## 4 CFD Simulations
### Boundary Conditions  
In general, air flow in an underground cavern is a complex three-dimensional turbulent flow [17; 24; 25]. However, the length of most caverns is much larger than their hydraulic diameter. It can be considered that the wind flow parameters are approximately evenly distributed in the cavern section, that is, the parameters such as the pressure, flow velocity and density of the wind flow are regarded as constants in the same section [26]. During construction ventilation, the air velocity in the cavern is under 7 m/s, and the air velocity in the ventilation duct is no more than 30 m/s. The diffusion of harmful gas caused by drilling and blasting is an unsteady process over time. Therefore, the following basic assumptions are used in the numerical calculation. (1) The air airflow field in the cavern is assumed as a three-dimensional, viscous and incompressible Newtonian fluid. (2) The air leakage rate of the duct and the pressure loss of the airflow in the duct are not considered. (3) The heat exchange between the airflow and the cavern wall is not considered and the heat dissipation caused by viscous fluid work is ignored. (4) The disturbance of the flow field induced by the equipment and obstacles in the cavern is not considered.  
According to the actual project situation, there are three types of boundary conditions in the simulations: wall, inlet and outlet [27; 28; 29]. (1) A non-slip stationary wall boundary is applied to the cavern surface, heading face and air duct surface, and the standard wall function method is used. The roughness constant (\(C_{s}\)) is set as 0.57 and the equivalent sand-grain roughness height (\(K_{s}\)) is used for roughness height, \(K_{s}=0.07\) m [30]. The distance from point \(P\) in the first near-wall cell to the wall (\(y_{p}\)) is 0.52 m. (2) The outlet of the air duct is considered to be a velocity-inlet with an airflow velocity of 15 m/s in Figure 5a. There are four velocity-inlets located at the entrance of the shaft in Figure 5b, the value is 5 m/s in the oil-inlet shaft and 10 m/s in the oil-outlet shaft. (3) The pressure outlet boundary is adopted at the entrance of the construction roadway in the two schemes.  
A large amount of gas is generated after blasting, and carbon monoxide (CO) and oxides of nitrogen are highly toxic. Because of the superior stability of chemical and physical properties of CO compared to nitrogen oxide, CO was used as an evaluation index in ventilation calculation. The amount of nitrogen oxide was converted to the equivalent carbon monoxide at a ratio of 1:6.5 [31]. The initial throwing length and average CO concentration were calculated by the following equation and are shown in Table 2.  
(1) The CO throwing length:  
\[L_{\text{OT}}=15+G/5 \tag{3}\]  
(2) The initial average CO concentration:  
\[C_{0}=\frac{G\times b}{L_{\text{OT}}\times A} \tag{4}\]  
Figure 6: Local grids. (**a**) Forced ventilation scheme; (**b**) Axial-flow gallery ventilation scheme.  
where, \(C_{0}\) is the initial CO concentration (mg/m\({}^{3}\)), \(G\) is the amount of explosive (kg), \(L_{\mathrm{OT}}\) is the throwing length (m), \(b\) is toxic gas produced per kilogram of explosive (m\({}^{3}\)/kg), \(b\) is generally valued at 0.04, and \(A\) is the excavation area (m\({}^{2}\)).