FLUENT-Intro 14.0 L08 MovingZones

7/24/2019 FLUENT-Intro 14.0 L08 MovingZones

1/37

2011ANSYS,Inc. January19,20121 Release14.0

14.0Release

IntroductiontoANSYS

FLUENT

Lecture8NonConformalInterfaces&MovingZones


2/37


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


3/37


Acrossaninterfacebetweentwocellzones,thenodesmayormaynotexactlyalign

Ifthenodesmatchperfectly,thisisaConformalmesh

IfusingDesignModeler,combiningbodiesintoasinglepartwillgiveaconformalmesh Ifthenodesdonotmatchup,thisisaNonConformalmesh

FLUENTcaninterpolateacrosstheinterface,butthismustbedefinedintheGUI.

Ifnot,FLUENTwilltreattheinterfaceasawall,andnofluidcanflowthrough.

Overview

NonconformalInterfaces

Conformal NonConformal



4/37


NonconformalInterfacescanbeusedfor: Connectionofmismatchedmeshes(hextotet forexample)

asinglemeshfilemaycontainnonmatchingmeshregionsandrequirenonconformalinterfaces

Changesinreferenceframesbetweencellzones

evenifthemeshmatches

Connectdifferenttypesofcellzonestogether(e.g.FluidandSolid)

Createperiodicregionswithinadomain

Overview




5/37


InsertingNonconformalInterfaces

Tocreateanonconformalinterface:

Step1:Define/BoundaryConditions

Changethetypeofeachpairofzonesthatcomprisesthenonconformal

boundarytointerface

Step2:Define/MeshInterfaces

EnteranamefortheinterfaceintheMeshInterfacetextentrybox

Specifythetwointerfacezonesthatcomprisethemeshinterfacebyselectingoneormore

zonesintheInterfaceZone1listandoneormorezonesintheInterfaceZone2list Ifoneofyourinterfacezonesismuchsmallerthantheother,youshouldspecifythesmallerzone

asInterfaceZone1toimprovetheaccuracyoftheintersectioncalculation

EnablethedesiredInterfaceOptions ifappropriate




6/37


TocreateaPeriodicboundarycondition EnablethePeriodicBoundaryConditionoptionintheMeshinterfacespanel SelecteitherTranslationalorRotationalastheperiodicboundaryconditionType

RetaintheenableddefaultsettingofAutoComputeOffsetifyouwantANSYSFLUENTto

automaticallycomputetheoffset Meshcanbenonconformal

PeriodicBoundaryCondition

TranslationalPeriodicity Simulatesgeometriesthathavetranslational

periodicity

Allowsforeitherthemassflowrateorthepressurechangeacrosstheinterfacetobespecified

Thequantitynotspecifiedwillbepartofthesolution

RotationalPeriodicity

Simulatesrotationallyperiodicgeometries

Beforeproceeding,youhavetocorrectlyentertherotationalaxisforthecorrespondingcellzoneintheBCpanel




7/37


8/37


x

y

MovingReferenceFramesvs.MeshMotion

MovingReferenceFrame Domain moveswithcoordinatesystem

Tofollowthemotionofthebody,topologyofthemeshdoesnotneedtobeupdated

Rotation/TranslationoftheMovingdomain

MeshMotion Domainchangesshapeasafunctionoftime

Tofollowthemotionofthebody,topologyofthemeshneedtobeupdated

Smoothing/Remeshing ofthedomain

MovingZones



9/37


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



10/37


Singlevs.MultipleReferenceFrameModeling

stationarywall

MRFis necessarySRFis sufficient

WhendomainsrotateatdifferentratesorwhenstationarywallsdonotformsurfacesofrevolutionMultipleReferenceFrames(MRF)areneeded

stationary wall

MRFis necessary

stationarywall

baffle

MovingZones



11/37


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



12/37


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



13/37


DefiningaSlidingMeshproblem

Inotherproblems,onemustactuallymovethemeshcomponents. Thesolutionisthereforetransient.

DefinethelinearorrotationalmotionofeachzonetouseaslidingzonebysettingMeshMotion

Rememberthatinthiscasethedifferentcellzonesareactuallymovedateachtimestep. Makesurethemodelisalwayssavedbeforetestingthemotion! Otherusefultipsaboutrunningatransientsimulation(likegeneratingimagesontheflywillbegivenin

alaterlecture.

MovingZones



14/37


MeshDeformation

MeshDeformationcanbeappliedinsimulationswhereboundariesorobjects

aremoved Thesolvercalculatesnodaldisplacementsoftheseregions

andadjuststhesurroundingmeshtoaccommodatethem

Examplesofdeformingmeshesinclude Automotivepistonmovinginsideacylinder

Aflapmovingonanairplanewing

Avalveopeningandclosing

Anarteryexpandingandcontracting

MovingZones



15/37


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



16/37


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



17/37


TheDynamicMesh(DM)Model

Combinationofapproaches:

Initialmeshneedsproperdecomposition

Layering: Valvetravelregion

Lowercylinderregion

Remeshing: Uppercylinderregion

Nonconformalinterface

betweenzones

AnadvancedtrainingcourseonDynamicMeshisavailable

MovingZones



18/37


6DOFCoupledMotion

Objectsmoveasaresultof aerodynamicforcesandmomentsactingtogetherwith

otherforces,suchasthegravityforce,

thrustforces,orejectorforces

Insuchcases,themotionandtheflowfieldarethuscoupled,andwecallthiscoupled

motion

FLUENTprovidesthe6DOFModel Thetrajectoryofanobjectiscomputedbased

ontheaerodynamicforces/moments,

gravitationalforce,

and

ejector

forces

The6DOFUDFisfullyparallelized

MovingZones



19/37


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



20/37


21/37


22/37


ES SThU

t

h

)()(

MotioninSolidZones

Insolidzones,theconservationoftheenergyequationcanaccountforheattransportduetomotionofthesolid,conduction

and volumetricheatsources

Notethatthesolidisneverphysicallymovedwhenusingthisapproach,thereisonlyanadditionaladvectiontermaddedtotheenergyequation

