热能与动力工程专业英语翻译李瑞扬

1.3T h e C h a r a c t e r i s t i c s o f F l u i d s流体的特征
constituent：组成的；tangential：切向的；restrain：限制、约束；equilibrium：平衡，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：察觉不到的，
细微的；
restore：恢复；
subside：下沉、沉淀、减退、
衰减；
hypothetically：假设地、假想
地；
sphere：球、球体；
microvolume：微元体积；
rarest：最稀罕的，虽珍贵的
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. This 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 continue to move indefinitely. In a fluid, however, shear forces are 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 forces which in some way restrain its movement. 流体是可以流动的物质，也就是说，组成流体的质点可以连续的改变它们的相对位置。

而且，不管层与层之间的相对位移有多大都不会产生持久的抵抗力。

这意味着流体在静止状态下是不会存在剪切力的（剪切力是与其作用表面相切的力）。

另一方面，固体在静止时却可以抵抗剪切力，其中的剪切力也可以使层与层之间发生相对位移，但是固体材料却不一定会有连续的运动。

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别还在于在特定的时刻，确定数量的流体其形状取决于承载它的容器，或者取决于一些限制其运动的力。

The 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 would take 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, certain solids may be made to ―flow ‖ when a sufficiently large force is applied; these are known as plastic solids. 固体和流体之间的区别通常是很明显的，但是也有些物质难于归类。

比如说有些流体并不易流动，如重油和树脂有时候会表现得像固体一样，像这样的一块物质如果放在地面上，虽然它的流动发生的非常缓慢，要经过很长的一段时间—也许要好几天，但是在重力的作用下它仍然会在地面上蔓延开来，也就是说，它的组成质点会改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“流动”，这就是我们所知的“塑性固体”。

Even so, the essential difference between solids and fluids remains. Any fluid, no matter how ―thick ‖ or viscous it is, begins to flow, even if 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 it exceeds a 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 applied, 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 applied force is then removed, the resisting forced will tend to restore the solid body to its original shape.即便如此，固体和流体之间依然有本质的差异。

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动，即便这种流动是极其细微的。

而且，只要这种力持续作用，流体
就会连续流动。

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一定数值后才会流动，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，抵抗这种变形的力也就越大。

因此，当一恒定力作用时，就会达到这样一个状态：即会产生一个抵抗这种层间相对运动的力以平衡所施加的外力，所以不会产生进一步的运动。

如果将所施加的外力移除，抵抗力将会使固体恢复到它的原始形状。

In a fluid, however, the forces 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 removed the shearing movement subsides and, as there are then no forces tending to return the particles 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止运动的力。

所以不会存在这种外力和抵抗力之间的静态平衡。

只要有剪切力作用，流体便会产生连续变形。

但是如果将外力移除，剪切运动便会减退，而且因为这时没有使流体质点回到它们初始位置的力，所以流体将保持它的“新”形状。

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 container, and it forms an interface separating if from its own vapour, the atmosphere or any other gas present. 流体可以划分为液体和气体。

一定数量的液体其所占据的体积也是一定的，它随温度和压力的变化很小。

如果容器的体积大于这个一定的体积，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 always expand until its volume equals that of the container. Only then can it be in equilibrium. In the analysis of the behaviour of fluids the most important difference between liquids and gases is that, whereas under ordinary conditions liquids are so difficult to compress that they may for most purposes be regarded as incompressible, gases may be compressed much more readily. Where conditions are such that
an amount of gas undergoes a negligible change of volume, its behaviour is similar to that of a liquid and it may then be regarded as incompressible. If, however, the change in volume is not negligible, the compressibility of the gas must be taken into account in examining its behaviour.而
另一方面，容器内一定数量的气体则通常都要膨胀到和容器相等的体积。

这样它才会达到平衡状态。

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可压缩流体，而气体则容易压缩的多。

当一定量的气体其体积变化可以被忽略的情况下，气体的表现和液体类似，因此也可以认为是不可压缩的。

然而，如果体积的变化不可忽略，那么在分析其行为特征时就必须考虑气体的压缩性。

In considering the action of forces on fluids, one can either account for the behavior of each and every molecule of fluid in a given field of flow or simplify the problem by considering the average effects of the molecules in a given volume. In most problems in fluid dynamics the latter approach is possible, which means that 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，可以用给定流动区域内的流体上每个分子的行为来解释，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。

在流体动力学的许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 physical dimensions of the body 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 of molecules in a volu me much smaller than the sphere’s is sufficiently great so that the average 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。

比如我们现在研究空气流过一个直径为1厘米的球体的问题。

如果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 number of molecules in a cubic meter of air at room temperature and sea-level pressure is about 1025. Thus the number of molecules in
a volume of 10-19 m3 (about the 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 constant. 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 not be valid for fluid flow except in the 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，如果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现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。

