The three-dimensional flow around railway trains

When a railway train is travelling in a cross-wind it experiences aerodynamic forces and moments which, in extreme cases, may cause a derailment. This paper presents a computer program which has been developed to predict these forces and moments in a steady wind, for effective incidences of up to 50°. The calculation method has been tested against measurements made on an idealised wind tunnel model of a railway train in a cross-wind. These measurements have been made for a range of incidences from 20–35° with a Reynolds number, based on the freestream velocity and a body cross-sectional dimension, of 3.7×105. The vortices in the wake on the leeside of the train were found to have a large effect on the pressure distribution on the model, and hence the overall forces. Consequently the computer program for the prediction of the pressure distribution incorporates a method for calculating the positions and strengths of the wake vortices. For an idealised model of a train with no wheels the vortex wake is like a three-dimensional Karman Vortex Street, with vortices being shed from successive points on both the roof and underside of the train. However, when wheels are simulated the vortices shed from the underside of the train are disrupted and reduced in strength, increasing the overall forces on the train. The calculation method has been modified to account for this effect and applied to a more realistic flowfield, using the cross-sectional shape of the Advanced Passenger Train.