1、Unit27Fluid MechanicsTextFluid mechanics is the branch of science concerned with moving and stationary fluids.Given that the vast majority of the observable mass in the universe exists in a fluid state,that life as we know it is not possible without fluids,and that the atmosphere and oceans covering
2、 this planet are fluids,fluid mechanics has unquestioned scientific and practical importance.Its allure crosses disciplinary boundaries,in part because it is described by a nonlinear field theory and also because it is readily observed.Mathematicians,physicists,biologists,geologists,oceanographers,a
3、tmospheric scientists,engineers of many types,and even artists have been drawn to study,harness,and exploit fluid mechanics to develop and test formal and computational techniques,to better understand the natural world,and to attempt to improve the human condition.The importance of fluid mechanics c
4、annot be overstated for applications involving transportation,power generation and conversion,materials processing and manufacturing,food production,and civil infrastructure.For example,in the twentieth century,life expectancy in the United States approximately doubled.About half of this increase ca
5、n be traced to advances in medical practice,particularly antibiotic therapies.TextThe other half largely resulted from a steep decline in childhood mortality from water-borne diseases,a decline that occurred because of widespread delivery of clean water to nearly the entire population a fluids-engin
6、eering and public-works achievement.Yet,the pursuits of mathematicians,scientists,and engineers are interconnected:Engineers need to understand natural phenomena to be successful,scientists strive to provide this understanding,and mathematicians pursue the formal and computational tools that support
7、 these efforts.Advances in fluid mechanics,like any other branch of physical science,may arise from mathematical analyses,computer simulations,or experiments1.Analytical approaches are often successful for finding solutions to idealized and simplified problems and such solutions can be of immense va
8、lue for developing insight and understanding,and for comparisons with numerical and experimental results.Thus,some fluency in mathematics,especially multivariable calculus,is helpful in the study of fluid mechanics.In practice,drastic simplifications are frequently necessary to find analytical solut
9、ions because of the complexity of real fluid flow phenomena.Furthermore,it is probably fair to say that some of the greatest theoretical contributions have come from people who depended rather strongly on their physical intuition.Ludwig Prandtl,one of the founders of modern fluid mechanics,first con
10、ceived the idea of a boundary layer based solely on physical intuition.TextHis knowledge of mathematics was rather limited,as his famous student Theodore von Karman testifies.Interestingly,the boundary layer concept has since been expanded into a general method in applied mathematics.As in other sci
11、entific fields,mankinds mathematical abilities are often too limited to tackle the full complexity of real fluid flows.Therefore,whether we are primarily interested in understanding flow physics or in developing fluid-flow applications,we often must depend on observations,computer simulations,or exp
12、erimental measurements to test hypotheses and analyses,and develop insights into the phenomena under study.Properties of LiquidsOne of the first questions we need to explore is what is a fluid or we might ask,what is the difference between a solid and a fluid We have a general,vague idea of the diff
13、erence.A solid is“hard”and not easily deformed;whereas a fluid is“soft”and is easily deformed(we can readily move through air).Although quite descriptive,these casual observations of the differences between solids and fluids are not very satisfactory from a scientific or engineering point of view.Te
14、xtA closer look at the molecular structure of materials reveals that matter that we commonly think of as a solid(steel,concrete,etc.)has densely spaced molecules with large intermolecular cohesive forces that allow the solid to maintain its shape,and to not be easily deformed.However,for matter that
15、 we normally think of as a liquid(water,oil,etc.),the molecules are spaced farther apart,the intermolecular forces are smaller than for solids,and the molecules have more freedom of movement.Thus,liquids can be easily deformed(but not easily compressed)and can be poured into containers or forced thr
16、ough a tube.Gases(air,oxygen,etc.)have even greater molecular spacing and freedom of motion with negligible cohesive and as a consequence are easily deformed(and compressed)and will completely fill the volume of any container in which they are placed.Both liquids and gases are fluids.The study of fl
17、uid mechanics involves the same fundamental laws you have encountered in physics and other mechanics courses.These laws include Newtons laws of motion,conservation of mass,and the first and second laws of thermodynamics2.Thus,there are strong similarities between the general approach to fluid mechan
18、ics and to rigid-body and deformable-body solid mechanics.TextThis is indeed helpful since many of the concepts and techniques of analysis used in fluid mechanics will be ones you have encountered before in other courses.The broad subject of fluid mechanics can be generally subdivided into fluid sta
