第五章 液体在管道中流动的基础知识

is called laminar flow, which is characterized by the fluid flowing in smooth layers or laminas. In this type of flow, a particle of fluid in a given layer stays in that layer, as shown in the figure. This type of fluid motion is called streamline flow because all the particles of fluid are moving in parallel paths. Therefore laminar flow is smooth with essentially no collision of particles. For laminar flow, the friction is caused by the sliding of one layer or particle of fluid over another in a smooth continuous fashion）。 2.紊流（turbulent flow）：如果流速达到足够高的数 值，流动就中止层流而变成紊 流。如图所示，在紊流中，微粒 的运动变成了无规则并在与指定 的流动方向垂直和平行的方向上
essentially a measure of the viscosity of the fluid. The greater the viscosity of a fluid, the less readily it flows and the more energy is required to move it. This energy is loss because it is dissipated into heat and thus represents wasted energy）。 能量损失总是出现在被称为管接头的管道收缩部位， 管接头是一个输送和控制液体的元件（与直管不同）。 例如阀、三通管接头、弯头和节流口。通过管接头流动 的路径性质确定了能量损失的多少。一般来说，路径越 弯曲，损失越大。在许多液体传动的使用中，管接头造 成的能量损失超过了管道中因粘性流动的损失（Energy losses also occur in pipeline restrictions called fittings. A fitting is a component（other than a straight pipe）that is use to carry or control the fluid. Examples are valves, tees, elbows, and orifices. The nature of the
then be used to perform a complete analysis of a fluid power system. This includes calculating the pressure drops, flow rates, and horsepower losses for all components of the fluid power system）。
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层流和紊流的区别可以通过使用水龙头来看出。当 水龙头部分打开时，少量的水流出，这个流动形态是平 滑的层流。然而，当水龙头全开时，流动扰动并变成紊 流（The difference between laminar and turbulent flow can be seen when using a water faucet. When the faucet is turned only partially open, with just a small amount of flow, the flow pattern observed is a smooth laminar one. However, when the faucet is opened wide, the flow mixes and becomes turbulent）。
5.2 层流和紊流（LAMIMAR AND TURBULENT FLOW） 我们在第3章中讨论液体在管道中流动时，假定在 任何位置其速度都为一定值。然而，当液体在管道中流 动时，其与管壁接触的流层速度为0。这是由于粘度， 导致液体微粒粘着在管壁上。流层的速度随着与管壁的 距离的增大而提高，其最高速度出现在管道中心，如图 所示（In our discussions of fluid flow in pipes in Chapter 3, we assumed a constant velocity at any one station. However, when a fluid flows through a pipe, the layer of fluid at wall has zero velocity. This is due to viscosity, which causes fluid particles to cling to the wall. Layers of fluid at the progressively greater distances from the pipe
第五章 液体在管道中流动的基础 知识（Basics of Hydraulic Flow in Pipes）
5.1概述（INTRODUCTION） 迄今为止我们还没有研究液体在管道中流动时由于 摩擦而产生的能量损失的机理。液体是直观的，像水和 汽油，它们比像油液这样高粘度的液体容易流动。流动 的这个阻尼实质上是液体粘度的度量标准。粘度越大的 流体越不容易流动也就是流动所需的能量越大。这些能 量的减少是因为它散失成了热及代表了损耗的能量 （Up to now we have not investigated the mechanism of energy losses duo to friction associated with the flow of a fluid inside a pipe. It is intuitive that liquid, such as water or gasoline, flow much more readily than do heavier liquids such as oil. The resistance to flow is
pipe diameter or fitting size, the greater the losses. However, using large-diameter pipes and fittings results in greater cost and poor space utilization. Thus, the selection of component sizes represents a compromise between energy losses and component cost and space requirements）。 油管和管接头的阻尼可以由根据实验得出的经验公 式确定。这些公式可以计算任何系统元件的能量损失。 伯努利方程和连续方程可以用来完成液压传动系统的分 析。这包括计算液压传动系统所有元件的压力降、流量 和功率损失（The resistance of pipes and fitting can be determined using empirical formulas which have been developed by experimentation. This permits the calculation of energy losses for any system component. Bernoulli’s equation and the continuity equation can
下波动。这个混合作用由于液体微粒的碰撞而产生扰动。 这引起了相当大的流动阻尼以及比层流产生的能量损失 更大（If the velocity of flow reaches a high enough value, the flow ceases to be laminar and becomes turbulent. As shown in the figure, in turbulent flow the movement of a particle becomes random and fluctuates up and down in a direction perpendicular as well as parallel to the mean flow direction. This mixing action generates turbulence due to the colliding fluid particles. This causes considerably more resistance to flow and thus greater energy losses than that produced by laminar flow）。
flow path through a fitting determines the amount of energy losses. Generally speaking, the more torturous the path, the greater the losses. In many fluid power applications, energy losses due to fittings exceed those due to viscous flow in pipes）。 在液压传动系统中所有的能量损失保持在最小的容 许范围是非常重要的。这要求适当选择组成系统的油管 和管接头的尺寸。通常，油管直径或管接头尺寸越小， 损失越大。然而，使用大直径管道和管接头会增大成本 以及对空间利用不利。因此，元件尺寸的选择就代表在 能量损失、元件成本和空间占用三者之间的平衡（It is very important to keep all energy losses in a fluid power system to a minimum acceptable level. This requires the proper selection of the sizes of the pipes and fittings which make up the system. In general, the smaller the
surface have higher velocities, with the maximum velocity occurring at the pipe centerline, as illustrated in the figure）。 实际上，管道中的流动有两种基本形态，这取决于 影响流动的不同因素（Actually there are two basic types of flow in pipes, depending on the nature of the different factors which affect the flow）。 1.层流（laminar flow）：第一种形态称为层流，它 表明液体以平滑层或薄片流动。在这种流态中，一个给 定层的液体微粒始终停留在这一层中，如图所示。因为 液体所有的微粒都以平行路线运动，这种类型的液体运 动称为层流。因此层流对微粒在本质上没有碰撞而平滑。 对于层流，摩擦是由流体的一 层或微粒以平滑连续的形态在另一 层上滑动所产生的（The first type