Introdução ao CFD
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Universidade Federal de ItajubUniversidade Federal de ItajubInstituto de Engenharia MecnicaInstituto de Engenharia Mecnica
Grupo de Estudos em Tecnologias de Converso de EnergiaGrupo de Estudos em Tecnologias de Converso de Energia
Coordenador: ProfCoordenador: Prof. Dr. Marco Antnio R. Nascimento. Dr. Marco Antnio R. Nascimento
INTRODUO DINMICA DOSINTRODUO DINMICA DOSFLUIDOS COMPUTACIONAISFLUIDOS COMPUTACIONAIS -- CFDCFD
SIMULAO COMPUTACIONALSIMULAO COMPUTACIONAL
Eraldo Cruz dos SantosEraldo Cruz dos Santos
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TPICOS DA APRESENTAOTPICOS DA APRESENTAO
O QUE O CFD?;
MODELAGEM;
MTODOS NUMRICOS; TIPOS DE CDIGOS DO CFD;
INTERFACE EDUCACIONAL DO CFD;
PROCESSOS DO CFD;
EXEMPLOS DE PROCESSOS;
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O QUE O CFD?
CFD is thesimulationoffluidsengineeringsystemsusingmodeling
(mathematical physical problemformulation)andnumericalmethods (discretization methods,solvers,numerical parameters,andgridgenerations,etc.)
Historicallyonly Analytical Fluid Dynamics (AFD)andExperimental Fluid Dynamics (EFD).
CFD made possibleby theadvent ofdigitalcomputerandadvancing with improvementsofcomputerresources(500 flops, 194720 teraflops,2003)
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PORQUE USAR CFD?
Analysisand Design1. Simulation-baseddesigninsteadof build & test Morecost effectiveandmorerapid than EFD CFD provides high-fidelitydatabasefordiagnosingflow
field
2. Simulationof physicalfluid phenomena that aredifficult forexperiments Fullscalesimulations (e.g.,shipsandairplanes) Environmentaleffects (wind, weather,etc.)
Hazards (e.g.,explosions,radiation, pollution) Physics (e.g., planetaryboundarylayer,stellarevolution)
Knowledgeandexplorationofflow physics
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Aplicaes do CFD
Onde usadoo CFD? Aeroespacial Automotiva
Biomdica Processos qumicos HVAC Hidrulica Martimas
leo & Gs Geraode Energia Esportes
F18 Store Separation
Temperature and naturalconvection currents in the eyefollowing laser heating.
Aeroespacial
Automotiva
Biomdico
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Aplicao do CFD
Polymerization reactor vessel - predictionof flow separation and residence timeeffects.
Streamlines for workstation
ventilation
Onde usado o CFD? Aerospace Automotive
Biomedical Chemical Processing HVAC Hydraulics Marine
Oil & Gas Power Generation Sports
HVAC
Chemical Processing
Hydraulics
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Aplicaes do CFD
Where is CFD used?
Aerospace
Automotive
Biomedical
Chemical Processing HVAC
Hydraulics
Marine
Oil & Gas
Power Generation Sports
Flow of lubricating
mud over drill bitFlow around cooling
towers
Marine
Oil & Gas
Sports
Power Generation
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Modelagem
Modelingis themathematical physics problemformulationin termsofacontinuousinitialboundaryvalue problem (IBVP)
IBVPisin theformofPartial Differential Equations(PDEs) with appropriateboundaryconditionsandinitialconditions.
Modelingincludes:1. Geometryanddomain2. Coordinates3. Governingequations
4. Flow conditions5. Initialandboundaryconditions6. Selectionofmodelsfordifferent applications
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Modeling (geometryanddomain)
Simplegeometries canbeeasilycreatedbyfewgeometric parameters (e.g.circular pipe) Complex geometries must becreatedby the partial
differentialequationsorimporting thedatabaseof thegeometry (e.g.airfoil)intocommercialsoftware
Domain:sizeandshape Typicalapproaches
Geometryapproximation CAD/CAE integration: useofindustrystandardssuch asParasolid, ACIS, STEP,or IGES,etc.
