Abstract: The turbulent underbody flow developed around a high-speed train is the crucial factor for ballast flight. Based on the delayed detached-eddy simulation, the flow behaviour around a scaled high-speed train with ground underneath is investigated. Results show that due to flow separations and vortex interactions, the highly unsteady flow is generated around the train, especially at the regions of the nose, the bogies, the inter-carriage gaps and the tail car. A significant pressure pulse produced at the nose area is expected to play a key role in the initiation of the ballast flight and make it happen more easily. Compared to the areas underneath the rail running surface, the flow fluctuations above the rail are larger. Thus, the ballast particles are conductive to be projected by the fluctuating flow beneath the train after separated from the trackbed surface under the strong wheel-rail excitement. Due to the special location within the inter-car gaps, the strong vortical flow is developed and distributed around the articulated bogie region. The train tail generates the large trailing vortices within the train wake which act on the ballasted trackbed surface and may induce the ballast flight. Therefore, the streamlined design of the nose, the smoothing of the inter-carriage gaps and the control of the train wake, which can reduce the interaction between the train underbody flow and the trackbed surface, and thus reduce the occurring of the ballast flight.

Key words: high-speed train, train aerodynamics, ballast flight, delayed detached-eddy simulation, flow behavior