




下载本文档
版权说明:本文档由用户提供并上传,收益归属内容提供方,若内容存在侵权,请进行举报或认领
文档简介
Chapter1 MainInthischapter,wefocusonthefollowingconceptsandDefinitionofaContinuumUnitsandBasic ysisTechniques,controlvolumeandEulerianandLagrangianThermodynamicpropertiesofaViscosityandothersecondaryproperties(Newtonian/non-Newtonian;no-slipcondition,turbulence;surfacetension)FlowVisualizationFlowpatterns--Streamlines,streaklines, DefinitionofaFluidisasubstancethatdeforms变形)continuouslyundertheapplicationofashear(tangential)stressnomatterhowsmallorlargetheshearstressmaybe.3Figure1.1Behaviorof(a)solidand(b)fluid,undertheactionofaconstantshear 与时间有4MaindifferencesbetweenthebehaviorofsolidsandfluidsunderanappliedforceForasolidthestrain(应变)ordeformationisafunctionoftheappliedstressandindependentofthetimeoverwhichtheforceisapplied,providingthattheelasticlimitisnotForafluid,thedeformationisdependentonForasolid,iftheelasticlimitisnotexceeded,thedeformationdisappearswhentheforceisremoved;Afluidcontinuestoflowaslongastheforceisappliedandwillnotrecoveritsoriginalformwhentheforceisremoved.5Fluidsunderashearstressmustbeinakindof Howtokeepthefluidatrest (FWFluidtakesshapeofContinuum连续介质概Allfluidsarecomposedofmoleculesinconstantmotion.Howeverinmostengineeringapplicationsweareinterestedintheaverageormacroscopiceffectsthatweordinarilyperceiveandmeasure.Wethustreatafluidasaninfini ydivisiblesubstance,acontinuum,thusfrommacropointofview,wedonotneedtoconcernwiththebehaviorofindividualmolecules.Thisistheso-calledcontinuumconceptinclassicalfluidmechanics.在经典流体力学中,只考虑分子平均或作用,不考虑单独分子的性能 ValidityofcontinuumTheconceptofacontinuumisthebasisofclassicalfluidmechanics.Thecontinuumassumptionisvalidintreatingthebehavioroffluidsundernormalconditions.Itbreaksdownwheneverthemeanfreepathofthemolecules(
7 esthesameorderasthesmallestsignificantcharacteristicdimension(特征长度)oftheproblem.Forrarefiedgasflow稀薄气体(e.g.,asencounteredinflightsintotheupperreachesoftheatmosphere);micro-scalechannelflows(inMEMSandevenNEMS,etc.8HowtorepresentflowpropertyatapointinaForexample,thedensityatpointWhenVisshrank(收缩)toaverysmallsize,itthenwillrepresentafluidparticle/element,andthisvolumeistermedelementalvolume. CCVV Inacontinuum,volumeofafluidparticleorelementsatisfy(连续介质中的流体质点、流体微团满足下Largeenoughinmicroscope(微观足够大Smallenoughinmacroscope(宏观足够小),i.e.109
ofairatstandardconditions y
SignificanceofcontinuumAsaconsequenceofthecontinuumassumption,eachfluidpropertyisassumedtohaveadefinitevalueateverypointinspace.Thereforefluidpropertiessuchasdensity,temperature,velocity,andsoon,areconsideredtobecontinuousfunctionsofpositionandtime,andthisleadtoafield*descriptionoffluidflow.InparticularthevelocityfieldisgivenbyrrNOTE:稀薄微小空间(尺度)“Continuumconcept”breaksdownwheneverthemeanfreepathofthemolecules(分子自由行程) esthesamemagnitudeorder(相同数量级)asthesmallestsignificantcharacteristicdimension(特征长度)oftheproblem.若连续介质不适用,应如何处理呢?Dimensionand单位与量Dimensionsarepropertiesthatcanbemeasured,e.g.length,velocity,area,volume,accelerationetc.