The threecoordinates: Cartesiansystem (x,y,z),cylindricalsystem (r, ,z),andsphericalsystem(r, , )shouldbeappropriatelychosenforabetterresolutionof thegeometry(e.g.cylindricalforcircular pipe).
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Modeling (coordinates)
x
y
z
x
y
z
x
y
z
(r,U,z)
z
rU
(r,U,J)
rU
J(x,y,z)
Cartesian Cylindrical Spherical
General Curvilinear Coordinates General orthogonalCoordinates
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Modeling (governingequations)
Navier-Stokes equations (3D in Cartesian coordinates)
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xx
xx
xx
xx!
xx
xx
xx
xx
2
2
2
2
2
2
zu
yu
xu
xp
zuw
yuv
xuu
tu QVVVV
-
x
x
x
x
x
x
x
x!
x
x
x
x
x
x
x
x2
2
2
2
2
2
z
v
y
v
x
v
y
p
z
vw
y
vv
x
vu
t
vQVVVV
0!xxxxxxxx zw
yv
xu
tVVVV
RTp V!
L
v pp
Dt
DR
Dt
RDR
V
!
2
2
2
)(2
3
Convection Piezometric pressure gradientViscous termsLocalacceleration
Continuity equation
Equation of state
Rayleigh Equation
-
x
xx
xx
xx
x!x
xx
xx
xx
x2
2
2
2
2
2
z
w
y
w
x
w
z
p
z
ww
y
wv
x
wu
t
wQVVVV
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Modeling (flow conditions)
Based on the physics of the fluids phenomena, CFDcan be distinguished into different categories usingdifferent criteria
Viscous vs. inviscid (Re) External flow or internal flow (wall bounded or not) Turbulent vs. laminar (Re) Incompressible vs. compressible (Ma) Single- vs. multi-phase (Ca)
Thermal/density effects (Pr, g, Gr, Ec) Free-surface flow (Fr) and surface tension (We) Chemical reactions and combustion (Pe, Da) etc
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Modeling (initialconditions)
Initialconditions (ICS,steady/unsteadyflows) ICsshouldnot affect finalresultsandonlyaffect convergence path,i.e.numberofiterations (steady)or timesteps (unsteady)needtoreach convergedsolutions.
Morereasonableguesscanspeedup theconvergence
Forcomplicatedunsteadyflow problems, CFDcodesareusuallyrunin thesteadymodeforafew iterationsforgettingabetterinitialconditions
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Modeling(boundaryconditions)
Boundary conditions: No-slip orslip-freeon walls, periodic,inlet (velocityinlet,massflow rate,constant pressure,etc.),outlet (constant pressure,velocityconvective,numericalbeach,zero-gradient),andnon-reflecting (forcompressibleflows,such asacoustics),etc.
No-slip walls: u=0,v=0
v=0, dp/dr=0,du/dr=0
Inlet ,u=c,v=0 Outlet, p=c
Periodic boundary condition inspanwise direction of an airfoilo
r
xAxisymmetric
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Modeling (selectionofmodels)
CFD codes typically designed for solving certain fluidphenomenon by applying different models
Viscous vs. inviscid (Re) Turbulent vs. laminar (Re, Turbulent models)
Incompressible vs. compressible (Ma, equation of state) Single- vs. multi-phase (Ca, cavitation model, two-fluid model) Thermal/density effects and energy equation
(Pr, g, Gr, Ec, conservation of energy) Free-surface flow (Fr, level-set & surface tracking model) and
surface tension (We, bubble dynamic model) Chemical reactions and combustion (Chemical reaction model) etc
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Modeling (Turbulenceandfreesurfacemodels)
Turbulent models: DNS: most accurately solve NS equations, but too expensive
for turbulent flows
RANS: predict mean flow structures, efficient inside BL but excessivediffusion in the separated region.
LES: accurate in separation region and unaffordable for resolving BL DES: RANS inside BL, LES in separated regions.
