Tyre aerodynamics of passenger vehicles

Licentiate thesis, 2022

To decelerate climate change and limit global warming, there is a need for reducing the environmental impact of vehicles, which can be achieved by increasing their energy efficiency. For a passenger vehicle, one of the largest resistive forces is aerodynamic drag. A significant contribution to the total drag originates from the wheels, making the understanding of their flows essential for creating efficient vehicles. However, wheel flows are complex and challenging to comprehend due to factors such as the rotation, high level of geometrical details and tyre deformation, all of which have been shown to affect drag. For a better understanding, numerical simulations can be used. However, to determine the accuracy of the simulations, these must be correlated to experiments.

In this work, both the correlation between experiments and simulations as well as the flow field effect of varying the tyre tread pattern are investigated. Wind tunnel tests using a full-scale DrivAer model have been performed for four tyre tread patterns and two rim designs. First, the numerical simulations are compared to the experiments, where the interference of the wind tunnel on wheel flows is investigated. By performing simulations using both an open road domain and a domain containing a detailed model of the wind tunnel, it was found that the inclusion of the wind tunnel improves predictions of both absolute drag values as well as the drag deltas between configurations.

Then, the effects of the different tyre tread patterns are analysed in more detail. Results showed that adding rain grooves typically reduces drag compared to a slick tyre, whereas the effect of lateral grooves is dependent on the rim configuration. In addition to the drag, lift variations were considered. There, the largest effects were obtained at the front axle and, in general, the lift is reduced by the rain grooves and increased by the lateral grooves, most clearly for the closed rim. Also, the influence of the parasitic lift forces acting on the wheel drive units was demonstrated.