## 4 Results and Discussion
### Flow Mechanism in Crosswind Conditions
To be specific, TP2's Cs is slightly greater than that of TP1 and of TP3 in terms of average and maximum. TP4's Cs is apparently lower than the other three. Additionally, from Figure 22b we notice that the side force on the tail points windward, and the tail Cs drops by an order of magnitude compared to the head Cs, implying that the tail side force is less influenced by the streamlined structure of the streamline. Figure 21: Velocity contours of different sections and the pressure contours of the train body (head section: x = \(-\)99 m, x = \(-\)95 m, x = \(-\)90 m, x = \(-\)85 m; tail section: x = 85 m, x = 90 m, x = 95 m, x = 99 m). Figure 23 shows the time histories of lift coefficients on the head and tail for four models. In crosswind conditions, lift on the head and on the tail are positive with the former slightly larger than the latter. Compared to the lift in wind-free conditions, the head lift in crosswind conditions is three times larger and the tail lift grows by an order of magnitude except TP4. In terms of the head lift, TP1 has the greatest magnitude while TP4 has the least. As for the lift of the trailing car, TP4 owns the least lift, with its time-averaged value being half of the others. TP3 has the greatest time-averaged value and TP1 has the most significant magnitude. Specifically, the peak of the lift of TP4 is near to the trough of the lift of TP1. The side force and lift on the head and tail in crosswind conditions are mainly determined by the ambient flow field features. Figure 24 shows the vorticity contours at different sections of four noses. It could be discovered that the energy of the flow field mainly gathers around the leeward side and vortex structures leeward are extraordinarily complicated and unsteady. A tiny vortex V1 is formed around the bottom of the head in the leeward side and as it moves towards the tail it gradually vanishes. Meanwhile the flow separation occurs around the transition from the leeward side to the top and another vortex V2 is generated. V2 also develops towards the tail and deviates from the leeward train. The vortex core of V2 core grows larger and larger, and then breaks down into smaller vortices with different intensity around the tail. The flow separation near the transition from the top of the main body to the leeward side breeds a vortex V3 around the leeward side of the train. V3 attaches closely to the train as it travels towards the tail. This behavior is the most influential to the train's aerodynamic performance. However, vortices at the leeward side of the head are not fully developed and are less significant in size and intensity. They are less important to the performance of the train. Yet vortices at the leeward side of the middle and the tail are maturely developed and are vital to the pressure distribution on the leeward train, thus, further affect the performance of the middle carriages and the trailing car. Due to TP2's shortest nose, V2 is formed at the closest place to the head nose tip. On the contrary, TP4 has the longest nose, so the V2's generation in this case is the other way around. Besides, V2's core grows up at the slowest speed as it moves to TP4's  
Figure 23: Cl on the head and tail in time domain: (**a**) head; (**b**) tail. Figure 22: Cs on the head and tail in time domain: (**a**) head; (**b**) tail. tail. In the middle of the train, vortices at the leeward side of the train are developed for all cases, regardless of different nose shapes. To further illustrate vortices at the leeward side of the train, Figure 25 shows the iso-surface of Q (Q = 100) for the four models, in which vortex labels are the same as in Figure 24. It could be found that V1 is generated in the leeward side around the first bogie of the leading car and grows larger as it travels downstream. It nearly disappears around the conjunction of CAR2 and CAR3. Therefore, V1 only affects the aerodynamic performance of CAR1 and CAR2 and its influence on other compartments can be ignored.