The automotive industry is facing new emission regulations, changing customer preferences and technology disruptions. All have in common, that external aerodynamics plays a crucial role to achieve emission limits, reduce fuel consumption and extend electric driving range. Probably the most challenging components in terms of numerical aerodynamic drag prediction are the wheels. Their contribution to the overall pressure distribution is significant, and the flow topology around the wheels is extremely complicated. Furthermore, deltas between different rim designs can be very small, normally in the range of only a few drag counts. Therefore, highly accurate numerical methods are needed to predict rim rankings and deltas.
This paper presents experimental results of four different production rim designs, mounted to a modified production car. An accurate representation of the loaded, deformed tire geometry is used in all calculations for comparable conditions between wind tunnel and CFD. Different simulation approaches are compared and analyzed to measured rim rankings and deltas. A special meshing strategy is introduced to reduce the influence of mesh changes on the flow field to a minimum.
A steady state simulation approach in combination with a moving reference frame model is able to capture the delta between the best and worst rim design. The rim position has a non-negligible influence when using this frozen rotor method and needs to be considered. Transient scale resolving simulations with real motion of the rims remove the limitations of steady state. Ranking and deltas are accurately predicted for all four rims by the simulations.