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Freeshearflowillustratinglaminar,transitionandturbulentphases

PostedByLEAPCFDTeamonMar20,2013|10commentsTheReynoldsnumber(Re)isthesinglemostimportantnondimensionalnumberinfluiddynamicsandisrecommendedtobecalculatedbeforeyoubegin

anynewCFDmodellingproject.TheReynoldsNumberisdefinedasthedimensionlessratiooftheinertialforcestoviscousforcesandquantifiestheir

relevancefortheprescribedflowcondition:

WhereU andLarethecharacteristicvelocityandlengthscaleoftheproblem,isthefluiddensityandisthedynamicviscosity.Theuseofthe

ReynoldsnumberfrequentlyariseswhenperformingadimensionalanalysisandisknownasReynoldsprincipleofsimilarity.Forexample,airflowofU

=1[m/s]overaflatplateofL=1[m]willexhibitthesameflowpatternasairflowofU =10[m/s]overaflatplateofL=0.1[m].Thisconceptholdssincethe

Reisequalandtheflowisincompressible(i.e.theMachnumberislow).

TheRealsoallowsustocharacterizewhetheraflowislaminarorturbulent.LaminarflowischaracterizedbylowerReandhighdiffusionoverconvection.

TurbulentflowontheotherhandischaracterizedbyhigherRewhereinertialforcesdominateconsiderably,resultinginlargelychaoticflow.Theflowmay

alsoundergoatransitioningphasewherebytheflowexhibitsneithercompletelylaminarnorcompletelyturbulentcharacteristics.

Note:Inertialforcesareproportionaltothesquareofvelocity,whileviscousforcesvary

linearlywithvelocity.ThereforeastheRe0itisreasonabletoneglectconvectiveterms(e.g.creepingflow).Similarly,astheRetheviscousterms

ofthemomentumequationcanbeneglectedandtheproblemcanbecharacterizedpurelybythegenerationofinertialforces(e.g.highsupersonicand

hypersonicflows).

ForflowsinapipeofdiameterLornoncircularpipeswithhydraulicdiameterL,empiricalstudieshaveshownthatlaminarflowoccursforRe 4000.Intheintervalbetween,transitionoccurs.Forexternalflows(e.g.flowoveraflatplate),laminarflowis

observeduptoapproximatelyReD~110 to510 (whereDisthedistancefromtheleadingedge)beforetransitiontoturbulencetakesplaceand

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Animationillustratingcycliclaminarvortexshedding

Figure1:Re



FIgure5:Re~1e6steadyfullyturbulentflowfield(streamlinesofvelocity)

Figure6:Re~5e6threedimensionaltranscriticalflowshowingturbulentvorticalstructures

AtRe~10 (seeFigure5)theflowtransitionstoturbulencebeforeitseparates.Theuseofatransitionmodelhereisnecessarytoobtaincorrectviscous

dragpredictionsbutwillultimatelynotaffectthepointofseparationandsowecanassumethattheinfluenceofthelaminarboundarylayerisonly

additive.Furthermore,theflowcanbeapproximatedtobesteadystate,asthedelayedseparationduetothehigherstressescausedbyturbulent

fluctuationsresultsinanarrowerwakeregion.AtevenhigherRe(whileremainingincompressibleseeFigure6)ahighlyunsteadyturbulentvortexsheet

appearsandinstabilitiesintheboundarylayerresultintheseparationpointmovingfurtherupstream.Thisiscalledthetranscriticalregime.This

phenomenonmustbecapturedusingahighorderturbulentscaleresolvingsimulation.

Thissmallexampleshouldprovidesomebasisfordeterminingwhethertheuseofatransitionmodelisnecessary,providedwecalculatethecharacteristic

Reynoldsnumberandareawareoftheflowpatternthatwewouldliketocapture.Forfurtherclarification,orforsomeinsightintoyourownparticular

problem,pleaseleaveacommentbeloworalternativelycontactoursupportline.

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Author:LEAPCFDTeam

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StephenAPRIL14,2013

Iamachemeng.studentcurrentlyusingANSYStostudyflowconditioninreactorsaspartofmyresearch.Whenyousaytransitionare

youreferringtothetransitionsstturbulencemodel?Thankyou!Thisblogisawesome!

LEAP Support Team

modelswallfunctions



APRIL22,2013

Inthispostwearedirectlyreferringtothephysicsoflaminartoturbulenttransition.Tocapturethisphenomenon,youcan

utilizeeitherthetransitionSSTmodelorthekklomegamodelwhichareavailableinANSYSCFDproducts.These

modelsallowyoutomodelbothlaminarandturbulentregionsoftheflowaswellastheintermittentturbulencewhichis

inherentwiththephysicsoftransition.Thesemodelsareespeciallyusefulforaerodynamicflowswherecapturingthe

laminarandtransitioningregionsoftheflowarecrucialforaccuratedragprediction

Post a Reply

kumarAUGUST13,2013

Hi,

Iamdoingresearchworkonfan,apipeof9mlongandthefanislocatedat3mfromtheinlet.Ineedtosimulatethevolumeflowrange

from2m3/sto22m3/s.