在这种情况下，除了像在外层空间这种空气非常稀薄的情况之外，流体流动的连续性假设都是无效的。

1.4 Scope Significance and Trend of Fluid Mechanics
Fluid 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 rigid bodies and dynamics of non-rigid bodies. The latter is usually further divided into two general classifications--elasticity(solid 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，物体运动的研究，可以分为两部分——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）和流动力学。

The subject of fluid mechanics can be subdivided into two broad categories: hydrodynamics and 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 category. 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动。

例如液体和气体在低速情况下的流动。

水动力学，例如在管道和开放式通道中液体流动的研究，属于这一类别。

对浸入流动液体和低速流动其他的物体所受流体力的研究也可以分类为水动力学。

Gas dynamics,on the other 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，处理经历有效密度变化的流体。

高速气体流动通过一个喷嘴或者掠过一个物体，发生化学反应气体的流动，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 compressible 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动，是否它是低速不可压缩流动或者是高速可压缩流动。

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 is 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。

第二个是，我们经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 understanding of the basic principles and concepts of fluid mechanics are essential in the analysis and design of any system in which a fluid is the working medium. Many applications of fluid mechanics make it one of the most vital and fundamental of all engineering and applied scientific studies. The flow of 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, heat exchangers,jet and rocket engines, and the like, makes fluid mechanics of importance to mechanical engineers.Lubrication is an area of considerable importance in fluid mechanics.The flow of air over objects,aerodynamics ,is of fundamental interest to aeronautical and space engineers in the design of aircraft,missiles and rockets.In meteorology,hydrology and oceanography the study of fluids is basic since the atmosphere and the ocean are fluids. And today in modern engineering many new disciplines combine fluid mechanics with classical disciplines. For example,fluid mechanics and electromagnetic theory are studied together as
magnetogas-dynamics. In new types of energy conversion devices and in the study of stellar and ionospheric phenomena, magnetogasdynamics is vital.
流体力学最基本的原理和概念的知识和理解在以流体作为工作介质的任何系统的分析和设计中是十分重要的。

很多流体力学的应用是它成为所有工程和运用科学研究中最重要的基础的一环。

流体在管子和隧道中的流动使流体力学对土木工程师很重要。

流体机械的研究，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。

掠过物体的空气流动，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，流体的研究是基本因为大气和海洋是流体。

在当今现代工程中，许多新学科是结合了流体力学和传统学科。

例如，流体力学和电磁理论放在一起研究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研究以及电离现象中，磁性气体动力学很重要。

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 all a lesson.The wind could not even be considered strong on that day,but it happened to disturb the great Tacoma Narrows suspension bridge cyclically with a frequency close to the bridg e’s natural frequency of vibration.The entire bridge started to dance. Traffic was stopped;and an astonished public watched the bridge itself to pieces. 相反地，美国塔科马海峡吊桥的垮塌是忽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，大自然决定教我们一堂课，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。

整座桥开始摇晃，交通停止，一个震惊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 probably 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 mastered more advanced books and research literature may be studied to incr ease one’s understanding of more specialized as pects of fluid mechanics.
我们看到，很好的熟悉流体力学对现代工程师和科学家至关重要。

并且很显然，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。

我们应该做的是掌握最基本的流体力学的概念和原理。

一旦这
些基础的东西被掌握，更多先进的书籍和研究文献可以被研究用来增加流体力学更多专业化方面的理解
The significance of fluid mechanics becomes apparent when we consider the vital role it plays in 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 the flow of water through turbines or from a thermal power source derived from the flow of steam past turbine blades.
流体力学的意义变得很明显，当我们思考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，阀和泵中的流动。

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动操作的水力源或者来自起源于流过涡轮叶片蒸汽流动的热力源。

When we drive our cars, pneumatic tires provide suspension, hydraulic shock absorbers reduce road shocks,gasoline is pumped 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 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，气动轮胎提供悬浮，水力振动吸收装置减小路面振动，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创造了一个拖力。

并且当我们停车时，我们对水力刹车的运行有信心。

很复杂的流体过程还牵涉到了这本书打印所用纸的制造，并且，我们完整的生命依赖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动。

