Numerical study of the effect of aspect ratio on the flow characteristics of the Ground Transportation System

The effect of varying the aspect ratio of a 1:8 scaled Ground Transportation System (GTS) was numerically investigated using the steady formulation of the RANS k−ω SST model. Cases of different aspect ratios (defined as the ratio of the varying width to the baseline width (Eq. (1)), with and without a boat-tail, were considered. For the GTS without boat-tail, a quasi-linear decline in the drag coefficient was found with increasing aspect ratio, resulting from a variation in the strength of the side shear layers, and the consequent change in the vertical symmetry of the near-wake. The average length of the wake was found to increase with increasing aspect ratios, resulting from an increase in the size of high Reynolds shear stress regions in the wake. For the GTS with boat-tail, variation of aspect ratio was found to have no effect on the critical boat-tail angle, albeit the magnitude of reduction in the drag coefficient differed between low and high aspect ratio configurations. Variation of the ground clearance (G ^∗) of the GTS (without boat-tail) concluded a G ^∗ value corresponding to minimum drag coefficient (G ^∗=0.083), while retaining of vertical quasi-symmetry was observed for G ^∗≥0.29.