Large-eddy simulation of the flow around a freight wagon subjected to a crosswind

Abstract

The purpose of the research reported in this paper is to investigate the aerodynamic response of a freight train subjected to a crosswind at 90° yaw angle using large-eddy simulation (LES). The freight train under investigation consists of many identical wagons. The wagons of the train are of the single-stacked container type that is having a box-like shape. In order to reduce the computational cost, attention is confined to the aerodynamic loads and flow structures around a selected single wagon from the middle of the train. The influences of the neighbouring wagons are simulated by imposing spanwise periodicity. The Reynolds number of the flow, based on the time-averaged speed of the crosswind and the height of the wagon from the ground, is 300,000. The standard Smagorinsky model with model constant of 0.1 and the Van Driest damping function, implemented in the commercial CFD package CFX, are used. The assessment of LES accuracy was carried out by performing three different computations using three different meshes; coarse, medium and fine. The fine mesh simulation gives results similar to that of the medium mesh simulation. The LES results showed that the flow moves in the gaps between the containers and under the wagon with high speed resulting in a much more complex flow topology in the wake of the wagon compared to that previously published around passenger trains. The flow separates at the sharp windward edge of the container to form a large separated flow region on the roof of the container. Aerodynamic forces on the container and freight wagon were computed and their time history was used to reveal the characteristic frequencies of the flow motion around the body. Further, the effect of the moving ground on aerodynamic coefficients is investigated by performing simulation on a moving ground. As a result of the floor motion, the side force coefficient was reduced by about 2.5% while the lift force was increased by about 11%.