# Effects of Bogies on the Wake Flow of a High-Speed Train
## 1 Introduction
Aside from various methods for identifying the wake vortex structures, such as the PIV method in wind tunnel tests, the hot-wire anemometer (HWA) and surface pressure measurements in real trains, and moving-model tests and the instantaneous flow field analyses in numerical simulations, some modern decomposition methods based on the instantaneous flow field data have also been applied to flow structure identifications recently. [PERSON] et al. [15] used the proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) methods to extract the wake-flow structures of the aerodynamic train model (ATM) high-speed trains; the authors identified a pair of counter-rotating streamwise vortices as the dominant wake feature and these vortices were periodically generated and shed at a frequency of St = 0.085. [PERSON] et al. [6] analyzed the surface pressure of the ICE3 trailing car with the POD method and found that the vortex shedding frequency was St = 0.2. The coherent correlation of several dominant modes and the corresponding power spectral density distributions were also investigated. [PERSON] et al. [16] decomposed the PIV data of the CRH3 high-speed train using the POD method and analyzed the coherent structures. Their results clarified the physical meaning of each mode and the dynamic characteristics of the near wake. It is well known that high-speed trains have a unique geometry with a large number of complex and sophisticated accessory parts, such as bogies, pantographs, inter-carriage gaps, air conditioning, and so on, making it impossible to fully replicate the flow fields. Therefore, high-speed trains are always simplified to certain degrees in model tests and numerical simulations. For example, [PERSON] et al. [6; 7] used a 1/10 th-scale model of an ICE3, which had four sets of bogies but no pantograph, no inter-carriage gap, and no air conditioning in wind-tunnel tests; [PERSON] et al. [15; 17] used a 1/50 th-scale model of an ATM in their numerical simulations with simple bogies but no pantographs. However, several studies [9; 12; 18] have demonstrated that the accessory parts, especially the bogies, not only have large impacts on the nearby flows, but also on the wake of the train. However, the unique effects of bogies on the generation and evolution of vortices in the wake flow of high-speed trains are not fully understood. A better understanding of these effects will help researchers appropriately simplify high-speed train models in experiments or numerical simulations to investigate the wake flow of high-speed trains accurately and effectively. In this paper, the unsteady characteristics of the wake flows of three 1:1 China Railway High-Speed 380A (CRH380A) models with different degrees of simplification are numerically studied using the IDDES method. First, the numerical details are introduced in Section 2, followed by the description of the DMD method in Section 3 and the validations and grid convergence tests in Section 4. Next, the results of the analysis of the aerodynamic forces, train-induced slipstream, and turbulent kinetic energy (TKE) are presented in Sections 5.1 and 5.2 and we discuss the effects of the bogies on the wake flow of the high-speed train. The instantaneous wake flow fields and their coherent flow structures obtained by the DMD method are presented in Sections 5.3 and 5.4, respectively. Finally, the detailed mechanism of the generation and evolution of trailing vortices in a real CRH380A high-speed train is explained in Section 5.5.