Effects of Moving Ground and Rotating Wheels on Aerodynamic Drag of a Two-Box Vehicle 2018-01-0730

Previous studies and recent practical aerodynamic evaluations have shown that aerodynamic drag of passenger vehicles with “ground simulation” with moving ground and rotating wheels may increase in some cases and decrease in other cases relative to the fixed ground and stationary wheel conditions. Accordingly, the effects of the ground simulation on the aerodynamic drag should be deeply understood for further drag reduction. Although the previous studies demonstrated what is changed by the ground simulation, the reason for the change has not been fully understood. In this article, the effects of wheels and wheel houses attachment and those by the ground simulation with ground movement and wheel rotation on the aerodynamic drag were investigated by quantification of the underfloor flow that plays a crucially important role on the formation of vortical structure around vehicles. Wall-resolved large eddy simulations (WR-LES), which used approximately 5 billion grid elements, were conducted for two hatchback vehicle models with a backlight slant angle of 30 degrees measured from the horizontal line: one with flattened rear underfloor (non-swept case) and the other with an upward slope with a swept angle of 8 degrees at the rear underfloor (upswept case). Although attachment of the wheels and wheel houses increased the overall drag both for the non-swept and upswept cases, it considerably increased the overall drag for the upswept case. For the upswept case, it formed large rear-wheel wakes inside the underfloor. The enlarged wake reduced the underfloor flow volume and considerably increased the overall drag for the upswept case. The ground simulation decreased the overall drag for the non-swept case, while it increased the overall drag for the upswept case. The change in the rear-wheel wakes and their influence are remarkably different between the non-swept and upswept cases, which caused the difference in the overall drag for the two cases.