Investigation of aerodynamic wheel design

Abstract

The passenger car has become a vital part of modern society, meaning that it accounts for 10 percent of the worldwide energy consumption. With an increasing number of vehicles produced, it is therefore necessary to utilise clean energy sources and to minimise the energy consumption of the vehicles. The aerodynamic drag is the dominating resistive force above 60 km/h and must therefore be reduced to increase the efficiency of the vehicles, especially since the new WLTP and CO2 emission regulations put stricter demands on the automotive industry. Furthermore, the wheels and wheel housings accounts for up to 25 percent of the aerodynamic drag, making them important components for optimisation. This thesis investigated the aerodynamic effect of different wheel designs. Both experimental and numerical tools were used to evaluate the importance of 14 wheel parameters. The experiments used a statistical design of experiments (DOE) setup and were performed in the Volvo Aerodynamic Wind Tunnel, where force measurements were taken along with flow field and surface pressure measurements in order to describe the flow around the wheels. The numerical CFD simulations implemented a transient IDDES turbulence model with a sliding mesh setup for the wheel rotation. It was found that the coverage area was by far the most important design parameter of the wheel. The rim cover and the depth of centre were also found to be significant parameters. The flow field measurements described the important parameters’ effects on the flow field at the wheels, where it was found that minimising the nearground vortex at the front wheel could significantly decrease the aerodynamic drag. The numerical simulations proved to accurately model the flow at the front wheels and aided the understanding of the strong near-ground vortex formation. In conclusion, it was found that a covered wheel with the cover surfaces flush to the tire wall (bead), was preferred for decreasing the aerodynamic drag and making the vehicle more energy efficient. Furthermore, the open wheels and the wheels with low outer rim coverage had an outflow through the lower part of the wheel, feeding energy to and strengthening the near-ground vortex. Thus, it could be concluded that limiting this outflow is one way of decreasing the aerodynamic drag caused by the wheels.