Passenger Car Wheel Aerodynamics

Doctoral thesis, 2011

Current and future requirements for efficiency force the automotive industry to consider all aspects of reducing the energy consumption of road going vehicles. An important aspect of reducing fuel consumption is reduced aerodynamic drag, which at highway cruising speeds is dominating the total drag force for both cars and heavy goods vehicles. Furthermore, on a modern car, up to 25 percent of the aerodynamic drag originates from the wheels and wheel housings, making them important aerodynamic components. This thesis investigates the aerodynamic effects of wheels and tyres further, with focus on reducing aerodynamic drag. Both experimental and numerical tools were used in order to increase the understanding of how the wheels interact with the surrounding flow field regions, and how such interactions are affected by wheel design. Experiments were performed in the Volvo Aerodynamic Wind Tunnel and RANS and U-RANS were used in the numerical investigations. With regards to CFD, the performance of MRF for modelling wheel rotation was investigated in detail, and compared with both experimental data and sliding mesh simulations for some configurations. The steady state RANS approach using MRF was found to correlate well with experiments for several configurations, but in the case of highly closed front wheels, large discrepancies were identified. Using sliding mesh showed a potential for improved predictions of wake structures compared with experiments, but further investigations of numerical methods, suitable for modelling wheel aerodynamics in a reliable way, was recommended. Several experimental investigations on wheel design resulted in an increased understanding of important wheel design parameters, and their effect on both drag and local flow field. Radial wheel covers was found to be one of the most efficient means of reducing the drag contributions from the wheels. The effect of tyre model was found to be of equal importance as the wheel design. Despite identical size definitions, the tyre geometries were found noticeably different, giving several possible reasons for the differences in drag. Furthermore, effects on drag due to the tyre geometry changing with velocity were also investigated.