IhavehavedonethesimulationatdesignpointwithhighRemodel(kepsilon),Igotniceresults.howeverItriedwithkomegaSST(lowRe

model).itstakingmassiveamountoftimewhencomparingtokepsilon,aswellitisnotconvergingfully.

InmycasehowcanIdecidetheturbulencemodel.

Ihavedonesmallcalculation,at2m3/stheReynoldsnumberis0.465*10^5atdesignpoint(16m3/s)3.73*10^5.

Thanks,

Kumar

LEAP Support TeamSEPTEMBER27,2013

Thechoiceoftheturbulencemodelisproblemdependentandwillrequiregenerationofasuitablemesh.Inyour

particularcaseSSTwithautomaticwalltreatmentshouldworkfineiftherestofthesetupiscorrect.Youmayalsobe

capturingtransientbehaviour,hencethe(non)convergingbehaviouryouhaveobserved.Youcaninvestigatefurtherby

settingupadditionalmonitorsofinterestforyourproblem(ie.quantifiedvaluessuchaspressuredrop,forcesonfan

blades,minimums/maximums,etc)andobservinganytransientoscillationsinthesemonitorsforyoursteadystate

simulation.Youcanthentestbyreducingthepseudotimestepusedandrerunning,thenobserveifthischangealso

causesanassociatedchangeintheperiodofoscillationinthemonitors.Ifitremovestheoscillations,thenitwasa

numericalissue.Ifitcontinuestooscillate,thenitislikelytobeaninherentlytransientandyouneedtorunyoursimulation

transiently(withasuitabletimestep)insteadofsteadystate.

Post a Reply

HansPeter SteinereAUGUST13,2013

IamaphysicianinIVF

IinventedanewNeedleforeggcollectionandIwouldgreatful

tocomeinmailcontactwithyou.

pleaseseeatYouTubeSteinerTanNeedle

Mymainquestionis:isitpossibletocategorizedifferntneedles

(singlelumen,doublelumenwithReynoldsNumber

concerningcapacityofflushingtheeggoutoffollicles?

Kindregards

HansPeterSteiner,Graz,Austria

LEAP Support Team



AUGUST21,2013

ThanksHansPeterforyourinterest.CFDcanbeusedtoevaluatetheabilityofdifferentneedleconfigurations(single,

doublelumen)toachievethesegoals.IfyoudonthaveexistingexperiencewithCFD,wesuggestinyourcasetocontact

CADFEMAustriainVienna:http://[email protected]

Post a Reply

VR IKkurthiMARCH3,2014

Thanksforyourtips.Iamaphysicist,juststaringCFDsimulations.HowdowecalculateRebeforehand,whenwedonotknowtheflow

velocity.i.e.sayIhaveapipeandIknowtheinletandoutletpressurevalues,butnotinletflowvelocity.

AlsohowdoIcalculatey+beforehandanddecidewhethertousewallfunctionsorresolvetheboundarylayer.

Thanksinadvance

ramana

LEAP Support TeamMARCH7,2014

Youcanestimateboththelikelyflowvelocities(forRe)andY+valuesbeforehand,andthenrefinethesecalculations

basedonaninitialCFDsimulation.Thedecisiontousewallfunctionsorfullyresolvetheboundarylayersismadein

termsoftheapplication(Reestimates,likelyflowseparation,whetherornotconjugateheattransferisimportant,etc)and

theoveralllevelofaccuracyrequired.

Post a Reply

armaghanMAY18,2014

DearLEAPSupportTeam

Thankyouforyourdecentwebsite.ItisreallyhelpfulandusefulespeciallyforthosewhoarenewinCFDtogetthegreattipsandtricks.

Iamsimulating2Dflowinsidearectangularchannelwithhieghof0.015meterandlengthof0.400meteratdifferentvelocityofflowing

(0.4m/s,.,1.5m/s).theflowisturbulentandIhavebeenusingKemodel(standard,Enhancedwalltreatment,thermaleffects).Mygoalis

tofindpressuredropthroughoutarectangularchannel.TheamountofpressuredropchangeswithincreasingofRebutitdoesnotfollow

aconsistenttrend.ForexamplewhenforRe=6000Ihavedp=18201.156Pa

andforRe=10000Ihavedp=4625.089Pa



andforRe=22000Ihavedp=1959.609Pa.

Idonotknowwhymyresultsareincorrect.PleasehelpmewithdetailexplainingasIamnewinCFD.

thankyouforyourtimeandconcern

LEAP CFD TeamJUNE16,2014

ItseemsthatyoucouldbenefitfromsomeintroductorytraininginCFDthisforumisaimedataddressingspecific

questionsthatusersmayhaverelatedtoCFDbestpractice.Howsureareyouthata2Dchannelisappropriately

reflectingtheexperimentandthatyourresidualsarefullyconvergedateachdesignpoint?



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