The aerodynamic flow beneath a high speed train: A numerical and experimental analysis

Abstract eng:
In the operation of high-speed trains, ballast stones on the track bed sporadically lift off which damages the underside of trains, trackside barriers and rail heads. This phenomenon is known as ‘ballast projection’. Because ballast projection occurs intermittently it is difficult to observe during operation. The exact mechanisms that cause ballast projection are not fully understood, although from the available evidence it is thought to be due to a combination of sleeper movement and aerodynamic effects. To investigate the aerodynamic aspect of ballast projection the flow beneath a high speed train has been investigated using full-scale tests, moving-model experiments and numerical simulations in order to better understand the flow beneath the train. Results show excellent agreement between the full-scale test results and physical modelling results. However the results from the CFD simulations show some overshoot in both pressure and velocity in comparison to the experimental work. This is hypothesised to be due to the influence of the modelled wall roughness in the physical model that was not replicated in the CFD simulations. INTRODUCTION High speed railways now exist in the majority of developed nations and allow trains to operate at speeds of over 300 kph. In countries such as South Korea and China train speeds have steadily increased over time and the infrastructure is now capable of coping with train speeds approaching 400 kph. Aerodynamic effects increase proportionally with train speed squared, consequently at higher speeds these effects will be significantly greater than for trains travelling at lower speeds. Ballast projection is the phenomenon by which ballast particles become airborne during the passage of a high speed train and was first observed on Japanese and Korean high speed railways [1]. Flying ballast particles can cause damage to the underbody of trains and to the rail head if crushed between a wheel and the rail. Figure 1, replicated from Quinn et al. [2], shows ballast pitting damage as a result of a ballast particle being crushed between the wheel and the rail head. Ballast flight is also known to cause damaged wheel sets, broken glass in stations and damage to trackside acoustic screens. The work presented in this paper is part of a collaborative project with the University of Southampton to investigate the geotechnical and aerodynamic influences of the ballast projection phenonenon. This paper contains a description of the physical modelling, numerical simulations and full-scale experimental tests.

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