Wheel Housing Aerodynamics of Heavy Trucks

Doctoral thesis, 2012

For trucks in long-haul operation the aerodynamic drag constitutes more than half of the total driving resistance, and by optimising the aerodynamics of the vehicle it is possible to reduce the fuel consumption significantly. The drag of the underbody, including wheels, wheel housings and engine compartment, accounts for a large part of that aerodynamic drag, and very little aerodynamic optimisation has been carried out in this specific area on heavy trucks. When studying this area of the vehicle it is of utmost importance to utilise the correct boundary conditions, that is, moving ground and rotating wheels. It has been shown for passenger cars that ground simulation has a significant influence on both the predicted drag coefficient as well as on the flow field. This thesis investigates the influence of ground simulation on heavy trucks, as well as thoroughly analysing the flow field around the front wheels. In this project both wind tunnel experiments and numerical simulations (CFD) were used for acquiring knowledge about the flow field. The focus being on the front wheel housings; and the aim was to understand the flow field around the front wheels and in the wheel housing area under the influence of moving ground and rotating wheels. This knowledge was then to be used to control the local flow field by changing the wheel housing geometry, in order to reduce the aerodynamic drag of the vehicle. Amongst the parameters that were investigated were the wheel housing volume and different wheel housing opening shapes, and it was seen that it was possible to reduce the aerodynamic drag by changing the wheel housing geometry. The influence of ground simulation was investigated, and it was seen that the local flow field was altered significantly with changed boundary conditions, and that the wheel rotation had the greatest impact on the wheel housing aerodynamics. The predicted drag coefficient tended to increase with moving ground and rotating wheels, which correlates with previously published studies on heavy trucks. A wind tunnel model for studying wheel housing aerodynamics was also developed in this thesis. The model was used both to study the local flow field, and also for investigating different modelling strategies of the wheel rotation in the numerical simulations. It was seen that the ground simulation altered the flow field around the wheel, and that there was good correlation between the experiments and simulations.