# Impact of Different Nose Lengths on Flow-Field Structure around a High-Speed Train
## 4 Results and Analysis
### Time-Averaged Flow  
#### 4.2.1 Time-Averaged Slipstream Velocity  
A slipstream is generated because a HST will drive the surrounding air to produce a complicated flow due to the boundary-layer effect. At present, HSTs can run at speeds of more than 300 km/h, and the resulting slipstream will threaten the safety of passengers on the platform and track-side workers, and cause damage to the trackside infrastructure [17; 22]. Therefore, a large number of scholars have studied the slipstream [15; 17; 21; 22; 23; 21; 23]. According to the Technical Specifications for Interoperability (TSI) (European Union Agency for Railways 2014) [32] and EN CEN European Standard 2009) [33], the horizontal velocity is harmful to persons and infrastructure along the line, so in this study, we mainly consider the horizontal velocity. The horizontal velocity is herein referred to as the \"slipstream velocity\" [17], and the dimensionless \"slipstream velocity\" \(U\) is defined as:  
\[U=\frac{\sqrt{\left(u-u_{train}\right)^{2}+v^{2}}}{u_{train}}, \tag{7}\]  
where \(u\) is the longitudinal velocity, \(v\) is the lateral velocity, and \(u_{\text{train}}\) is the speed of the HST. Figure 9 shows the positions of different measuring points relative to the TOR and COT, where in four points with heights of 1.6-2.8 m are the positions of the measuring points of the time-averaged pressure and the measuring points of \(U\) at heights of 0.2 and 1.4 m. At \(y\) = 3 m, \(z\) = 0.2 m and \(y\) = 3 m, \(z\) = 1.4 m, the two measurement positions are specified by the TSI (European Union Agency for Railways 2014) [32] and EN CEN European Standard 2009) [33]. Figure 10 shows a comparison of the time-averaged \(U\) at different nose lengths at a height of \(z\) = 0.2 m from the TOR and at different positions from the COT. As can be seen from the figure, there will be a peak value of \(U\) in the nose region, and with increasing nose length, the peak value in different  
Figure 8: Time-averaged aerodynamic (**a**) drag coefficients, and (**b**) lift force coefficients of the head and tail cars. Figure 9: Position of the measurement point relative to center of track (COT) and top of rail (TOR). positions will show a decreasing trend. In the near-wake region, there also will be a larger peak. Similarly, with increasing nose length, the peak value of different positions shows a decreasing trend. In the boundary-layer region, when the nose length is 4 m, \(U\) is larger at \(y\) = 2.5, 3, and 3.5 m, while, at \(y\) = 4 m, \(U\) is basically the same with different nose lengths. At different positions from the COT, the peak value of \(U\) with a nose length of 4 m is obviously larger than that of the other lengths. When the nose length increases to 9 m, the influence on the peak value of \(U\) in the near-wake region decreases. With increasing nose length, the peak value of the wake region of \(U\) tends to move backward. When the nose length increases from 4 m to 12 m, the peak value of the near wake region decreases by 55.4% at \(y\) = 2.5 m, 57% at \(y\) = 3 m, 53.7% at \(y\) = 3.5 m, and 52.2% at \(y\) = 4 m. With increasing nose length, the effect of weakening \(U\) in the near-wake region is obvious. To study the influence of position on the peak value of \(U\), a nose length of 4 m is taken as an example. The peak value of \(U\) is 0.307 at y = 2.5 m and 0.159 at y = 4 m, and the distance from the COT increases from 2.5 m to 4 m, decreases by 48.2%, and the value of the \(U\) decreases obviously, which indicates that the \(U\) decays faster with the increase of distance from the COT. Figure 11 shows a comparison of \(U\) at different nose lengths at a height of \(z\) = 1.4 m from the TOR and at different positions from the COT. Similar to \(U\) at the trackside position, \(U\) at the platform position also peaks in the nose region and near the wake region. However, the position of the maximum peak value of \(U\) is opposite to \(U\) on the trackside position, and the maximum peak value of \(U\) at the platform position appears in the nose region.