Fluid PartideandIts Mainly Phical characters. Seeing from mechanics properties, solid can resist pressure pull and shear force, so it only occurs lesser distortion and stops deforming in a certain degree under the action of external force Liquid cant keep a definite shape because the gravitation between the molecules is small, so it can only resist pressure but cant resist pull and shear force. Liquid will deform continuously and flow when it is acted by a shear force These are the remarkable differences between solid and fluid
11 Seeing from mechanics properties, solid can resist pressure, pull and shear force, so it only occurs lesser distortion and stops deforming in a certain degree under the action of external force. Liquid can’t keep a definite shape because the gravitation between the molecules is small, so it can only resist pressure but can’t resist pull and shear force. Liquid will deform continuously and flow when it is acted by a shear force. These are the remarkable differences between solid and fluid
流你及其物质 从力学性质来看,固体具有抵抗压力、拉力和剪切 力三种能力,因而在外力作用下,通常只发生较小的变 形,而且到一定程度后变形就停止。 流体由于分子间引力小,不能保持一定的形状,所 以它仅能抵抗压力而不能抵抗拉力和剪切力,当它受到 剪切力作用时,就要发生连续不断的变形既流动。 这就是固体与流体的显著区别
12 从力学性质来看，固体具有抵抗压力、拉力和剪切 力三种能力，因而在外力作用下，通常只发生较小的变 形，而且到一定程度后变形就停止。 流体由于分子间引力小，不能保持一定的形状，所 以它仅能抵抗压力而不能抵抗拉力和剪切力，当它受到 剪切力作用时，就要发生连续不断的变形既流动。 这就是固体与流体的显著区别
Fluid PartideandIts Mainly Phical characters. 2. The concepts of fluid particle Seeing from microstructure, there are big gaps between fluid molecules and which are not continuous. But to fluid mechanics which researching macroscopical rules it is to model the physical bodies and is not to discuss molecular microstructure Definition fluid parcel -a physical body in the fluid whose macroscopical scale is very small but microcosmic scale is big enough and has its volume and mass fluid particlethe least unit in fluid whose linear scale can be e ignore d 13
13 2. The concepts of fluid particle Definition： Seeing from microstructure, there are big gaps between fluid molecules and which are not continuous . But to fluid mechanics which researching macroscopical rules it is to model the physical bodies and is not to discuss molecular microstructure. fluid parcel ——a physical body in the fluid whose macroscopical scale is very small but microcosmic scale is big enough and has its volume and mass. fluid particle——the least unit in fluid whose linear scale can be ignored
流你及其物质 二、流体质点的概念 从微观结构来看,流体分子与分子之间存在着较大间 隙,是不连续的但是对于研究宏观规律的流体力学来说,一般 不需要讨论分子的微观结构,而是对流体的物理实体加以模型 化 定义: 流体微团——流体中宏观尺寸非常小而微观尺寸又足够大 的任意一个物理实体,具有自己的体积和质量 流体质点—流体中可以忽略线性尺度的最小单元。 14
14 二、流体质点的概念 定义： 从微观结构来看，流体分子与分子之间存在着较大间 隙，是不连续的但是对于研究宏观规律的流体力学来说，一般 不需要讨论分子的微观结构，而是对流体的物理实体加以 模型 化。 流体微团——流体中宏观尺寸非常小而微观尺寸又足够大 的任意一个物理实体，具有自己的体积和质量。 流体质点——流体中可以忽略线性尺度的最小单元
Fluid PartideandIts Mainly Phical characters. 3. Continuous medium hypothesis Euler put forward the hypothesis of continuous medium mechanical model in 1753 a. Leave out of account the gaps of molecules, fluid is considered to be composed of micro groups which have no gaps each other and distribute continuously throughout the whole region where the liquid lies in b. All the physical variables such as density, speed, pressure, shear stress and temperature which represent liquid properties are single values and continuous differentiable functions when fluid flows continuously. So the scalar quantity fields and vector fields flow fields) of all kinds of physical variables come into being In this way we can use continuous function and field theory to study the problems about fluid movement and balance successfully 15
15 3. Continuous medium hypothesis In this way ,we can use continuous function and field theory to study the problems about fluid movement and balance successfully. Euler put forward the hypothesis of continuous medium mechanical model in 1753. a. Leave out of account the gaps of molecules, fluid is considered to be composed of micro groups which have no gaps each other and distribute continuously throughout the whole region where the liquid lies in . b. All the physical variables such as density, speed, pressure, shear stress and temperature which represent liquid properties are single values and continuous differentiable functions when fluid flows continuously. So the scalar quantity fields and vector fields ( flow fields) of all kinds of physical variables come into being