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1、LaminarandTurbulentFlowObservationshowsthattwoentirelydifferenttypesoffluidflowexist.Thiswasdemon-stratedbyOsborneReynoldsin1883throughanexperimentinwhichwaterwasdischargedfromatankthroughaglasstube.Therateofflowcouldbecontrolledbyavalveattheoutlet,andafinefilamentofdye
2、injectedattheentrancetothetube.Atlowvelocities,itwasfoundthatthedyefilamentremainedintactthroughoutthelengthofthetube,showingthattheparticlesofwatermovedinparallellines.Thistypeofflowisknownaslaminar,viscousorstreamline,theparticlesoffluidmovinginanorderlymannerandretai
3、ningthesamerelativepositionsinsuccessivecross-sections.Asthevelocityinthetubewasincreasedbyopeningtheoutletvalve,apointwaseventuallyreachedatwhichthedyefilamentatfirstbegantooscillateandthenbrokeupsothatthecolourwasdiffusedoverthewholecross-section,showingthattheparticl
4、esoffluidnolongermovedinanorderlymannerbutoccupieddifferentrelativepositioninsuccessivecross-sections.Thistypeofflowisknownasturbulentandischaracterizedbycontinuoussmallfluctuationsinthemagnitudeanddirectionofthevelocityofthefluidparticles,whichareaccompaniedbycorrespon
5、dingsmallfluctuationsofpressure.Whenthemotionofafluidparticleinastreamisdisturbed,itsinertiawilltendtocarryitoninthenewdirection,buttheviscousforcesduetothesurroundingfluidwilltendtomakeitconformtothemotionoftherestofthestream.Inviscousflow,theviscousshearstressesaresuf
6、ficienttoeliminatetheeffectsofanydeviation,butinturbulentflowtheyareinadequate.Thecriterionwhichdetermineswhetherflowwillbeviscousofturbulentisthereforetheratiooftheinertialforcetotheviscousforceactingontheparticle.TheratioThus,thecriterionwhichdetermineswhetherflowisvi
7、scousorturbulentisthequantityρvl/μ,knownastheReynoldsnumber.Itisaratioofforcesand,therefore,apurenumberandmayalsobewrittenasul/vwhereisthekinematicviscosity(v=μ/ρ).Experimentscarriedoutwithanumberofdifferentfluidsinstraightpipesofdifferentdiametershaveestablishedthatift
8、heReynoldsnumberiscalculatedbymaking1equaltothepipediameterandusingthemeanvelocityv,then,belowacriticalvalueof
