Passenger vehicle tyre aerodynamics

Doktorsavhandling, 2024

Road vehicles are an essential part of society, enabling the movement of people and goods. They are, however, responsible for a large part of greenhouse gas emissions. Therefore, automotive companies strive to constantly increase the energy efficiency of their fleets. For cars, aerodynamic drag is one of the main resistive forces and a substantial part of the total energy consumption. A significant contribution to the total drag originates from the wheels, making the understanding of their flows essential for creating efficient vehicles. Factors such as the wheels' rotation, bluff-body shape and small geometrical details result in complex flow fields that are sensitive to, for example, wind tunnel setup, vehicle velocity and small geometrical changes. Due to this sensitivity, the experimental and numerical methods used to assess wheel aerodynamics should be carefully evaluated.

Full-scale wind tunnel tests are compared to simulations with an open road domain and with a domain containing a detailed model of the wind tunnel. Including the wind tunnel improves the prediction of both absolute drag values and the drag delta between configurations. The importance of considering the parasitic lift acting on the wheel drive units (WDUs) of the wind tunnel is also evaluated. Although the parasitic lift reduces for smaller belts, it is shown that leakage flows from the gaps around the WDUs can interfere with the flow around the wheels if the belts are too narrow. Finally, the velocity dependency of the drag coefficient, C_D, is studied. With rotating wheels, C_D reduces for increased speeds, whereas it is constant with stationary wheels. It is shown that the drag reduction likely occurs locally at the wheels and that the separations at the outer tyre shoulders of the front wheels are one of the main factors.

In the second part of the thesis, the effects of various geometrical tyre features and their impact on the total drag and flow field around the vehicle are considered. Using full-scale wind tunnel tests with the DrivAer model and a production vehicle, the influence of the tyre tread patterns is studied for two rim configurations. Adding rain grooves consistently reduces the drag whereas the trend is less clear for lateral grooves due to a stronger rim and vehicle dependency. A tyre profile with a narrower sidewall generally reduces drag. With a protruding rim protector, a large drag increase is found when combined with an open rim. Generally, the largest variations between the tyres are observed around the unshielded parts of the wheels, next to the ground.