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

Sammanfattning: Environmental issues and increased fuel prices are driving forces for the automotive manufacturesto develop more fuel efficient vehicles with lower emissions. Large investments are aimed atminimizing power needed for propulsion i.e. new downsized engines with new aerodynamic devicesfor drag reduction. For passenger cars the aerodynamic drag force is the dominating resistanceforce at higher velocity. The car body is often optimized for reducing the drag resistance but oneregion where the aerodynamic development has not reached its full potential is the design of theunderbody.To explain the aerodynamic force in a simplified manner the resisting drag originates from thepressure 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 consumptionwill be reduced. A device to improve the aerodynamics that is used on sports- and racing-carsis 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, willimprove the pressure recovery on the underbody and the base pressure will be increased. To getan effective diffuser a flat underbody is preferred which also contributes to reducing the resistingdrag force.In this study the drag reduction effect of a diffuser with panels on the underbody have beenstudied on the SAAB 9-3 Sport sedan and the SAAB 9-3 Sports wagon. To measure the effectof altering the underbody Computational Fluid Dynamics (CFD) simulations has been performedfor the analysis, i.e. no wind tunnel tests have been performed. The simulations showed that agreat improvement of the aerodynamic drag force can be achieved with a flat underbody and adiffuser. It was also found that the rear-end of the vehicles has an effect of the diffusers effect, asteeper 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 mosteffective drag reduction setup, it was found that the most effective configuration consisted of adiffuser with covered rear rims. A reason for the drag reduction was found that the turbulentcrossflow through the rims was prevented which was advantageous for the pressure recovery andoverall 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 dragresistance can significantly be reduced and hence a lower fuel consumption.