Analysis of the Base Wake on Passenger Vehicles

Licentiate thesis, 2013

Car manufacturers are presented by a continuous increase in fuel prices and by ever more strict legislations regarding CO2-emissions. Decreasing the driving resistance and hence the fuel consumption of their vehicles helps to improve their market share. Aerodynamics plays an important role since it is a significant contributor to the driving resistance at typical driving speeds of 70 km/h and above. Passenger vehicles are defined as bluff bodies of which the main source of air resistance is pressure drag. This drag is caused by the wake a car creates and can be explained by the pressure difference between the front and the rear of the vehicle. Therefore, understanding the behaviour of the wake in time and in space is crucial in order to accomplish the goal of drag reduction. Experimental and numerical investigations have been conducted on a 2WD and 4WD Volvo XC60 to study the effects of geometrical changes to the base pressure and wake shape of the car. Surface pressure measurement equipment and two omni-directional pressure probes were used for the base and wake measurements, respectively. Results show a base pressure reduction with an increased standard deviation for a reduction in drag caused by covering the upper and lower front grille. In the flow field this effect is visible as a smaller wake size and high-energy underbody flow enhancing the total pressure recovery in the far wake. Improved energy recovery in the far wake can also be realised by having an extended roof spoiler, that causes a delayed separation and a more downward inclination of the shear layer. The corresponding numerical simulations in general show good agreement with the experiments in force trend predictions. The configuration changes have a similar flow field effect in CFD compared to the wind tunnel. However some significant differences in wake shape behind the vehicle can also be observed. A new visualisation technique has been developed to differentiate the contribution of the different geometrical sections of the car to the overall air resistance. This creates the opportunity to quantify the drag of each section in relation to the total drag of the vehicle.