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IntroductiontoANSYS
FLUENT
Lecture8NonConformalInterfaces&MovingZones
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LectureTheme:
ManyCFDapplicationsacrossindustriesinvolvesystemsordeviceswithmovingparts. FLUENToffersmanydifferentmodelsforrotatingmachinery,
forarbitrary
prescribed
motion
and
for
objects
whose
path
isdetermined
by
theflow.
LearningAims:Youwilllearn:
Howtodefinenonconformalinterfacesandperiodicboundaryconditions
Themodelsavailableforrotatingmachinerysuchasthemultiplereferenceframeandslidingmeshmodels
Thedifferentdynamicmeshingmethodsforarbitrarymotion,includingthecoupled6DOFsolver
LearningObjectives:
YouwillbeabletousenonconformalmeshesandwillbecomefamiliarwithFLUENTsmodelsforsystemswithmovingpartsandwhenaparticularmodelisapplicable.
Introduction
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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Acrossaninterfacebetweentwocellzones,thenodesmayormaynotexactlyalign
Ifthenodesmatchperfectly,thisisaConformalmesh
IfusingDesignModeler,combiningbodiesintoasinglepartwillgiveaconformalmesh Ifthenodesdonotmatchup,thisisaNonConformalmesh
FLUENTcaninterpolateacrosstheinterface,butthismustbedefinedintheGUI.
Ifnot,FLUENTwilltreattheinterfaceasawall,andnofluidcanflowthrough.
Overview
NonconformalInterfaces
Conformal NonConformal
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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NonconformalInterfacescanbeusedfor: Connectionofmismatchedmeshes(hextotet forexample)
asinglemeshfilemaycontainnonmatchingmeshregionsandrequirenonconformalinterfaces
Changesinreferenceframesbetweencellzones
evenifthemeshmatches
Connectdifferenttypesofcellzonestogether(e.g.FluidandSolid)
Createperiodicregionswithinadomain
Overview
NonconformalInterfaces
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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InsertingNonconformalInterfaces
Tocreateanonconformalinterface:
Step1:Define/BoundaryConditions
Changethetypeofeachpairofzonesthatcomprisesthenonconformal
boundarytointerface
Step2:Define/MeshInterfaces
EnteranamefortheinterfaceintheMeshInterfacetextentrybox
Specifythetwointerfacezonesthatcomprisethemeshinterfacebyselectingoneormore
zonesintheInterfaceZone1listandoneormorezonesintheInterfaceZone2list Ifoneofyourinterfacezonesismuchsmallerthantheother,youshouldspecifythesmallerzone
asInterfaceZone1toimprovetheaccuracyoftheintersectioncalculation
EnablethedesiredInterfaceOptions ifappropriate
NonconformalInterfaces
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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TocreateaPeriodicboundarycondition EnablethePeriodicBoundaryConditionoptionintheMeshinterfacespanel SelecteitherTranslationalorRotationalastheperiodicboundaryconditionType
RetaintheenableddefaultsettingofAutoComputeOffsetifyouwantANSYSFLUENTto
automaticallycomputetheoffset Meshcanbenonconformal
PeriodicBoundaryCondition
TranslationalPeriodicity Simulatesgeometriesthathavetranslational
periodicity
Allowsforeitherthemassflowrateorthepressurechangeacrosstheinterfacetobespecified
Thequantitynotspecifiedwillbepartofthesolution
RotationalPeriodicity
Simulatesrotationallyperiodicgeometries
Beforeproceeding,youhavetocorrectlyentertherotationalaxisforthecorrespondingcellzoneintheBCpanel
NonconformalInterfaces
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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x
y
MovingReferenceFramesvs.MeshMotion
MovingReferenceFrame Domain moveswithcoordinatesystem
Tofollowthemotionofthebody,topologyofthemeshdoesnotneedtobeupdated
Rotation/TranslationoftheMovingdomain
MeshMotion Domainchangesshapeasafunctionoftime
Tofollowthemotionofthebody,topologyofthemeshneedtobeupdated
Smoothing/Remeshing ofthedomain
MovingZones
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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RotatingEquipment
Whyusearotatingreferenceframe?
Aflowfieldwhichisunsteadywhenviewedinastationaryframecanbecomesteadywhenviewedinarotatingframe
Steadystateproblemsareeasiertosolve... Additionalaccelerationtermsareaddedtothemomentumequations
SimplerBCs
Lowcomputationalcost
Easiertopostprocessandanalyze
Limitation: Youmaystillhaveunsteadinessintherotatingframedueto
turbulence,circumferentiallynonuniformvariationsinflow,separation,etc.
