航空發(fā)動機專業(yè)英語之流體的物理特性.ppt

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Pressure,Thepressureofafluidisanotherimportantconsiderationforcomputingaerodynamicforces.Whenafluidmovesoverorthroughanobject,itexertssmallpushesonthesurfaceoftheobject.Thesepushesaredefinedasthepressureexertedontheobject.Pressureismeasuredasforceperunitarea,becausethepushesoccurovertheentiresurfaceoftheobject.Theunitsforpressureincludemasstimesalengthdividedbytimesquared(theunitsforaforce)dividedbytheareaofthesurface(lengthsquared).exert施加力，作用力，發(fā)揮，盡力,Inmetricunits,pressureismeasuredinNewtonspersquaremeter(N/m2)orPascals(Pa).ANewton,aforceunit,is1kilogramtimes1meterdividedbysecondssquared.APascalis1Newtonpermetersquared.IntheEnglishsystem,pressureismostcommonlymeasuredinpoundspersquareinch(psi).Ineithersystem,pressureissometimesmeasuredinatmospheres,ameasureofhowthepressurediffersfromtheatmosphericpressureinatypicalroomontheearth’ssurface.Atmosphericpressurehasbeendefinedexperimentallyas14.7psior101kPa.Akilopascal(kPa)is1000pascals.,Sometimespressureisgivenininchesofmercury(inHg)ormillimetersofmercury(mmHg)becauseold-stylemanometersandbarometers(instrumentstomeasurepressure)usedtomeasurethelengthofathincolumnofmercurytocomputethepressures.metric度量poundspersquareinch磅每平方英寸atmospheres大氣atmospheric大氣壓的，常壓的,experimentally實驗上，用實驗方法mercury汞，水銀manometers（液體）壓力計barometers氣壓表（計）column柱，柱狀物deceive欺騙，蒙蔽，偽裝，掩飾govern管理，支配，調(diào)整，控制，操縱運輸,Pressurecanbeapowerfulquantity.Itcanalsobedeceiving.Asmallpressurespreadoveraverylargeareacanadduptobeaverylargeforce.Airpressuresdecreaseasthealtitudeincreases;pressuresalsodecreasewhenthespeedofthefluidincreases.Whenthetemperatureofafluidincreases,sodoesthepressure.Therulesthatgovernthesechangesarepartofthestudyofaerodynamics,andthepressuresonanairplanecontributedirectlytoitsflightcapabilities!,DensityDensityisameasureofhowmuchmass(theamountofmolecules)isincludedinagivenvolume.Anotherwaytothinkaboutitisthatitcanbeconsideredthemeasureofhowtightlythemoleculesarepackedinavolumeorobject.Theunitsfordensityaremasspervolume,orkg/m3,forexample,Whenwetalkaboutthedensityofafluid,weoftencomputethedensityusingaunitvolume,say1mby1mby1m(1m3).,Wedothisfortworeasons:first,wedon’thavetoknowexactlyhowlargeourvolumeis(couldbetoughintheearth’satmosphere,orinalargeocean!),andsecond,itallowsustocomparedensitiesatdifferentconditions–ahotdayversusacoldday,orhighaltitudesversusloweraltitudes.tough堅韌的，強韌的，堅強的，結(jié)實的,Theunitsfordensityaremasspervolume,andthemostcommonmeasurementsarekg/m3,slugspergallon,orslugspercubicfoot(slugs/ft).Whatisaslug?TheUSusestermsformassandweightinterchangeably,butsciencerequiresprecisedefinitionsofpropertiesandtheirunits.Aslugisequalto32.174poundsmass.Scientistsandengineershavedefinedthetermspoundsforce(1bf)andpoundsmass(1bm)tohelpdifferentiatebetweenamassandaforcelikeweight.,slug斯（拉格）質(zhì)量單位＝32.2磅Afluidwithalotofmoleculestightlypackedtogetherhasahighdensity;onethathasfewermoleculeswouldhavealowerdensity.Water,forexample,hasamuchhigherdensitythanair.A10gallonfishtankwithwaterinithasmuchmoremassinitthana10gallontankwithairinit.Sinceithasmoremass,itwillweighmore(moreonthatinalatersection).Inaddition,thedensityisusedtodefinewhetherafluidisincompressibleorcompressible.,Ifthedensityofthefluidisfixed(constant),thefluidisincompressible;neitherthemassorthevolumecanchange.Water(anincompressibleliquid)pouredfroman8ouncecylindricalglasswillstillhave8ouncesinalargeroundbowl.theamountofmassandthevolumestaythesame.Ifthedensitycanchange,thefluidiscompressible.Gasesarecompressiblefluids;theywillexpandtofillanewvolume.Themassdoesn’tchange,butthevolumeincreases,sothedensityofthegasdecreasesinthenewvolume.,tank水箱，大容器compressible可壓縮的，可濃縮的Allofthepropertiesarelinkedtogether.Ifthepressureorthetemperatureofafluidchanges,itsdensitywillusuallychange,too.Thedensityofaironahotdayislowerthanthedensityofaironacoldday.Athighaltitudes,wherethepressureislower,thedensityisalsolower.Thisiswhyanaerodynamicistmustpayattentiontoallofthepropertiesofthefluidwhentryingtodefineflowconditions.,Viscosity,Thisisoneofthemostdifficultpropertiesonthislisttodefine.Itisameasureofhowmuchafluidwillresistflowing.Ifyouspillwateronaninclinedboard,itwillrunquicklydowntheboard.However,ifyouSpillhoneyonthesameboard,itwilltraveldowntheboardmuchmoreslowly.Honeyhasamuchhigherviscositythanwater.Itissaidthathoneyisamoreviscousfluidthanwater.,viscosity粘性，粘度resist阻礙，反抗，抵抗spill流出，溢出honey蜂蜜viscous粘性的Theunitsforviscositycomefromthemathematicaldefinitionoftheproperty.Whenafluidflowsoverasurface,itexertsaforce(measuredinNewton’s,forexample)onit.,Theviscosityismeasuredbydividingthisforcebythespeedofthefluidflowing(meterspersecond,m/s)andthethicknessofthefluidlayer(meters).SotheunitsfortheviscosityareNewton-secondspersquaremeter(Ns/m).However,theconventionusedbyscientistsandengineersistodefinethepropertyusingunitsofmass.SinceaNewtonisequaltoakilogrampersecondsquared(kgm/s),themorecommonlyusedunitsarekilogrampermetersecond(kg/ms)forthemetricsystem,andpoundsmassperfootsecond(1bm/fts)intheEnglishsystem.,mathematical數(shù)學的，數(shù)學上的，精確的convention條約，協(xié)約，集會，會議,Force力WeightandGravity重量和重力Velocity速度Acceleration加速性Differentwaysairmoves空氣運動的不同方式Speedofsound聲速MachNumber馬赫數(shù)Friction摩擦力BoundaryLayer附面層，邊界層,LaminarBoundaryLayer層流邊界層TurbulentBoundaryLayer紊流邊界層Transition過渡，轉(zhuǎn)變FlowSeparation氣流分離Buoyancy浮力，浮性，彈性，恢復力StreamlinesandFlowPatterns流線和氣流特性曲線Shocks沖擊震蕩PerfectGaslaw理想氣體定律Bernoulli’stheorem伯努力定理,