Large Eddy Simulation of the Flow around a Simplified Train Moving through a Crosswind Flow

Artikel i vetenskaplig tidskrift, 2012

This paper presents the results from large eddy simulation (LES) of the flow around a simplified train model moving through a crosswind flow. The results are compared with the experimental data from previous experimental study by Suzuki et al. [1] who used a model train with 1/60 scale that was moved along a 4m linear guide rail placed in front of a wind tunnel nozzle. The train in the present numerical work is stationary and the influence of the train movement is simulated by changing the boundary conditions. In addition to the simulation of the flow around the moving model, another simulation of the flow around a steady model influenced by the maximum crosswind from the moving case was made and the results are compared with the moving case. Comparison of the LES results with the experimental data from [1] showed good agreement proving that LES is capable of accurately predicting the dynamic change of the aerodynamic coefficients caused by the motion of the model through the wind profile. The aerodynamic moments and forces are computed and the results from the dynamic and the steady case were compared. Overshoots in several aerodynamic coefficients of the moving case over the steady case were observed. The yawing moment coefficient displayed an overshoot of some 30% indicating the importance of performing the dynamic tests for the purpose of satisfying the safety conditions. The large difference between the front and the rear part of the body in the distribution of the flow structures attached to the lee side of the body were found to cause the overshoot in the yawing moment signal during the dynamic case simulations. The instantaneous trailing vortices responsible for much of the aerodynamic stability of the vehicle when it enters the crosswind region were analysed. Several differences between the trailing vortices resulting from the dynamic and the steady cases were observed. These flow structures were found have a shape that changed during the cross flow passage. During their evolution, some of them obtained, for some time instances, the shape of the vortices in the steady case while other (such as the trailing vortex around the lower front edge) never reached exactly the same shape of the structure as in the steady case.