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1、1,Chapter 1 Fluid Properties,Why Study Fluid Mechanics ?,For Civil Engineers : Water Supply : Dams, reservoirs, Treatment and Distribution network systems Drainage and Irrigation Systems : Open Channel Hydraulics Environmental Hydraulics : Sewerage Systems, Pollutant Dispersion Modelling in Air and
2、Water For Mechanical Engineers : Aerodynamics Airfoil design, Lift and Drag, CFD Rotodynamic Machinery Pumps and Turbines Hydraulic/Pneumatic Control Systems Industrial Hydraulics : Piping Systems Gas and Oil Industries,3,Fluid mechanics is the study of fluids either in motion (fluid dynamics) or at
3、 rest (fluid statics) and the subsequent effects of the fluid upon the boundaries, which may be either solid surfaces or interfaces with other fluids.,What is the fluid mechanics study?,Subsequent: 随后的，后来的，并发的,4, The differences between solids and fluids can be explained qualitatively(定性地) on the ba
4、sis of molecular structure.,1.1 Definition of a Fluid,What is a fluid? from a scientific or engineering point of view., A more specific distinction is based on how they deform under the action of an external load.,5,A solid has densely spaced molecules with large intermolecular cohesive forces that
5、allow the solid to maintain its shape, and to not be easily deformed.,1.1 Definition of a Fluid,A liquid, the molecules are spaced farther apart, the intermolecular forces are smaller than for solids, and the molecules have more freedom of movement.,6,Specifically, a fluid is defined as a substance
6、that deforms continuously when acted on by a shearing stress of any magnitude. A shearing stress (force per unit area) is created whenever a tangential(切向的) force acts on a surface.,1.1 Definition of a Fluid,7,1.1 Definition of a Fluid,When common solids such as steel or other metals are acted on by
7、 a shearing stress, they will initially deform (usually a very small deformation), but they will not continuously deform (flow).,8,1.1 Definition of a Fluid,9,Some materials, such as slurries(泥浆), tar(柏油), putty(油 灰, 灰泥), toothpaste, and so on, are not easily classified since they will behave as a s
8、olid if the applied shearing stress is small, but if the stress exceeds some critical value, the substance will flow. The study of such materials is called rheology(流变学) and does not fall within the province of classical fluid mechanics.,1.1 Definition of a Fluid,10,Although the molecular structure
9、of fluids is important in distinguishing one fluid from another, it is not possible to study the behavior of individual molecules when trying to describe the behavior of fluids at rest or in motion.,1.1 Definition of a Fluid,11,1.1 Definition of a Fluid,For gases at normal pressures and temperatures
10、, the spacing is on the order of 10-6 mm and for liquids it is on the order of 10-7mm . The number of molecules per cubic millimeter is on the order of 1018 for gases and 1021 for liquids.,10 to the power of minus 6 或 the minus 6th power of 10 minus 可用negative换 plus / positive,12,We thus assume that
11、 all the fluid characteristics we are interested in (pressure, velocity, etc.) vary continuously throughout the fluidthat is, we treat the fluid as a continuum (连续体). This concept will certainly be valid for all the circumstances considered in this text.,1.1 Definition of a Fluid,etc,这个是来源于拉丁语，全称etc
12、etera. n.等等之人（或物），附加的人（或物）； 加s：附加（或额外）的项目；零星杂物。 或者分开的et cetera, adv. 等等，以及其他等等（略作etc.或c.） 但得注意，人名后不宜用，要用and others。,13,14,1.2.1 System of units The International System of Units U.S. customary system units , an Inch Units Centimeter-gram-second system of units,1.2 Units and Conversion,15,1.2.2 Units
13、 Conversion Table1.1 SI units, U.S. customary system units, and conversion,1.2 Units and Conversion,16,1.2.3 Abbreviations of SI Units Abbreviations of SI units are written in lowercase letters for terms. hours - h meters - m seconds s When a units is named after a person, the abbreviation is capita
14、lized. watt W pascal - Pa,1.2 Units and Conversion,17,1.3 Fluid Properties,1.3.1 Force on the fluid mass forces and surface forces 1.3.1.1 Mass Force the gravitational force the inertia force centrifugal force,18,1.3 Fluid Properties,The above three kinds of forces are directly proportional to the m
15、ass of fluid m and act on the center of mass.,19,1.3 Fluid Properties,Figure 1.2,问题1： 比较重力场（质量力只有重力）中，水和水银所受的单位质 量力f水和f水银的大小？,A. f水f水银； B. f水=f水银； D、不一定。,问题2： 试问自由落体和加速度a向x方向运动状态下的液体所受的单 位质量力大小（fX. fY. fZ）分别为多少？,自由落体：XY=0,Z=0，加速运动：X=-a, Y=0,Z=-g。,22,1.3 Fluid Properties,1.3.1.2 Suface Force pressure
16、 stress shear stress,23,1.3.2 Density, Specific weight, relative density,1.3.2.1 Density,The density of a fluid, designated by the Greek symbol (rho), is defined as its mass per unit volume. Density is typically used to characterize the mass of a fluid system. In SI the units are kg/m3.,24,1.3.2 Density, Specific weight, relative density,The specific weight of a fluid, designated by the Greek symbol (gamma), is defined as its weight per unit volume. Thus
