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Aerodynamic analysis of drag reduction devices on the underbody for SAAB 9-3 by using CFD

Johan Levin ; Rikard Rigdal
Göteborg : Chalmers tekniska högskola, 2011. Diploma work - Department of Applied Mechanics, Chalmers University of Technology, Göteborg, Sweden, ISSN 1652-8557; 2011:30, 2011.
[Examensarbete på avancerad nivå]

Environmental issues and increased fuel prices are driving forces for the automotive manufactures to develop more fuel efficient vehicles with lower emissions. Large investments are aimed at minimizing power needed for propulsion i.e. new downsized engines with new aerodynamic devices for drag reduction. For passenger cars the aerodynamic drag force is the dominating resistance force at higher velocity. The car body is often optimized for reducing the drag resistance but one region where the aerodynamic development has not reached its full potential is the design of the underbody. To explain the aerodynamic force in a simplified manner the resisting drag originates from the pressure difference between the stagnation pressure in the front and the base pressure at the rear. By reducing the difference in pressure the drag force will be reduced hence the fuel consumption will be reduced. A device to improve the aerodynamics that is used on sports- and racing-cars is a diffuser, with lower ground clearance the diffuser generates downforce and aid the braking, cornering and acceleration. Using a diffuser on a passenger car, with higher ground clearance, will improve the pressure recovery on the underbody and the base pressure will be increased. To get an effective diffuser a flat underbody is preferred which also contributes to reducing the resisting drag force. In this study the drag reduction effect of a diffuser with panels on the underbody have been studied on the SAAB 9-3 Sport sedan and the SAAB 9-3 Sports wagon. To measure the effect of altering the underbody Computational Fluid Dynamics (CFD) simulations has been performed for the analysis, i.e. no wind tunnel tests have been performed. The simulations showed that a great improvement of the aerodynamic drag force can be achieved with a flat underbody and a diffuser. It was also found that the rear-end of the vehicles has an effect of the diffusers effect, a steeper diffuser is to prefer on a rear-end with steep rear-windscreen e.g. the sedan. Different additional aerodynamic devices and diffuser designs was simulated to find the most effective drag reduction setup, it was found that the most effective configuration consisted of a diffuser with covered rear rims. A reason for the drag reduction was found that the turbulent crossflow through the rims was prevented which was advantageous for the pressure recovery and overall streamlining of the pressure wake behind the vehicle. This study has shown that there are still possibilities to improve the aerodynamics of vehicles, especially at the underbody. By implementing panels at the underbody and a diffuser the drag resistance can significantly be reduced and hence a lower fuel consumption.

Nyckelord: CFD, Diffuser, Drag reduction, Underbody, Aerodynamics, Pressure recovery, Wake

Publikationen registrerades 2011-07-07. Den ändrades senast 2013-04-04

CPL ID: 143210

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