武科大流体力学Cha.ppt

流 体 力 学 FLUID MECHANICS,双 语 课 程,Teacher: 雷 斌 Cellphone:E-mail: ,武汉科技大学-机械自动化学院-流体传动及控制研究所,2019/6/18,A Easy Problem: Could we live without water or air?,2019/6/18,Fluid everywhere,2019/6/18,Whats fluid mechanics?,It is the branch of applied mechanics concerned with the statics and dynamics of fluids. The analysis of the behavior of fluids is based on the fundamental laws of mechanics which relate continuity of mass and energy with force and momentum. Fluid mechanics vs. Solid mechanics: 1. The nature is much different to that of a solid, 2. In fluids we usually deal with continuous streams of fluid. In solids we only consider individual elements.,2019/6/18,Applications of Fluid Mechanics,App-1 : infusion apparatus 静脉输液器,2019/6/18,App-2 : toilet bowl 抽水马桶,2019/6/18,App-3 :hydraulic system 液压系统,2019/6/18,Wind energy,Tidal energy,App-4 : Renewable energy 可再生能源,2019/6/18,App-5 :,2019/6/18,Course Notes,Period: 28 （Lecture：24 Practice：4） Test: Exam 70%,Experiment 15%,Others 15% Book:FLUID MECHANICS, 金晓宏 等主编, 中国电力出版社, 2011 液压传动实验指导书,金晓宏 等主编, 中国电力出版社, 2009,2019/6/18,Content and period,Chapter 1. FLUID PROPERTIES 第一章 流体性质,2019/6/18,Outline,1.1 Definition of a fluid 1.2 Units and Conversion 1.3 Fluid Properties + 1.4 Perfect Gas # Homework,+: Detailed #: Brief,2019/6/18,1.1 流体定义,1.1 Definition of a fluid,2019/6/18,Whats fluid?,chocolate,water,cloud,oil,2019/6/18,In physics, a fluid is a substance that continually deforms under an applied shear stress, no matter how small it is.,流体是一种一受到切力作用（不论多么小）就会连续变形的物体。,Definition of a fluid,2019/6/18,Fluid classification: Liquid or Gas. The intermolecular ,int(:)mlekjul attraction of fluid is very small and its molecular motion is very intense, so the arrangement of its molecules is very loose.,Character of fluid: can not maintain certain shape and has the fluidity(流动性).,Classification & Character,2019/6/18,Solid & Fluid,Solid: withstanding the pressure, tension and shear force. Fluid: can only resist the pressure, can not resist any small tension and shear force.,Fluid can not maintain its certain shape, and will deform continuously.,2019/6/18,Liquid: The distance between molecules of liquid is almost equal to the effective diameter （有效直径）of molecule. Liquid is often called incompressible fluid（不可压缩流体）.,Gas: The distance between the molecules is very large(3107), the order of magnitude of its molecules effective diameter is 108 centimeter. Gas is often called compressible fluid（可压缩流体）.,10-8: 10 to the power minus 8,Liquid & Gas,2019/6/18,Attention: When the compressibility is not referred, the laws in fluid mechanics are suitable to liquid as well as gas. When the compressibility is involved, the problem of gas and liquid must be dealt with individually.,2019/6/18,Continuum Hypothesis haipisis (连续性假设 ),In fluid mechanics, the physical quantities of macrofluid (宏观流体) (such as pressure, velocity and density) is the statistical average of the action and the behavior of most fluid molecules. In 1753, the continuous media was first proposed as macrofluid model by Euler（欧拉）：,The real fluid is considered as no-gap continuous media, called the basic assumption of continuity of fluid, or the continuum hypothesis of fluid.,2019/6/18,1.2 Units and Conversion,1.2 单位及换算,2019/6/18,International System of Units (SI, 国际单位制) newton (N) kilogram (kg) meter (m) second (s),The U.S. customary system of units, an Inch units pound (lb) - force slug(斯勒格，质量单位) - mass foot (ft) - length second (s) - time,(1.1),(1.2),1.2.1 System of Units,2019/6/18,Centimeter-gram-second system ( universal CGS unit 通用物理单位制，厘米-克-秒制),dyne(达因) - force gram (g) - mass centimeter (cm) - length second (s) - time,(1.3),2019/6/18,Table 1.1 SI unit, U.S. customary system unit, and conversion,1.2.2 Unit Conversion,2019/6/18,Abbreviations of SI units are written in lowercase letters (小写字母) for terms like hours (h), meters (m), and seconds (s). When a unit is named after