Related to the train underfloor carbody aerodynamics, a phenomenon of ballast flight occurs frequently associated with the operation of high-speed railways. Ballast flight will cause rail defect known as “ballast pitting” which is formed by the small ballast particles becoming trapped between the rail running surface and the vehicle wheels. Generally, the ballast particles have irregular shapes and are distributed on the trackbed which has a permeable surface. The flow developed beneath a high-speed train running on a ballasted track and around the ballast particles is turbulent and has strong influence on the ballast flight. This paper investigates the aerodynamic behaviour of the flow past the ballasted trackbed with permeable surfaces using computational fluid dynamics based on the delayed detached-eddy simulation. It is found that the flow field is highly unsteady due to strong flow separations and interactions developed around the gaps of the permeable surfaces. The ballast particles located on the permeable surfaces are situated in the stronger turbulent flow and are subject to larger fluctuating lift force than the particles located on solid surfaces. Moreover, the fluctuating lift, drag and side forces increase when the ballast particles are located beneath the bogie cavity area. When the ballast particles become airborne, the fluctuating forces generated around them increase greatly since the particles are situated in the unsteady flow with more flow interactions. Therefore, the stronger unsteady flow developed around the trackbed with a permeable surface will produce larger fluctuating forces on the ballast particles, which will be more likely to cause ballast flights for high-speed railways.

ballast flight; flow behaviour; ballasted trackbed; permeable surface; aerodynamic forces