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1、精品学习资源1.3 The Characteristics of Fluids流体的特点欢迎下载精品学习资源constituent ：组成的； tangential ：切向的； restrain ：限制、约束； equilibrium ：平稳， 均衡；interface ：相互关系、分界面；molecule ：微小颗粒、分子；continuum ：连续体；vessel：容器； tar ：焦油、柏油； pitch ：树脂；imperceptibly：发觉不到的， 微小的；restore ：复原；subside ：下沉、沉淀、减退、衰减； hypothetically ：假设地、假想地； sphe
2、re ：球、球体；microvolume ：微元体积；rarest ：最稀奇的，虽宝贵的欢迎下载精品学习资源A fluid is a substance which may flow; that is, its constituent particles may continuously change their positions relative to one another. Moreover, it offers no lasting resistance to the displacement, however great, of one layer over another. Th
3、is means that, if the fluid is at rest, no shear force that is a force tangential to the surface on which it acts can exist in it. A solid, on the other hand, can resist a shear force while at rest; the shear force may cause some displacement of one layer over another, but the material does not cont
4、inue to move indefinitely. In a fluid, however, shear forcesare possible only while relative movement between layers is actually taking place. A fluid is further distinguished from a solid in that a given amount of it owes its shape at any particular time to that of a vessel containing it, or to for
5、ces which in some way restrain its movement.流体是可以流淌的物质，也就是说，组成流体的质点可以连续的转变它们的相对位置；而且，不管层与层之间的相对位移有多大都不会产生长久的抗击力；这意味着流体在静止状态下是不会存在剪切力的 剪切力是与其作用外表相切的力；另一方面， 固体在静止时却可以抗击剪切力，其中的剪切力也可以使层与层之间发生相对位移，但是固体材料却不肯定会有连续的运动；然而在流体中，只有当层与层之间有相对运动产生时才会有剪切力存在；流体和固体的进一步区分仍在于在特定的时刻，确定数量的流体其外形取决于承载它的容器，或者取决于一些限制其运动的力；Th
6、e distinction between solids and fluids is usually clear, but there are some substances not easily classified. Some fluids, for example, do not flow easily: thick tar or pitch may at times appear to behave like a solid. A block of such a substance may be placed on the ground, but, although its flow
7、wouldtake place very slowly, yet over a period of time-perhaps several days-it would spread over the ground by the action of gravity, that is, its constituent particles would change their relative positions. On the other hand, certainsolids may be made to flow when a sufficiently large force is appl
8、ied; these are known as plastic solids. 固体和流体之间的区分通常是很明显的，但是也有些物质难于归类；比方说有些流体并不易流淌，如重油和树脂有时候会表现得像固体一样，像这样的一块物质假如放在地面上，虽然它的流淌发生的特别缓慢，要经过很长的一段时间或许要好几天，但是在重力的作用下它仍旧会在地面上扩散开来，也就是说，它的组成质点会转变它们之间的相对位置；另一方面，某些固体当足够大的力作用时也会“流淌”，这就是我们所知的“塑性固体”； Even so, the essentialdifference between solids and fluids remai
9、ns. Any fluid, no matter how thickis, boergviinsscotousfloitw, evenif imperceptibly, under the action of the slightest net shear force. Moreover, a fluid continues to flow as long as such a force is applied. A solid, however, no matter how plastic it is, does not flow unless the net shear force on i
10、t exceedsa certain value. For forces less than this value the layers of the solid move over one another only by a certain amount. The more the layers are displaced from their original relative positions, however, the greater are the forces resisting the displacement. Thus, if a steady force is appli
11、ed, a state will be reached in which the force resisting the movement of one layer over another balance the force applied and so no further movement of this kind can occur. If the appliedforce is then removed, the resisting forced will tend to restore the solid body to its original shape.即便如此，固体和流体之
12、间依旧有本质的差异；任何流体，无论多“稠”或者粘性多大，在最微小的剪切力作用下都会流淌，即便这种流淌是极其微小的；而且，只要这种力连续作用，流体就会连续流淌；然而对于固体，不管它的可塑性有多强，只 有当作用其上的净剪切力超过肯定数值后才会流淌，而小于该值的力所引起的固体层之间的相对移动是有限的；层 偏离原始位置的程度越大，抗击这种变形的力也就越大；因此，当一恒定力作用时，就会到达这样一个状态：即会产生一个抗击这种层间相对运动的力以平稳所施加的外力，所以不会产生进一步的运动；假如将所施加的外力移除，抗击力将会使固体复原到它的原始外形； In a fluid, however, the force
