《专业英语》课件Unit_27.pptx
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1、Unit27Fluid MechanicsTextFluid mechanics is the branch of science concerned with moving and stationary fluids.Given that the vast majority of the observable mass in the universe exists in a fluid state,that life as we know it is not possible without fluids,and that the atmosphere and oceans covering
2、 this planet are fluids,fluid mechanics has unquestioned scientific and practical importance.Its allure crosses disciplinary boundaries,in part because it is described by a nonlinear field theory and also because it is readily observed.Mathematicians,physicists,biologists,geologists,oceanographers,a
3、tmospheric scientists,engineers of many types,and even artists have been drawn to study,harness,and exploit fluid mechanics to develop and test formal and computational techniques,to better understand the natural world,and to attempt to improve the human condition.The importance of fluid mechanics c
4、annot be overstated for applications involving transportation,power generation and conversion,materials processing and manufacturing,food production,and civil infrastructure.For example,in the twentieth century,life expectancy in the United States approximately doubled.About half of this increase ca
5、n be traced to advances in medical practice,particularly antibiotic therapies.TextThe other half largely resulted from a steep decline in childhood mortality from water-borne diseases,a decline that occurred because of widespread delivery of clean water to nearly the entire population a fluids-engin
6、eering and public-works achievement.Yet,the pursuits of mathematicians,scientists,and engineers are interconnected:Engineers need to understand natural phenomena to be successful,scientists strive to provide this understanding,and mathematicians pursue the formal and computational tools that support
7、 these efforts.Advances in fluid mechanics,like any other branch of physical science,may arise from mathematical analyses,computer simulations,or experiments1.Analytical approaches are often successful for finding solutions to idealized and simplified problems and such solutions can be of immense va
8、lue for developing insight and understanding,and for comparisons with numerical and experimental results.Thus,some fluency in mathematics,especially multivariable calculus,is helpful in the study of fluid mechanics.In practice,drastic simplifications are frequently necessary to find analytical solut
9、ions because of the complexity of real fluid flow phenomena.Furthermore,it is probably fair to say that some of the greatest theoretical contributions have come from people who depended rather strongly on their physical intuition.Ludwig Prandtl,one of the founders of modern fluid mechanics,first con
10、ceived the idea of a boundary layer based solely on physical intuition.TextHis knowledge of mathematics was rather limited,as his famous student Theodore von Karman testifies.Interestingly,the boundary layer concept has since been expanded into a general method in applied mathematics.As in other sci
11、entific fields,mankinds mathematical abilities are often too limited to tackle the full complexity of real fluid flows.Therefore,whether we are primarily interested in understanding flow physics or in developing fluid-flow applications,we often must depend on observations,computer simulations,or exp
12、erimental measurements to test hypotheses and analyses,and develop insights into the phenomena under study.Properties of LiquidsOne of the first questions we need to explore is what is a fluid or we might ask,what is the difference between a solid and a fluid We have a general,vague idea of the diff
13、erence.A solid is“hard”and not easily deformed;whereas a fluid is“soft”and is easily deformed(we can readily move through air).Although quite descriptive,these casual observations of the differences between solids and fluids are not very satisfactory from a scientific or engineering point of view.Te
14、xtA closer look at the molecular structure of materials reveals that matter that we commonly think of as a solid(steel,concrete,etc.)has densely spaced molecules with large intermolecular cohesive forces that allow the solid to maintain its shape,and to not be easily deformed.However,for matter that
15、 we normally think of as a liquid(water,oil,etc.),the molecules are spaced farther apart,the intermolecular forces are smaller than for solids,and the molecules have more freedom of movement.Thus,liquids can be easily deformed(but not easily compressed)and can be poured into containers or forced thr
16、ough a tube.Gases(air,oxygen,etc.)have even greater molecular spacing and freedom of motion with negligible cohesive and as a consequence are easily deformed(and compressed)and will completely fill the volume of any container in which they are placed.Both liquids and gases are fluids.The study of fl
17、uid mechanics involves the same fundamental laws you have encountered in physics and other mechanics courses.These laws include Newtons laws of motion,conservation of mass,and the first and second laws of thermodynamics2.Thus,there are strong similarities between the general approach to fluid mechan
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