On the influence of Reynolds number and ground conditions on the scaling of the aerodynamic drag of trains

The present study examines the possibilities of transferring drag measurement results on reduced-scale train models to the respective full-scale vehicle. A comprehensive experimental and numerical study of the boundary layer and skin friction along trains is performed, focusing on Reynolds number effects. The data are supplemented by an extensive literature study and compared with different approaches from flat plate theory. Good agreement can be found when using the appropriate empiric coefficients and boundary conditions. Simultaneously, the difficulties in determining the skin friction drag of trains due to three-dimensional effects and surface roughness become apparent. The ground simulation analysis, including the effects of ground roughness due to ballast and sleepers, reveals a significant effect of the ground conditions on the vehicle’s aerodynamic drag. Additionally, the effects of elements mounted on the train roof are investigated for different upstream flow conditions. Finally, a scaling method is proposed to transfer drag results from model-scale to full-scale trains based on the findings