## 2 Methodology
### Model Setting and Validation  
For the model setting, experiment results from former research were used for the validation of the model. Previously, the parameters of a typical subway platform were analyzed for the experimental model which was scaled down to \(1/20\) of the actual platform [35]. The underground subway station model that was used to measure the level of inundation in a flood situation is depicted in Figure 1a. Stairs from the ground to B1F are shown as P1234, two staircases to B2F are at the center of the model, and a flow supply device to simulate the rainwater inflow make up the experimental subway model. The total length of the model was 2.0 m and the width were 0.5 m, with the staircases leading to B1F having a width of 0.1 m and the staircases to B2F having a width of 0.2 m. Since typical subway station entrances are immediately connected to ground level, it was assumed in the creation of this experimental model that precipitation from urban roads would enter directly through the staircases. For the numerical simulations, the OpenFOAM model with RANS was used to run the two-phase flow using a Volume of Fluid (VOF) method to capture the interface between the two fluids which are air and water. The VOF method is used to track the interface between two immiscible fluids. It involves solving a transport equation for the volume fraction of one of the phases typically denoted as alpha. The volume fraction alpha varies between 0 and 1, where 0 indicates pure air and 1 indicates pure water. The momentum equation is solved to update the velocity field \(U\), which handles the coupling between velocity and pressure through the PIMPLE (merged PISO-SIMPLE) algorithm to ensure numerical stability and convergence. For the mesh setup, the former experiment was recreated with the same size with mesh resolution size at 5 to 10 mm, consisting of the top stairways to the B1F area shown in  
Figure 1: Plan view of the experimental and numerical model. Figure 1b. This accounted for the cells to be a find grid resolution (340 \(\times\) 100 \(\times\) 26), which discretization satisfies that the dimensionless wall height between 39.1 and 78.1 falls within 30 \(<\)\(z^{+}\) <200, which condition is prerequisite for using wall function for the solid-fluid boundaries of sidewalls and a channel bed. \(z^{+}=z_{b}\sqrt{\tau_{w}/\rho}/\
u\) is the dimensionless wall distance, where \(\tau_{w}\) is the wall shear stress, and \(z_{b}\) is the distance from the walls and channel bed to the center of the first grid cell from these wall boundaries. For this research, the flowrate of 3.78 \(\times\) 10\({}^{-4}\) m\({}^{3}\)/s for Q1 discharge and 4.92 \(\times\) 10\({}^{-4}\) m\({}^{3}\)/s for Q2 discharge that was used in the former experiment was used for the boundary conditions for the inlet for the comparison and validation of the numerical model. The measurement sections (Sec. A, Sec. B, and Sec. C) in Figure 1 are indicated by green lines which used the digital point gauge (PH-340, KENEK, Tokyo, Japan) to measure the water depth in the former experiment. These lines were chosen in order to examine the flooding characteristics of the subway station model for comparison between the experiment and the simulation. The boundary conditions for the numerical model used the specified flow rate above for the inlet, and the outlet for the central stairways which originally led to B2F used a zero-gradient boundary condition for the velocity and turbulence properties, and the pressure outlet boundary condition was assigned to be zero. At the walls, a non-slip boundary condition and wall functions were used for the velocity and turbulence variables, respectively. The time step was set to maintain the Courant number (\(Cr=U_{i}\Delta t/\Delta x_{i}\)) to be less than 0.5, and for model validation, the flow simulations ran until the variables of flow velocity and depth converged to a steady-state condition.