可以测量的性质叫Unitsarethestandardelementsweuse (量化thesedimensionssuchasameter,afoot用来量化量纲的标Itisnotedthat“dimension’isnotapropertyoftheindividualunits,butit lswhattheunitrepresentsAllmeasurable tiescanbedividedintotwogroups: tiesandsecondary PrimaryunitprimarydimensionMLTMMSITL ty–orsecondarydimensionorsecondary ThePrincipleofDimensional量纲一致性additivemustbedimensionallyhomogeneousandsimultaneouslybeconsistentinunitsDifferentsystemsandconversionBritishGravitationalunitsChineseEngineeringUnits(中国工程单位).metricSIsystem公制或国际单位制TheSISEEpage850–AppendixC:Conversion1.41.4Basicysiscontrolvolumeandsystem,EulerianLagrangianBasic ysisTherearethreedifferentwaystotackleafluidflow ysis(积分分析lookingatgrosseffectoffluidparticlesincontrolvolumeorsystem–ToobtainsomeintegralDifferentialysis(微分分析)lookingatinfinitesimal(极微小的)systemorcontrolvolume(localindividualbehaviour)–Toobtaindifferentialequations ysis(量纲分析)isusedinexperimentalstudyoffluidflowtorearrangeflowparametersandobtaindimensionlessparametergroups,suchasRe,Ma,etc.throughwhichwecannotobtainanexactflowsolution.ControlvolumeandTheabovementionedflow ysiscanbecarriedoutforafluidsystemoracontrolvolume.Asystemisdefinedasafixed tyandfixedidentityofmass;thesystemboundariesseparatethesystemfromthesurroundings.Theboundariesofthesystemmaybefixedormovable,butthereisnomasstransferacrossthesystemboundaries.Piston-cylinderAfluidsystemfixedmassandfixedidentityofItisnotedalltheequationsareestablishedforafixedmassintegrity.DifficultyarisesfortreatingafluidInfact,weveryoftenconcernedwiththeflowoffluidsthroughadevice,suchascompressor,turbines,pipelines,nozzles.Inthesecases,itisdifficulttofocusourattentiononafixedidentifiable tyofmass.Itismuchmoreconvenientto yzingavolumeinspacethroughwhichthefluidflows.Controlvolumeisanarbitraryvolumeinspace,chosenby yst,withopenboundariesthroughwhichmass,momentum,andenergyareallowedtocross.ItsboundaryiscalledcontrolPPipFlowFluidflowthroughaItisnotedThecontrolvolumemaybefixed,movingordeformable(固定、运动、变形).Controlvolumecanbebothfiniteand ysisisusedforafinitevolume,whiledifferential usedforaninfinitesimalvolumeCanweusetheconservationequations(suchasenergyconservationequation,momentumconservationequation)introducedinpreviouscoursesdirectlytoacontrolvolume?Itisnotedthatallconservationequationsthatyoulearnedinpreviouscoursesareestablishedforasystem(afixedmassintegrity), suchasNewton’slaw,lawsofthermodynamics,etc.;theseequationscannotbedirectlyusedforacontrolvolume.Sincethecontrolvolumedoesnothaveafixed tyandidentityofmass!3Lagrangian&EulerTherearetwodistinctwaystodescribefluidflow(toestablishtheequationsofmotion),andtheyareLarangianandEulerianmethod.Lagrangianapproachwedealwithasystemandapplybasicequationstoa tyandidentityofmass(namelysystemasdefinedabove),forinstance,theapplicationofNewton’ssecondlawtoaparticleoffixedmass.SuchaapproachisoftentermedLagrangianapproach.Indifferential ysis,Lagrangianapproachfocusesona tyofmass,andtracethehistoryofpropertyforindividualfluidparticles(orelementalbodyofmassFrvtr(rr)rrrardd2rdrr(a,b,c)v.r(a,b,rrV a,andrrrSincea,V a,andrrr
ItisnotedthatinLagrangianframe(在日框架下),flowparametersareexpressedasrV(a,b,c,t)P(a,b,c,t)(a,b,c,t)Wherea,b,&careconstants,andtheyrepresentinitialposition初始位置ofthefluidApplicationofLagrangianLagrangianapproachspecifieshistoryofpropertiesforindividualfluidparticles.Forexample,itisusedtotrackdiscreteparticlesanddroplets(离散粒子和液滴)inacontinuousfluid.Lagrangianapproachtracesallparticles(e.g.1,2,and3…..N)atinitialandsuccessivetimeinstant(a1,b1,c1,
,
,
,particle particle3
(a3,b3,c3,ty
