Influences of Different Front and Rear Wheel Designs on Aerodynamic Drag of a Sedan Type Passenger Car

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Efforts towards ever more energy efficient passenger cars have become one of the largest challenges of the automotive industry. This involves numerous different fields of engineering, and every finished model is always a compromise between different requirements. Passenger car aerodynamics is no exception; the shape of the exterior is often dictated by styling, engine bay region by packaging issues etcetera. Wheel design is also a compromise between different requirements such as aerodynamic drag and brake cooling, but as the wheels and wheel housings are responsible for up to a quarter of the overall aerodynamic drag on a modern passenger car, it is not surprising that efforts are put towards improving the wheel aerodynamics. The actual force on the wheels is typically not a full quarter of the overall drag, but as the wheels strongly interact with several other key flow features such as cooling air flow, underbody flow and the base wake, the wheels have a large influence on the overall aerodynamic performance of the vehicle. This study investigates the potential of different wheel design parameters focusing on reduced aerodynamic drag. A correlation with experimental measurements on a full size vehicle is presented and several additional configurations are analyzed numerically using a standard automotive CFD approach. Furthermore, the potential of optimizing the front and rear wheels individually is investigated to some extend. Results show that closing most of the rear wheels results in local drag reductions along the rear end underbody, rear wheels and vehicle base. The fully covered rear wheel typically reduced base drag between 6-7 drag counts. Effects of covering the front wheels were more complex as both upstream and downstream flow regions were affected, and it was shown that for the vehicle investigated in this study a limited amount of outer radial covering of the wheel gave the largest drag reduction. The investigation of using different front and rear wheel designs showed that this concept have potential in reducing overall drag as it generated the largest drag reduction in this study of approximately 22 drag counts.