Some of the most significant environmental problems facing society today involve fluid mechanics,For example, coastal cities often discharge their wastewater(usually treated) into the sea, near the sea bed, far enough from shore so that the wastes become sufficiently diluted with
the ambient sea water to render the resulting mixture harmless. The process involves mixing the wastewater with the ambient liquid, a complex turbulence phenomenon,The degree of mixing is a function of the characteristics of the wastewater and the ambient liquid (such as density ) as well as the discharge velocity of the wastewater,Also involved in this process are the velocity and pattern of coastal currents.In addition to the fluid mechanics of such a problem, the contaminants in the mixture may change both chemically and biologically in the process.Thus sophisticated models linking the basic flow model with other aspects of the problem are required to design a satisfactory waste disposal system,Such models are generally developed and used by multidisciplinary teams that may include engineers, mathematicians, chemists,and bioscientists.There is an increasing need for engineers who have the ability and mathematical skills to assist in the generation of, and to use,sophisticated computational models of this type. Other problems, similar in nature, that involve fluid mechanics include air pollution and underground hazardous waste problems.
今天，一些有价值的直面社会的环境问题涉及流体力学，例如沿海城市经常排放他们的废水（通常被处理）到大海中，海床附近，离海滨足够远以便于这些废物在周围海水作用下充分稀释来缓解最终混合物的危害。

这个过程涉及了废水和周围液体的混合，一个很复杂的紊流现象。

混合的程度是周围液体和废水性质的一个指标（例如密度）还有废水的排放速率，还有涉及到这个过程的是沿海气流的速率和形式。

另外，这样的一个问题的流体力学，在这个过程中混合物的成分可能发生化学性和生物性的改变。

因此，连着问题其他方面的基本模型的复杂模型被要求设计一个符合要求的废物处理系统，这些模型发展普遍并且被训练过的团队使用，可能包括工程师，数学家，化学家，和生物学家。

对那些有能力并且具有帮组构建和使用这种复杂计算模型的数学技巧的工程师的需要也越来越大。

其他问题，本质上类似，涉及流体力学包括空气污染和地下危险废物问题。

Modern developments in fluid mechanics, as in all fields, involve the use of high-speed computers in the solution of problems. Remarkable progress is being made in this area, and the use of the computer in fluid dynamic design is increasing.In the design of aircraft, computers are used to
predict the flow over engine nacelles and appendages in order to select configurations that minimize aerodynamic drag.The NASA publication on wind tunnels(1) explains the role of computers in aircraft design,Computational solutions for wind forces on buildings and structures are used to complement measurements on wind tunnel models to insure the safety and structural integrity of the full-scale structures.
流体力学的现代发展，和所有领域一样，在问题解决方案中涉及高速计算机的使用。

在这个领域取得了巨大进步，并且电脑在流体动力学设计中的使用不断增多。

在飞机设计中，电脑被用来预测掠过发动机机舱和附属物的流动为了选择结构来最小化航空动力拉力。

NASA出版物关于风道说明了电脑在飞机设计的作用，对于作用在建筑物和结构的风力计算解决方案被采用来补充完善风道模型的测量来确保安全性和全规模结构的完整性。

The ever-increasing speed and memory capacity of modern computers are leading to even more exciting applications of computers in fluid puter solutions for the motion of terrestrial winds and weather fronts are leading to more accurate forecasting of local weather conditions.The coupling of fluid mechanics with heat transfer and chemical kinetics in computational solutions will lead to improved designs for industrial power and propulsion systems.As space stations and space travel become more feasible, computers will play a vital role in the design of flow systems in microgravity environments that are difficult to examine through terrestrial experimentation.The application of computers to the analysis of flows in biological systems is only beginning but it will continue to grow as the mechanics of flows in these systems becomes better understood.
现代计算机的高速和记忆功能正引领流体力学中计算机更加振奋的应用。

关于大陆风移动和天气朝向的计算机解决方法正促进更多精确的当地气候条件的预测。

计算机化的带有热量转换和化学运动的流体机械的联轴器将促进提升工业动力和推进系统的设计。

当宇宙空间站和宇宙旅游具有更多可能性，
计算机将会在通过陆地实验很难检测的微重力环境中的流动系统的设计起到重要作用。

生物系统中的流动分析的计算机应用只是开始，但他会不断发展，当这些系统中的流体力学规律被更加了解。

The science of fluid mechanics is developing at a rapid rate. Armed with more detailed measurements and numerical models, fluid mechanicians have developed higher levels of understanding that have led to sophisticated designs and applications of fluid systems. Still,there are many areas in which only rudimentary information and physical models are available. Turbulence is a prime example. Even though we presently have high_speed computers at our disposal, the solutions are only as valid as the equations we use to describe the basic flow phenomena. And there is currently no general analytic model that completely describes the nature of turbulence.We have good data on turbulence in straight pipes, so reliable empirical formulas have been developed to describe the turbulence in such a simple case. But turbulence in
high-shear flows,buoyant flows,and compressible flows is still the subject of extensive
study.Analyses of the flow of multiphase mixtures such as solids in a liquid (slurries )and bubbles in a liquid still rely heavily on empiricism.In oil recovery operable liquids,such as oil in water, which is not well understood. These are areas which represent exciting challenges to current and future practitioners of fluid mechanics.
流体力学技术正快速发展。