a person, the abbreviation is capitalized(大写的）; examples are watt (W), pascal (Pa), and newton (N).,Table 1.2 Selected prefixes for powers of 10 in SI units,1.2.3 Abbreviations,bri:viein of SI units,2019/6/18,1.3 Fluid Properties,1.3 流体性质（重点）,2019/6/18,Mass force acting on the center of mass in a substance is proportional to the mass. m=V,(1) Mass forces,(a) The fluid is under the action of gravity （重力）G,(1.4),1.3.1 Forces on the Fluid,Figure 1.1 Free body in the fluid,2019/6/18,(b) The inertia force （惯性力）I is:,(1.5),Figure 1.2 A container filled with liquid in a linear motion,2019/6/18,(c) The centrifugal acceleration for point A of mass m is 2r and its centrifugal force (离心力) R is,(1.6),Figure 1.3 A rotating container,2019/6/18,The total mass force and resultant acceleration are expressed by W and q， respectively.,(1.7),(1.8),(1.9),the components（分量） of force,2019/6/18,If the mass force is only gravity and inertia force, the components of force per unit mass are：,2019/6/18,(2) Surface Force,Surface force is the force acting on the surface (internal or external), and it is proportional to the area of surface. In fluid, they are pressure P and friction force T.,(1.10),is average force per unit area, or pressure stress(压应力), usually called pressure（压强）.,(1.11),is mean friction per unit area, or mean shear stress（剪应力）.,Figure 1.1 Free body in the fluid,2019/6/18,(1) Density （密度）,Density is the ratio of the mass of fluid to its volume.,(1.12),Specific volume (比容) : volume occupied by unit mass.,(1.13),The specific volume is the reciprocal (倒数) of density.,(kg/m3 ),(m3/kg ),1.3.2 Density, specific weight, relative density+,2019/6/18,(2) Specific Weight （重度）,Its the weight per unit volume,(1.14),in which is the specific weight of fluid - N/m3; G is the weight of fluid - N.,(1.14a),Or the product of density and acceleration of gravity g.,2019/6/18,(3) Relative density and specific gravity,The relative density (相对密度) RD of a fluid is the ratio of its density to the density of a given reference material.,The reference material is water at 4C i.e., ref=water.,(dimensionless quantity无量纲),The specific gravity (比重) SG of a fluid is the ratio of its weight to the weight of an equal volume of water at standard conditions(标准状态).,(dimensionless quantity无量纲),2019/6/18,(1) Compressibility（压缩性） +,As the temperature is constant, the magnitude of compressibility is expressed by coefficient of volume compressibility (体积压缩系数) p , a relative variation rate（相对变化率） of volume per unit pressure.,(1.15),The bulk modulus of elasticity (体积弹性模量) K is the reciprocal of coefficient of volume compressibility p.,(1.16),（Pa）,（Pa1）,1.3.3 Compressibility and expansibility,2019/6/18,Table1.3 The bulk modulus of elasticity of commonly used materials at 20,(2) Thermal Expansibility(热膨胀性) #,The coefficient of cubical expansion (体积热膨胀系数) t is the relative variation rate of volume per unit temperature change.,(1.17),C1,2019/6/18,Reviews,Fluid classification: Liquid or Gas. Character of fluid: can not maintain certain shape and has the fluidity. Forces on the Fluid: mass forces-gravity, inertia force, centrifugal force; surface forces-pressure, friction force. Compressibility,2019/6/18,A mineral oil in cylinder has a volume of 1000cm3 at 0.1MN/m2 and a volume of 998 cm3 at 3.1MN/m2. What is its bulk modulus of elasticity?,Example 1.1,Solution:,2019/6/18,A pressure gauge tester(压力表检测器） is shown in Fig.1.4. The cylinder is filled with oil, p=4751012Pa1. The diameter of piston is D=10 mm, the screw pitch (螺距) t=2mm. At the atmospheric pressure the volume of oil in the cylinder including pipeline is V=200cm3. For the reading of 20.0MPa in the pressure gauge, how many revolutions do the handwheel must screw in?,Figure 1.4 A pressure gauge tester,Example 1.2,1. handwheel; 2. screw; 3. piston; 4. cylinder; 5. oil; 6. pipeline; 7. standard gauge; 8. gauge to be tested,2019/6/18,The variation of cylinder volume as the handwheel screws in n revolutions is,When dV1=dV, the desired pressure dp is reached, so,The desired revolution of screwing in is,From Eq. (1.15), the decrease of volume of oil is,Solution:,2019/6/18,Viscosity is an internal property of a fluid that offers resistance to shear deformation. It describes a fluids internal resistance to flow and may be thought as a measure of fluid friction.,1.3.4 Viscosity visksiti: (黏性,粘滞性)+,流体在运动状态下抵抗剪切变形的性质，称为黏性或粘滞性。