热能与动力工程专业英语翻译(李瑞扬)

1.3 The Characteristics of Fluids 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 gas