A computational fluid dynamics study of the influence of sleeper shape and ballast depth on ballast flight during passage of a simplified train

The paper assesses the effect on the air flow regime underneath a simplified high-speed train of changing the ballast depth and the sleeper shape, with regard to its potential for causing ballast flight or pickup. The study was carried out numerically using the commercial Computational Fluid Dynamics (CFD) software AnSys Fluent. The flow profile beneath the underbody of the train was generated by means of a moving wall above the track. The Delayed Detached Eddy Simulation (DDES) with the SST k - ω turbulence model was used to simulate turbulent flow, and the ballast bed roughness was applied parametrically using the wall roughness feature when resolving the boundary layer. CFD simulations were validated for flow over a cube, showing good agreement with experimental results. Up to three different depths to the ballast surface and three different sleeper profiles were investigated. Velocity profiles and aerodynamic forces on cubes placed between or on top of the sleeper blocks were used to assess the propensity of individual ballast grains for movement. For a standard G44 sleeper, increasing the ballast depth and/or the ballast bed roughness was found to reduce aerodynamic loads on an individual ballast grain. A ballast grain on top of the sleeper is more prone to uplift than a grain on the surface of the ballast bed in the crib. A curved upper surface to the sleeper is beneficial in that it prevents ballast from settling on top, the most vulnerable position. However, the reduced flow separation associated with the curved top may increase the likelihood of ballast pickup from the crib. Hence new sleeper shapes intended to reduce the potential for ballast flight should not only prevent ballast from settling on top, but also increase flow separation through the provision of a sharp surface. A prismatic sleeper shape that achieves both is suggested.

本文评估了改变道砟深度和轨枕形状对简化高速列车下方气流状况的影响，以及其导致道砟飞起或被卷起的可能性。该研究使用商业计算流体动力学（CFD）软件AnSys Fluent进行数值模拟。列车底部下方的流场轮廓是通过轨道上方的移动壁面生成的。采用带SST k - ω湍流模型的延迟分离涡模拟（DDES）来模拟湍流，在求解边界层时使用壁面粗糙度特性参数化地应用道砟床粗糙度。CFD模拟通过对立方体上方流动的验证，显示与实验结果吻合良好。对多达三种不同的道砟表面深度和三种不同的轨枕外形进行了研究。通过轨枕块之间或顶部放置的立方体的速度剖面和空气动力，来评估单个道砟颗粒移动的倾向。对于标准的G44轨枕，发现增加道砟深度和/或道砟床粗糙度会降低单个道砟颗粒上的空气动力载荷。在轨枕顶部的道砟颗粒比在轨枕间道砟床表面的颗粒更容易被掀起。轨枕上表面弯曲是有益的，因为它可防止道砟堆积在轨枕顶部这个最脆弱的位置。然而，与弯曲顶部相关的流动分离减少可能会增加从轨枕间卷起道砟的可能性。因此，旨在降低道砟飞起可能性的新型轨枕形状不仅应防止道砟堆积在轨枕顶部，还应通过设置尖锐表面来增加流动分离。建议采用一种能同时实现这两点的棱柱形轨枕形状。