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1.3TheCharacteristicsofFluids流体的特征constituent:组成的;tangential:切向的;restrain:限制、约束;equilibrium:平衡,均衡;interface:相互关系、分界面;molecule:微小颗粒、分子;continuum:连续体;vessel:容器;tar:焦油、柏油;pitch:树脂;imperceptibly:察觉不到的,细微的;restore:恢复;subside:下沉、沉淀、减退、衰减;hypothetically:假设地、假想地;sphere:球、球体;microvolume:微元体积;rarest:最稀罕的,虽珍贵的Afluidisasubstancewhichmayflow;thatis,itsconstituentparticlesmaycontinuouslychangetheirpositionsrelativetooneanother.Moreover,itoffersnolastingresistancetothedisplacement,howevergreat,ofonelayeroveranother.Thismeansthat,ifthefluidisatrest,noshearforce(thatisaforcetangentialtothesurfaceonwhichitacts)canexistinit.Asolid,ontheotherhand,canresistashearforcewhileatrest;theshearforcemaycausesomedisplacementofonelayeroveranother,butthematerialdoesnotcontinuetomoveindefinitely.Inafluid,however,shearforcesarepossibleonlywhilerelativemovementbetweenlayersisactuallytakingplace.Afluidisfurtherdistinguishedfromasolidinthatagivenamountofitowesitsshapeatanyparticulartimetothatofavesselcontainingit,ortoforceswhichinsomewayrestrainitsmovement.流体是可以流动的物质,也就是说,组成流体的质点可以连续的改变它们的相对位置。而且,不管层与层之间的相对位移有多大都不会产生持久的抵抗力。这意味着流体在静止状态下是不会存在剪切力的(剪切力是与其作用表面相切的力)。另一方面,固体在静止时却可以抵抗剪切力,其中的剪切力也可以使层与层之间发生相对位移,但是固体材料却不一定会有连续的运动。然而在流体中,只有当层与层之间有相对运动产生时才会有剪切力存在。流体和固体的进一步区别还在于在特定的时刻,确定数量的流体其形状取决于承载它的容器,或者取决于一些限制其运动的力。Thedistinctionbetweensolidsandfluidsisusuallyclear,buttherearesomesubstancesnoteasilyclassified.Somefluids,forexample,donotfloweasily:thicktarorpitchmayattimesappeartobehavelikeasolid.Ablockofsuchasubstancemaybeplacedontheground,but,althoughitsflowwouldtakeplaceveryslowly,yetoveraperiodoftime--perhapsseveraldays--itwouldspreadoverthegroundbytheactionofgravity,thatis,itsconstituentparticleswouldchangetheirrelativepositions.Ontheotherhand,certainsolidsmaybemadeto―flow‖whenasufficientlylargeforceisapplied;theseareknownasplasticsolids.固体和流体之间的区别通常是很明显的,但是也有些物质难于归类。比如说有些流体并不易流动,如重油和树脂有时候会表现得像固体一样,像这样的一块物质如果放在地面上,虽然它的流动发生的非常缓慢,要经过很长的一段时间—也许要好几天,但是在重力的作用下它仍然会在地面上蔓延开来,也就是说,它的组成质点会改变它们之间的相对位置。另一方面,某些固体当足够大的力作用时也会“流动”,这就是我们所知的“塑性固体”。Evenso,theessentialdifferencebetweensolidsandfluidsremains.Anyfluid,nomatterhow―thick‖orviscousitis,beginstoflow,evenifimperceptibly,undertheactionoftheslightestnetshearforce.Moreover,afluidcontinuestoflowaslongassuchaforceisapplied.Asolid,however,nomatterhowplasticitis,doesnotflowunlessthenetshearforceonitexceedsacertainvalue.Forforceslessthanthisvaluethelayersofthesolidmoveoveroneanotheronlybyacertainamount.Themorethelayersaredisplacedfromtheiroriginalrelativepositions,however,thegreateraretheforcesresistingthedisplacement.Thus,ifasteadyforceisapplied,astatewillbereachedinwhichtheforceresistingthemovementofonelayeroveranotherbalancetheforceappliedandsonofurthermovementofthiskindcanoccur.Iftheappliedforceisthenremoved,theresistingforcedwilltendtorestorethesolidbodytoitsoriginalshape.