Example:vortex
shedding
from
fan
blade
trailing
edge
Rotationallyperiodicboundariescanbeemployedforefficiency(reduceddomainsize)
Centrifugal
Compressor
(single blade passage)
MovingZones
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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Singlevs.MultipleReferenceFrameModeling
stationarywall
MRFis necessarySRFis sufficient
WhendomainsrotateatdifferentratesorwhenstationarywallsdonotformsurfacesofrevolutionMultipleReferenceFrames(MRF)areneeded
stationary wall
MRFis necessary
stationarywall
baffle
MovingZones
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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DifferentApproaches:
OverviewofModelingApproaches: SingleReferenceFrame(SRF)
Entirecomputationaldomainisreferredtoamovingreferenceframe
steadystate
MultipleReferenceFrame(MRF) Selectedregionsofthedomainarereferredtomovingreferenceframes
Interactioneffectsareignored
steadystate
Mixing
Plane
(MPM) Influenceofneighboringregionsaccountedforthroughuseofamixingplanemodelatrotating/stationarydomaininterfaces
Circumferentialnonuniformitiesintheflowareignored
steadystate
SlidingMesh(SMM) Motionofspecificregionsaccountedforbyameshmotionalgorithm
Flowvariablesinterpolatedacrossaslidinginterface
Unsteadyproblemcancaptureallinteractioneffectswithcompletefidelity,but
morecomputationallyexpensivethanSRF,MRF,orMPM
MovingZones
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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DefiningaMRFZone
ThesimplestapproachtosetupandsolveisusingaMovingReferenceFrame Solutionissteadystate
Meshneveractuallymoves,localaccelerationsappliedtoeachgridcell
This
is
applicable
if
there
is
a
steady
state
solution
to
the
problem,
so: Exactrelativepositionsofmovingandstationary(rotor/stator)partsdoesnotmatter
Novortexsheddingorothertransientphenomena
Foreachcellzone,enable
FrameMotionandsetthe
detailsofthemotion.
This
motion
can
be
defined
relative
to
another
zone,
it
doesnt
have
to
be
set
to
absolute
coordinates
MovingZones
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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DefiningaSlidingMeshproblem
Inotherproblems,onemustactuallymovethemeshcomponents. Thesolutionisthereforetransient.
DefinethelinearorrotationalmotionofeachzonetouseaslidingzonebysettingMeshMotion
Rememberthatinthiscasethedifferentcellzonesareactuallymovedateachtimestep. Makesurethemodelisalwayssavedbeforetestingthemotion! Otherusefultipsaboutrunningatransientsimulation(likegeneratingimagesontheflywillbegivenin
alaterlecture.
MovingZones
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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MeshDeformation
MeshDeformationcanbeappliedinsimulationswhereboundariesorobjects
aremoved Thesolvercalculatesnodaldisplacementsoftheseregions
andadjuststhesurroundingmeshtoaccommodatethem
Examplesofdeformingmeshesinclude Automotivepistonmovinginsideacylinder
Aflapmovingonanairplanewing
Avalveopeningandclosing
Anarteryexpandingandcontracting
MovingZones
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DynamicMesh(DM)Methods
Internalnodepositionsareautomaticallycalculatedbasedonuserspecifiedboundary/objectmotion,celltype,andmeshingschemes
BasicSchemes Springanalogy(smoothing)
Localremeshing
Layering
OtherMethods 2.5D
Userdefinedmeshmotion
Incylindermotion(RPM,strokelength,crankangle,)
PrescribedmotionviaprofilesorUDF
Coupledmotionbasedonhydrodynamicforcesfromtheflowsolution,viaFLUENTssixdegreeoffreedom(6DOF)solver
MovingZones
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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DynamicMeshMethods
LayeringLayersofcellsaregeneratedand
collapsedasthey
are
overrun
by
themovingboundary. Layeringis
appropriateforquad/hex/prism
mesheswithlinearorrotational
motionandcantoleratesmallor
largeboundarydeflections
LocalRemeshingInlocalremeshing,ascellsbecome
skewedduetomovingboundaries,cellsarecollapsedandtheskewed
regionisremeshed. Localremeshing
isappropriatefortri/tet mesheswith
largerangeofboundarymotion
SpringAnalogySpringanalogyisusefulwhenthere
aresmallboundarydeformations.Theconnectivityandcellcountis
unchangedduringmotion. Spring
analogyisappropriatefortri/tet
mesheswithsmalldeformations
MovingZones
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TheDynamicMesh(DM)Model
Combinationofapproaches:
Initialmeshneedsproperdecomposition
Layering: Valvetravelregion
Lowercylinderregion
Remeshing: Uppercylinderregion
Nonconformalinterface
betweenzones
AnadvancedtrainingcourseonDynamicMeshisavailable
MovingZones
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6DOFCoupledMotion
Objectsmoveasaresultof aerodynamicforcesandmomentsactingtogetherwith
otherforces,suchasthegravityforce,
thrustforces,orejectorforces
Insuchcases,themotionandtheflowfieldarethuscoupled,andwecallthiscoupled
motion
FLUENTprovidesthe6DOFModel Thetrajectoryofanobjectiscomputedbased
ontheaerodynamicforces/moments,
gravitationalforce,
and
ejector
forces
The6DOFUDFisfullyparallelized
MovingZones
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Summary
Fivedifferentapproachesmaybeusedtomodelflowsovermovingparts Single(Rotating)ReferenceFrameModel
MultipleReferenceFrameModel
MixingPlane
Model
SlidingMeshModel
DynamicMeshModel(ConsiderourAdvancedTrainings)
Thefirstthreemethodsareprimarilysteadystateapproacheswhilesliding
meshis
inherently
unsteady
Enablingthesemodelsinvolvesinpart,changingthestationaryfluidzonestoeitherMovingReferenceFrameorMovingMesh
Mostphysicalmodelsarecompatiblewithmovingreferenceframesormovingmeshes(e.g.multiphase,combustion,heattransfer,etc.)