它表示流体的内部流动阻力，也可当做一个流体摩擦力量。,Whats viscosity?,2019/6/18,The viscosity of a gas increases with temperature, the viscosity of a liquid decreases with temperature.,Predominant Cause: Cohesion (内聚力) is the cause of viscosity of liquid. Transfer of molecular momentum (分子动量交换) is the cause of viscosity of gas.,When fluid is at rest, there is no relative motion of adjacent fluid layer, so no matter how much the viscosity of fluid is, no shear forces can be generated.,(1) Cause of viscosity,2019/6/18,(2) Newtons law of viscosity,Figure 1.5 Deformation resulting from application of constant shear force,In Fig.1.5, a substance is filled to the space between two closely spaced parallel plates（平行板）. The lower plate is fixed, the upper plate with area A move with a constant velocity V, a force F is applied to the upper plate.,2019/6/18,Experiment shows that F is directly proportional to A and to V and is inversely proportional （反比）to thickness h.,(1.18),If the shear stress is =F/A, it can be expressed as,The ratio V/h is the angular velocity of line ab, or it is the rate of angular deformation of the fluid.,2019/6/18,(1.19),The angular velocity may also be written as du/dz, so Newtons law of viscosity is,The proportionality factor (比例因子） is called the viscosity coefficient（黏性系数，黏度）.,Fluids may be classified as Newtonian or non-Newtonian. Newtonian fluid: is constant. (gases and thin liquids稀液) Non-Newtonian fluid: is not constant. (thick稠的, long- chained hydrocarbons长链碳氢化合物),2019/6/18,(3) Dynamic viscosity and Kinematic viscosity,The dynamic viscosity（动力黏度）is also called absolute viscosity（绝对黏度）. From （1.19）,SI unit ：kg/(ms) or Ns/m2 U.S. customary unit：dynes/cm2 （达因秒/厘米2） cgs unit： Poise (P, 泊). 1P=100cP (厘泊) 1P=0.1Pas (帕秒),The kinematic viscosity （运动黏度）is the ratio of dynamic viscosity to density.,SI unit：m2/s U.S. customary unit：ft2/s （英尺2/秒） cgs unit：stokes (St, 斯).,1cm2/s=1St 1mm2/s=1cSt（厘斯）,2019/6/18,While pressure is below atmospheric pressure, the air dissolved（溶化的） in liquid at atmospheric pressure be in a state of over-saturation (过饱和), oversaturated air will separate from the liquid and become bubble, this pressure is as air apart pressure.,1.3.5 Air apart pressure, vapor veip pressure and cavitation,(1) Air apart pressure (空气分离压),2019/6/18,Liquids evaporate（蒸发）because of molecules escaping from the liquid surface. The vapor（水汽）molecules exert a partial pressure(气体分压) in the space, known as vapor pressure. The vapor pressure of liquid depends upon temperature and increases with temperature. When the pressure on the surface of a liquid equals the vapor pressure of the liquid, boiling occurs.,(2) Vapor pressure (汽化压),2019/6/18,When the pressure is below the air apart pressure or even the vapor pressure, the air will separate out or even the liquid flashes into vapor. This is the phenomenon of cavitation.,(3) Cavitation (空穴，汽穴）,2019/6/18,The cavity in liquid collapses（破碎）, it will result in the erosion（腐蚀） of metal parts. The vapor pressure is much lower than the air apart pressure, so in hydraulic technique, whether the absolute pressure is lower than the air apart pressure is being used as a criterion for cavitation.,Cavitation damage on a valve plate for an hydraulic pump.,2019/6/18,1.3.6 Ideal Fluid,Real fluid is viscous. So while fluid moves, the inner friction is produced. In convenient for the analysis, the fluid is considered non-viscous, and called ideal fluid.,2019/6/18,1.4 Perfect Gas,1.4 完全气体 #,2019/6