## 4 Results and Discussion
### Flow Mechanism for Trains Running in the Open Air without Crosswind
Figure 14 shows the velocity contour of the longitudinal section around the first bogie region and pressure contour of the first bogie. It could be seen that after flowing by the stagnant zone ahead of the nose tip and the coovcather, the flow accelerates within the clearance. Due to the sudden expansion of free space in the cabin, more flow separation could be observed. Stagnation could also be observed on the front of the bogie while most of the flow gets inside the cabin and becomes more turbulent. A high pressure region is generated at the rear of the cabin due to the impingement of the flow. Specifically, the guidance effect of the coovcatcher design plays an important role in the aerodynamic performance of the system. The flow is guided along the train surface until it reaches the bogie region. If the sides of the cabin shrink too much, just as TP1 shows, it would be much easier for the guided flow going inside the cabin and further generating larger drag on the cabin. For TP2, the sides of the bogie cabin shrink less compared to TP1. Consequently, less flow could impinge on the cabin, and less high pressure region around the cabin could be observed. Due to the integration design of the coovcatcher and the streamline for TP3,  
Figure 13: Time-averaged Cd of the bogie system. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Models** & \(\mathbf{C_{dw}}\) & \(\mathbf{C_{d1}}\) & \(\mathbf{C_{d2}}\) & \(\mathbf{C_{d3}}\) & \(\mathbf{C_{d4}}\) & \(\mathbf{C_{d5}}\) & \(\mathbf{C_{d6}}\) & \(\mathbf{C  
the stagnation point lowers down to the region near the bottom of the streamline. Thus, more flow gathers around the clearance and lateral sides of the train, resulting in much higher pressure and a larger high pressure zone in the rear of the cabin. For TP4, it could be seen clearly that the flow is guided away from the bogie cabin, so that no high pressure region could be observed in the cabin. Three key design variables that influence the flow details around the bogie region could be found out, which are the streamlined structure of the streamline, the height of the clearance, and the distance between the front of the cowcatcher and the front of the bogie cabin. When designing the bottom structures of the nose, the main idea should be decreasing the amount of air entering the bogie cabin. For many engineering practices a design of a fully-wrapped apron board around the bogie is applied, yet this scheme hinders the detection and maintenance of the bogie. A more practical scheme should be trying to find the balance point among the three key variables. The train-induced wind is also a key index to evaluate the aerodynamic design of a high-speed train [33; 34] and the nose shape streamlined structure affects the train-induced wind substantially. This study refers to the train-induced wind regulated in TSI [33] at standard positions (z = 0.2 m, y = 3 m; z = 1.44 m, y = 3 m) to contrastively analyze how train-induced wind reacts to the nose streamlined structures. Figure 15 shows the curves of time-averaged velocity of the train-induced wind at standard positions for the four models. It is obvious that the maximum wind appears in the wake flow field. At the same position, the velocity of the train-induced wind grows when near the ground. The streamlined structure of the streamline does not evidently affect the train-induced wind around the head but significantly disturbs that around the tail. At z = 0.2 m, the wind velocity firstly oscillates at the streamlined region on the head. After that the train-induced wind grows steadily until reaching a peak as near as the maximum at the middle of the train. Then, the wind velocity unsteadily drops to the secondary minimum around the trailing nose tip. The wind again ultimately speeds up to the maximum in the wake flow field about 25 m away from the trailing nose tip. The maximum velocities of the train-induced wind of TP1, TP2, TP3, and TP4 are 12.6, 15.1, 15.1, and 17.3 m/s, respectively.