race car aerodynamics designing for speed 空气动力学.docx

1、race car aerodynamics designing for speed 空气动力学Race car aerodynamics,designing for speedFig.1-7. Trends showing the increase of the maximum cornering acceleration over the past years for race cars with and without aerodynamic downforce.Fig.1-11 Schematic description of the “ground effect” that incre

2、ases the aerodynamic lift of wings when placed near the ground.Fig.2-1. Streamlines in a steady-state flow over an airfoil.By observing several streamline traces in the flow(as in Fig.2-2), it is possible to see if the flow follows the vehicles body shape close to its surface. When the streamlines n

3、ear the solid surface follow exactly the shape of the body(as in the upper portion of Fig.2-3) the flow is considered to be attached. If the flow does not follow the shape of the surface(as seen behind the vehicle in Fig.2-2 and in the lower part of Fig.2-3) then the flow is considered detached or s

4、eparated. Usually such separated flows behind the vehicle will result in an unsteady wake flow, which can be felt up to large distances behind the vehicle. As we shall see later, having attached flow fields is extremely important in reducing aerodynamic drag and/or increasing downforce.Fig.2-2. Visu

5、alization of streamlines(by smoke injection) during a wind-tunnel test. Courtesy of Volkswagen AG.Fig.2-3. Attached flow over a streamlined car(A), and the locally separated flow behind a more realistic automobile shape(B).Fig.2-4. Side view of the velocity distribution near a flat plate in a free s

6、tream, V(velocity profile on the upper surface is described by the V vs z graph)Now, leaving momentarily the discussion about the basic definitions, we can observe a couple of interesting features in Fig.2-4. First, the air velocity near the surface comes to a halt! This is known as the “no-slip-con

7、dition.” The fluid particles touching the body will stick to the surface; they have no relative velocity. Farther away from the solid body the velocity increases, until it equals the local free-stream value. This thin boundary is termed the boundary layer and will be discussed with more detail later

8、.Fig.2-5. Fluid particle traces in laminar and in turbulent flow.Fluid Properties: e.g., temperature, pressure, density, viscosity., etc. Viscosity is, in a very generic sense, a measure of fluid resistance to motion (similar to friction), and is designated by the Greek symbol . The effect of viscos

9、ity in a fluid can be demonstrated by the simple example shown in Fig.2-6(following the analogy to dry friction) where a viscous fluid is placed between two parallel, solid surfaces. The lower surface is stationary, while the upper one is moving to the right at a constant speed. The fluid particles

10、near the two walls tend to stick to the solid surface and maintain a zero relative velocity (this is the previously mentioned no-slip condition). Fig.2-6. Velocity distribution between two parallel plates, caused by the motion of the upper plate. The lower plate is stationary, and the upper one is m

11、oved by the force F at a constant speed V. The magnitude of the shear force F can be connected to the speed of the upper plate and to the viscosity of the fluid by the relation: As an example, assume that the upper plate with an area of A=1m2 is being pulled at a speed of 5m/s. The fluid between the

12、 two surface is water, and the separation distance is 0.02m. Taking the value of the viscosity coefficient from Table 2.1 we can calculate the force F required to pull the plate as: The Reynolds number: The Re number represents the ratio between inertial and viscous (friction) forces created in the

13、air and is defined by the following formula:Here (pronounced rho) is the fluid density, is the viscosity, V represents the velocity, and L is some characteristic length (of the velocity, for example ).An important feature of this number is that it is nondimensional, that is, the units cancel out (ev

14、en if we use British, USA, or European units). For a typical numerical value of the Reynolds number we can assume a car length of 4m and a speed of 30m/sec, and use the properties of air from Table 2.1, thus; For example, for Reynolds number values(base on the car length) of less than 105 the flow o

15、ver wings will be laminar and the drag and the lift obtained at this rang may be considerably different than at the higher values of the Reynolds number. Returning to the case of a race car piercing its way through air(with very small viscosity) we find that the Reynolds number will be on the order

16、of several millions. But if the same vehicle moves through a highly viscous fluid such as motor oil then the Reynolds number will be far less. 雷诺数代表流场中物体所受的惯性力与粘性力的比。因此，Re数越小的流动，粘性作用越大（相对于惯性力来说）；Re数越大的流动，粘性作用越小。 Another interesting feature of the Reynolds number is that two different flows can be co

