Wheel Modelling and Cooling Flow Effects on Car Aerodynamics
Doctoral thesis, 2018
In recent years, the automotive industry has focused on computer simulations as an alternative to physical testing. This allows for early vehicle optimization, short turn around times, and significant cost savings. Aerodynamics is an important vehicle attribute where Computational Fluid Dynamics (CFD) simulations are being used to improve vehicle design. In this context, predicting the aerodynamic forces in CFD is challenging, given the complexity of both the geometry and the flow. Two main areas where discrepancies between CFD and wind tunnel tests are present are wheel aerodynamics and cooling flow.
Wheel aerodynamics has been the focus of CFD research for several years, especially with the expected introduction of the Worldwide harmonized Light vehicles Test Procedure (WLTP) regulations. Car manufactures will need to evaluate the drag of the vehicle for all different rim and tyre combinations sold with the car, in order to determine its official fuel consumption. Thus, accurate modelling of tyre and rims in CFD is a high priority for vehicle manufacturers, in order to optimize vehicle design without large increases in testing costs. This thesis investigates the effects of different wheel geometries, as well as different wheel rotation modelling techniques and their effect on overall vehicle forces. A hybrid approach for modelling tyre rotation, named MRFg, has been proposed and analysed. It showed significant improvements in numerical prediction, when compared to experimental results. MRFg can be applied on the loaded and deformed tyres with no significant cost increase. Investigations into the tyre geometry also showed that the CFD accuracy can be improved by reproducing the rain grooves' continuity at the contact patch in the virtual model. The rain grooves have been show to reduce overall vehicle drag in both the wind tunnel tests and the simulations.
The cooling flow is an important parameter to consider when validating simulations to experiments, as it changes the flow field around the vehicle and has a large impact on the aerodynamic forces. In this thesis, a simple and quick method for measuring cooling flow is introduced. It is a force based approach where the force acting on the radiator core is used to calculate the air mass flow. It allows for non-intrusive monitoring of the cooling flow and forces during aerodynamic development with good accuracy. A comparison of CFD and wind tunnel tests is also performed with focus on parameters influencing cooling flow and force measurements. The effects of wind tunnel blockage and measurement grid have been shown to have significant effects on mass flow predictions.