《建筑工程及给排水专业中英文对照翻译毕业设计用》Word文档格式.docx
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Observationshowsthattwoentirelydifferenttypesoffluidflowexist.Thiswasdemon-stratedbyOsborneReynoldsin1883throughanexperimentinwhichwaterwasdischargedfromatankthroughaglasstube.Therateofflowcouldbecontrolledbyavalveattheoutlet,andafinefilamentofdyeinjectedattheentrancetothetube.Atlowvelocities,itwasfoundthatthedyefilamentremainedintactthroughoutthelengthofthetube,showingthattheparticlesofwatermovedinparallellines.Thistypeofflowisknownaslaminar,viscousorstreamline,theparticlesoffluidmovinginanorderlymannerandretainingthesamerelativepositionsinsuccessivecross-sections.
Asthevelocityinthetubewasincreasedbyopeningtheoutletvalve,apointwaseventuallyreachedatwhichthedyefilamentatfirstbegantooscillateandthenbrokeupsothatthecolourwasdiffusedoverthewholecross-section,showingthattheparticlesoffluidnolongermovedinanorderlymannerbutoccupieddifferentrelativepositioninsuccessivecross-sections.Thistypeofflowisknownasturbulentandischaracterizedbycontinuoussmallfluctuationsinthemagnitudeanddirectionofthevelocityofthefluidparticles,whichareaccompaniedbycorrespondingsmallfluctuationsofpressure.
Whenthemotionofafluidparticleinastreamisdisturbed,itsinertiawilltendtocarryitoninthenewdirection,buttheviscousforcesduetothesurroundingfluidwilltendtomakeitconformtothemotionoftherestofthestream.Inviscousflow,theviscousshearstressesaresufficienttoeliminatetheeffectsofanydeviation,butinturbulentflowtheyareinadequate.Thecriterionwhichdetermineswhetherflowwillbeviscousofturbulentisthereforetheratiooftheinertialforcetotheviscousforceactingontheparticle.
Theratio
Thus,thecriterionwhichdetermineswhetherflowisviscousorturbulentisthequantityρvl/μ,knownastheReynoldsnumber.Itisaratioofforcesand,therefore,apurenumberandmayalsobewrittenasul/vwhereisthekinematicviscosity(v=μ/ρ).
ExperimentscarriedoutwithanumberofdifferentfluidsinstraightpipesofdifferentdiametershaveestablishedthatiftheReynoldsnumberiscalculatedbymaking1equaltothepipediameterandusingthemeanvelocityv,then,belowacriticalvalueofρvd/μ=2000,flowwillnormallybelaminar(viscous),anytendencytoturbulencebeingdampedoutbyviscousfriction.ThisvalueoftheReynoldsnumberappliesonlytoflowinpipes,butcriticalvaluesoftheReynoldsnumbercanbeestablishedforothertypesofflow,choosingasuitablecharacteristiclengthsuchasthechordofanaerofoilinplaceofthepipediameter.Foragivenfluidflowinginapipeofagivendiameter,therewillbeacriticalvelocityofflowcorrespondingtothecriticalvalueoftheReynoldsnumber,belowwhichflowwillbeviscous.
Inpipes,atvaluesoftheReynoldsnumber>
2000,flowwillnotnecessarilybeturbulent.LaminarflowhasbeenmaintaineduptoRe=50,000,butconditionsareunstableandanydisturbancewillcausereversiontonormalturbulentflow.Instraightpipesofconstantdiameter,flowcanbeassumedtobeturbulentiftheReynoldsnumberexceeds4000.
PipeNetworks
Anextensionofcompoundpipesinparallelisacasefrequentlyencounteredinmunicipaldistributionsystem,inwhichthepipesareinterconnectedsothattheflowtoagivenoutletmaycomebyseveraldifferentpaths.Indeed,itisfrequentlyimpossibletotellbyinspectionwhichwaytheflowtravels.Nevertheless,theflowinanynetworks,howevercomplicated,mustsatisfythebasicrelationsofcontinuityandenergyasfollows:
1.Theflowintoanyjunctionmustequaltheflowoutofit.
2.Theflowineachpipemustsatisfythepipe-frictionlawsforflowinasinglepipe.
3.Thealgebraicsumoftheheadlossesaroundanyclosedcircuitmustbezero.
Pipenetworksaregenerallytoocomplicatedtosolveanalytically,aswaspossibleinthesimplercasesofparallelpipes.Apracticalprocedureisthemethodofsuccessiveapproximations,introducedbyCross.Itconsistsofthefollowingelements,inorder:
1.Bycarefulinspectionassumethemostreasonabledistributionofflowsthatsatisfiescondition1.
2.Writecondition2foreachpipeintheform
hL=KQn(7.5)
whereKisaconstantforeachpipe.Forexample,thestandardpipe-frictionequationwouldyieldK=1/C2andn=2forconstantf.Minorlosseswithinanycircuitmaybeincluded,butminorlossesatthejunctionpointsareneglected.
3.Toinvestigatecondition3,computethealgebraicsumoftheheadlossesaroundeachelementarycircuit.∑hL=∑KQn.Considerlossesfromclockwiseflowsaspositive,counterclockwisenegative.Onlybygoodluckwilltheseaddtozeroonthefirsttrial.
4.Adjustth