Investigation of ballast flight under aerodynamic flow using computational fluid dynamics

Abstract

This study focuses on the aerodynamics underneath a passing High-Speed Train that can cause ballast flight. Numerical studies have been carried out on simplified particles at different orientations and elevations (levels), to observe changes in aerodynamic forces. A cube and a hemisphere(0.06 m in diameter) were simulated at inlet flow speed up to 120 m/s. Studies using moving wall techniques were also carried out, to observe changes in aerodynamic forces on isolated ballast grains and different track sections. Results demonstrated that URANS has produced meaningful results but not as detailed as DDES at a higher computational cost. Certain ballast characteristics are identified that can trigger ballast movements such as the particle shape, its orientation and the mass of the ballast grain. It was determined that shapes with flatter surfaces (i.e. cube) facing the flow are more likely to move than smoothly shaped ballast (i.e. hemisphere) and ballast of smaller mass is also prone to ballast movement. Further studies included some simple wind tunnel experiments, in which the critical velocities for ballast were measured. CFD studies of different sleeper blocks were also conducted using periodic and moving wall boundary conditions,to determine the risk of ballast movement at various locations across the track at certain ballast depths. Aerodynamic forces and flow behaviour are presented for each case study isolating it from mechanical vibrations caused by the passing High-Speed Trains. These numerical studies show that be lowering the ballast bed depth and having a rough ballast bed surface can provide benefits by reducing the aerodynamic load on the ballast bed. These computational studies suggest that improper use of aerodynamic sleepers can increase the likelihood of ballast movement due to reduced flow separation, which is why new sleeper shapes should be considered to not only prevent ballast from settling on top of sleeper blocks but also increase the flow separation.

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ORCID for William Powrie: orcid.org/0000-0002-2271-0826

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