## 4 Results and Discussion
### Flow Mechanism in Crosswind Conditions
The apparent high pressure zone only exists around the tail nose. Its low pressure zone around the windward side of the tail streamlined region and on the top is clearly larger than its high pressure zone. These facts are disadvantageous for reducing the side force and the lift. For TP3 who has a double-arch spindle structure with a front-mounted cowcatcher, the front of the cab glass is almost wide-flat, causing the flow to separate at the leeward side after passing by this region. As a result, the size of the high pressure zone around the cab glass grows and the low pressure zone leeward shrinks, decreasing the side force and the lift on the head. Lastly, for TP4 which is built in a wide-flat double-arch structure, its nose as long as 16 m can direct the air from the nose tip to the leeward side well. The low pressure zone at the leeward side of the head almost disappears and the high pressure zone moves back to the windward side of the cab glass. The flow separation line on top of the tail travels across the cab glass and then goes straight backwards. The area of the low pressure zone in the windward side drops and the high pressure zone on top of the streamlined region increases, which significantly restrains the lift and the side force both on the head and tail. Overall, the wide-flat double-arch structure is more feasible under crosswind circumstances, with its pressure distribution more beneficial to the reduction of the side force and lift on the head and tail. To explain why the pressure around the head streamlined region and around the tail streamlined region are asymmetrical, Figure 21 presents the instantaneous velocity  
Figure 20: Time-averaged pressure contours and streamlines on the head and the tail. contours of different sections and pressure contours of the head and tail for four models. Around the head streamlined region, the air is impeded by the windward side around the nose and gets stagnant. This phenomenon clearly slows down the air and creates a high pressure zone around the windward side of the stagnant point. After that the air carries on its way to the top at a growing speed, leading to a decrease of surface pressure to the negative zone. When the air arrives at the bottom of the leeward side, it interacts with the flow from the bottom and generates vortices. The nose streamlined structure determines how the air flows around the streamlined regions. Like the wide-flat structure, it is not an ideal barrier to the air flow in front of the nose and permits the air to go freely to the leeward side. It also hinders the air to accelerate on the top, narrowing down the low pressure zone leeward. On the other hand, for the single-arch ellipsoid which has a larger exposure to the wind, it expands the size of the low pressure zone on top of the train and the leeward side as the air flow speeds up quickly on top of the train. For the trailing streamlined region, the separation position deviates from the transition of the leeward side and the top to the windward side. As far as to the tail nose tip, the separation is located on the top of the windward side. This flow pattern with great separation forces the air to speed up windward and the velocity windward is far greater than that leeward, causing a powerful negative pressure zone windward and a positive pressure zone on top of the streamlined region and at its leeward side. In general, how the nose structure influences the flow mechanism around the tail copies the way it influences the head. In wind-free conditions, the side force of the head fluctuates around zero and the time-averaged side force on the tail is close to zero. However, in crosswind conditions, the wind destroys the flow field symmetry along the central axis of the train, resulting in the increase of the aerodynamic loads. Figure 22 depicts the side force in time domain on the head and tail for four models. It is clear that coefficients of side force (Cs) for all models' heads are considerable and pointing leeward. Their actual side force is nearly 60 kN.