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PARTIFUNDAMENTALPRINCIPLES(基本原理)InpartI,wecoversomeofthebasicprinciplesthatapplytoaerodynamicsingeneral.ThesearethepillarsonwhichallofaerodynamicsisbasedChapter1Aerodynamics:SomeIntroductoryThoughtsTheterm“aerodynamics”isgenerallyusedforproblemsarisingfromflightandothertopicsinvolvingtheflowofair.LudwigPrandtl,1949Aerodynamics:Thedynamicsofgases,especiallyofatmosphericinteractionswithmovingobjects.TheAmericanHeritageDictionaryofEnglishLanguage,19691.1ImportanceofAerodynamics:
HistoricalExamplesSeabattlebetweenEnglishfleetandSpanishfleet,Englishchannel,8-8-1588(英國與西班牙海戰(zhàn),英吉利海峽)FirstflightofWrightbrothers,12-27-1903(懷特兄弟首次飛行)MinimizingofaerodynamicheatingofICBMs(洲際彈道導(dǎo)彈氣動熱降低問題)Impetustothestudyoffluidmechnics(流體力學(xué)研究的推動力)1.Newton’ssine-squarelaw2.ExperimentscarriedoutbyD’Alembert3.Euler’sdescriptionoftheflowmodel1.Newton’ssine-squarelawa)Newtonconsideredafluidflowasauniform,rectilinearstreamofparticles,muchlikeacloudofpelletsfromashotgunblast.b)Newtonassumedthatuponstrikingasurfaceinclinedataangletothestream,theparticleswouldtransfertheirnormalmomentumtothesurfacebuttheirtangentialmomentumwouldbepreserved.Hence,aftercollisionwiththesurface,theparticleswouldthenmovealongthesurface.Thisledtoanexpressionforthehydrodynamicsforceonthesurfacewhichvariesas2.D’Alembert
Theexperimentresultsshow:therulethatforobliqueresistancevarieswiththesinesquareoftheangleoftheincidenceholdsgoodonlyforanglebetween50and90degandmustbeabandonedforlesserangles3.Eulernoted
Thefluidmovingtowardabody“beforereachingthelatter,bendsitsdirectionanditsvelocitysothatwhenitreachesthebodyitflowspassitalongthesurface,andexercisenootherforceonthebodyexceptthepressurecorrespondingtothesinglepointsofthecontact.”4.Realcaseforfluidapproachingabody
Allthefluidparticlesareinrandommotion,andhasaaveragevelocity.Duringtheirmotion,theycollidewitheachother.
Themoleculesstrikeontothesolidsurfacewillberebounded,andthesereboundedmoleculeswillmakecollisiontoothermolecules.
Thisprocesstransfersthemessageoftheexistenceofthebody,andmostoftheparticleswillgootherround.Afterthecollisionbetweenfluidparticlesandsolidsurface,the
momentumchangeoftheparticlesisintheperpendiculardirectionofthesurface.FirstflightofWrightbrothersDec.17,1903WilburandOrvilleWright'sWrightFlyerwasthefirstsuccessfulairplane.OnDecember17,1903,atKittyHawk,NorthCarolina,OrvilleWrightflewthefirstheavier-than-airmachineinapowered,controlled,andsustainedflight.TheFlyer,constructedofwood,wire,andmuslin,wentadistanceof120feetin12seconds.Itwasatremendoussuccess,comingfromalongseriesofaeronauticsexperimentsthattheWrightBrothersstartedin1899withakite.Attherearofthe1903WrightFlyeronefindsapairofpusherpropellers.Thepropellersarelong,thin,twistedpiecesofwoodwhicharespunathighspeed.Controlofroll:WINGWARPOverviewofWrightBrothersDiscoveriesAerodynamicheatingofthereentryvehicle
ICBMsreentrytheatmosphereatthespeedsoffrom6to6.7km/s.Theaerodynamicheatingofthereentryvehiclesbecomessevere,thecoverofthewarheadwillbeheatedupto10,000K.Bluntreentrybodydesigncanminimizetheaerodynamicheatingproblem.1.2Aerodynamics:ClassificationandPracticalObjectives
