## 3 Results, Analysis, and Discussion
### Transient Flow Structures
#### 3.2.2 Flow Structures in the WT Region: \(t^{*}=65\)-\(48\)  
At \(t^{*}=65\), when the head car operated in the region of WT, as shown in Figure 11a, there was a region of strong airflow that rushed into the railway in region A and induced an intense positive pressure on the head car's WWS. The head car's LWS was surrounded by negative pressures, so the \(C_{Fy}\) value of the head car (Figure 7(a1)) suddenly became a positive peak value during this time. However, the middle car and tail car were still under the protection effect of the windbreak, so the \(C_{Fy}\) of the middle car was negative and similar to that at \(t^{*}=47\), and the \(C_{Fy}\) of the tail car was also negative and slightly larger than that at \(t^{*}=47\) due to the impact of extra airflow from the WT region. Figure 10: Top view of streamlines at a height \(0.5\) h.  
Figure 9: Flow structures around the train at \(t^{*}=47\): (**a**) vortex distribution (\(Q=5000\)) and (**b**) space streamlines. The detailed pressure distribution on the train is shown in Figure 11b. Overall, in WWS regions A and B of the head car, the pressure was higher than that in LWS region C. The \(C_{Fy}\) of the head car was positive. For the middle car (region D) and tail car (region E), it was evident that the LWS pressure was higher than that at the WWS, so their \(C_{Fy}\) values were still negative. However, because the tail car's LWS nose part was dominated by strong negative pressures, the \(C_{Fy}\) of the tail car was less than that of the middle car. The head car's top pressure became negative and lower than that at the bottom. Thus, the \(C_{Fz}\) of the head car suddenly became positive in the WT region. In region G of the middle car, the difference between the top and bottom was smaller, and \(C_{Fz}\) of the middle car was still near zero, similar to that at \(t^{*}\) = 47. For the tail car, although the pressure difference for the car body was slight, the pressure on the top of the nose was less than that at the bottom. This resulted in a higher positive \(C_{Fz}\) value for the tail car, similar to that at \(t^{*}\) = 47. Figure 12a depicts flow structures derived from the train when the head car ran in the WT region At \(t^{*}\) = 65. There were new vortices induced by the WT, as indicated by the circle. Furthermore, Figure 12b shows the source of the airflow from the WT. There were mainly two sources. One was from the right-angle region. The direction and magnitude of this part of the airflow changed suddenly near the right-angle structure, and this part of the airflow directly impacted the train, generating vortex \(V10\). The other source is the top position of the slope. The height of the windbreak was insufficient, and this part of airflow rushed into the railway and directly impacted the train, generating vortex \(V11\). Combined with the information presented in Figure 12b, the positions of vortices \(V10\) and \(V11\) corresponded to regions B and A of the head car, respectively. The impact of these two vortices explains why the head car's \(C_{Fy}\) showed a sudden peak value in the WT region. For the detailed flow structures, Figure 13 shows a top view of streamlines around the train and further explains why the head car's \(C_{Fy}\) presented a positive change, whereas those of the middle car and the tail car had negative values. Because the entire train was surrounded by a region of backflow from the WT area, the \(C_{Fy}\) values of the middle car and the tail car were higher than their corresponding values at time \(t^{*}\) = 47. Figure 11: (**a**) The distribution of the wind speed coefficient (\(U\)) at a height of 0.5 h and (**b**) pressure distribution on the train surface. Figure 12: Flow structures around the train at \(t^{*}\) = 65: (**a**) vortex distribution (\(Q\) = 5000) and (**b**) space streamlines. Figure 14 shows the flow field at \(t^{*}=75\) and \(t^{*}=84\), respectively, as the train moved through the WT region.