# Impact of Different Nose Lengths on Flow-Field Structure around a High-Speed Train
## 4 Results and Analysis
### Flow-Field Visualization  
#### 4.4.1 Slipstream and Pressure  
The slipstream velocities at 0.2 and 1.4 m from the TOR and different positions from the COT are quantitatively analyzed. Next, the contour plots of \(U\) at 0.2 m from the TOR will be studied. The time-averaged \(U\) and instantaneous \(U\) at a distance of 0.2 m from the TOR are shown in Figure 16. The flow field is divided into two regions: the boundary-layer region (covering the train) and the wake region. It can be seen from the Figure 16a that the time-averaged \(U\) and the instantaneous \(U\) near the nose of the tail car are relatively large. The nose length has a great influence on the development of the flow field along the train, especially on the wake-flow field. When the nose length is 4 m, the time-averaged \(U\) appears as a \"bifurcation\" in the wake region, but when the nose length is 7, 9, and 12 m the \"bifurcation\" is not obvious. This may be why the peak value of the time-averaged \(U\) for a nose length of 4 m is significantly larger than that of the other lengths in the wake region. Figure 16b shows an instantaneous \(U\) flow field. Similarly, in the wake region, a nose length of 4 m fluctuates more violently in the lateral direction, and the lateral pulsation decreases obviously with increasing nose length. Figure 16: Slipstream velocity at height of 0.2 m from TOR. (**a**) Time-averaged \(U\), and (**b**) instantaneous \(U\) (from top to bottom: 4, 7, 9, and 12 m. Black dashed line is 3 m from COT. ). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Length (m)**} & \multicolumn{3}{c}{**Without 1s MA**} & \multicolumn{3}{c}{**With 1s MA**} \\ \cline{2-7}  & - & \(\mathbf{U_{\text{P}}}\) & \(\mathbf{\sigma}\) & - & - & - \\ \hline
4 & 0.2643 & 0.1762 & 0.6167 & 0.1665 & 0.0471 & 0.2607 \\
7 & 0.1995 & 0.1579 & 0.5189 & 0.1447 & 0.0372 & 0.2191 \\
9 & 0.1275 & 0.1085 & 0.3445 & 0.113 & 0.0441 & 0.2009 \\
12 & 0.127 & 0.1015 & 0.33 & 0.1127 & 0.0372 & 0.1874 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Peak values of gust measurement without and with 1 s MA. Figure 17 shows the time-averaged \(U\) of vertical planes 1 \(H\), 2 \(H\), 3 \(H\), 4 \(H\), 5 \(H\), and 6 \(H\) from the tail of the train. Because the left- and right-hand sides are symmetrical, half of the plane is selected for the study. It can be seen from the figure that the time-averaged \(U\) in the vertical plane near the COT (RA) is larger than other regions (RB) with different nose lengths, and the RA shows the law of continuous downward and outward development, which is similar to the development of wake vortices [17]. At 1\(H\), when the nose length is 4 m, the area of the RA is obviously larger than that of other nose lengths, so that the RA is closer to the trackside position and platform position, and causes the time-averaged \(U\) of the train with a nose length of 4 m to be greater than that of the other nose lengths. At 2 \(H\), 3 \(H\), 4 \(H\), 5 \(H\), and 6 \(H\), there is a similar law. When the nose length of the train is 4 m, the RA already reaches the platform position, while the other nose length of the RA is only constantly approaching. This is also the reason for the law illustrated in Figure 10, in which the time-averaged \(U\) with a nose length of 4 m is significantly larger in the wake region. Figure 18 shows the contour plots of pressure on the surface of the train and plane at 1.6 m from the TOR. It can be seen from the figure that the streamlined length has a great influence on the distribution and amplitude of the surface pressure in the nose area of the train. With increasing nose length, the negative pressure concentration area at the junction of the streamlined and the non-streamlined nose  
Figure 17: Time-averaged \(U\) of vertical planes 1 \(H\), 2 \(H\), 3 \(H\), 4 \(H\), 5 \(H\), and 6 \(H\) from tail of train (hollow pink square: platform position; hollow pink circle: trackside position.). of the head and tail cars moves backward.