即便如此,固体和流体之间依然有本质的差异。任何流体,无论多“稠”或者粘性多大,在最微小的剪切力作用下都会流动,即便这种流动是极其细微的。而且,只要这种力持续作用,流体就会连续流动。然而对于固体,不管它的可塑性有多强,只有当作用其上的净剪切力超过一定数值后才会流动,而小于该值的力所引起的固体层之间的相对移动是有限的。层偏离原始位置的程度越大,抵抗这种变形的力也就越大。因此,当一恒定力作用时,就会达到这样一个状态:即会产生一个抵抗这种层间相对运动的力以平衡所施加的外力,所以不会产生进一步的运动。如果将所施加的外力移除,抵抗力将会使固体恢复到它的原始形状。Inafluid,however,theforcesopposingthemovementofonelayeroveranotherexistonlywhilethemovementistakingplace,andsostaticequilibriumbetweenappliedforceandresistancetoshearneveroccurs.Deformationofthefluidtakesplacecontinuouslysolongasashearforceisapplied.Butifthisappliedforceisremovedtheshearingmovementsubsidesand,astherearethennoforcestendingtoreturntheparticlesoffluidtotheiroriginalrelativepositions,thefluidkeepsitsnewshape.然而,在流体中,只有当层间相对运动发生时才会存在这种阻止运动的力。所以不会存在这种外力和抵抗力之间的静态平衡。只要有剪切力作用,流体便会产生连续变形。但是如果将外力移除,剪切运动便会减退,而且因为这时没有使流体质点回到它们初始位置的力,所以流体将保持它的“新”形状。Fluidsmaybesub-dividedintoliquidsandgases.Afixedamountofaliquidhasadefinitevolumewhichvariesonlyslightlywithtemperatureandpressure.Ifthecapacityofthecontainingvesselisgreaterthanthisdefinitevolume,theliquidoccupiesonlypartofthecontainer,anditformsaninterfaceseparatingiffromitsownvapour,theatmosphereoranyothergaspresent.流体可以划分为液体和气体。一定数量的液体其所占据的体积也是一定的,它随温度和压力的变化很小。如果容器的体积大于这个一定的体积,那么液体占据的只是容器的一部分,而且会形成一个分界面将液体与该液体的蒸汽、空气或其它存在的气体分开。Ontheotherhand,afixedamountofagas,byitselfinacontainer,willalwaysexpanduntilitsvolumeequalsthatofthecontainer.Onlythencanitbeinequilibrium.Intheanalysisofthebehaviouroffluidsthemostimportantdifferencebetweenliquidsandgasesisthat,whereasunderordinaryconditionsliquidsaresodifficulttocompressthattheymayformostpurposesberegardedasincompressible,gasesmaybecompressedmuchmorereadily.Whereconditionsaresuchthatanamountofgasundergoesanegligiblechangeofvolume,itsbehaviourissimilartothatofaliquidanditmaythenberegardedasincompressible.If,however,thechangeinvolumeisnotnegligible,thecompressibilityofthegasmustbetakenintoaccountinexaminingitsbehaviour.而另一方面,容器内一定数量的气体则通常都要膨胀到和容器相等的体积。这样它才会达到平衡状态。在分析流体特性时,液体和气体之间最重要的差异在于:鉴于在通常的条件下液体是难以压缩的,因此常常将液体认为是不可压缩流体,而气体则容易压缩的多。当一定量的气体其体积变化可以被忽略的情况下,气体的表现和液体类似,因此也可以认为是不可压缩的。然而,如果体积的变化不可忽略,那么在分析其行为特征时就必须考虑气体的压缩性。Inconsideringtheactionofforcesonfluids,onecaneitheraccountforthebeh
本文标题:热能与动力工程专业英语-翻译(李瑞扬)
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