空气动力学复习资料完整版.docx

1、空气动力学复习资料完整版Aerodynamic drag is usually insignificant at low vehicle speed but the magnitude of air resistance becomes considerable with rising speed.(气动阻力在低速时并不重要，但随着车速的升高，它将越来越重要)14.1 Viscous air flow fundamentals (粘性气流基本原理) When air flows over any surface, air particles in intimate contact with t

2、he surface loosely attach themselves so that relative air velocity at the surface becomes zero. (与物体表面亲密接触的空气的相对速度为零) The relative air velocity further out from the surface rises progressively between air layers until it attains the unrestricted main airstream speed.（相对空气速度会随着离物体表面距离的增加而增加，直到它达到空气来时

3、的速度）14.1.1Boundary layer(边界层) 边界层又叫附面层，是指贴近固壁附近的一部分流动区域，在这部分区域中，沿着固壁面切向速度由固壁处的0速度发展到接近来流的速度，一般定义为在边界处的流速达到来流流速的99%。在这部分区域中，由于厚度很小，故速度急剧变化，速度梯度很大，流体的粘性效应也主要体现在这一区域中。边界层内的流动也分为层流和紊流，相应的也分为层流边界层和紊流边界层。对于又有层流又有紊流的边界层又称为混合边界层。14.1.2 Skin friction(表面摩擦) The skin friction is the viscous resistance generate

4、d within the boundary layer when air flows over a solid surface. （表面摩擦其实就是空气流过固体表面边界层产生的粘性阻力） Skin friction is dependent on the surface area over which the air flows, the degree of surface roughness or smoothness and the air speed.(表面摩擦取决于空气流过的表面积、表面粗糙或平滑度以及气流速度)14.1.3 Surface finish（表面粗糙度） The thic

5、kness of the boundary layer is influenced by the surface finish. (边界层厚度受表面粗糙度影响) The thicker boundary layer associated with a rough surface will cause more adjacent layers of air to be sheared, accordingly there will be more resistance to air movement compared with a smooth surface.（越光滑的表面边界层越薄，而较粗糙

6、表面的边界层更厚，产生的相邻层切变更多，因而阻力越大。）14.1.4 Venturi(喉管) ：截面收敛处流速增大，压力随之减小。14.1.5 Air streamlines (空气流线) These streamlines broadly follow the contour of the body but any sudden change in the cars shape compels the streamlines to deviate, leaving zones of stagnant air pockets. The further these streamlines are

7、 from the body the more they will tend to straighten out. (车身形状的突变将带来流线的破坏，产生滞流气孔区；流线远离车身后将趋于伸直。)14.1.6 Relative air speed and pressure conditions over the upper profile of a moving car(运动车辆外部轮廓的相对气流速度与压力情况)可以参考喉管效应来解释，车辆中间上面部分可视为喉管的收敛部分；车辆上部轮廓：发动机罩后半部和前挡风窗前部为正压，后挡风窗中部到行李箱后部为正压（The pressure over the

8、 rear half of the bonnet to the mid-front windscreen region where the airstream speed is slower is positive）,其余部分为负压(the pressure over the front region of the bonnet and particularly over the windscreen/roof leading edge and the horizontal roof area where the airstream speed is at its highest has a

9、negative pressure)车辆下部轮廓：基本为一个较小的负压.14.1.7 Lamina boundary layer(层流边界层)在低速流动时，液体质点互不干扰，液体的流动呈线性或层状，且平行于管道轴线，此种流动状态称为在层流14.1.8 Turbulent boundary layer（紊流边界层）当流速大时，液体质点的运动杂乱无章，除了平行于管道轴线的运动外，还存在着剧烈的横向运动，此种流动状态称为紊流。14.1.9 Lamina/turbulent boundary layer transition point 层流紊流突变点（TP） As the speed of the

10、vehicle rises the transition point tends to move further to the front , therefore less of the boundary layer will be lamina and more will become turbulent; accordingly this will correspond to a higher level of skin friction. (车速上升TP前移层流减少，紊流增加表面摩擦阻力上升)气流再附着14.1.10 Flow sepration and reattachment （气流

