# Effects of Bogies on the Wake Flow of a High-Speed Train
## 1 Introduction  
The wake region of high-speed trains is an area of very complex turbulent flow. Vortices of different strengths are rapidly generated and shed, especially in the near wake area, which has adverse effects on high-speed trains: not only does it increase the pressure drag, but also seriously affects the lift and side forces of the trailing car. This problem causes discomfort to the passengers and affects the stability and safety of the train [1; 2; 3; 4; 5; 6; 7]. Therefore, the optimization of the wake flow is an important aspect of high-speed train research. It is imperative to understand the detailed mechanism of the generation of trailing vortices as a result of the complex geometry of high-speed trains. Many studies have been conducted on the wake flow of high-speed trains. [PERSON] et al. [1] conducted experiments on a 1/25-scale moving model rig to analyze the wakes and slipstreams of high-speed trains and to provide an understanding of the motions of the wake-vortex structures. [PERSON] et al. [5; 6; 7] used Cobra probes, surface pressure measurements, and particle image velocimetry (PIV) flow visualization technique in wind tunnel tests to determine the topology structure of wake vortices of an ICE3 high-speed train; oscillations in the spanwise and vertical directions were obtained. [PERSON] et al. [8] used oil paint and smoke visualization techniques in wind tunnel tests to analyze the near wake of high-speed trains; it was found that the type of flow structure, i.e., a separationbubble or vortex shedding, depended on the geometry of the trailing car. [PERSON] et al. [9] compared the unsteady Reynolds average Navier-Stokes (URANS) and detached eddy simulation (DES) methods in wake-flow simulations of a high-speed train with different running speeds. The results of these studies indicated that the wake of a modern high-speed train is highly complex, three-dimensional, and consists of shear layers, separation and recirculation regions, and shedding vortices. The wake is always characterized by a pair of large counter-rotating streamwise vortices; the vortices move towards the ground and meanwhile separate and subsequently break up as a result of their mutual inductions and the interactions with the ground. The oscillations of the shedding vortices in both the spanwise and vertical directions of an ICE2 high-speed train were identified in URANS simulations by [PERSON] et al. [10]; it was found that the oscillations coincided with the periodic oscillations of the side and lift forces. [PERSON] et al. [2; 3] conducted full-scale train tests of an ICE2 train and determined that the dimensionless frequency of vortex shedding in the wake was St = 0.11 (St is the Strouhal number). Numerical simulations using an improved delayed detached eddy simulation (IDDES) were conducted by [PERSON] et al. [11] to investigate the effects of the ground configurations on the near wake; the frequency in the wake was St = 0.118. [PERSON] et al. [12] identified vortex shedding at frequencies of St = 0.15-0.18; the vortices originated from the underbody due to the interaction with the bogies, as determined by the Lattice Boltzmann Method (LBM). The aforementioned results show that the near wake of high-speed trains is characterized by strong periodicity, resulting in periodic oscillations of the side and lift forces and the vortex shedding frequency is in the range of St = 0.085-0.21 and depends on the geometry of the trailing car. Moreover, it has been demonstrated via full-scale experiments [3; 4], wind tunnel experiments [5] and numerical simulations [13; 14] that a strong slipstream is always induced by the highly unsteady wake flow and may endanger passengers waiting at platforms and track-side workers.