Solid Velocity


23/37


MotioninSolidZones

Solidzonemotioncanbeclassifiedintotwoareas:

TranslationalMotion Forexample,aprocesswherea

solidmovescontinuouslyinalinear

directionwhilecooling

Thesolidmustextendcompletely

throughthedomain

RotationalMotion

Forexample,abrakerotorwhichisheatedbybrakepads

q

q=0

q=0

q=0

Tin= Tspec


24/37


25/37


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


26/37


TheVelocityTriangle

Therelationshipbetweentheabsoluteandrelativevelocitiesisgivenby

Inturbomachinery,thisrelationshipcanbeillustratedusingthelawsofvectoraddition.ThisisknownastheVelocityTriangle

V

W

U

VelocityRelative

VelocityAbsolute

W

V


27/37


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


28/37


IntroductiontotheMRFModel

Thedomainissubdividedintostationaryandrotatingfluidzones Morethanonerotatingzoneispermitted

Zonescanrotateatdifferentspeeds

Governingequationsaresolvedineachfluidzone SRFequationsusedinrotatingzones

Attheinterfacesbetweentherotatingandstationaryzones,appropriatetransformationsofthevelocityvectorandvelocitygradientsareperformedto

computefluxesofmass,momentum,energy,andotherscalars

Flowisassumedtobesteadyineachzone(clearlyanapproximation)

MRFignorestherelativemotionsofthezoneswithrespecttoeachother Doesnotaccountforfluiddynamicinteractionbetweenstationaryandrotating

components

ForthisreasonMRFisoftenreferredtoasthefrozenrotorapproach

Ideally,theflowattheMRFinterfacesshouldberelativelyuniformormixedout


29/37


TheMixingPlaneModel(MPM)

TheMPMisatechniquewhichpermitssteadystatesolutionsformultistageaxialandcentrifugalturbomachines whereupstreamanddownstreamperiodicdomainsdonotmatchattheconnection

Advantage:

MPMrequiresonlyasinglebladepassageperbladerowregardlessofthenumberofblades,becauseofcircumferentialaveragingnonuniformitiesintheflowatthemixingplaneinterface

MPMcanhandledifferentnumbersofbladesatbothsidesofmixingplane

Mixing plane interface

Fan (9 blades) Vane (12 blades)


30/37


MPMvs.MRF

MRFcanbeusedonlyifwehaveequalperiodicanglesforeachrow

Formultistageturbomachinery problems Thestageboundaryconditionsareoftenknown(e.g.inlettotalpressureandtemperatureandstageoutletstaticpressure)butnottheinterstageconditions

Bladecountswillgenerallynotbethesame fromonerowtothenext

TheMPMrequiresonlyasinglebladepassageperbladerowregardlessofthenumberofblades

Thisisaccomplishedbymixingout(averaging)the

circumferentialnonuniformitiesintheflowattheinterstage(mixingplane)interface


31/37


Axialvs.RadialMixingPlanes

SRFsolutionsareobtainedineachdomain,withthedomainslinkedbypassing

boundaryconditionsfromonezonetoanother

Theinlet/outletboundariesmustbeassignedBCtypesinoneofthefollowingcombinations:

Pressureoutlet/Pressureinlet

Pressureoutlet/Velocityinlet

Pressureoutlet/Massflowinlet

Axial machines Radial machines


32/37


MixingPlane SetUp

MixingPlaneModel

GUI:Define MixingPlanes

p

drrpz

),(1

)(

p

dzzp

r),(

1)(

MixingPlaneGeometrydeterminesmethodofcircumferentialaveragingChooseRadialforaxialflowmachinesChooseAxialforradialflowmachines


33/37


TheSlidingMeshModel(SMM)

Therelativemotionofstationaryandrotatingcomponentsinaturbomachinewillgiverisetounsteadyinteractions

Theseinteractionsaregenerallyclassifiedasfollows: Potentialinteractions

(pressurewaveinteractions)

Wakeinteractions

Shockinteractions

BothMRFandMPMneglectunsteadyinteractionentirelyandthusarelimitedtoflowswheretheseeffectsareweak

Ifunsteadyinteractioncannotbeneglected,wecanemploytheSlidingMeshmodel(SMM)toaccountfortherelativemotionbetweenthestationaryandrotatingcomponents

wake interaction

Shock

interactionpotential

interaction

StatorRotor


34/37


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


35/37


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)


36/37


SMMSetup

Enabletheunsteadysolver

DefineslidingzonesasInterfaceBCtypes

Foreachinterfacezonepair,createanonconformalinterface

EnablePeriodicoptionifsliding/rotatingmotionisperiodic.

EnableCoupledforconjugateheattransfer

Formovingzones,selectMoving MeshasMotionTypeinFluid BCpanel

OtherBCsaresameasSRF,MRFmodels


37/37


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

FLUENT-Intro 14.0 L08 MovingZones

Documents

Transcript of FLUENT-Intro 14.0 L08 MovingZones