An Investigation of the Aerodynamic Drag Mechanisms Due to Ground Simulation in Yawed Flow Conditions for Heavy Trucks

The drag from the underbody, including wheels and wheel housings, constitute a significant amount of the total aerodynamic drag of heavy trucks. A correct simulation of the underbody boundary conditions, including rotating wheels and moving ground, has turned out to be of great importance in the minimising of the aerodynamic drag. Earlier studies on passenger cars have described the drag mechanisms involved when implementing proper ground simulation. However, model scale wind tunnel tests of heavy trucks have shown an opposite trend on the drag coefficient with ground simulation compared to passenger cars. An important aspect of truck aerodynamics is the yaw dependency of the drag coefficient when the vehicle is exposed to crosswinds. Therefore it is of outmost importance to evaluate the performance of the vehicle in yawed flow conditions during the development process. In the current study the influence of ground simulation, including moving ground and rotating wheels, on the flow has been investigated at several yaw angles using Computational Fluid Dynamics. For the simulations a tractor trailer geometry corresponding to a standard European configuration was used, and the geometry included a fully detailed underbody and engine compartment. It was found that there was a difference in the wheel wake structure and this was mainly due to the implementation of wheel rotation. It was also shown that the drag coefficient increased due to the utilisation of ground simulation in yawed flow conditions.