# Influence of Posture Change on Train Running Safety under Crosswind
## 4 Results  
### Aerodynamic Responses  
Figure 7 shows the time-varying aerodynamic coefficients of each car during the train's run, from 100 m before the crosswind region to 100 m after the tail car exits. The speeds of the train and the crosswind were 83.33 m/s and 20 m/s, respectively, giving a yaw angle of 13.5\({}^{\circ}\). The time points when the corresponding car entered and left the crosswind region were given in each figure to explore the transient influence of the crosswind on the aerodynamics of the train. The results of the train with and without the consideration of posture were included in each figure to compare the aerodynamic differences. The range of the coefficient values of the sub-figures was maintained to facilitate the analysis of the different performances among various aerodynamic coefficients. The peak-to-peak value of the aerodynamic coefficients obtained when each car of the train entered and left the crosswind region was also calculated and shown in Table 2 and Figure 8, which quantified the transient fluctuations. The period when each car entered and left the crosswind region was captured--from 0.1 s before the corresponding car started to enter the crosswind region, to 0.1 s after it exited. Based on this, the periods for the leading, middle, and trailing cars entering the crosswind region were 1.100-1.609 s, 1.409-1.913 s, and 1.713-2.223 s, respectively, while the periods for exiting the region were at 3.500-4.010 s, 3.810-4.313 s, and 4.113-4.624 s, respectively. As can be seen in Figure 7a-c, the aerodynamic side force of different cars showed great differences. The value of \(C_{y}\) received from the head car to the tail car decreased in order. The \(C_{y}\) value of the head car in the crosswind region was increasing, and the growth rate was the fastest when it had just entered this region due to the increase in the blocking area against the crosswind. When the whole head car was in the crosswind region, the growth rate significantly decreased. When it left the crosswind region, the \(C_{y}\) value quickly dropped to 0, which was the same as before entering the region. The posture of the car head was proven to have a global effect on the \(C_{y}\) value of the leading car, whose rotations gave it a higher side force especially in the crosswind region; however, the \(C_{y}\) values during its exit from the crosswind region were almost the same. The same difference was found in the performance of the middle car, where the original car presented a slightly higher \(C_{y}\) value beyond the crosswind region, but showed a lower \(C_{y}\) value when it was within the crosswind region. Due to the development of the longitudinally flowing boundary layers, the intrusion of crosswind on the trailing car was protected to some extent, so that the trailing car gave much slighter fluctuations when it entered and left the crosswind region. From Figure 8, the peak-to-peak transient fluctuations of the \(C_{y}\) value of the separated cars were always higher when the posture was considered, except for when the tail car exited the crosswind region. There was no noticeable difference in the maximum lift acting on the leading, middle, and trailing cars, which was similar to that coming from the side force. A default posture combination weakened the aerodynamic lift force of the leading and middle cars, while it strengthened that of the tailing car. Additionally, the difference of \(C_{z}\) values between the trains with and without posture increased from the leading car to the trailing car. The peak-to-peak transient fluctuations of the \(C_{z}\) value of the separated cars were similar or slightly lower with a default posture, while that when the leading car entered the crosswind region increased, as can be seen in Figure 8.  
As a combined effect of the aerodynamic side and lift forces, the value of \(C_{M_{x}}\) acting on different cars passing through the crosswind region presented a similar trend to \(C_{y}\) values, as exhibited in Figure 7g-i.