Unsteady aerodynamics and underbody flow characteristics of high-speed trains with bogie covers at 400 km/h

Abstract

Although high-speed train (HST) bogie covers effectively reduce aerodynamic drag, they raise safety concerns due to lift-induced oscillations caused by unsteady underbody flow. This study investigates the effects of various bogie covering structures on aerodynamic load pulsations, pressure fluctuations, and underlying flow mechanisms in HSTs using an improved delayed detached-eddy simulation at 400 km/h. Three covering configurations are considered: fully enclosed covers (FECs), separated-type covers (STCs), and skirts-only. The numerical results show that all covering configurations significantly reduce the total aerodynamic drag. Specifically, FECs achieve the largest reduction at 19.85%, while the STC and skirt-only configurations achieve reductions of 16.22% and 14.45%, respectively. STCs perform better than FECs in suppressing lift fluctuations of car bodies, effectively reducing the root-mean-square lift coefficient by up to 44%. While both STCs and FECs significantly attenuate pressure fluctuations by up to 98% in the head bogie cabins and 83% in the tail cabins, they intensify fluctuations in the middle cabins by 43% for STCs and 162% for FECs. Spectral analysis reveals that the dominant lift fluctuation frequencies for STCs and FECs primarily fall within 7–14 Hz, with a secondary range of 63–72 Hz. These fluctuations are attributed to cavity flow-induced resonance within the bogie cabin, which couples the internal and external airflows via the wheel gaps. Additionally, FECs exhibit larger fluctuation amplitudes than STCs. Furthermore, boundary layer instability on the lower surface of the head cover induces periodic high-frequency flow field fluctuations with a dominant frequency exceeding 115 Hz.