# Dynamic Response of Outer Windshield Structure in Different Schemes under Aerodynamic Load
## 2 Methodology of Simulation
### Meshing Strategy and Brief Description of the Solver  
In order to reduce the influence of grid settings on the aerodynamic effect analysis of the outer windshield of different schemes, the grid settings are consistent for the calculation models of the outer windshield of the three schemes. The solid model and the fluid model were discretized using the elements that are available in the software Abaqus and STAR-CCM, respectively. Details of the computational grid used for the fluid model and the solid model are shown in Figure 4. Ten prism layers were applied at the wall of windshield and train model, with a total thickness of four millimeters [32; 33]. As shown in Figure 4a, a refinement box was built around the full-scale train model, and the external windshield area for local mesh encryption. For the full-scale train model, the smallest mesh used on the outer windshield surface had a size of 4 mm, the y+ around the train ranging from 0.1 to 10 [16; 19]. To verify the accuracy of the grids and to avoid the influence of mesh resolution on the resulting analysis, a grid sensitivity test was performed for a train speed of 350 km/h [14; 19].  
Figure 3: Boundary conditions and computational domain: (**a**) fluid model, (**b**) boundary conditions of FSI.  
This study carries out a mechanical co-simulation using STAR-CCM+ and Abaqus. The mechanical co-simulation differs from the file-based method; in the co-simulation, data are automatically exchanged between Abaqus and STAR-CCM+ using the SIMULIA Co-Simulation Engine. Co-simulation involves a strong coupling between the two codes. Data are exchanged at frequent intervals that are called coupling steps. This level of communication between the solvers allows for obtaining a full solution across the fluid-solid interface. STAR-CCM+ and Abaqus run simultaneously. In mechanical coupling, the fluid domain passes traction loads to the solid domain (pressure and wall shear stress), and the solid domain passes displacements to the fluid domain. In the solid domain, the traction loads are applied to the surface of the solid structure. In the fluid domain, the displacements are used as an input to the mesh morber [34]. For the FSI simulation, the relevant parameters are shown in Table 1. The simulation time step is determined according to the minimum volume grid distribution around the train body and the incoming flow velocity, which ensures that the CFL of the numerical calculation in this study is less than 1. Owing to the large deformation of the U-shaped rubber external windshield structure under the aerodynamic load, the relative movement between molecules in the structure is small, that is, the rubber molecular chain of the external windshield only bears a relatively small deformation, and the corresponding stress can be well approximated from the traditional elastic analysis. Based on this, we assume that the rubber material used in the external windshield is linear elastic and isotropic [35]. The comparison between the full-scale test results and the numerical simulation results shows that the small deformation assumption for the rubber external windshield has certain applicability in the problems studied in this paper. This study assumes that the rubber external windshield structure has linear elastic characteristics under the aerodynamic load, and takes into account the influence of structural damping. Additionally, the Rayleigh damping factor is considered to be alpha = 0.125 and beta = 0.00159.