## 1 Introduction  
Under crosswind, the airflow structure surrounding the train changes, and the train's aerodynamic force rapidly increases, which affects the running stability and safety. The nose length and nose shape of the train have been demonstrated to have a significant impact on aerodynamics [1, 2]. However, the current research mainly focuses on the influence of the yaw angle on the pressure distribution or aerodynamic force in the streamlined head car [3, 4, 5], but the mutual mapping law between the shape of the train and aerodynamic force is unclear. Random turbulence in crosswinds leads to disorderly turbulence in the flow field around the train, which also changes the train's aerodynamic performance. [PERSON] et al. [2] adopted large-eddy simulation (LES) to examine the effect of the nose length on the ambient flow field. The effect of nose length on the aerodynamic property was studied by the DES method, and it was found that the drag coefficient, vortex shedding strength and wake strength decreased with the nose length increase [1]. [PERSON] et al. [7] investigated the effect of nose length on aerodynamic performance using the IDDES method and found that the increase in nose length suppressed the pulsation of drag and lift. In [PERSON] et al. [8], The effects of four types of train nose structures on aerodynamic forces and slipstreams were studied. The effect of three types of tail car on the tail flow was numerically investigated by [PERSON] et al. [9], and found that the spread distribution of wake varied greatly with the length of nose. Wind tunnel experiment is one of the most convenient ways to study the aerodynamic performances of high-speed trains. [PERSON] et al. [10] measured the unsteady flow field on the leeward side of the train using Particle Image Velocimetry (PIV) in a wind tunnel. In the studies of [PERSON] et al. [11; 12], the wake flow of TGV and ICE train models was obtained through wind tunnel experiments. [PERSON] et al. [13; 14; 15; 16; 17] carried out a wind tunnel experiment with large quantities of scaled models, investigated the effect of tail car geometry on the tail vortex structure, and drew many conclusions about wake flow dynamics. Visualizations were conducted in a wind tunnel with a high-speed train (HST) model at various yaw angles [18]. [PERSON] et al. [19] carried out detailed wind tunnel experiments for four different lengths of high-speed trains to investigate the train aerodynamic forces. The aerodynamic characteristics of the train on the slope were analyzed, and the aerodynamic forces and moments of the train increased dramatically [20]. Nevertheless, the aforementioned studies have not yet given a detailed account of the correlation between leeward longitudinal flow characteristics and high-speed trains with different nose lengths through experiments or numerical simulation, especially the influence of turbulence intensity on the leeward longitudinal flow field. Thus, the correlation of aerodynamic lift and side forces with the corresponding leeward longitudinal vortex is beneficial for us to optimize the aerodynamic performance of the train, which motivates the current study. Previous experiments and simulations have used less turbulent flow conditions to study the inflow field changes around high-speed trains and have not yet explored in detail the effect of turbulence on train bypass, and the effect of head geometry combined with turbulence on flow field structure has not been reported. By aiming at a three-car high-speed train with different nose lengths, this paper numerically investigates the influence of nose lengths on the aerodynamic force. The PIV was applied to research the leeward longitudinal flow dynamics with three different nose lengths trains.