MovingZones
Introduction NonConformalInterfaces RotatingZones DynamicMesh Summary
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ES SThU
t
h
)()(
MotioninSolidZones
Insolidzones,theconservationoftheenergyequationcanaccountforheattransportduetomotionofthesolid,conduction
and volumetricheatsources
Notethatthesolidisneverphysicallymovedwhenusingthisapproach,thereisonlyanadditionaladvectiontermaddedtotheenergyequation
Solid Velocity
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MotioninSolidZones
Solidzonemotioncanbeclassifiedintotwoareas:
TranslationalMotion Forexample,aprocesswherea
solidmovescontinuouslyinalinear
directionwhilecooling
Thesolidmustextendcompletely
throughthedomain
RotationalMotion
Forexample,abrakerotorwhichisheatedbybrakepads
q
q=0
q=0
q=0
Tin= Tspec
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NavierStokesEquations:RotatingReference
Frames Equationscanbesolvedinabsoluteorrotating(relative)
referenceframe
RelativeVelocityFormulation ObtainedbytransformingthestationaryframeNSequationstoarotating
referenceframe
Usestherelativevelocityasthedependentvariable
CanbeselectedundertheGeneraltabinProblemSetup
AbsoluteVelocityFormulation Derivedfromtherelativevelocityformulation
Usestheabsolute
velocity
asthedependentvariable
DefaultformulationforrotatingzonesinFLUENT
Rotationalsourcetermsappearinmomentumequations
x
y
z
z
y
x
stationary
frame
rotating
frame
axis of
rotation
r
CFD domain
or R
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TheVelocityTriangle
Therelationshipbetweentheabsoluteandrelativevelocitiesisgivenby
Inturbomachinery,thisrelationshipcanbeillustratedusingthelawsofvectoraddition.ThisisknownastheVelocityTriangle
V
W
U
VelocityRelative
VelocityAbsolute
W
V
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ComparisonofFormulations
2 rWx
p
wWt
w
vrxx
x
Relative Velocity Formulation: x-momentum equation
Vx
pvW
t
vvxx
x
Absolute Velocity Formulation: x-momentum equation
Coriolis acceleration Centripetal acceleration
Coriolis + Centripetal accelerations
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IntroductiontotheMRFModel
Thedomainissubdividedintostationaryandrotatingfluidzones Morethanonerotatingzoneispermitted
Zonescanrotateatdifferentspeeds
Governingequationsaresolvedineachfluidzone SRFequationsusedinrotatingzones
Attheinterfacesbetweentherotatingandstationaryzones,appropriatetransformationsofthevelocityvectorandvelocitygradientsareperformedto
computefluxesofmass,momentum,energy,andotherscalars
Flowisassumedtobesteadyineachzone(clearlyanapproximation)
MRFignorestherelativemotionsofthezoneswithrespecttoeachother Doesnotaccountforfluiddynamicinteractionbetweenstationaryandrotating
components
ForthisreasonMRFisoftenreferredtoasthefrozenrotorapproach
Ideally,theflowattheMRFinterfacesshouldberelativelyuniformormixedout
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TheMixingPlaneModel(MPM)
TheMPMisatechniquewhichpermitssteadystatesolutionsformultistageaxialandcentrifugalturbomachines whereupstreamanddownstreamperiodicdomainsdonotmatchattheconnection
Advantage:
MPMrequiresonlyasinglebladepassageperbladerowregardlessofthenumberofblades,becauseofcircumferentialaveragingnonuniformitiesintheflowatthemixingplaneinterface
MPMcanhandledifferentnumbersofbladesatbothsidesofmixingplane
Mixing plane interface
Fan (9 blades) Vane (12 blades)
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MPMvs.MRF
MRFcanbeusedonlyifwehaveequalperiodicanglesforeachrow