(空氣動力學(xué):分類和應(yīng)用目標)Distinctionofsolids,liquids,andgasesPracticalapplicationsinengineeringSolids,liquids,andgasesinacontainerThesolidobjectwillnotchange:itsshapeandboundarieswillremindthesame.Theliquidwillchangeitsshapetoconformtothatofthecontainerandwilltaketakeonthesameboundariesasthecontaineruptothemaximumdepthoftheliquid.Thegaswillcompletelyfillthecontainer,takingonthesameboundariesasthecontainer.Solidand“fluid”(aliquidoragas)underatangentialforce==deformation固體和流體在受到剪應(yīng)力時,各自形狀所發(fā)生的變化方式截然不同。Underaforceappliedtangentiallytothesurfaceofasolidbody,thesolidbodywillundergoafinitedeformation,andthetangentialforceperunitarea—theshearstress—willusuallybeproportionaltotheamountofdeformation.Ifthecasehappensforafluid,then,thefluidwillexperienceacontinuouslyincreasingdeformationandtheshearstresswillusuallybeproportionaltotherateofthedeformation.Solid:fluid:Shearstress剪應(yīng)力Deformation變形Rateofdeformation變形率Mechanicsdistinctionofsolids,liquids,andgasesDistinctionofsolids,liquids,andgasesrespectstotheintermolecularforcesFluiddynamicsissubdividedintothreeareas:
Hydrodynamics---flowofliquidsGasdynamics---flowofgases
Aerodynamics---flowofairPracticalobjectivesofAerodynamics1.Thepredictionofforcesandmomentsonandheattransferto,bodiesmovingthroughafluid.2.Determinationofflowsmovinginternallythroughducts3.Externalaerodynamics4.Internalaerodynamics1.3RoadMapofthischapterWhat’stheusageoftheroadmapAtthebeginningofeachchapter,roadmapgiveyouthesenseforyougettoknowwhereyouare,whereyouaregoing,andhowcanyougetthereShowtheinterrelationshipofthematerialsinthechapterAttheendofthechapter,afteryoulookbackovertheroadmap,youwillseewhereyoustarted,whereyouarenow,andwhatyoulearnedinbetween.1.4SomefundamentalAerodynamicVariablesAerodynamicvariablesaresomethingliketechnicalvocabularyforthephysicalscienceandengineeringunderstandingFirstintroducedaerodynamicvariables:
pressure,density,temperature,andflowvelocityThevelocitydescriptionofafluidisquitedifferenttothatofasolidbody.VelocityofaflowinggasatanyfixedpointBinspaceisthevelocityofasmallfluidelementasitsweepsthroughB.1.5AerodynamicforcesandmomentsAerodynamicforcesandmomentsonamovingbodyareduetoonlytwobasicsources:1.Pressuredistributionoverthebodysurface2.ShearstressdistributionoverthebodysurfaceBothpressureandshearstresshavedimensionsofforceperunitarea.
pressureactsnormaltothebodysurface.shearstressactstangentialtothesurface.TheneteffectofthepressureandshearstressdistributionresultsinaaerodynamicforceRandmomentMonthebody.TheresultantforceRcanbesplitintocomponentsL=lift:componentofRperpendiculartoD=drag:componentsofRparallelto(windsystem)N=normalforce:componentofRperpendiculartoc
A=axialforce:componentsofRparalleltoc
(bodysystem)Afterthe
pressureandshearstress
distributionsbeingdefined,andthegeometryshapeofthebodybeingknown,theresultantaerodynamicforcecanbeobtainedbytheintegrationofthepressureandshearstress
distributionsalongthesurfaceofthebody.FromEqs.(1.7),(1.8)and(1.11),wecanseeclearly,thatthesourcesoftheaerodynamiclift,drag,andmomentsonabodyarethepressureandshearstressdistributionintegratedoverthebody.Thebasictaskoftheoreticalaerodynamicsistocalculatep(s)andτ(s)foragivenbodyshapeandfreestreamconditions,andthenobtaintheaerodynamicforcesandmomentswiththeuseofEqs.(1.7),(1.8)and(1.11)Dimensionlessaerodynamicforceandmomentcoefficientsareevenmoreimportantthantheaerodynamicforcesandmoments.Definitionofanddensityandvelocityinthefreestream,whichisfaraheadofthebody.Definitionofdynamicpressure
