Flow field investigation of rotating wheels on passenger cars

Licentiate thesis, 2009

The importance of aerodynamics of road-going vehicles on the total driving resistance and hence fuel consumption is well known to vehicle manufactures. At present, upper body aerodynamics of passenger cars is relatively well understood. However, upper body aerodynamics only represents half of the total aerodynamic resistance on a typical passenger car. It has been shown that as much as 25 percent of the total aerodynamic drag of a passenger car originates from the wheels. Consequently it is possible to achieve important reductions in driving resistance by optimising the flow around the wheels and underbody. At present, advanced ground simulations techniques exist. Several car manufacturers perform wind tunnel experiments with moving ground and other systems for boundary layer treatment. It has been shown that such system have a large impact on the results and are essential to create accurate boundary conditions when investigating road-going vehicles in wind tunnels. A full width moving ground system has been developed for the L2 Aerodynamic Wind Tunnel at Chalmers University of Technology. It will be used for future correlation investigations between full size and scale model experiments. Detailed flow field investigations around the front and rear wheels on a full size Volvo C30 DRIVe™ passenger car have been performed in the Volvo Aerodynamic Wind Tunnel. Omni-directional pressure probes were used to map the local flow field using an automated traversing and data acquisition system. General flow field structures were identified both at the front and the rear wheels. Two large scale wakes were found at the front wheel and one at the rear wheel. A significant dependence on ground simulation was found for both the front and rear wheel flow structures as well as for the global drag. Some dependency on wheel geometry was found at the front wheel whereas the differences at the rear wheel were insignificant for the investigated configurations. A qualitative agreement between front wheel lower wake size and global drag was found. However, integrated microdrag showed that even though there was a quantitative difference in the local flow field, it did not fully explain the differences between configurations. Consequently, it is necessary to expand the experimental investigation in order to fully explain the reasons for changes in global drag. Computational Fluid Dynamics (CFD) analysis of an equivalent vehicle showed good qualitative agreement in the local flow field with some minor exceptions that need to be further investigated. The difference in global drag between the investigated wheel configurations differed by only 1 drag count between experiments and CFD, thus showing good correlation as well.