# Impact of Different Nose Lengths on Flow-Field Structure around a High-Speed Train
## 1 Introduction  
The aerodynamic performance of a high-speed train (HST) is closely related to its nose shape [1], such as wake flow, train surface pressure, and aerodynamic force. According to this characteristic, the aerodynamic performance of the train can be improved by optimizing the nose shape of the train, especially to reduce the aerodynamic drag force of the train, which will be beneficial to the increase of train speed and the reduction of energy consumption [2; 3]. Therefore, the research on the influence of nose shape on train aerodynamic performance has attracted the attention of a large number of scholars. With the improvement of computing power, some scholars have studied the evolution mechanism of the flow field with different nose shapes by numerical simulation and revealed the basic law of the influence of train nose shape on flow field. For example, [PERSON] et al. [3] used the improved delayed detached-eddy simulation (IDDES) method to study the aerodynamic performance of 8 and 12 m nose trains. It was found that nose length has a great influence on train drag, lift force, development of the boundary layer, pressure, and velocity around the train, and the wake vortex. [PERSON] et al. [4] used the large eddy-simulation (LES) method to study the time-averaged flow and instantaneous flow oftrains with different nose lengths under crosswind conditions. The results showed that the flow on the leeward side and top of the train is different with different nose lengths. [PERSON] et al. [5] also studied the aerodynamic performance of trains with different nose lengths under crosswind conditions. It was found that the change of nose length had effects on train surface pressure, horizontal slice pressure, and vortex structure on the leeward side of the train. The vortex intensity on the leeward side and tail can be weakened by increasing the nose length of the train. [PERSON] et al. [6] studied the pressure on the rail side of different nose shapes (bulge-wide, ellipsoid, spindle, and flat-wide types), and found that the peak value of trackside pressure caused by the bulge-wide shape was the largest. Numerical simulation is one of the important methods used to study the influence of train nose shape on train aerodynamic performance. However, there are many factors that affect the train nose shape, and the numerical simulation of each will increase the cost of nose shape design. Therefore, some scholars have used some optimization methods to improve the efficiency of nose shape optimization. For example, [PERSON] [7] and [PERSON] et al. [8] used the response surface method (RSM); [PERSON] [9], [PERSON] [10], and [PERSON] et al. [11] used the genetic algorithm (GA), and [PERSON] and [PERSON] [12] and [PERSON] et al. [13] utilized the adjoint method (AM). After, the numerical simulation of the optimized model is carried out with these methods, and the results of the numerical simulation are used to judge whether the optimization goal is achieved, including improving the drag, lift force, and lateral stability of the train, among other characteristics. In addition to the numerical simulation method, some scholars have used wind tunnels to study the influence of nose shape on the aerodynamic performance of trains. [PERSON] et al. [14] studied the EMUV250 train model with different nose shapes by wind tunnel testing, and it was found that the lift force, lateral force, and overturning moment of the new model were reduced. [PERSON] and [PERSON] [15] used wind tunnel tests to compare the aerodynamic forces and moments of different nose shapes under different yaw angles, and the results showed that the nose shape has a significant impact on aerodynamic forces and moments. [PERSON] et al. [16] studied the slipstream and wake flows of simplified trains with different roof slant angles by wind tunnel experiments, and it was found that the nose length had a great influence on the separation of wake vortex.