Investigation of Aerodynamic Resistance of Rotating Wheels on Passenger Cars

Licentiate thesis, 2013

With the current conditions of rising fuel prices and the toughening of environmental legislation for CO2 and other emission gases, vehicle manufacturers around the world are looking into the ways to make their vehicles more energy efficient and therefore more environmentally friendly. One of the ways to achieve this goal is to improve aerodynamic design of vehicles in order to decrease aerodynamic resistance forces. These forces are especially important for any vehicles capable of 50km/hour and higher, and that is basically any passenger or commercial vehicle. In this thesis the focus is on passenger vehicle aerodynamics and more specifically on the area of the wheels and wheelhouses. There have been a number of studies of the aerodynamic performance of this area, which have shown that wheels and wheel-housing flows generate a significant part of the aerodynamic drag on a passenger car and can relate to as much as 25% of it. The studies also show the relative importance of tyre and rim design in having better aerodynamic characteristics. Usually when speaking about aerodynamic resistance of vehicles one thinks about the aerodynamic drag force. Certainly it is the largest contribution to the overall resistance that the vehicle has to overcome when moving; but it is not the only contribution. The resistance moment acting on the wheels rotating in the air, commonly referred as ventilation resistance, plays an important role as well. This moment is not as easy to measure in a standard aerodynamic wind-tunnel as aerodynamic drag force, and therefore it is usually left unaccounted for. In this thesis a closer look at ventilation resistance is taken, and different effects contributing to this resistance moment are discussed. In order to be able to measure ventilation resistance various modifications to a normal wind-tunnel set-up are presented. The results and analysis of one of the studies investigating ventilation resistance, and its dependency on rim design, are also given in this thesis. It is shown that ventilation resistance has a significant effect on total aerodynamic performance and it should be taken into account when designing a vehicle.