A Numerical Wind Tunnel Study On Distance Between Tail Wing And Rear Hood And Angle Of Attack Of Tail Wing Of A Car Model Using CFD

The aerodynamic characteristics is one of the most important parameters for road vehicles (especially for passenger cars and racing car), which has a great effect on the performance, fuel economy and handling quality. As the roads condition improved, vehicles have more and more time driving in high velocity With the increase of vehicle's velocity, the aerodynamic lift produced by this kind of varies will become heavier, which will reduce the ground adhesion and can do harm to the performance and lower the passengers'security. To reduce the aerodynamic lift of the car with high velocity, we could set a tail wing on the rear hood of the car. Because different position of the tail wing to set and different shape of the tail wing have different influence in aerodynamics. So, it is important to choose the suitable location parameter and shape parameter to improve the aerodynamics characteristics of the car with high velocity.In this article, we do aerodynamic characteristics analysis on two position parameters which are distance between tail wing and rear hood and angle of attack of the tail wing to research on the influence of the distance and the angle of attack in aerodynamic characteristics. The distance to research on is h=0.6c,h=0.8c和h=1.0c,additional, the c is the chord length of the tail wing. As is known in the past research, the distance h has tight relation with the chord length c, so they present together in this article. The chord length is a fixed value 170 mm in the article; the angle of attack to research on is 0°,15°,30°,45°and 75°.In this article, we research on the influence of the different tail wings in aerodynamic characteristics of the car with high velocity by CFD. The same to the wind tunnel experiment, CFD is an effectual method to investigate the flow field around the car body and the influence of the flow field, further more, CFD has faster speed and lower cost.In this article we build three wagon type notch back 1:1 models with three different distance and five different angle of attack by Rhinoceros and do some predigestions to the rearview mirror , doorknob and so on.We mesh the models by ANSYS ICEM CFD. The computational domain must large enough to simulate the real drive condition better. In this research, we set the computational domain as a cuboids; the inlet before the front of the car three times of the length of the car, the outlet behind the back of the car seven times of the length of the car, the total length is eleven times of the length of the car; the width of the cuboids is seven times of the length of the car. The width of two sides of the car is three times of the width of the car; the height is the five times of the height of the car. Because of the complicated bend surface of the car body, we mesh the tetrahedral which has a good adaptability on the car surface and six layers prism upon the car surface to simulate the boundary layer, the total height of the prism is 20 mm. we mesh hexahedral between tetrahedral and the big computational domain to reduce the total number of the mesh ,save the computational time and advance the precision. The total number of the mesh is two millions, more or less.As followed, we lead the mesh to FLUENT and use it to simulate. FLUENT is based on Finite Volume Method to disperse, this simulation adopt implicit segregated solver which bases on pressure. The turbulent model use Standard k ?εmodel. The coupling method of pressure and velocity is SIMPLE corrected pressure method and all the correlative equations dispersing format is Second Order Upwind. Near wall domain use Standard Wall Function to simulate. Residual is 0.001. The inlet boundary condition of the computational domain is upright velocity inlet and the value is 30m/s(108km/h)to simulate a running car which velocity is 30m/s. Turbulence parameters are turbulent kinetic energy k and turbulent dissipationε, the value uses the experiment formula to obtain, they are 0.03375 and 0.01593; The outlet boundary condition is pressure outlet, its value is standard atmosphere. To simulate the relative motion between the car and the ground, the ground boundary condition is moving with the same velocity of air flow.After about 300 iterations in FLUENT software, the computation converged. Aerodynamic force and coefficient could be directly obtained. We could contrast and analyze the pressure and velocity distribution with the module of post processing.By analyzing aerodynamic force and distribution of pressure and velocity on car and wing surface of three different distance models, we could know that the tail wing obtains the max down force when the distance is 0.8c; in the same way, the tail wing obtains the max down force when the angle is 15°in the five different angle of attack models and the drag force of the car is smallest, so, 15°is the suitable and widely used. There are relation between the distance and the angle in influence of aerodynamics of car. After integrally analyze simulation result. So, the following points can be concluded:1. The sedan installing tail wing will produce much greater aerodynamics forces than that without the tail wing;2. As the angel of attack turns wide, the wake will get bigger and bigger, take on curved tendency,15 degree is the suitable angle;3. We get the maxim pressure of the tail wing when the distance is 0.8C and the minimum drag coefficient when the distance is 0.6C; 4. Under the other parameters unchanged, with the increase of angle of attack, the drag and negative lift is also increase. But the side force has no change;5. CFD can provide us the reasonable qualitative results about the exterior flow to some extent. So when short of test conditions, we can use CFD as a method of studying the theory, which may lay definite foundations for the future tests.