The aerodynamic drag of high speed trains

Abstract

The resistance to forward motion of a high speed passenger train at cruising speed is dominated by aerodynamic drag. To measure train aerodynamic drag a model must be of a small scale in order to fit a standard wind tunnel working section. Due to the scale dependency of the skin friction component of total aerodynamic drag the results of wind tunnel tests cannot be directly related to train performance at full scale Reynolds number. It is thus necessary to quantify the relationship between skin friction drag and Reynolds number. A series of boundary layer and total aerodynamic drag measurements were made using small scale model high speed passenger trains at zero yaw. Boundary layer measurements were also made at full scale to determine skin friction. A simulation program was used to model the recorded boundary layer development and the train boundary layer was found to be strongly three-dimensional. The dependence of train skin friction drag, and hence the total aerodynamic drag, on Reynolds number was clearly demonstrated but the complexity of the train flow regime at full scale prevented an accurate quantification of this. The nature of the train boundary layer has been clarified and this knowledge will assist future investigations of high speed passenger train aerodynamic drag.