A Parametric Study On The Diffuser And Ground Clearance Of A Simplified Car Model Using CFD

As the roads condition improved, vehicles have more and more timedriving in high speed, which ask higher demands for vehicles' handlingstability, safety,economy and comfort. Aerodynamics is one of the mostimportant factors for vehicles' capability of handling stability, safety,economy and comfort. Therefore, it is very important that studying on theaerodynamics of vehicles and through this to improve vehicles' handlingstability, safety,economy and comfort.In the past, lots of researches are focus on aerodynamic charactersof vehicles' head, top and tail. And researches on vehicles' bottom arequit rare. However vehicles' shape optimization is limited by modern cars'styling. Based on the research of some researchers, the results showedthat drag caused by vehicles' bottom, wheels and wheels' room cost moreof total drag. They think that the drag of modern vehicle mainly come fromthe flow of underbody. So it is reasonable to do more research on theseplaces to improved aerodynamic characters.To well understand the air flow in the bottom of vehicles and theinteraction between the air flow and underbody, providing the foundationof improving the whole vehicles' aerodynamic characters, it is needed thatstudying on many of kind parameters about underbody of vehicle throughexperiment. The parameters of Ground Clearance, Diffuser Angle, thewidth of Tire, Track, Wheel housing radius, Front Overhang and RearOverhang under the bottom of vehicle have influence on the aerodynamiccharacters of vehicle. The researchers of the Pinifanina wind tunnel havedone wind tunnel experiment about above parameters. In this paper, bysimulation of CFD, the parameter of diffuser angle and ground clearancewould been studied, and the results of CFD would been compared withthose of wind tunnel.To compare with the results of wind tunnel, the SAE model that hasbeen used in wind tunnel was selected as the model of simulation. Thedifferent flows around whole vehicle with the diffuser angle of 0°, 6°and 12° and the ground clearance of 200, 150, 100 and 50 millimeterhave been simulated. The ground clearance is fixed at 200 millimeterwhen studying the vary of diffuser angle and the diffuser is fixed at 12°as the vary of ground clearance. The surface of underbody is simplifiedas plane except the diffuser.First, using UG (Unigraphics) software to set up all kinds of modelsand the domains of simulation, these models with domain were input themeshing software ICEM-CFD. Finally, the mesh of simulation is input theCFD software Star-CD to simulate. By comparing with the results of windtunnel, the post processing is obtained.To simulate the station of vehicle on road, the domain should beenough spacious. In this paper, the length between inlet and the front endof vehicle is 3 times of the over length of vehicle, and between the outletand the rear end is 7 times. The width of domain is 3 times of the width ofmodel in both sides. The height of domain is 5 times of the over height ofmodel.The mesh is hybrid mesh to make easier and more precision of theresult because there have tire and wheel housing. There is a small cubearound the vehicle whose surface offset from the car surface 500millimeter. Between this cube and vehicle is tetrahedral mesh, andbetween the whole domain and this cube is hexahedra mesh. Finally, thesurface of vehicle is tetrahedral mesh to simulate the vary of curvature ofsurface of vehicle.The k ?ε high Renault turbulent model is used in this simulation.The velocity of inlet is 39m/s, and the same velocity is given to the ground.The top and sides are fixed.By analyzing the distributing of the pressure and the velocity, and thecoefficient of aerodynamics that obtained from the simulation, as well ascompared with the results of wind tunnel experiment, the influence of thevary of diffuser angle and ground clearance to the aerodynamiccharacters of whole vehicle is obtained as following:In the station of the underbody surface simplified as plane, with thediffuser angle increasing from 0° to 12°, the lift decrease, and the dragdecreased firstly then increased.Further research about four different gap distances between groundand vehicles with 12 degree diffuser is made to analyze vehicles flow field.By comparing with the results of wind tunnel tests, the influence aboutground gap distance is considered. The results show that, as ground gapdistance decreasing, lift decreased firstly. When the gap becomes smallerand smaller so that the viscous of the air is considered as a main factor,the lift started to increase as the gap decreased. And for the drag, thenarrower the gap is, the smaller the drag is.