fans and 1 set of top exhaust axial fans arranged in the model, as shown in Figure 5.  
Figure 6a shows the distribution of wind speed and the CO\({}_{2}\) concentration measurement points in the experimental model. In the experiment of pollutant diffusion from the vertical discharge outlet, a horizontal concentration sampling line was arranged at a height of 0.5 m from the ground, with a spacing of 0.05 m between the sampling points, for a total of 12 sampling points. When conducting the pollutant diffusion experiment at the horizontal discharge outlet, a horizontal concentration sampling line was arranged 0.09 m away from the ground, with a spacing of 0.3 m between the sampling points, using 10 sampling points in total, as shown in Figure 6b. Figure 6c shows the distribution of the wind speed measuring points on tunnel cross sections 1-1 and 2-2. Figure 5: Schematic diagram of the scale model: (**a**) pollutant release device; (**b**) axial fan; (**c**) jet fan simulator. During the test, there was no ambient wind outside the model tunnel. The test process involved several steps: (1) the jet fans number 1 and 2 were turned on to ensure the air flow into a region of quadratic resistance law, and various values of \(V_{0}\) were obtained by regulating the speed of the axial-flow fan. Then, we waited a few minutes to make sure the airflow speed in the tunnel was stable; (2) in the tunnel, CO2 was released 10 m away from the upper vent to ensure that it was evenly mixed at the upper vent, and various values of discharge concentrations were obtained by changing the CO2 releasing flow; (3) the Testo 425 thermal anemometer (Testo AG, Baden-Wurttemberg, Germany) with accuracy of \(\pm\)(0.03 m/s \(\pm\) 5% reading) was used to collect 2 min speed data at each measuring point, and the average value of the 2 min data was taken as the speed of the measuring point.