Free-surface models: Surface-tracking method: mesh moving to capture free surface,limited to small and medium wave slopes
Single/two phase level-set method: mesh fixed and level-setfunction used to capture the gas/liquid interface, capable of
studying steep or breaking waves.
Turbulent flows at high Re usually involve both large and small scale
vortical structures and very thin turbulent boundary layer (BL) near the wall
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Examplesofmodeling (Turbulenceandfreesurfacemodels)
DES, Re=105, Iso-surface of Qcriterion (0.4) for turbulent flow aroundNACA12 with angle of attack 60 degrees
URANS, Re=105, contour of vorticityfor turbulent flow around NACA12 withangle of attack 60 degrees
DES, Athena barehull
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Numericalmethods
Thecontinuous Initial Boundary ValueProblems(IBVPs)arediscretized intoalgebraicequationsusingnumericalmethods. Assemble thesystemofalgebraicequationsandsolve thesystem toget approximatesolutions Numericalmethodsinclude:1. Discretization methods2. Solversandnumerical parameters
3. Gridgenerationand transformation4. High Performance Computation (HPC)and post-processing
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Discretizationmethods
Finitedifferencemethods (straightforward toapply,usuallyforregulargrid)andfinitevolumes andfiniteelement methods (usuallyforirregularmeshes)
Each typeofmethodsaboveyields thesamesolutionifthegridisfineenough. However,somemethodsaremoresuitable tosomecases thanothers
Finitedifferencemethodsforspatialderivatives withdifferent orderofaccuraciescanbederivedusingTaylorexpansions,such as2nd orderupwindscheme,centraldifferencesschemes,etc.
Higherordernumericalmethodsusually predict higherorderofaccuracyfor CFD,but morelikelyunstabledue tolessnumericaldissipation Temporalderivatives canbeintegratedeitherby theexplicit method (Euler, Runge-Kutta,etc.)orimplicitmethod (e.g. Beam-Warmingmethod)
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Discretization methods (Contd)
Explicit methods canbeeasilyappliedbut yieldconditionallystable Finite Different Equations (FDEs),which arerestrictedby the timestep; Implicit methodsareunconditionallystable,but needeffortsonefficiency.
Usually, higher-order temporaldiscretization isusedwhen thespatialdiscretization isalsoof higherorder.
Stability: A discretization methodissaid tobestableifit doesnot magnify theerrors that appearin thecourseofnumericalsolution process.
Pre-conditioning methodisused when thematrix of thelinearalgebraicsystemisill-posed,such asmulti-phaseflows,flows with abroadrangeofMach numbers,etc. Selectionofdiscretization methods shouldconsiderefficiency,accuracyandspecialrequirements,such asshock wave tracking.
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Discretization methods (example)
0!x
x
x
x
y
v
x
u
2
2
y
u
e
p
xy
u
vx
u
u x
x
x
x!
x
x
x
x
Q
2D incompressiblelaminarflow boundarylayer
m=0m=1
L-1 L
y
x
m=MMm=MM+1
(L,m-1)
(L,m)
(L,m+1)
(L-1,m)
1l
l lm
m m
uuu u u
x x
x ! - x (
1
l
l lm
m mvuv u u
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1
l
l lm
m m
vu u
y ! - (
FD Sign( )0
2
1 12 22l l l
m m m
uu u u
y y
QQ
x ! - x (
2nd order central differencei.e., theoretical order ofaccuracyPkest= 2.
1st
order upwind scheme, i.e., theoretical order of accuracy Pkest= 1
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Discretization methods (example)
1 12 2 2
12
1
l l l
l l l l m m m
m m m m
FDu v vy
v u F D u BD u x y y y y y
BDy
Q Q Q
( ( ( ( ( ( ( - -
(-
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l
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uu p e
x x
x!
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m m m m m B u B u B u B u p e
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0 0 0 0 0 0
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l
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mm l
mm
mm
pB u
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B B B
B B B
B B u pB u
x e
x x
yy v !y y y y yy
yy - x-
x -
Solveit usingThomasalgorithm
To be stable, Matrix has to be
Diagonally dominant.