t2Eulerapproach-focusesonflowpropertiesatagivenpointinspaceoragivenfinitecontrolvolume(throughwhichdifferentfluidparticlesmaypassatdifferenttimeinstants.)Itisnotedthatinanintegral ysisweshouldconsiderafinitecontrolvolumewithfixedboundary,whileinadifferential ysisweshouldconsideraninfinitesimalcontrolvolumeofagivenpointinspace.EulerEulerapproachfordifferential ysisfocusingonflowpropertyatanarbitrarypoint(ofaninfinitesimalcontrolvolume,微小的容积).ItisnotedthatinEulerframe(在框架下vv(x,y,z,tp(x,y,z,t(x,y,z,trTheapproachisbasedonfieldIngeneral,velocityisavectorfunctionofpositionandthushasthreecomponents,u,v,andw,andwritten 3Eulerapproachforintegral ysisfocusingonafinitecontrolvolume(有限大容积)withfixed3PPipControlFlowFluidflowthroughaThermodynamicpropertiesofaVelocityarethemostimportantfluidproperty,anditinteractscloselywiththethermodynamicpropertiesofthefluid.Thefollowing9tiesarethermodynamicpropertiesdeterminedbythermodynamicconditionorstate.Basic(intensive) OtherintensivePressureDensityTransportCoefficientofviscosity
Enthalpyh=ˆp/Entropy
weightVgg limmVV'(g)liquidV(V1000(g)gasV( 1.205Specificgravity(SG,)比引力或比(waterat(airat20°C&Internal,PotentialandKineticInternalmolecularbondingPotential
molecularactivityKineticfludmechanicssumofthree eˆ1/2V2 Wedefinezasupward,
gr
andweeˆ1/2V2Viscosityandothersecondary tiessuchaspressure,temperature,anddensityareprimarythermodynamicproperties(variables).Certainsecondaryproperties(variables)alsocharacterizespecificfluidmechanicalbehavior.Viscosityisthemostimportantsecondaryproperty,whichrelatesthelocalstressestothestrainrateofthemovingfluidelement.Itisa tativemeasureofafluid's toflow,inparticular,itdeterminesthefluidstrainrategeneratedbyagivenappliedshearViscosityisthepropertyofafluid,duetocohesionandinteractionbetweenmolecules,whichoffers sheardeformation.粘DifferentfluidsdeformatdifferentratesunderthesameshearTherelationshipamongstress,strainordeformationrate,viscosityisdifferentfordifferentDeformationofa 平板作用于流体上的切应力 A0 )流体的变形率(deformation)
limt0MM'之间的距 l lu故t 流体的变形率(deformationdd
Evenamongfluids,therecarewidedifferencesinthebehaviorunderstress.Accordingtotherelationbetweentheappliedshearstress,andtherateofdeformation,fluidsareclassifiedintoNewtonianandnon-newtonian流体与 NewtonianFluidsinwhichappliedshearstressisdirectlyproportionalto(正比于)rateofdeformationaretermedNewtonianfluids.
SuchapropertyoffluidsisaconstantforallNewtonianfluids,termeddynamicviscosity(动力粘性)/orabsoluteviscosity Newton’slawofviscosity(内摩擦定律 Dynamicviscosityorabsolute
m2//Non-Newtonianfluids(非流体non-NewtonianfluidsdonotsatisfyNewton’slawofviscosity. Fornon-Newtonianfluids,theviscositycommonlyisnotaconstant. Figure1.9Rheologicalbehaviorofvariousmaterials:(a)Stressversusstrainrate--Comparisonsofnewtonianand newtonianfluids;(b)Effectoftimeonappliedstress--Tomaintainaconstantstrainratewithtime,thestressrequiredisdifferentforcommonfluids,rehopecticfluids,andthixotropicfluidsViscosityvarieswith