13、s opposing the movement of one layer over another exist only while the movement is taking place, and so static equilibrium between applied force and resistance to shear never occurs. Deformation of the fluid takes place continuously so long as a shear force is applied. But if this applied force is r
14、emoved the shearing movement subsides and, as there are then no forces tending to return theparticles of fluid to their original relative positions, the fluid keeps its new shape.然而，在流体中，只有当层间相对运动发生时才会存在这种阻挡运动的力；所以不会存在这种外力和抗击力之间的静态平稳；只要有剪切力作用，流欢迎下载精品学习资源体便会产生连续变形；但是假如将外力移除，剪切运动便会减退，而且由于这时没有使流体质点回到它们
15、初始位置的力，所以流体将保持它的“新”外形； Fluids may be sub-divided into liquids and gases. A fixed amount of a liquid has a definite volume which varies only slightly with temperature and pressure. If the capacity of the containing vessel is greater than this definite volume, the liquid occupies only part of the cont
16、ainer, and it forms an interface separatingif from its own vapour, the atmosphere or any other gas present.流体可以划分为液体和气体；肯定数量的液体其所占据的体积也是肯定的，它随温度和压力的变化很小；假如容器的体积大于这个肯定的体积，那么液体占据的只是容器的一部分，而且会形成一个分界面将液体与该液体的蒸汽、空气或其它存在的气体分开；On the other hand, a fixed amount of a gas, by itself in a container, will alway
17、s expand until its volume equals that of the container. Onlythen can it be in equilibrium. In the analysis of the behaviour of fluids the most important difference between liquidsand gases is that, whereas underordinary conditionsliquidsare sodifficultto compressthat they may for most purposes be re
18、garded as incompressible, gases may be compressed much more readily. Where conditions are suchthat an amount of gas undergoes a negligible change of volume, its behaviour is similar to that of a liquid and it maythen be regarded as incompressible. If, however, the change in volume is not negligible,
19、 the compressibility of thegas must be taken into account in examining its behaviour.而另一方面，容器内肯定数量的气体就通常都要膨胀到和容器相等的体积；这样它才会到达平稳状态；在分析流体特性时，液体和气体之间最重要的差异在于：鉴于在通常的条件下液体是难以压缩的，因此常常将液体认为是不行压缩流体，而气体就简洁压缩的多；当肯定量的气体其体积变化可以被忽视的情形下，气体的表现和液体类似，因此也可以认为是不行压缩的；然而，假如体积的变化不行忽视，那么在分析其行为特点时就必需考虑气体的压缩性；In considering
20、 the action of forces on fluids, one can either accountfor thebehavior ofeach and every molecule offluidin a given field offlowor simplify theproblembyconsidering the average effects of the molecules in a given volume. In most problems in fluid dynamics the latter approach is possible, which means t
21、hat the fluid can be regarded as a continuum-that is, a hypothetically continuous substance.在考虑作用于流体上的力时，可以用给定流淌区域内的流体上每个分子的行为来说明，也可以只考虑给定体积内分子的平均效应而使问题得以简化；在流体动力学的很多问题中后一种方法是可行的，这意味着将流体看作是连续介质即一种假想的连续介质；The justification for treating a fluid as a continuum depends on the physicaldimensions of the b
22、ody immersed in the fluid and on the number of molecules in a given volume. Let us say that we are studying the flow of air past a sphere with a diameter of 1 cm. A continuum is said to prevail if the number ofmolecules in a volume much smaller than the spheres is sufficiently great so that the aver
23、age effects pressure,density, and so on within the volume either are constant or change smoothly with time.是否可以将流体看作是连续介质取决于浸入流体的物体的尺寸和给定体积内的分子数目；比方我们现在讨论空气流过一个直径为1 厘米的球体的问题；假如在比球的体积小得多的体积内流体分子数目足够多以至于该体积的平均效应压力、密度等等为常数或者随时间缓慢变化，就认为该流体是连续介质；The numberof moleculesin a cubic meterof air at room temper
24、ature and sea-level pressure is about 1025. Thus the number of molecules in a volume of 10-19 m3 aboutthe size of a dust particle, which is very much smaller than the sphere would be 106 . This number of molecules is so large that the average effects within the microvolume are indeed virtually const
