## 4 Results and Discussion  
### Flow Mechanism for Trains Running in the Open Air without Crosswind  
The most frequent circumstance where a high-speed train operates is the static open environment. In this situation, drag of the whole train, the side force, and train-induced air flow represent the key variables to the aerodynamic performance of the train. Figure 8: The grids on the longitudinal section of the domain and the local components. Figure 7: Computational domain. The aerodynamic drag is firstly focused on. As a major part in the total drag of the train [3], the inviscid drag is determined by the pressure distribution on the train body. Figure 9 shows the contours of the time-averaged pressure on the head and the tail of the four models. The nose tip of the leading streamline is the stagnation point, which owns the highest pressure. However, for different streamlined structures, the distribution and area of high pressure in the stagnation zone differ from each other. The high pressure region of TP2 is the largest, while that of TP3 is the least due to the integrated design between the cowcatcher and the nose. Yet, for TP4 whose cowcatcher is moved backwards to behind the nose tip, a rather large high pressure zone is induced around the cowcatcher. To alleviate this problem an airflow-guiding design is adopted around the cowcatcher for E6's nose. As seen from Figure 10, it is obvious that there is a secondary high pressure zone at the transition from the nose to the cab window for the trains with double-arch structures. In contrast, TP2 with the single-arch shape has only one high pressure zone at the nose tip. Due to great bending of the cab window, the airflow accelerates rapidly, resulting in significant negative pressure over this zone, which is the reason why negative drag appears around this region for TP2. The third high pressure zone appears at the rear of the first bogie for each nose shape. That is because the air flows quickly underneath and then impinges hard on the back of the bogie cabin, generating a high pressure zone consequently. The pressure distribution of this area is mainly determined by how the air flows by the cowcatcher. For TP2 and TP4 whose air flows bilaterally by the train with the guidance of cowcatchers, there is less air impingement around the bogie thus less impact upon the rear of the bogie cabin. In light of this, to reduce the pressure force around the bogie region, some pumping devices could be mounted at the rear of the bogie cabin according to the pressure distribution. The pressure distributions on the head and tail remain the same, except that the pressure on the head varies more drastically. This kind of distribution also explains why the drag along the head and that along the tail act oppositely as mentioned above. Viscous force is the other major part of the total drag of the high-speed train. The smoother and straighter the train body is, the more viscous force contributes to the total resistance. Owing to great slenderness, the development of a turbulent boundary layer on the train surface affects the distribution of shear force to a large degree. When the nose  
Figure 10: Plots of pressure drag and shear force drag: (**a**) plots of drag on TP1 at t = 2 s; (**b**) distributions of shear force coefficients of four models along the train body. Figure 9: Contours of the time-averaged pressure on the head and the tail for different models. streamlined structure changes, the location where the flow separation occurs also alters, thus, the turbulent boundary layer will be influenced. Figure 10a shows the distributions of accumulated pressure force, shear force, and total drag along the train body of TP1. It can be seen that on regions with irregular components pressure force contributes more, while on regions where the cross section of the train remains constant, shear force takes the lead. The variation of the nose streamlined structure does not relate to obvious changes of the proportion that pressure force and shear force make up the total drag.