## 3 Results, Analysis, and Discussion
### Transient Flow Structures  
#### 3.2.1 Flow Structures on Flat Ground: \(t^{*}=47\)  
Before the position of WT at \(t^{*}=47\) (Figure 8a), the pressure profile on the train body is shown in Figure 8b. As shown in regions A and B, the pressure on the LWS was higher than that on the WWS, especially for the middle car. Thus, the \(C_{Fy}\) of the middle car had a maximum value of about \(-0.7\) at this time. For the head and tail cars, the \(C_{Fy}\) values were about \(-0.4\) and \(-0.2\), respectively. Based on the top and bottom pressure distributions, in regions C and D, the pressure at the top was higher than that at the bottom. Thus, the head car's \(C_{Fz}\) presented a negative value of about \(-0.1\) at \(t^{*}=47\). For the middle car in region E, the total pressure difference between the top and bottom was smaller, and the \(C_{Fz}\) value was close to 0. For the tail car in region F, the pressure of the top position was significantly less than that at the bottom. Consequently, the \(C_{Fz}\) of the tail car was positive and the largest of the three cars, with a value of approximately 0.4 at this time.  
The vortex around the train at \(t^{*}=47\) is shown in Figure 9. In Figure 9a, the second invariant of the velocity gradient, \(Q\), is used to show the vortices, which are coloured according to the vorticity concentration (\(\omega\)) [42]. Four vortices are evident around the train, combined with the space streamlines in Figure 9b, the vortex \(V1\) formed due to the space between the WWS of the head car, and the LWS of the windbreak. Vortex \(V2\), which  
Figure 8: Pressure distribution on the train surface at \(t^{*}=47\): (**a**) train position and (**b**) pressure distribution.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & & & & \multicolumn{6}{c}{**Transition Region:**} \\  & & & & & **Maximum Sudden** & & & **Cutting** & \\  & & & & & & **Peak Value** & & & \\ \cline{2-10}  & **Head Car** & **Middle Car** & **Tail Car** & **Head Car** & **Middle Car** & **Tail Car** & **Head Car** & **Middle Car** & **Tail Car** \\ \hline \(C_{Fy}\) & \(-0.20\) & \(-0.50\) & \(-0.10\) & \(1.00\) & \(0.30\) & \(0.48\) & \(-0.40\) & \(-0.22\) & \(0.00\) \\ \(C_{Fz}\) & \(-0.14\) & \(-0.02\) & \(0.38\) & \(0.21\) & \(0.36\) & \(0.40\) & \(0.02\)was generated from the lower position of the WWS of the train and \(V2\) in the rear half of the train, was stronger than that in the front half. After the tail car, the \(V3\)-\(V4\) generated from the tail car's nose position and \(V3\) was smoother than \(V4\). After a certain distance, vortices \(V2\), \(V3\), and \(V4\) mixed. A detailed view is shown in Figure 10; strong vortices were induced by the WT, and these vortices increased the side forces on the head and middle cars; however, at this moment, the tail car was influenced by backflows derived from the windbreak walls. Therefore, as shown in Figure 7(a1-a3), at \(t^{*}=47\), the \(C_{Fy}\) values of the head and middle cars increased after a step change. Figure 10 shows that the step change for the head car is more sudden, whereas that for the middle car is smoother. In particular, the impact of the vortex induced by the WT on the head car was the strongest and fastest. After the head car, the vortex streamlines impacted the middle car gradually, so the step change of \(C_{Fy}\) for the middle car was smoother. This phenomenon can be seen clearly in Figure 7.