ThedynamicpressurehastheunitofpressureDefinitionofdimensionlessforceandmomentcoefficientsLiftcoefficient:
Dragcoefficient:
Normalforcecoefficient:
Axialforcecoefficient:Momentcoefficient:
:reference
area:reference
length
Definitionofandmaybedifferentfordifferentshapesofthebodybeingconcerned.Thesymbolsincapitalletters,suchasrepresentstheforceandmomentcoefficientsforathree-dimensionalbody.Thesymbolsinlowercaselettersdenotetheforceandmomentcoefficientsforatwo-dimensionalbody
areforceandmomentsperunitspanTwoadditionaldimensionlessquantitiesofimmediateusearePressurecoefficientSkinfrictioncoefficientWhereisthe
freestreampressure1.6Centerofpressure(壓力中心)Thecenterofthepressureisapointonthebodyaboutwhichtheaerodynamicmomentcontributedbythepressureandshearstressdistributionsisequaltozero.Ifisdefinedasthemomentgeneratedbythedistributedloads,andisthecomponentoftheresultantforce,thenthepressurecentermustbelocateddownstreamoftheleadingedgeIftheangleofattackissmall,,thusItiscleartoseethatasliftapproachestozero,thecenterofpressuremovestoinfinity.So,thecenterofpressureisnotalwaysaconvenientconceptinaerodynamics.Thereareotherwaystodefinetheforce-and-momentsystemonanairfoil1.7Dimensionalanalysis:TheBuchinghamPItheorem(量綱分析:PI定理)※Whatphysicalquantitiesdeterminethevariationoftheaerodynamicforcesandmoments?Onaphysical,intuitivebasis,weexpectRisdependon:1.Freestreamvelocity2.Freestreamdensity3.Viscosityofthefluid4.Thesizeofthebody5.Thecompressibilityofthefluid※
Howtofindaprecisefunctionalrelationfortheequationabove?Executehugeamountofwindtunnelexperimentmightbeoneway.Isthereanyotherwaycandomoreeffectively?Methodofdimensionalanalysis※AnobviousfactforthedimensionalanalysisAllthetermsinthisphysicalrelationmusthavethesamedimensions※BuckinghamPItheorem1.LetKtobethenumberoffundamentaldimensionsrequiredtodescribethephysicalvariables2.LetrepresentNphysicalvariablesinthephysicalrelation3.Thenthephysicalrelationcanbereexpressedasarelationof(N-K)dimensionlessproducts.4.EveryproductisadimensionlessproductofasetofK
physicalvariablesplusoneotherphysicalvariable.5.iscalledrepeatingvariables.Thesevariablesshouldincludeall
theKdimensionsusedintheproblem.※Aerodynamicforceonagivenbodyatagivenangleofattack.1.Eq.(1.23)canbeexpressedas2.FollowingBuckinghamtheoremandourphysicalintuition,thefundamentaldimensionsarem,landt.Hence,
K=33.Thephysicalvariablesandtheirdimensionsareand
N=64.AsexplainedbyBuckinghamtheorem,Eq.(1.27)canbereexpressedintermsofN-K=3
dimensionlessproducts,thatis5.Now,wechoseasrepeatingvariables,fromEq.(1.26),theseproductsare5.Assume
indimensionalform6.Asisdimensionless,then7.TheaboveEquationsgived=-1,b=-2,ande=-2,thenwehaveor
whereS
isdefinedasreferencearea8.Inthesameway,wecanobtaintheremainingproductsasfollowsReynoldsNumber雷諾數(shù)
isaforcecoefficient,definedasMachNumber馬赫數(shù)9.InsertingalltheproductsintoEq.(1.28)
oror10.Importantconclusion:Inthegeneralfunctionform,RisexpressedwithfiveindependentphysicalvariablesAfterourdimensionalanalysis,Rcanbeexpressedwithonly
twoindependentvariables
RcanbeexpressedintermsofadimensionlessforcecoefficientisafunctionofonlyReand11.ImportantapplicationsofReand.
