The effect of bogies on high-speed train slipstream and wake

Slipstream, which is the induced air movement generated by a high-speed train (HST) as it passes, is both a safety hazard to commuters and trackside workers, and can cause damage to infrastructure along track lines. Bogies have been shown to exert a strong influence on HST aerodynamics by altering the underbody flow, and their effects have been extensively studied previously from the perspectives of ballast flight and drag reduction. In contrast, the effect of bogies on slipstream characteristics, and especially on the structure of the wake, is less well understood. This study explicitly investigates the effect of bogies on HST slipstream characteristics based on two generic train configurations: a Simplified Train Model (M1) with the bogies covered by a flat surface, and a Full-featured Train Model (M2) with simplified bogie-sets. The bogie effects are revealed through a systematic comparison of flow structures, slipstream characteristics and aerodynamic forces between these configurations. Remarkably, this study discovers that the generation of the strong spanwise oscillation of the wake, observed especially in the presence of bogies, can be interpreted as due to seeding and amplification of a natural instability of the time-mean pair of counter-rotating vortices behind the tail, rather than through direct side-to-side vortex shedding from the bogie geometry. This paper also documents how the altered wake flow affects slipstream characteristics through statistical and gust analyses, and the effect of the bogies on aerodynamic loading by comparing the train surface pressure distributions between the configurations.