On the Scale Size of the Aerodynamic Characteristics of a High-Speed Train

In the wind tunnel test of trains, the scale size changes the Reynolds numbers of trains, which may affect the aerodynamic characteristics of the train. Based on computational fluid dynamics (CFD), numerical models of train aerodynamics with five different scale sizes are established. The five different scale sizes are lambda=1/1, 1/2, 1/8, 1/16 and 1/25, respectively, and the aerodynamic characteristics of trains running in the open-air operating condition and crosswind operating condition with different scale sizes are numerically simulated. The results show that the pressure drag coefficients and pressure lift coefficients of the train tend to decrease with the decrease of the scale size. In the open-air operating condition, compared with the full-size train, the pressure drag coefficient of the 1/25th scaled train is less by 14.4%, and the pressure lift coefficients of the head car, middle car and tail car change 16.1%, 46.6% and 12.3%, respectively. The scale size affects the velocity gradient near the train surface and the position of flow separation changes. The decrease of the scale size leads to the decrease of Reynolds numbers and the increase of viscous drag coefficient. When the scale size is 1/25, the viscous drag coefficient of the train is 0.186, which is 48.6% larger than the one of the full-size train. Compared with the open-air operating condition, the trend of the pressure drag coefficients and viscous drag coefficients is consistent except for the head car in crosswind operating condition when the scale size decreases. In the range of scale size lambda between 1/1 and 1/25, the aerodynamic drag coefficient of the head car, middle car and tail car increase with the decrease of scale size, and the difference in the aerodynamic drag coefficient of the train is 12.9%. In addition, the train's aerodynamic lift coefficient shows an increasing trend with the decrease of scale size.