similarityparameters
12.Asliftanddragarecomponentsoftheresultantforce,thentheliftanddragcoefficientsarealsofunctionsofonlyRe
and.Moreover,arelationsimilartoaerodynamicforcesholdsforaerodynamicmoments,anddimensionanalysisyields13.Iftheangleofattackisallowedtovary,then,thelift,dragandmomentcoefficientswillingeneraldependonthevalueof.14.Othersimilarityparametersassociatedwiththermodynamicsandheattransfer.Physicalvariablesshouldbeaddedtemperature,specificheat,thermalconductivity,temperatureofthebodysurfaceFundamentaldimensionshouldbeaddedunitofthetemperature(K)Similarityparameterscreated1.8Flowsimilarity(流動相似)※DefinitionofflowsimilarityDifferentflowsaredynamicallysimilarif:Thestreamlinepatternsaregeometricallysimilar2.Thedistributionsofetc.,throughouttheflowfieldarethesamewhenplottedagainstcommonnondimensionalcoordinates.3.Theforcecoefficientsarethesame※CriteriatoensureflowsimilarityThebodiesandanyothersolidboundariesaregeometricallysimilarforbothflows.2.Thesimilarityparametersareidenticalforbothflows.3.ReynoldsandMachnumberarethemostdominantsimilarityparametersformanyaerodynamicproblems.※Examples1.4and1.51.9FluidStatics:BuoyancyForce
(流體靜力學(xué):浮力)Skippedover1.10TypesofFlow(流動類型)1.Thepurposeforcategorizingdifferenttypesofflow.2.Thestrategytosimplifytheflowproblems.3.Itemizationandcomparisonofdifferenttypesofflow,andbriefdescriptionoftheirmostimportantphysicalphenomena.1.10.1Continuumversusfreemoleculeflow1.Definitionofmean-free
path.2.Continuumflow.3.Freemoleculeflow4.Inmostaerodynamicproblems,wewillalwaystreatthefluidascontinuumflow.1.10.2Inviscidversusviscousflow1.Therandommotionofthemoleculewilltransporttheirmass,momentum,andenergyfromonelocationtoanotherinthefluid.Thistransportonamoleculescalegivesrisetothephenomenaofmassdiffusion,viscosity,andthermalconduction.Allrealflowsexhibittheeffectofthesetransportphenomena;suchflowsarecallviscousflows.2.Aflowthatisassumedfreewithallthesephenomenaaboveiscalledinviscidflow.3.InviscidflowisapproachedinthelimitastheReynoldsnumbergoestoinfinity.4.TheflowwithhighReynoldsnumber,canbeassumedtobeinviscid.Andtheinfluenceof
friction,thermalconduction,anddiffusionislimitedintheboundarylayer.5.Theinviscidtheorycanbeusedtopredictsthepressuredistributionandlift.However,itcannotpredictstotaldrag.6.Flowsdominatedbyviscouseffects.
FlowaroundairfoilathighangleofattackFlowaroundbluntbody7.Noinviscidtheorycanindependentlypredicttheaerodynamicsofsuchflows.
1.10.3IncompressibleversuscompressibleFlowsAflowinwhichthedensityisconstantiscalledincompressible.Incontrast,aflowwherethedensityisvariableiscalledcompressible.
2.Alltheflowsarecompressible,moreorless3.Thereareanumberofaerodynamicproblemsthatcanbemodeled
asbeingincompressible
withoutanydetrimentallossofaccuracy.4.Inmanycases,whetherthecompressibilityshouldbeconsideredornot,ismanlybasedon
theMachnumberoftheflow.1.10.4MachnumberregimesLocaldefinitionSubsonicifSonicif
Supersonicif
WhereisthelocalMachnumberatanarbitrarypointinaflowfield.2.Definitionforwholeflowfield3.Blockdiagramcategorizingthetypesofaerodynamicflows1.11Appliedaerodynamics:Theaerodynamiccoefficients—TheirmagnitudeandvariationsDifferencebetweenthefundamentals
andapplicationsofaerodynamics.
2.Aerodynamiccoefficients,suchaslift,drag,andmomentcoefficients,aretheprimarylanguageofapplicationexternalaerodynamics.3.Typicalvaluesfortheaerodynamiccoefficientsforsomecommonaerodynamicshapesandit’svariationwithMachnumberandReynoldsnumber.4.Sometypicaldragcoefficientsforvariousaerodynamicconfigurationsinlowspeedflows.
Comparisonthroughcaseatoc:
theReynoldsnumbersforallthesethreecasesarethesamebasedond(diameter).thewakesaregettingsmallerinsizefromatoc
alsobecomessmallerfromcase
atoc
Comparisonbetweencasebandd:
theReynoldsnumberincaseb:theReynoldsnumberincased
:isthesameforcasebtod
foracircularcylinderisrelativelyindependentofReynoldsnumberbetweenRe=andComparisonbetweencasebtoe:
theReynoldsnumberincaseb:theReynoldsnumberincasee:incaseeis0.6
smallerwakebehindthecylinderincasee
comparedtothatincase
b.Note:Withbasedonthefrontalprojectedarea(S=d(1)perunitspan),thevalueofrangefromamaximum2tonumbersaslowas0.12.MagnitudeofReynoldsnumberofaflowaroundacircularcylinderatstandardsealevel,where,
Th
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