工程流体力学(英文版)第一章.pdf

Chapter1BasicConceptsandPropertiesofFluidMechanicscontents1ThedefinitionoffluidsandcontinuumHypothesis(2PhysicalProperties3Forcesactedonafluid4Flowofafluid5Viscosity(ofafluid**6Compressibilityandincompressibleflow7SurfaceTension1.Definitionofafluid:Intheearththerearethreemainsubstancestates1ThedefinitionoffluidsandcontinuumHypothesisSolidliquidgas.FluidAfluidisasubstancetheparticlesofwhicheasilymoveandchangetheirrelativepositionOr:asubstancethatwilldeformorflowfluidsolidτPdeformation,noshape•DistinctionBetweenaSolidandaFluidSolidcannotonlybearpressureandpullingforce,butalsoresistingtangentialstress.Fluidcanbearpressure,butcannotbearpullingdeformationforceandresisttangentialstress.MicrocosmicAfluidiscomposedofalargeamountofmoleculesinirregularmotionwithgapbetweenmolecules.Macroscopicmuchlargerthanintermoleculardistancemeanfreepathyxz0·P(x,y,z)δδ
δδ
δ
δ
Ifδδρ=δδIfδδδδchangeso,ρisdefinedas

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lim
mΩΩρΩ→=AssumptionofContinuumModelAssumptionofContinuumModelContinuum/ContinuoussubstanceFluidsorsolidparticlesoccupyspacecontinuouslywithintermoleculardistanceandmolecularmotionignored2DefinitionofContinuumModelδδ
δ
δ
1DefinitionfluidparticleAfliudelementwiththevolumeδisdefinedasafluidparticle,δisavolumethatissmallenoughwithenoughmolestomakesurethatthemacroscopicmeandensityhasdefinitevalue.1cm3liquid:3.3x1022molecules1cm3gas:2.7x1019molecules10-19cm3gas:2.7x1010moleculesContinuumModelAnassumptionmodelthatassumesafluidasacontinuum.Underthisassumption,allphysicalpropertiesarethecontinuousfunctionofspacecoordinatesandtimeExerciseAccordingtotheconceptofcontinuum,fluidparticleisAfluidmoleculeBsolidparticleinfluidCgeometricpointDdifferentialelementwhosegeometricscaleisinfinitesimal,yetincludesalargeamountofmolecules.return1Excludingcomplexibilityofmoleculesinmotion.2Physicalpropertiesarethecontinuousfunctionofspaceandtime,sowecanusemathematicaltoolofcontinuousfunctiontosolveproblems.return3AdvantagesDensityismassofthesubstanceperunitvolumeUnitkg/m3.1.DensityForhomogenousfluid,eachpointhasthesamedensity2PhysicalProperties(Density,Relativedensity,SpecificVolumeandDensityofmixturegas)m=ρΩpRTρ=Foridealgas01ttρρα=+δδ
δ
δ


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lim
m→=ΩΩΩρ2.Relativedensitywhereρw——densityof4waterkg/m31.2PhysicalPropertieswdρρ=1υρ=3.SpecificVolumeofGas(SpecificVolumeisthevolumeoccupiedbyamassofgas,specificvolumeisthereciprocalofdensity,m3/kgRelativedensityofafluidistheratioofitsdensitytothatofpurewaterat4Ithasnounitanddimensionless.4.DensityofmixturegasDensityofmixturegasiscalculatedasvolumefraction:whereρi——densityofgasi——volumefractionofgasi1122331niiiρραραραρα==++⋅⋅⋅⋅⋅⋅=
iα1.2PhysicalPropertiesSpecificWeightisweightperunitvolume.Unit30limGΔΩΔγΔΩ→=Forhomogenousfluid,eachpointhasthesamespecificweightγ=G/V=ρgForwater,thenominalvalueofspecificweightisγ=9800N/m35.SpecificWeight***1.2PhysicalProperties2Accordingtoactingmethod:1Classification1Accordingtophysicalpropertiesoffluids:3ForcesonFluids(Massforceandsurfaceforce)gravity,frictionforce,inertiaforce,elasticforceandsurfacetension.Massforceandsurfaceforce.1MassForce:Theforceactingoneveryfluidmassparticlewithinthecontrolbody.Itsvalueisproportionaltoitsmass.Forhomogeniousfluids(eachpointwithinthefluidhasthesamedensity)massforceisproportionaltofluidvolume.Massforceisalsoknownasbodyforce.Unit:N2Massforce2Unitmassforceisthemassforceactingonperunitmassfluid.Unitofunitmassforce[m/s2]It’sthesameastheunitofaccelerationunit.dFfdρ=Ω

xyxffifjfk=++



3ForcesonFluids(Massforceandsurfaceforce)1SurfaceForceistheforceexertedonthecontactsurfacebythecontactedfluidorotherbody.Itsvalueisproportionaltocontactarea.3SurfaceForce2/mNorPa
nFAσ=
0lim
nAFAσ→=NormalCompressiveForce:perpendiculartoactingsurfaceShearForce:paralleltoactingsurfaceAccordingtotheactingdirections,surfaceforcescanbeclassifiedinto:2Stressisthesurfaceforceonperunitarea,unit:
FAττ=
0lim
AFAττ→=ShearingstressPressurenFFFτ=+


1.Mechanicalcharacteristics:Solidcannotonlybearpressureandpullingforce,butalsoresistingtangentialstress.Fluidcanbearpressure,butcannotbearpullingdeformationforceandresisttangentialstress.4Flowofafluidpressurepullingforcetangentialforce.Solid:Fluid:√√√√√√√√√√√√√√√√fluidsolidτPF2.Relationshipbetweendeformationandforce:4FlowofafluidFx∝ΔreturnxΔF2.Relationshipbetweendeformationandforce:4Flowofafluid0,Fx≠Δ→∞returnChineseText:P16:1-1,1-2,1-4,1-7,1-8ExerciseEnglisheText:P2:1.1.1,P17:1.7.1,1.7.2,1.7.35ViscosityofaFluid1.Thedefinitionofviscosity2.Newton’sViscosityLaw*3.Newtonianandnon-NewtonianFluids4.CoefficientofViscosity5.5.AffectingFactorsofViscosityAffectingFactorsofViscosity5.1DefinitionofviscosityViscosityisthatpropertyofafluidbyvirtueofwhichitoffersresistancetoshear.(thefluidstickingaction)Themostimportantpropertyofafluid,anyrealfluidisviscousfluid.Kinescope2Kinescope1Accordingtowhetherfluidshaveviscosity,fluidscanbeclassifiedintoIdealfluid1.μ0.2.du/dy=0.RealfluidViscousfluid,μ≠0.1.Viscosity:2.Causeofviscosity:(a)attractive,cohesiveforcesbetweenthemolecules(b)molecularinterchangeQuestion:Whatisidealgas?return5.2Newton’sViscosityL