第五章 液体在管道中流动的基础知识.

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 is called laminar
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 representsΒιβλιοθήκη Baiduwasted 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 flow path through a fitting determines the
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）。
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）。
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 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
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
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下波动。 这个混合作用由于液体微粒的碰撞而产生
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 pipe diameter or fitting size, the greater the losses. However, using large-diameter pipes and fittings results in greater
power system）。
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 surface have higher velocities, with the