CFD Simulation Of The External Flow Around A Car Body

With the increasing of velocity of cars, aerodynamic drag on the cars increase rapidly. Experiments show that at the speed of 65 kilometers per hour, aerodynamic drag contributes to 1/2 of the total drag on the car-body. If the velocity increases more, aerodynamic drag and aerodynamic moment due to the exterior airflow increases dramatically, which influence greatly the dynamic properties, the fuel economy and the operating performance of cars.At present, there are two methods in the research of car aerodynamics: experiment and numerical calculation. Traditional aerodynamic development employs the use of partial-scale or full-scale models of the proposed car-body configuration in wind tunnel test, which is a main technology of aerodynamic test. In the past few decades,with the development of the calculation technique and the advance of the computer, computational fluid dynamics (CFD) has become more and more as a modern alternative for reducing the use of wind tunnels in automotive engineering. Now CFD is widely applied to various stages of aerodynamic design of automobiles. By contrast to the tunnel test, CFD is cheaper and takes less time to complete. However, the present CFD technology has still many problems to be solved for turbulence, and it seems to be difficult to find an appropriate turbulence model to universal situations. Numerical analysis can not take the place of tunnel test, and tunnel test is very important to revise the CFD method and check the result of CFD.In this paper, firstly, the aerodynamics influence on the performances of the car and the characters of the airflow around car-bodies are introduced. In chapter 3, the mathematic and mechanic theories of the numerical calculation for the aerodynamics are presented. In chapter 4, the software of Star-cd and its characteristics are outlined. In chapter 5, using the Star-cd software with UG18.0, the 3-dimensional airflow around a car-body is numerically simulated, and the results are analyzed in detail and the predicted aerodynamic drag coefficient and aerodynamic lift coefficient of the car are 0.306 and 0.176 respectively. Then some advices on the shape of car-body are given. Through this simulation, the exterior air flows around the car-body are understood in detail. The air flow above the upper surface of the car-body separates earlier at the very front of the rear window, which increases the pressure difference drag, so smoothness of the transition area between the roof and the rear window is critical to reduce the total drag on the car. The air flow below the chassis turns to be turbulent and the thickness of the turbulent boundary layer adjacent to the chassis increases rapidly due to the disturbance at the very start and the influence of the ground. Therefore, arranging the clearance suitably and designing the shape of the head of the car to distribute the flow below the chassis properly are the main methods to solve the problem of too large lift on the car.The flow characters of the car in this simulation can apply to most homemade analogous cars.