Numerical Simulation Of Sedan's Aerodynamic Characteristics Improved By Spoiler

The aerodynamic characteristics is one of the most important parameters forroad vehicles (especially for passenger cars), which has a great effect on theperformance, fuel economy and handling quality. And the main factor of affectingthese performances is the flow characteristics around the vehicle, whichdetermines the forces and moments in the six degrees of freedom.With the rapid development of our national highway and the advance ofvehicle performance, the automobile travels at higher speed. People put forwardhigher and higher requirements to the handling stability, safety and comfort of aroad vehicle. With the increase of vehicle's velocity, the flow and pressure fieldaround the vehicle varies continuously. As a result, the aerodynamic lift producedby this kind of varies will become heavier, which will reduce the ground adhesionand can do harm to the performance and lower the passengers' security. Theaerodynamic lift mainly influences the automobile stable quality on two aspects:Firstly, at high speed, the automobile will generate enough lift which can causethe vehicle "float and unstable" and the ability of maintaining prearranged courseand handling quality will decline evidently. Secondly, when airflow appearsvelocity heft in the landscape orientation relative to the automobile (for exampleside wind or turning), the automobile will depart the original route if the pressurecenter of wind locates in the front of the body, and with the influence of liftproduced at high speed, the situation of the automobile will become dangerous.Considering the security aspect, the reduction of the aerodynamic lift perhaps ismore important than that of the aerodynamic resistance which influences thevehicle's performance and economy. Therefore, it is beneficial to do someresearch on the relationship between the factors that affect the aerodynamic lift ofthe specific model.Many factors influence the lift, such as rolling wheels, gap betweenautomobile and ground, part shape of the body and so on. To reduce the lift, weshould firstly consider from the whole mold of the automobile and try our best tochange the unreasonable aerodynamic lift distribution. What's more, we canacquire negative lift through designing special aerodynamic attachments, of whicha negative lift wing is a very important one and it will produce much greateraerodynamic forces than the automobile itself. The working principle of thenegative lift wing is an upside-down airplane wing. This upside-down wing willproduce a downward force that will counteract a part of aerodynamic lift, so that aadhere force between the wheels and the ground is increased, and the impetus andthe stableness of a high speed automobile is also improved. This paper is to makesome research on these aerodynamic attachments. When we do this job, designerssometimes are puzzled by various design details which influence each other. It isthese design details that often come into being incompatible result, but we still canfollow some basic principles and only these factors should be in consideration.However, even these basic principles for shape are often incompatible with eachother and published works in this domain also have the different view on thispoint. During the process of reducing aerodynamic lift, the reason of lackingdefinite and exact normativeness is possible the fact: the comprehensive effect ofvarious kinds of characteristic we concerned is deferent according to the shape,which will blur the argument.This paper firstly presents basic theory of automotive flow field and analyzesthe characteristics of automotive flow field and then introduces several arithmeticmethods. Then the writer will give a bird view of fluid dynamics control equation,turbulent model and grid discretization etc. Secondly, simply introduce the basictheory of negative lift.In this article, the computational model is k ?ε high Reynolds Numberturbulence model. Although it has many shortcomings, it is used widely becauseof higher solution ability, shorter computational time and lower requirement forthe mesh quality.And then on the basis of Ford Variable Geometry Model commended bySAE2004-01-1308, I research its aerodynamic lift by fixing the rear spoiler withdifferent degrees on the model. It needs high quality about the design andinstallation of the rear spoiler. And it is not only the angle of the installation butalso the length, the span, the shape of the wing that will take great effect on theflow field. Only from the point of angle, I illustrate how the obliquity of the rearspoiler influences the aerodynamic lift and the great effect in increasing theground adherence. This paper selects four rear spoilers with different obliquity by15deg, 30deg, 45deg and 60deg. So the following points can be concluded:1. The sedan installing rear spoiler will produce much greater aerodynamicsforces than that without the rear spoiler.2. Whether installing the rear spoiler or not, all the models take on the notchback characteristics on the wake.3. As the angel of the rear spoiler turns wide, the wake will get bigger andbigger, take on curved tendency.4. Under the other parameters unchanged, with the increase of obliquity ofthe rear spoiler, the drag and negative lift is also increase. This is coordinated withthe reality. But side force commonly is not to be effected. Flow in the wake reelsup obviously with the increase of obliquity of the rear spoiler, as a result the wakebecome larger.5. CFD can provide us the reasonable qualitative results about the exteriorflow to some extent. So when short of test conditions, we can use CFD as amethod of studying the theory, which may lay definite foundations for the futuretests.In the following research, we should pay our attention on the next severalaspects. Firstly, change the parameter of the velocity, and make sure that thevolume, direction and the turbulent flow will vary continuously with the timegoing on. Under this condition, the simulation is much alike to the reality so thatwe can reveal the characteristics of the flow field. Secondly, we should make acomprehensive research to the effects that the design of spoiler does on theaerodynamic characteristics.The results from the numerical simulation must be verified by tests. Atpresent because the numerical simulation of rear spoiler is not enough and thereare very few test's results to supply us. So in the future we must do wind tunneltests. On the one hand it can verify the results from numerical simulation, and onthe other hand can provide us with the meaningful conclusions, which willprovide theoretical foundation to the vehicle's safety.