Formultistageturbomachinery problems Thestageboundaryconditionsareoftenknown(e.g.inlettotalpressureandtemperatureandstageoutletstaticpressure)butnottheinterstageconditions
Bladecountswillgenerallynotbethesame fromonerowtothenext
TheMPMrequiresonlyasinglebladepassageperbladerowregardlessofthenumberofblades
Thisisaccomplishedbymixingout(averaging)the
circumferentialnonuniformitiesintheflowattheinterstage(mixingplane)interface
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Axialvs.RadialMixingPlanes
SRFsolutionsareobtainedineachdomain,withthedomainslinkedbypassing
boundaryconditionsfromonezonetoanother
Theinlet/outletboundariesmustbeassignedBCtypesinoneofthefollowingcombinations:
Pressureoutlet/Pressureinlet
Pressureoutlet/Velocityinlet
Pressureoutlet/Massflowinlet
Axial machines Radial machines
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MixingPlane SetUp
MixingPlaneModel
GUI:Define MixingPlanes
p
drrpz
),(1
)(
p
dzzp
r),(
1)(
MixingPlaneGeometrydeterminesmethodofcircumferentialaveragingChooseRadialforaxialflowmachinesChooseAxialforradialflowmachines
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TheSlidingMeshModel(SMM)
Therelativemotionofstationaryandrotatingcomponentsinaturbomachinewillgiverisetounsteadyinteractions
Theseinteractionsaregenerallyclassifiedasfollows: Potentialinteractions
(pressurewaveinteractions)
Wakeinteractions
Shockinteractions
BothMRFandMPMneglectunsteadyinteractionentirelyandthusarelimitedtoflowswheretheseeffectsareweak
Ifunsteadyinteractioncannotbeneglected,wecanemploytheSlidingMeshmodel(SMM)toaccountfortherelativemotionbetweenthestationaryandrotatingcomponents
wake interaction
Shock
interactionpotential
interaction
StatorRotor
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HowtheSlidingMeshModelWorks
LiketheMRFmodel,thedomainisdividedintomovingandstationaryzones,separatedbynonconformalinterfaces
UnliketheMRFmodel,eachmovingzonesmeshwillbeupdatedasafunction
of
time,
thus
making
the
mathematical
problem
inherently
unsteady.
AnotherdifferencewithMRFisthatthegoverningequationshaveanewmovingmeshform,andaresolvedinthestationaryreferenceframeforabsolutequantities
MovingreferenceframeformulationisNOTusedhere(i.e.noadditionalaccelerationsactingassourcestermsinthemomentumequations)
Equationsareaspecialcaseofthegeneralmoving/deformingmeshformulation Assumesrigidmeshmotionandsliding,nonconformalinterfaces
cells at time t cells at time t + t
moving mesh zone
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NSEquations:SlidingMesh
SdqvvvSdpeUVedt
d
SdkpvUVdVvdt
d
SdSdjpvUVdVvdt
d
SdSdipvUVdVvdt
d
UVdVdt
d
S
zvzyvyxvx
S
t
V
t
S
vz
S
z
V
z
S
vy
S
y
V
y
S
vx
S
x
V
x
SV
0)( (continuity)
(x momentum)
(y momentum)
(z momentum)
(energy)
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SMMSetup
Enabletheunsteadysolver
DefineslidingzonesasInterfaceBCtypes
Foreachinterfacezonepair,createanonconformalinterface
EnablePeriodicoptionifsliding/rotatingmotionisperiodic.
EnableCoupledforconjugateheattransfer
Formovingzones,selectMoving MeshasMotionTypeinFluid BCpanel
OtherBCsaresameasSRF,MRFmodels
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ChooseappropriateTime Step SizeandMax Iterationsper Time Steptoensuregoodconvergencewitheachtimestep
Time Step Sizeshouldbenolargerthanthetimeittakesfora
movingcelltoadvancepastastationarypoint:
Advancethesolutionuntiltheflowbecomestimeperiodic(pressures,velocities,etc.,oscillatewitharepeatingtimevariation).
Usuallyrequiresseveralrevolutionsofthegrid.
Goodinitialconditionscanreducethetimeneededtoachievetimeperiodicity`
SolvingSMMProblems
R
st
s = Average cell size R = Translational speed