Numerical investigation of the flow underneath a train and the effect of design changes

This thesis work was performed during Aug. 2006 – Feb. 2007 at Bombardier Transportation in Västerås, Sweden. Within this work the airflow underneath a high-speed train and the effect of design changes were studied with the help of Computational Fluid Dynamics (CFD). The aim of the work was to improve the knowledge about ballast projection and hopefully provide with helpful information to reduce ballast projection. The ballast projection mechanism is not entirely understood but is believed to have the following serial of steps. One ballast stone starts to move due to aerodynamic/track dynamic effects and is accelerated by the airflow underneath the train. The moving stone project itself on an obstacle, another stone, sleeper, rail etc. on the track bed and hits the train, the projected stone returns to the track bed with high momentum received from the impact with the train and starts a chain reaction of stones at the impact with the track bed [4]. Key parameters for this scenario are underfloor aerodynamics of the train, track and track bed dynamics and the track bed quality such as ballast properties, ballast position and distance between the ballast stones and the top of the sleeper [4]. The model in the CFD-simulations consisted of two halves of two cars capturing the arrangement of a motor bogie – inter car gap – trailer bogie of the train. This choice was based on measurements done on the airflow underneath trains. The chosen geometry captures the largest geometrical changes found on the underbelly of the train and it is here improvements can be made. Hence a comparison between the train and the train with modifications was made to investigate the effect of design changes. In particular additional plates were added to the bogie cavity and to the inter car gap edges to reduce the risk of ballast projection. To find an appropriate numerical model previously performed wind tunnel measurements on a 1:7 model of the motor bogie – inter car gap – trailer bogie region representing the train were post processed, analysed and compared to CFD-results. Reliable steady state solutions were achieved with a symmetry condition. To achieve the flow conditions that appears at the midsection of the train values at the outlet from one simulation was applied as the inlet condition for the next simulation, for in total three flow passages. By using this technique it was found that the modifications applied resulted in a reduced air velocity under the train corresponding to lower aerodynamically forces on the track bed and decreased ballast projection. The results pointed out that future high-speed trains should have tight bogie cavities with straight edges and fully covered inter car gaps. Protruding bogie details underneath the train and at the side should be avoided and if it is possible aligned with the train contours. Applying bogie skirts which not only lowers the drag of the entire train but also covers bogie details where air may enter is highly recommended. To summarise, by improving the aerodynamic properties of the train the risk of ballast projection can be reduced, but further studies have to be done to fully understand the physics behind it. Confidential until 2010-03-12Validerat; 20101217 (root