由于储备了更多详细的测量方法和数学模型，流体力学专家们发展了更高的理解水平，这种水平促进了产生流体系统的复杂设计和应用。

而且有许多只有基本信息和物理模型可使用的领域。

紊流就是最好的一个例子，即使我们目前有高速计算机可操作，但这种解决方式只与我们用来描述基本流动现象的等式一样有效。

并且当前没有一般的分析模型，可以完整描述紊流的本质。

我们有关于在直管中紊流的完整数据，因此，可靠的经验公式已经发展来描述在这样一个简单情况下的紊流。

但是，在高切变流动，浮升流动和可压缩流动中的紊流仍是扩展研究的学科。

多相混合的流动的分析例如固体在液体中和气泡在液体中仍要着重依赖于经验理论。

在油恢复操作中，工程师
遇到了不容混液体流动的问题例如油在水中，这些还不是很好理解。

这些是将带给现在和未来流体力学从业者激动挑战的领域。

1.5 The Principles of Fluid Machines 流体机械原理
vise versa：反过来也这样；reciprocate：互换位置、往复移动；
compressor：压缩机；blower：鼓风机、风扇；piston：活塞、柱塞；cylinder：汽缸、圆筒；rotor：转子、旋转部；hydro-static：流体静力学的；decelerate：减速；clearance：余隙；
churn：搅拌；
functioning：机能；
ensure：确保、保证；
diaphragm：横膈膜，振动膜；
impeller：叶轮；
momentum：动力、要素、动
量；
reduction：减少；
discharge：卸下、放出、流出；
casing：包装、保护性的外套；
bulky：大的，容量大的；
bring into play：发挥、发动；
positive-displacement：容积式
的；
rotodynamic：旋转式、转动
式；
diaphragm pump：膜片泵；
gear pump：齿轮泵；
close fitting：紧贴的；
exemplified：例证、例示，以….作为例子；
A fluid machine is a device either for converting the energy held by a fluid into mechanical energy or vice versa. The mechanical energy is usually transmitted by a rotating shaft : a machine in which energy from the fluid is converted directly to the mechanical energy of a rotating member is known as a turbine ( from the Latin turbo, a circular motion ); if, however, the initial mechanical movement is a reciprocating one the term engine or motor is used. A machine in which the converse process—the transfer of energy from moving parts to the fluid—takes place is given the general title of pump. When the fluid concerned is a gas other terms may be used. If the primary object is to increase the pressure of the gas, the machine is termed a compressor. On the other hand, a machine primarily used for causing the movement of a gas is known as a fan or blower. In this case the change in static pressure is quite small—usually sufficient only to overcome the resistance to
the motion—and so the variation of density is negligible and the fluid may be regarded as incompressible. No attempt will be made here to describe constructional details or the practical operation of any of these machines. Our concern is simply with the basic principles of mechanics of fluid machines which are brought into play.
流体机械是这样的一种装置：它将流体所具有的能量转化为机械能，或者反过来将机械能转化为流体的能量。

机械能通常通过旋转的轴来传输：将流体的能量直接转化为旋转部件的机械能的机器称为涡轮（该词起源于拉丁文字turbo，旋转运动的意思）；然而，如果初始的机械运动是一种往复的运动，则被称为马达或发动机。

相反的过程—即能量从运动部件传输给流体—所发生的机器则通常称为“泵”。

当所采用的流体为气体时，则要用到其它的名词。

如果主要的目的是为了提升气体的压力，那么这种机器称为压缩机。

而另一方面，主要用来使气体运动的机器则称为“鼓风机”或者“送风机”。

在这种情况下，静压的变化非常小—通常只够克服运动阻力—因此密度的变化也可以忽略，流体可以认为是不可压缩的。

在这里我们不会对以上任意一种机器进行构造细节和实际操作上的描述。

我们所关心的仅仅是这些流体机械运作的机械原理。

Although a great variety of fluid machines is to be found, any machine may be placed in one of two categories: the positive-displacement group or the rotodynamic group. The functioning of a
positive-displacement machine derives essentially from changes of the volume occupied by the
fluid within the machine. This type is most commonly exemplified by those machines, such as reciprocating pumps and engines, in which a piston moves to and from in a cylinder (a suitable arrangement of valves ensures that the fluid always moves in the direction appropriate to either a pump or an engine). Also in this category are diaphragm pumps, in which the change of volume is brought about by the deformation of flexible boundary surfaces (an animal heart is an example of this form of pump ), and gear pumps in which two rotors similar to gear wheels mesh together within a close-fitting housing. Although hydrodynamic effects may be associated with a。