11、分离和再附着）分离气泡气流分离除非有轮廓突变，否则气流将紧贴车辆轮廓运动。流线型车辆的气流在将要到达行李箱位置处才分离，而一般车辆将经历气流分离（发动机罩和前挡风玻璃上），再附着（前挡风玻璃顶部或者车顶的前端），再分离（车顶后端，后挡风玻璃顶部）的过程，从而在发动机罩和前挡风窗间以及后挡风窗和行李箱之间产生分离气泡。 The space between the separation and reattachment will then be occupied by circulating air which is referred to as a separation bubble, and

12、if this rotary motion is vigorous a transverse vortex will be established. (分离处和再附着处之间充斥着分离气泡，如果旋转运动是剧烈的，那么建立一个横向的漩涡) 这个后面会有提到14.2 Aerodynamic drag（气动阻力）14.2.1 Pressure drag（压差阻力）固体前的气流压力大气压力产生一个沿气流方向的阻力固体后的气流压力大气压力 涡流 Pressure drag can be reduced by streamlining any solid form exposed to the air fl

13、ow, for instance a round tube (Fig. 14.12(b) encourages the air to flow smoothly around the front half and part of the rear before flow separation occurs thereby reducing the resistance by about half that of the flat plate. The resistance of a tube can be further reduced to about 15% of the flat pla

14、te by extending the rear of the circulating tube in the form of a curved tapering lobe, see Fig. 14.12(c). Even bigger reductions in resistance can be achieved by proportioning the tube section (see Fig. 14.12(d) with a fineness ratio a/b of between 2 and 4 with the maximum thickness b set about one

15、-third back from the nose, see Fig. 14.12(e). This gives a flow resistance of roughly one-tenth of a round tube or 5% of a flat plate。压差阻力可以通过使暴露在空气流动中的固体外形流线化而降低的。例如，一个圆管可以使气流在发生分离前平滑地流过它的前半圆以及尾部的一部分，从而减少了大约一半的平板阻力（b图）。在这基础上延伸一下它的尾部还可以降低15%的平板阻力（c图）。选择合适的比例a/b，还可以进一步减少阻力。14.2.2 Air resistance oppos

16、ing the motion of a vehicle（阻碍车辆运动的空气阻力）压差阻力仪器 假设空气微粒撞到平板后速度变为零，通过撞击前后动量的变化求施加在板上的作用力，每秒动量损失为mv=pvAv=pAv ,故理论上板上的作用力为pAv N 实际上这个作用力F并非等于pAv ，而是与它成一定比例，即F=Cd*A*v ，Cd为空气阻力系数，取决于受到气流冲击的物体表面的形状。14.2.3 After flow wake（尾涡） This is the turbulent volume of air produced at the rear end of a forward moving ca

17、r and which tends to move with it. The wake has a cross-sectional are a equal approximately to that of the rear vertical boot panel plus the rearward projected area formed between the level at which the air flow separates from the downward sloping rear window panel and the top edge of the boot.（在运动中

18、的车辆尾部产生的涡流，趋于与车辆一起运动，从汽车侧面看，包括行李箱的垂直部分加上行李箱顶部与后挡风玻璃连接处对应的部分）14.2.4 Drag coefficient（空气阻力系数） The aerodynamic drag coefficient is a measure of the effectiveness of a streamline aerodynamic body shape in reducing the air resistance to the forward motion of a vehicle.（空气阻力系数是衡量流线型车身形状减少运动车辆空气阻力的效果） A lo

19、w drag coefficient implies that the streamline shape of the vehicles body is such as to enable it to move easily through the surrounding viscous air with the minimum of resistance; conversely a high drag coefficient is caused by poor streamlining of the body profile so that there is a high air resis

20、tance when the vehicle is in motion.（低的阻力系数表示它的车身轮廓流线型好，有利于汽车克服周围的粘性气体而运动，反之，高的阻力系数将增大前进阻力）14.2.5 Drag coefficients and various body shapes （不同表面形状的阻力系数）(a) Circular plate : Air flow is head on, and there is an immediate end on pressure difference. Flow separation takes place at the rim; this provid

21、es a large vortex wake and a correspondingly high drag coefficient of 1.15圆盘 气流垂直冲击板面，即刻产生一个指向后端的压力差，边缘发生气流的分离，形成较大的尾涡和一个相应较大的阻力系数 cd=1.15(b)Cube : Air flow is head on but a boundary layer around the sides delays the flow separation; nevertheless there is still a large vortex wake and a high drag co

