# Impact of Different Nose Lengths on Flow-Field Structure around a High-Speed Train
## 5 Conclusions  
In this study, the IDDES method is used to study the flow field around a HST with different nose lengths (4, 7, 9, and 12 m). The research includes time-averaged and instantaneous slipstream velocity, time-averaged pressure, wake flow, and aerodynamic force. From the results of the aforementioned factors, we can draw the following conclusions:  
1. When the nose length increases from 4 to 7 m, the drag and lift force coefficients change obviously, in which the head car drag coefficient decreases by 17.6%, the tail car drag coefficient decreases by 29.3%, the head car lift force coefficient decreases by 15.8%, and the tail car lift force coefficient decreases by 75.7%. When the nose length increased from 7 to 12 m, the difference between drag and lift force coefficients was not obvious.
2. With increasing nose length, the peak value of time-averaged \(U\) in the same position decreases. When the nose length increases from 4 to 12 m, the peak value of time-averaged \(U\) in the trackside position decreases by 57%, and the peak value of time-averaged \(U\) in the platform position decreases by 19.5%. With increasing distance from the COT, the peak value of time-averaged \(U\) decreases. The maximum \(U\) value of the trackside position appears in the near wake, while the maximum \(U\) value of the platform position appears in the nose region. The slipstream is usually evaluated by characteristic velocity _U\({}_{2\sigma}\)_. In this study, the characteristic velocity _U\({}_{2\sigma}\)_ of the trackside position is studied. It is found that the characteristic velocity _U\({}_{2\sigma}\)_ decreases with increasing nose length. When the nose length increases from 4 to 12 m, the _U\({}_{2\sigma}\)_ value decreases by 28.1%.
3. For the measuring points of trains with different nose lengths in different positions, the maximum value of _\(\Delta C_{P}\)_ appears in the nose region. With increasing nose length, the _\(\Delta C_{P}\)_ values of the measuring points at different positions decrease continuously. For _\(\Delta C_{P}\)_max, _\(|\Delta C_{P}\)_min, and _\(\Delta C_{P}\)_ at a height of 1.6 m from the TOR, the nose length increases from 4 to 12 m, which decreases by 26.5%, 58.5%, and 44.8%, respectively, with increasing nose length.  
Figure 20: Instantaneous iso-surface plot of Q criterion (Q = 5000) colored by time-averaged _U_: (**a**) top view and (**b**) side view (from top to bottom: 4, 7, 9, and 12 m. Black line is 3 m from COT).  
* The nose length also has a great impact on the wake flow of the train. When the nose length is 4 m, the lateral fluctuation of instantaneous \(U\) and wake vortex is more intense, and the lateral fluctuation is also wider and more intense. When the nose length is longer, the shedding distance of the wake vortex is longer in the longitudinal direction.  
[PERSON] conceived, designed, collected and analyzed data, and wrote and revised the paper; [PERSON] provided guidance on numerical simulation software; [PERSON] finalized the manuscript and provided the funding; [PERSON] provided the train model.  
This research was funded by the National Key R&D Program of China under grant numbers 2016 YFB1200601-B14; and the Project of Innovation driven Plan in Central South University, grant number 1053320182352.  
The authors acknowledge the computing resources provided by the High-Speed Train Research Centre of Central South University, China.  
The authors declare no conflict of interest.