## 4 Results and Discussion
### Flow Mechanism in Crosswind Conditions  
The environmental wind is an essential factor that should be taken into consideration when designing a high-speed train. Under windy circumstances, the ambient flow field of the train will behave quite distinctly [20,21], increasing the aerodynamic loads on the train to a large degree and posing a threat to the safe operation. To study the nose shape streamlined structures and their influence on the aerodynamic performance of the high-speed train subjected to strong crosswind, the velocity of the crosswind is set as 20 m/s perpendicular to the train motion. The high-speed train runs at a speed of 350 km/h. In crosswind conditions the side force of the leading and trailing cars significantly increases and tends to be the main aerodynamic load on the train. Figure 19 shows the distributions of coefficients of accumulated side force for the four models at t = 2 s. It could be found that side force around the streamlined region takes the major part as compared with other regions and the distribution forms of the side force of the leading car and that of the trailing car are adverse. In terms of magnitude, side force on the head is much stronger yet that on the middle carriages is weaker. Meanwhile, the side force increases at regions with the bogies and windshields. Around the streamlined region on the head, TP2\({}^{\prime}\)s side force shifts most drastically, which rises to the maximum soon after the nose then goes down slowly to zero. Due to its shortest nose among all models, the side force of TP2 owns the shortest route from the peak to zero. TP1, TP3, and TP4 all share a same pattern of side force distribution that the force firstly drops after the nose then goes up to the top, and eventually goes down to about zero. For trailing streamlined regions four noses share the same side force behavior that the absolute side force ascends then descends after the same-cross section region on the train. In particular, TP4 has a much steadier form of side force performance and its maximum of absolute side force around the head and around the tail are smaller than the other models due to its longest nose shape. Figure 19: Curves of accumulated Cs at t = 2 s.  
Figure 18: Streamlines on the tail surface at different timesteps. From Figure 20 which shows the time-averaged pressure contours of four nose shapes, it can be discovered that the high pressure zone around the head stagnant point moves windward and a significant low pressure zone appears at the leeward side of the stream-lined region, which promotes the side force on the head. At the tail there is also an intensive low pressure zone leeward and a high pressure zone occupies the cab glass surface as well as the tail nose tip. This kind of pressure distribution is opposite to the pattern on the head, so is the side force of the tail. For the main body region with a uniform section area, an obvious low pressure zone emerges at the transition from the windward side to the top but has a rather small size, which exerts a negligible impact on the side force. The nose shape streamlined structures determine the pattern of the pressure covers. For TP1 shaped in a double-arch ellipsoid structure, the elliptical cab glass is the main feature for this structure. It blocks the windward flow around the head streamlined region to some degree and causes the area of the windward high pressure zone and that of the leeward low pressure zone to expand at the same time. The flow separation line above the tail streamlined region travels across the cab glass and the elliptical nose tip, which also marks the boundary of the high pressure zone and the low pressure zone. The high pressure zone ahead of the tail nose tip is larger than the low pressure zone around the cab glass, which suppresses the lift on the tail. TP2 gets a short nose with a single-arch ellipsoid structure. Even though the high pressure zone in the windward side is small, the low pressure zone on top of the train and at the leeward side is dominant in size and intensity.