22、efficient of 1.05.立方块 表面受到气流的垂直冲击，侧面的边界层延缓了气流的分离，但仍有较大的尾涡和比较高的阻力系数 cd=1.05(c)Sixty degree cone: With the piecing cone shape air flows towards thecone apex and then spreads outwards parallel to the shape of the cone surface. Flow separation however still takes place at the periphery thereby producing

23、 a wide vortex wake.This profile halves the drag coefficient toabout 0.5 compared with the circular plateand the cube block.( 六十度圆锥 尖顶的存在使得气流沿圆锥的母线流动，但在底圆的边缘仍旧发生气流的分离，产生较大的尾涡，但其阻力系数相对于圆盘和立方块减少了约一半 cd=0.5)(e)Hemisphere:Air flow towards and outwards from the centre of the hemisphere. The curvature of

24、the hemisphere gradually aligns with the main direction of flow after which flow separation takes place on the periphery. For some unknown reason (possibly due to the very gradual alignment of surface curvature with the direction of air movement near the rim) the hemisphere provides a lower drag coe

25、fficient than the cone and the sphere shapes this, being of the order of 0.42半球 与球形的区别在于半球后部的轮廓曲面的走向与气流方向一致，因而其cd值进一步下降，cd=0.42(f)Tear drop (Fig. 14.15(f ) If the proportion of length to diameter is well chosen, for example 0.25, the streamline shape can maintain a boundary layer before flow separat

26、ion occurs almost to the end of its tail. Thus the resistance to body movement will be mainly due to viscous air flow and little to do with vortex wake suction. With these contours the drag coefficientcan be as low as 0.05.泪滴形 只要长度与直径的比例选择得当（0.25），其流线型可保证边界层在气流流经泪滴尾端时才发生分离，运动的阻力主要由气流的粘性作用产生，与尾涡基本无关，

27、cd=0.0514.2.6 Base drag 底部阻力 由尾涡的负压所引起的阻力 it is impractical to design a tear drop body with an extended tapering rear end, but if the tail is cut off (bobtailed) at the point where the air flow separates from the contour of the body (see Fig. 14.16(b), the same vortex (negative pressure) exists as i

28、f the tail was permitted to converge. (设计一个带有逐渐变细的尾部的泪滴形车身是不实际的，但是如果把气流分离处尾部的车身去掉，尾涡还是一样的。)14.2.7 Vortices（涡流）圆柱状的旋转气流，边缘的旋转速度最小，而随着半径的增大，旋转速度减小，中心附近的旋转速度最大；粘性切变产生于静态的中心和快速运动的外层气旋之间的相邻层上，因而涡流内的压力低于大气压，中心附近的压力要更低一些。14.2.8 Trailing vortex drag（后缘涡流阻力） The air pressure will therefore be higher in the s

29、lower underfloor airstream than that for the faster airstream moving over the top surface of the car. Now air moves from high to low pressure regions so that the high pressure airstream underneath the car will tend to move diagonally outwards and upwards towards the low pressure airstream flowing ov

30、er the top of the body surface (see Fig.14.18(b). Both the lower and the upper airstreams eventually interact along the side-to-top profile edges on opposite sides of the body to form an inward rotary air motion that continues to whirl for some distance beyond the rear end of the forward moving car,

31、 see Fig. 14.18(a and b). The magnitude and intensity of these vortices will to a great extent depend upon the rear styling of the car. The negative (below atmospheric) pressure created in the wake of the trailing vortices at the rear of the car attempts to draw it back in the opposite direction to

32、the forward propelling force; this resistance is therefore referred to as thetrailing vortex drag(车辆底部气流压力大于顶部，高压气流向低压区移动，于是底部气流斜向朝外往车顶运动，在车辆的侧面和顶面交际处产生一个向内的旋转气流，这样的气流在离开车辆后端一段距离之后仍能保持运动，其大小很大程度上取决于车辆的尾部样式，后缘涡流的负压将使车辆的运动受阻，这个阻力就是后缘涡流阻力。)14.2.9 Attached transverse vortices（附加横向涡） Separation bobbles which form between the bonnet(hood) and front windscreen, the rear screen and boot(trunk)lid and the boot and rear light panel ten