25、ant. On the other hand, if the 1 cm sphere were at an altitude of 305 km, there would be only one chance in 108 of finding a molecule in the microvolume, and the concept of an average condition would be meaningless. In this case, the continuum assumption would notbe valid for fluid flow except in th
26、e rarest conditions, such as those encountered in outer space.在室温、海平面压力下，每平方米空气中的分子数大约为1025 个，因此 10-19 m3体积中的分子数为106 个 10-19 m3大约是一个灰尘的体积， 这要远远小于上面所说的球体 ；如此多的分子数目使得在该微元体积内的平均效应实际上为常数；另一方面， 假如 1 厘米的球体被置于海拔305 千米处，那么在该微元体积内发觉一个分子的概率只有108 分之一， 所以平均状态的概念也就没有任何意义；在这种情形下，除了像在外层空间这种空气特别淡薄的情形之外，流体流淌的连续性假设都是
27、无效的；1.4 Scope Significance and Trend of Fluid MechanicsFluid mechanics,as the name indicates,is that branch of applied mechanics which is concerned with the statics and dynamics of liquids and gases.Dynamics,the study of motion of matter,may be divided into two part-dynamics of rigidbodiesand dynami
28、csof non-rigidbodies.The latteris usually furtherdividedintotwogeneralclassifications- elasticitysolid elastic body and fluid mechanics.流体力学，正如名字所指，是和液体，气体的静态和动态有关的应用力学的分支；动力学，物体运动的讨论，可以欢迎下载精品学习资源分为两部分刚性物体动力学和无刚性物体动力学；后者通常进一步分为两大一般类别；弹性固定弹性物体和流淌力学；The subjectof fluidmechanicscan be subdividedintotwo
29、broadcategories:hydrodynamicsand gas dynamics.Hydrodynamics deals primarily with the flow of fluids for which there is virtually no density change,such as liquid flow or the flow of gas at low speeds.Hydraulics,for example, the study of liquid flows in pipes or open channels, falls within this categ
30、ory. The study of fluid forces on bodies immersed in flowing liquids or in low-speed gas flows can also be classified as hydrodynamics.流体力学学科可以再分为两大广泛的类别：水动力学和气体动力学；水动力学主要处理的是实际没有密度变化的流体流淌；例如液体和气体在低速情形下的流淌；水动力学，例如在管道和开放式通道中液体流淌的讨论，属于这一类别；对浸入流淌液体和低速流淌其他的物体所受流体力的讨论也可以分类为水动力学；Gas dynamics,on the other
31、hand, deals with fluids that undergo significant density changes.High-speed gas flowing through a nozzle or over a body ,the flow of chemically reacting gases, or the movement of a body through the low density air of the upper atmosphere falls within the general category of gas dynamics.气体动力学，另一方面，处
32、理经受有效密度变化的流体；高速气体流淌通过一个喷嘴或者拂过一个物体，发生化学反应气体的流淌，物体通过更高大气中低密度空气中的运动属于气体动力学的一般类别；An area of fluid mechanics not classified as either hydrodynamics or gas dynamics is aerodynamics, which deals with the flow of air past aircraft or rockets,whether it be low-speed incompressible flow or high-speed compress
33、ible flow.流体力学中即不归类水动力学也不属于气体动力学的那一领域称为航空动力学，它处理穿过飞机和火箭的空气流淌， 是否它是低速不行压缩流淌或者是高速可压缩流淌；There are, however,two major aspects of fluid mechanics which differ from solid-body mechanics.The first is the nature and properties of the fluid itself, which are very different from those of a solid. The second i
34、s that, instead of dealing with individual bodies or elements of known mass, we are frequently concerned with the behavior of a continuous stream of fluid, without beginning or end.然而，流体力学不同于固体力学有两个主要方面；第一个是流体的本质和性能，他和那些固体有很大不同；其次个是，我们常常关注流体的一个连续单位的行为，没有首节和末端而不是处理单独物体或是已知固体的组分；Knowledge and understa
35、nding of the basic principles and concepts of fluid mechanics are essential in the analysisand design of any system in which a fluid is the working medium. Many applications of fluid mechanics make it oneof the most vital and fundamental of all engineeringand applied scientific studies. The flow of
36、fluids in pipe and channels makes fluid mechanics of importance to civil engineers. The study of fluid machinery such as pumps,fans,blowers,compressors,turbines,heatexchangers,jetand rocketengines,andthelike, makesfluidmechanicsofimportance to mechanical engineers.Lubrication is an area of considera
