## 4 Results and Discussion
### Flow Mechanism in Crosswind Conditions
V2 comes into being tightly around the main body with a uniform section, then moves away from the train. The vortex core of V2 magnifies gradually. Until the middle of the train it ceases to change its distance to the train and then travels parallel to the train and eventually enters the wake flow field. V3 emerges at the middle of CAR2 and stays close to the train as it propagates. It interacts with other vortices generated around the trailing streamlined region, creating an intricate trailing vortex system. V4 is generated out of the flow separation around the trailing nose and then moves leeward. V4 influences the aerodynamic performance of the trailing car the most while its impact on other parts of the train are so small that it can be neglected. The streamlined structure of the nose shape is the least important to V1. The longer the nose is, the nearer V2 is born to the tail. Its vortex core near the head gets smaller as the nose grows longer. As V3 comes out of the flow separation around the transition from the top of the main body to the leeward side, the streamlined structure of the nose does not affect V3 to a large degree. Yet, the nose streamlined structure is most crucial to V4. For example, after generation at the trailing nose tip on TP1 and TP4, V4 gets mixed up with vortices at the leeward side straightforward, resulting in the breakdown of large vortices into small ones at the rear of the wake flow field. As for TP2, V4 moves windward after being formed at the trailing nose tip, then mixes with vortices at the leeward side. Apart from that, large vortices of TP2 remain complete in the wake flow field. For TP3, after V4's generation around the trailing nose tip, V4 goes parallel to the wake flow field and does not interact that much with vortices leeward. Figure 24: Vorticity contours at different sections at t = 2 s.  
Figure 25: Q contours around the trains (Q = 100). In crosswind conditions the aerodynamic forces on the train are massively increased while the loads on the leading car keep the major part. As a consequence, the safety of the head is of great vitality to the overall design of the train. Table 4 lists the time-averaged coefficients of drag on the whole train (Cdw), the side force coefficient (Cs), and the lift coefficient (Cl) of the leading car. It can be discovered that Cdw under crosswind circumstances increases by 50% in contrast with that in a static environment for all four models. Cdw on each model does not differ from each other obviously. Cs of the leading car for TP2 is the greatest while that of TP4 is the least, with the latter decreasing by 9% compared to the former. Cs of the leading car for TP1 is almost the same as that of TP3. Meanwhile, Cl of the leading car is nearly half of Cs for the four models, among which TP2 is the biggest while TP3 is the smallest, dropping by 11.4% compared to the former. TP4 and TP3 have similar head Cl while TP1 and TP2 have close head Cl as well. Generally speaking, the wide-flat double-arch structure can better improve the aerodynamic performance of the high-speed train in crosswind conditions.