AUTOMOBILE AERODYNAMICS ON THE ROAD AND IN THE WIND TUNNEL – A CORRELATION STUDY - AND APPLICATION OF A NOVEL CORRECTION METHOD

Diploma Thesis Proposal for the degree of Master of Science in Engineering
1-2 students, start August 2007, end dec. 2007 - Jan. 2008.

Volvo Car Corporation
24 January 2007
Göteborg, Sweden

INTRODUCTION
The large automotive wind tunnel at Volvo Car Corporation (VCC) in Göteborg is currently being upgraded into a “moving ground” facility. The new wind tunnel provides a more accurate simulation of the air flow conditions on the road, especially with respect to the rotating wheels and the relative velocity between the ground and the car underbody. In addition, the ground plane boundary layer control system in the tunnel is greatly improved. Nevertheless, the road and the wind tunnel test method still have some major differences in the boundary conditions, in spite of the new ground simulation facility in the wind tunnel. For example, the moving ground plane is not fully simulated in the tunnel due to considerations of space and other limitations in the design. In addition, the confined or limited wind tunnel size and the design of the surrounding test section implies, among other things, a blockage of the fluid flow in the test section, which does not exist on the road. These remaining differences may affect the drag and lift of a test vehicle.

THESIS SUBJECT
The main subject of the present thesis is to compare automobile aerodynamic drag measurements obtained both in the wind tunnel and on the road. Do the boundary conditions (rotating wheels, moving ground plane, and blockage) affect the car and which condition dominates the result? The on-road data will be derived from pressure measurements. In addition, the findings of the present work shall be compared to existing coastdown test data. (Coastdown testing is an established technique for measuring the total vehicle drag on the road, without directly measuring the force acting on the car.)
Depending on the available resources and the final scope of the present thesis proposal it is possible to investigate a novel theoretical blockage correction method, presented at VCC during late 2006. The preliminary result of this correction looks very promising. We would like to know if the correction predicts the real aerodynamic road load or if it needs further adjustment.

METHOD
The sum of the aerodynamic pressure forces, acting normally to the upper and the under-body surfaces, dominate the drag and lift. As the surrounding boundary conditions or the car configuration change, it will most likely affect the pressure force. It is possible to determine a local surface drag and lift contribution by measuring the static surface pressure, over that particular surface, e.g. the rear and front bumper area, the wheelhouse or the rear screen, and so on. This will be accomplished by using a modern parallel processing, high-speed pressure block scanning system, an established measurement technique, frequently used in the wind tunnel. With some adjustments it is possible to use a similar mobile system inside the car while driving on the road. Here, care needs to be taken on the choice of reference pressure, or pressure variations due to the nature of the terrain or the atmospheric disturbances. Comparing data from tests in the wind tunnel and on the road will show if and where the pressure signature differs.

MILESTONES AND DELIVERABLES
o Project Start-up Graduate Project plan
o End of research assignment Literature study report
o End of master thesis project Final project report
o End of graduate period Oral presentation, hand over of project results

TIME SCHEDULE OF THE PROJECT
o Weekly recurring event: Evaluation/progress meeting with company supervisor
o 3-weekly recurring event: Evaluation/progress meeting with university supervisor
o August 2007. Start of the research assignment. Literature study and measurement technique preparations.
o September 2007. Start of first test round.
o October 2007. Start of second test round.
o October 2007. Delivery of draft pro