37、ble importance in fluid mechanics.The flow ofair overobjects,aerodynamics,is offundamentalinteresttoaeronauticaland space engineersin the designof aircraft,missilesandrockets.Inmeteorology,hydrologyandoceanographythe studyoffluidsis basic since the atmosphereand theoceanare fluids.Andtodayin moderne
38、ngineeringmanynewdisciplinescombinefluid mechanics with classical disciplines. For example,fluid mechanics and electromagnetic theory are studied together as magnetogas-dynamics.In newtypes of energy conversion devices and in the study of stellar and ionospheric phenomena, magnetogasdynamics is vita
39、l.流体力学最基本的原理和概念的学问和懂得在以流体作为工作介质的任何系统的分析和设计中是特别重要的；很多流体力学的应用是它成为全部工程和运用科学讨论中最重要的基础的一环；流体在管子和隧道中的流淌使流体力学对土木工程师很重要；流体机械的讨论，例如泵，风扇，鼓风机，压缩机，涡轮机，热交换器，喷气飞机和火箭引擎等等，使流体力学对机械工程师很重要；润滑在流体力学中是相当重要的一个领域；拂过物体的空气流淌，航空动力学是空气动力学和空间工程师在飞机，导弹，火箭的设计中的基本爱好；在气象学，水文学，海洋学中，流体的讨论是基本由于大气和海洋是流体；在当今现代工程中，很多新学科是结合了流体力学和传统学科；例如，
40、流体力学和电磁理论放在一起讨论称为磁性气体动力学；在能量转换装置的新类型和在星球的讨论以及电离现象中，磁性气体动力学很重要；欢迎下载精品学习资源On the contrary ,the collapse of the Tacoma Narrows Bridge in U,S.A. is evidence of the possible consequences of neglecting the basic principles of fluid mechanics .On a memorable day in November 1940.,Nature decided to teach us
41、 all a lesson.The wind could not even be considered strong on that day,but it happened to disturb the great TacomaNarrows suspension bridge cyclicallywith a frequency close to the bridge s natural frequency of vibration.The entirebridge started to dance. Traffic was stopped;and an astonished public
42、watched the bridge itself to pieces.相反地， 美国塔科马海峡吊桥的垮塌是无视流体动力学基本原理的可能结果的证明；在 1940 年 9 月，难忘的一天， 大自然打算教我们一堂课，在那天风据说不是很强，但是它刚好以一个接近桥自然振动频率的频率周期性的打扰宏伟的塔科马吊桥；整座桥开头摇动，交通停止，一个震动的公民看着桥身分裂成碎片；We see that a good familiarity with fluid mechanics is essential to the modern engineer and scientist,and it is probab
43、ly obvious that fluid mechanics and its applications is a broad subject with far-flung fields of specialization.What we should do is to master the basic concepts and principles of fluid mechanics.Once these fundamentals are masteredmore advanced books and research literature may be studied to increa
44、se one s understanding of more specialized aspects of fluid mechanics.我们看到，很好的熟识流体力学对现代工程师和科学家至关重要；并且很明显，流体力学和他的应用是具有专业户的长远领域的广泛学科；我们应当做的是把握最基本的流体力学的概念和原理；一旦这些基础的东西被把握，更多先进的书籍和讨论文献可以被讨论用来增加流体力学更多专业化方面的懂得The significance of fluid mechanics becomes apparent when we consider the vital role it plays in
45、our everyday lives.When we turn on our kitchen faucets,we activate flow in a complex hydraulic network of pipes,valves, and pumps.When we flick on a light switch ,we are drawing energy either from a hydroelectric source that operates by theflow of water through turbines or from a thermal power sourc
46、e derived from the flow of steam past turbine blades. 流体力学的意义变得很明显，当我们摸索它在我们日常生活中起到的重要作用；当我们打开我们厨房的插口，我们推动在一个复杂水力管网，阀和泵中的流淌；当我们弹开一个灯的开关时，我们正在引导自由通过涡轮机的水流淌操作的水力源或者来自起源于流过涡轮叶片蒸汽流淌的热力源；When we drive our cars, pneumatic tires provide suspension, hydraulic shock absorbers reduce road shocks,gasolineis pu
47、mped through tubes and later atomized,and air resistance creates a drag on the auto as a whole;and when we stop, we are confident in the operation of the hydraulic brakes,Very complex fluid processes are also involved in the manufacture of the paper on which this book is printed, And our very lives
48、depend on a very important fluid mechanic process-the flow of blood through our veins and arteries.当我们驾驶我们的车时，气动轮胎供应悬浮，水力振动吸取装置减小路面振动，汽油由泵的作用通过管束，并且分裂，空气阻力从总体来说自动制造了一个拖力；并且当我们停车时，我们对水力刹车的运行有信心；很复杂的流体过程仍牵涉到了这本书打印所用纸的制造，并且，我们完整的生命依靠一个很重要的流体力学过程通过我们静脉和动态的血液流淌；Some of the mostsignificantenvironmentalproblemsfacingsociety todayinvolvefluidmechanics,Forexample, coastal cities often discharge their wastewaterusually treated into the sea, near the sea bed, far enough from shoreso tha