Numerical and Experimental Investigations on Aerodynamic and Thermal Aspects of Rotating Wheels

Doktorsavhandling, 2015

With todays and future environmental legislations for CO2 and other emission gases, all vehicle manufacturers are forced to further develop tools and methods to make their vehicles more energy efficient and environmentally friendly. One option to achieve this is to reduce the aerodynamic resistance since it has a significant influence on the vehicle fuel consumption, especially for velocities higher than 50 km/h. Several methods can be used to assess the aerodynamic performance of passenger cars. Wind tunnel testing and numerical simulations are two of the most common, and both have evolved considerably over the last decades. Moreover, growing computational capacity and increasing general knowledge on the methods in context allow the vehicle manufacturers to advance even further in the development of their products. Hence, some effects, phenomena and processes, not accounted for earlier, can be studied thoroughly today. In this thesis a number of such effects and phenomena associated with rotating wheels of a passenger vehicle are addressed. The discussions focus on three main topics: deformation of tyres under different driving and testing scenarios; wheel aerodynamic resistance moment also known as ventilation resistance; and the possibility of combining aerodynamics and thermal management on studies of brake system performance. All topics are investigated experimentally in the Volvo aerodynamic wind tunnel as well as numerically using CFD simulations. From the results it can be concluded that the wheel ventilation resistance moment has a significant influence on the total aerodynamic resistance of a vehicle; however, both measurements and computations of this moment are quite challenging. Furthermore, it has been found that the inertial expansion of tyres is responsible for a substantial change in vehicle ride height and pitch angle. These movements are often restricted in wind tunnel studies, but can be captured using different types of fastening struts that allow vertical displacements, also known as floating struts. It has been shown that that there is a significant difference in the aerodynamic forces measured depending on the type of strut being used during the test. This is especially pronounced at high velocities. Lastly, the investigation has shown a large potential in combining aero- and thermodynamics for modelling and studying of the brake system performance under different load scenarios. However, further investigations are required to extend and develop the current simulation model.