19、tics,in which the fluid is at rest,and fluid dynamics,in which the fluid is moving.It is obvious that different fluids can have grossly different characteristics.For example,gases are light and compressible,whereas liquids are heavy(by comparison)and relatively incompressible.A syrup flows slowly fr
20、om a container,but water flows rapidly when poured from the same container.To quantify these differences,certain fluid properties are used.The properties of density and specific weight are measures of the“heaviness”of a fluid3.It is clear,however,that these properties are not sufficient to uniquely
21、characterize how fluids behave since two fluids(such as water and oil)can have approximately the same value of density but behave quite differently when flowing.There is apparently some additional property that is needed to describe the“fluidity”of the fluid.One of the most useful properties is the
22、viscosity,which is a measure of the resistance the fluid has to an externally applied shear stress.The viscosity indicates how a fluid will react under the action of an external shear stress.TextThe pressure is defined as a normal force per unit area existing in the fluid4 Fluids,whether at rest or
23、moving,exhibit some type of pressure variationeither with height or with horizontal distance.In closed-conduit flow,such as flow in pipes,differences in pressure from beginning to end of the conduit maintain the flow.The density of a fluid is its mass per unit volume,designated by the Greek symbol.I
24、t is typically used to characterize the mass of a fluid system.The value of density can vary widely between different fluids,but for liquids,variations in pressure and temperature generally have only a small effect on the value of.Unlike liquids,the density of a gas is strongly influenced by both pr
25、essure and temperature.The specific volume is the volume per unit mass and is therefore the reciprocal of the densitythat is,V=1/.This property is not commonly used in fluid mechanics but is used in thermodynamics.The specific weight of a fluid,designated by the Greek symbol (gamma),is defined as it
26、s weight per unit volume.Thus,specific weight is related to density through the equation=g,where g is the local acceleration of gravity.Just as density is used to characterize the mass of a fluid system,the specific weight is used to characterize the weight of the system.TextTextTextTextIt is known
27、to closely approximate the behavior of real gases under normal conditions when the gases are not approaching liquefaction.harnessv.治理,利用；尤指为产生能量而）控制并利用（自然资源）antibioticn.抗生素,抗菌素；破坏伤害生命的；抗生的;抗菌的mortalityn.致命性;死亡率；死亡率immensea.广大的;巨大的;无边的;无量的intuition n.直感,直觉知识（事物）,直觉力hypothesisp.假说,假设,前提vaguea.含糊的,不明确的
28、,不清楚的；模糊的descriptivea.描写的,叙述的;说明的,图形的immisciblea.不能混合的,不融和的；不相混的sphericala.球形的,球面的；天体的;天空的kinematic viscosityn.运动粘度；动粘滞率specific volumen.比容；比体积enthalpyn.焓;(热力学)热函;(单位质量的)热含量irreversiblea.不可更改的;不可挽回的,不可反转的isothermala.等温的,等温线的；等温线surface tensionn.表面张力New Words and Phrasesspecific heatn.比热；比热容internal
29、energy n.内能potential energy n.势能kinetic energy n.动能ideal gas law n.理想气体定律liquefaction v.（使）液化；液化(作用)civil infrastructure n.民用基础设施multivariable calculus n.多元微积分intermolecular cohesive force n.分子间内聚力New Words and PhrasesNotes（1）Advances in fluid mechanics,like any other branch of physical science,may
30、arise from mathematical analyses,computer simulations,or experiments.流体力学的发展如同物理学的其他任何分支一样，可能源自数学分析、计算机模拟或实验。（2）These laws include Newtons laws of motion,conservation of mass,and the first and second laws of thermodynamics.这些定律包括牛顿运动学定律、质量守恒定律以及热力学第一定律和第二定律。（3）The properties of density and specific
31、weight are measures of the“heaviness”of a fluid.密度和比重属性是衡量流体“重量”的指标。（4）Pressure is defined as a normal force per unit area existing in the fluid.压力是指流体中单位面积所受的力。（5）A density discontinuity may exist whenever two immiscible fluids are in contact,for example at the interface between water and air.密度变化可
32、能存在于两种不互溶流体相互接触的情况下，例如水和空气的界面。1.流动性可以通过不同的方式定义，例如，根据分子的间距和运动。在流体中分子运动的空间距离大于固体中，如果固体被充分加热，分子运动开始加剧，在边缘位置变得不再紧密，直到融化或变为流动状态。2.在实际流体流动的边界的表面上，相对切向速度是零。在流体紊流运动时，壁面速度是固定值或非零，这就意味着，在壁面处产生了流层滑动。3.Rotation is a characteristic of flow.Rotational flow occurs if a fluid element rotates while the fluid is flow
33、ing.4.Several devices are commercially available for measuring the viscosity of a fluid.Such meters contain the fluid and cause it to undergo a laminar motion by the imposition of a pressure drop or by the motion of a component.The laminar motion generated can usually be described by an analytic sol
34、ution with which viscosity can be calculated.5.Flow in closed conduits is a very important part of the study of fluid mechanicsprimarily because examples are so commonplace.Water for domestic use is distributed to all parts of the household in pipes;sewers and drains carry wastewater away.Crude oil
35、is pumped from well to refinery in pipes.Natural gas is brought to the user via pipes.Heated air is distributed to all parts of a house in circular and rectangular ducts.Many other examples of conduit flow can be found in everyday life.Exercises（Translate the following Chinese into English,or English into Chinese）