粘性随压力的变TheviscosityofNewtonianfluidsisatruethermodynamicpropertyandvarieswithtemperatureandGenerallyspeaking,theviscosityofafluidincreasesonlyweaklywithpressure.Forinstance,increasingpfrom1atmto50atmwillincreaseµofaironly10%.Itiscustomaryinmostengineeringworktoneglecttheinfluenceofpressurevariationonviscosityoffluids.Viscositymainlyvarieswith粘性随温度变Viscosityofgasesincreaseswithtemperature,whereasforliquids,viscositydecreaseswithincreasingThereasonisthatviscosityresultsfromthecombinedeffectofinteractionandcohesionofmolecules,andtheyplaydifferentrolesingasesandliquids.Forgases--Themoleculesofgasesareonlyweaklykeptinpositionbymolecularcohesion(astheyaresofarapart).Asadjacentlayersmovebyeachotherthereisacontinuousexchangeofmomentum.Moleculesofaslowerlayermovetofasterlayerscausingdrag,whilemoleculesmovingtheotherwayexertanaccelerationforce.气体聚合力小,分子动量交换强,间动量交换Iftemperatureofagasincreasesthemomentumexchangebetweenlayerswillincreasethusincreasingviscosity.Forliquids--Thoughthereissomemolecularinteraction,butasthemoleculesaresomuchcloserthaningases,thecohesiveforceholdthemoleculesinplacemuchmorerigidly.Thiscohesionplaysanimportantroleintheviscosityofliquids.Increasingthetemperatureofaliquidreducesthecohesiveforcesandthen todeformationdecreases,thusdecreasingviscosity.Deceasingthetemperatureofaliquidincreasecohesiveforcesthusincreasingviscosity.NotionsrelatedtoNo-slip
i.e.atsolidboundary,
Turbulent
Withoutviscosity,therewouldnotbeanyturbulentflowatall!Figure1.15:VelocityNo-slipconditioninwaterflowpastathinfixedLaminarSurfacetension表面张Surfacetensionisgeneratedattheinterface界面AmoleculeIintheinteriorofaliquidisunderattractiveforces(i.e.cohesion)inalldirectionsandthevectorsumoftheseforcesiszero.ButamoleculeSatthesurfaceofaliquidisactedbyanetinwardcohesiveforcethatisperpendiculartothesurface.Hencetheinterfacerequiresworktomovemoleculestothesurfaceagainstthisopposing(cohesion)force,andsurfacemoleculeshavemoreenergythaninteriorones
ofliquidsandSurfacetensionofaliquidistheworkthatmustbedonetobringenoughmoleculesfrominsidetheliquidtothesurfacetoformoneunitareaofthatsurface(J/m2=N/m).SurfacetensionmustsatisfyminimumenergySurfacetensionleadingtocapillarity毛细管现象Surfacetensioncausesdropsofliquidtotendtotakeasphericalshape.Thisisresponsibleforcapillaryactionandcausesaliquidtoriseordropinafinetubewhenitslowerendisinvertedinaliquid.ExampleExampleFigureRiseorLiquidsriseintubesiftheyadhesionofsolidsurface(粘附力)>cohesion(聚合力)andfallintubesifcohesion>adhesionNote:Whenuseacolumnofliquidtomeasurepressure,oneshouldremembertocorrecthis/herreadingerrorsresultedfromcapillarity!!FlowpatternsStreamlines,streaklines,andpathlinesForFlowVisualizationFlowpatternscanbevisualizedindifferentways,wecansketchesandphotographstodescribetheflowqualitativelyandFourbasictypesofline'spatternsareusedtovisualizethe1Pathline(迹线isatra
温馨提示
- 1. 本站所有资源如无特殊说明,都需要本地电脑安装OFFICE2007和PDF阅读器。图纸软件为CAD,CAXA,PROE,UG,SolidWorks等.压缩文件请下载最新的WinRAR软件解压。
- 2. 本站的文档不包含任何第三方提供的附件图纸等,如果需要附件,请联系上传者。文件的所有权益归上传用户所有。
- 3. 本站RAR压缩包中若带图纸,网页内容里面会有图纸预览,若没有图纸预览就没有图纸。
- 4. 未经权益所有人同意不得将文件中的内容挪作商业或盈利用途。
- 5. 人人文库网仅提供信息存储空间,仅对用户上传内容的表现方式做保护处理,对用户上传分享的文档内容本身不做任何修改或编辑,并不能对任何下载内容负责。
- 6. 下载文件中如有侵权或不适当内容,请与我们联系,我们立即纠正。
- 7. 本站不保证下载资源的准确性、安全性和完整性, 同时也不承担用户因使用这些下载资源对自己和他人造成任何形式的伤害或损失。
最新文档
- 2025年房地产土地开发合同范本
- 2025版军人离婚协议书范本:军人配偶再婚子女抚养权争议
- 2025版水利工程工地围挡及防汛抗洪合同范本
- 二零二五年度体育赛事策划与运营合同
- 二零二五年度矿产资源勘查开发项目矿产资源政策咨询服务协议
- 2025注册电气工程师考试电力系统优化设计与应用试题
- 二零二五版建筑砌体材料采购与施工协议
- 2025版汽车销售担保合同模板:以旧换新专项版
- 2025物流师职业技能鉴定实战案例分析试题集
- 骆驼祥子考试题库及答案
- 环境损害司法鉴定行业报告
- 神经内科常用特殊药物使用注意事项
- 医院智慧后勤管理平台建设方案
- 胸痛中心培训急性胸痛患者的早期快速甄别
- 黔东北土家族高腔山歌的特点
- 盐酸安全技术说明书(化学品安全技术说明书)
- GB/T 4728.8-2022电气简图用图形符号第8部分:测量仪表、灯和信号器件
- 等边三角形说课(ppt)
- 2023年普洱市辅警招聘笔试模拟试题及答案解析
- 设备点检管理标准
- 开标相关情况说明
评论
0/150
提交评论