## 2 Methods
### Details of Meshs and Calculations  
On the basis of the \"mosaic\" mesh technique, the computational domain of the model is meshed with a poly-hexcore mesh, and the final numbers of mesh cells for the two mesh models are approximately 46 and 71 million. Here, a mesh model comprising 46 million elements is utilized for the RNG \(k\)-\(\epsilon\) and IDDES simulations, while a mesh strategy with a total of 71 million elements is adopted in the LES computational model. This choice is based on the work in Section 2.5. Figure 3 presents the mesh model with a total number of meshes of 46 million in the IDDES method, and the zone away from the train is defined as the static mesh zone (A). In the dynamic zone (B), it is categorized into the fluid area (B2) adjacent to the sedan, the leading area (B1), and trailing layup areas (B3). The flow field data exchange is facilitated through the creation of interfaces between these zones. In the static zone, the smooth transition of the mesh size is realized by establishing two encrypted zones (C). The mesh sizes of encryption zones C1 and C2 do not exceed 0.014 \(H\) and 0.034 \(H\), respectively, and the mesh size in the most external zone is approximately 0.14 \(H\). The mesh size of the dune and the embankment is controlled within 0.034-0.068 \(H\), and 10 boundary layers are set on them.  
In the dynamic mesh zone, the majority of the area near surface of the sedan is occupied by a positive polygonal mesh, with an approximate size range of 6.81 \(\times\) 10\({}^{-4}\)-1.36 \(\times\) 10\({}^{-3}\)\(H\). Ten of the attached layers are set up on sedan surfaces (estimated, \(y+\) \(<\) 1). The \(y+\) value is used to distinguish the different velocity laws exhibited in different regions close to the wall in a turbulent flow. For the IDDES method, it is recommended to target a \(y+\) value within the range of 1 to 5 for the near-wall mesh cells. In practice, employing a \(y+\) value of around 1 for the IDDES simulations is often favored because it provides more accurate results in the near-wall region. However, a slightly higher \(y+\) value, around 5, can still maintain acceptable accuracy while reducing computational cost. In addition, to capture the flow characteristics near the wall, LES models generally require \(y+\) values close to 1. In the RNG \(k\)-\(\epsilon\) model, wall-modeled  
Figure 3: Mesh model applied to the IDDES computational models (B1, B2 and B3 are the leading area, the fluid area adjacent to the sedan, and the trailing layup areas, respectively; C1 and C2 indicate the #1 encryption zone and #2 encryption zone, respectively).  
turbulence modeling approaches generally require 30 < y+ < 300. Equations (1) and (2) provide the formulae for estimating the _y+_ value.  
\[y+=\frac{u_{*}y}{\
u} \tag{1}\]  
\[u_{*}=\sqrt{\frac{\tau_{\omega}}{\rho}} \tag{2}\]  
where \(u_{*}\), \(\
u\), and \(\rho\) are the friction velocity at the nearest wall (unit: m/s), the local kinematic viscosity (value: 1.8 \(\times\) 10\({}^{-5}\) Pa/s), and the density (value: 1.225 kg/m\({}^{3}\)) of the fluid, respectively; \(\tau_{\omega}\) and \(y\) represent the wall shear stress (unit: Pa) and distance to the nearest wall (unit: m), respectively.  
Table 1 presents the main parameter settings of the cases in this study. All simulations are conducted using a transient calculation method, with the crosswind velocity uniformly set at 20 m/s and the sedan's driving speed at 27.78 m/s. The time steps of the calculation are maintained at 5 \(\times\) 10\({}^{-4}\) s, and the Courant number is 0.694. Computational models are computed by commercial solvers, and the process is carried out at the Wuxi Supercomputing Center in China, using 144 cores for each model, taking a total of 26 days. The computation times of all cases are presented in Table 1.