17、nsidered similar if their Reynolds number are the same. A possible implementation of this principle may apply when exchanging water tunnel for wind tunnel testing, or vice versa. Typical gains are in reduced model size, or in lower test speeds. Foe example, the ratio of viscosity/density in air is a

18、bout 15 times larger than in water; therefore, in a water tow tank much slower speeds can be used to test the model at the same Reynolds number(and this has been done but seems not be very practical for automobile testing). A more practical application of this principle would be to test a 1/15-scale

19、 submarine model in a wind tunnel at true water-speed condition. Usually it is better to increase the speed in the wind tunnel and then even a smaller scale model can be tested. Boundary layer: The concept of a boundary layer can be described by considering the flow past a two-dimensional flat plate

20、 submerged in a uniform stream, similar to the one shown in Fig.2-4. This layer of rapid change in the tangential velocity(shown schematically by the velocity profile in Fig.2-4) is called the boundary layer, and its thickness (delta) increases with the distance along the plate. The boundary layer e

21、xists on more complicated shapes, as well,(e.g., the automobile shown in Fig.2-7).A typical velocity profile within this layer is described by the inset on this figure. The thickness of this boundary layer is only several mm at the front of a car traveling at 100 km/hr, and can be several cm thick t

22、oward the back of a streamlined car. As you will see, a thicker boundary layer creates more viscous friction drag. Furthermore, a too steep increase in this thickness can lead to flow separation, resulting in additional drag and a loss in the downforce created by a race cars wings.Fig.2-7. Boundary

23、layer near a vehicles surface, and typical velocity distribution within this layer Fig.2-8. Variation of the boundary layer thickness along a flat plate. Note the velocity distribution inside the boundary layer and its increase in thickness during the transition from laminar to turbulent flow. The S

24、kin-Friction Coefficient: The skin-Friction coefficient Cf is a nondimensional number (independent of units) indicating the level of friction between the vehicle surface and the air. It is defined as: Where is the surface shear force per unit surface(friction resistance) and it is nondimensionalized

25、 by the quantity (called the dynamic pressure) so that the numerical value of will be(almost) independent of speed. For example, if the friction coefficient is Cf = 0.002 (from Fig.2.9) and the air moves over the plate at a speed of 30m/sec(108km/hr) then the shear force per unit area(1m2) is: And t

26、he density of air was taken from Table 2.1. Now in terms of the effect of speed on friction, note that the boundary layer thickness decreases as airspeed increases. This is due to the larger momentum (the product of mass times velocity) of the free stream compared to the loss of momentum caused by t

27、he viscosity near the solid surface. Therefore, the friction coefficient (that contributes to the vehicles drag) will be reduced with increased flow speed.Fig.2-9. Skin-friction coefficient Cf values on a flat plate, placed parallel to the flow, for laminar and turbulent boundary layers, versus the

28、Reynolds number.The interesting observations on this figure are that there are two separate curves: one for laminar and one for turbulent flow, and that both decrease with increased Reynolds number. Furthermore, for a large range of the Reynolds number, both turbulent and laminar flows (sometimes 4

29、to 5 times less) which means that for the purpose of drag reduction, laminar flow is preferred.We can draw a few important conclusions about the boundary layer:1、Boundary layer thickness is larger for turbulent than for laminar boundary layers.2、The skin friction coefficient becomes smaller with inc

30、reased Reynolds number (mainly for laminar flow)3、At a certain Reynolds number range both laminar and turbulent boundary layers are possible. The nature of the actual boundary layer for a particular case depends on flow disturbances, surface roughness, etc.4、The skin friction coefficient is consider

31、ably larger for the turbulent boundary layer (larger skin friction results in larger friction drag).5、Because of the momentum transfer normal (perpendicular) to the direction of the average speed, in the case of a turbulent boundary layer, flow separations will be delayed somewhat compared to a lami

32、nar boundary layer. This is an important and indirect conclusion, but in many automotive applications it forces us to prefer turbulent boundary layers in order to delay flow separation. Fig.2-10. Schematic description of the laminar bubble and the transition from laminar to turbulent boundary layer A typical case is demonstrated in Fig.2-10, where the boundary layer on a streamlined hood is initially laminar. However, due to the large curvature of the upper surface the laminar boundary layer separates initially, then reattached