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Solversandnumerical parameters
Solvers include: tridiagonal, pentadiagonal solvers,PETSC solver,solution-adaptivesolver,multi-gridsolvers,etc.
Solvers canbeeitherdirect (Cramersrule, Gausselimination, LU decomposition)oriterative(Jacobimethod, Gauss-Seidelmethod, SOR method)
Numerical parameters need tobespecified tocontrolthecalculation. Underrelaxationfactor,convergencelimit,etc. Different numericalschemes
Monitorresiduals (changeofresultsbetweeniterations) Numberofiterationsforsteadyflow ornumberof
timestepsforunsteadyflow Single/double precisions
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Gridscaneitherbestructured
(hexahedral)orunstructured(tetrahedral). Dependsupon typeofdiscretization schemeandapplication Scheme
Finitedifferences: structured
Finitevolumeorfiniteelement:structuredorunstructured Application
Thinboundarylayersbestresolved with highly-stretchedstructuredgrids
Unstructuredgridsusefulforcomplex geometries Unstructuredgrids permit
automaticadaptiverefinementbasedon the pressuregradient,orregionsinterested (FLUENT)
Numericalmethods (gridgeneration)
structured
unstructured
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Numericalmethods (grid transformation)
y
xo oPhysical domain Computational domain
x x
f f f f f
x x x
\ L
\ L\ L \ L
x x x x x x x
! ! x x x x x x x
y y
f f f f f
y y y
\ L\ L
\ L \ L
x x x x x x x! !
x x x x x x x
Transformation between physical(x,y,z) and computational (\L^)domains, important for body-fitted grids. The partialderivatives at these two domainshave the relationship (2D as an
example)
L
\
Transform
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High performancecomputingand post-processing
CFD computations (e.g. 3D unsteadyflows)areusuallyveryexpensive
which requires parallel high performancesupercomputers (e.g. IBM
690) with theuseofmulti-block technique. Asrequiredby themulti-block technique, CFD codesneed tobe
developedusing theMassagePassing Interface (MPI) Standard totransferdatabetweendifferent blocks.
Post-processing: 1. Visualizethe CFD results (contour,velocityvectors,streamlines, pathlines,streaklines,andiso-surfacein 3D,etc.),and2. CFD UA:verificationandvalidationusing EFD data (moredetailslater)
Post-processingusually through usingcommercialsoftware
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Typesof CFD codes
Commercial CFD code: FLUENT, Star-
CD, CFDRC, CFX/AEA,etc. Research CFD code: CFDSHIP-IOWA Publicdomainsoftware (PHI3D,
HYDRO,andWinpipeD,etc.) Other CFD softwareincludes the Grid
generationsoftware (e.g. Gridgen,Gambit)andflow visualizationsoftware (e.g. Tecplot, FieldView)
CFDSHIPIOWA
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CFD Educational Interface
Lab1: Pipe Flow Lab 2: Airfoil Flow
CFD
1. Definition of CFD Process
2. Boundary conditions
3. Iterative error and grid convergence studies
4. Developing length of laminar and
turbulent pipe flows.
5. Validation using AFD/EFD
1. Inviscid vs. viscous flows
2. Boundary conditions
3. Effect of order of accuracy
4. Effect of angle of attack/turbulent
models on flow field
5. Validation and confidence of CFD
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CFD process
Purposes of CFD codes willbedifferent fordifferent
applications:investigationofbubble-fluidinteractionsforbubblyflows,studyof waveinducedmassivelyseparatedflowsforfree-surface,etc.
Dependon thespecific purposeandflow conditionsof theproblem,different CFD codes canbechosenfordifferentapplications (aerospace,marines,combustion,multi-phaseflows,etc.) Once purposesand CFD codeschosen, CFD process is thesteps toset up the IBVP problemandrun thecode:
1. Geometry2.Physics3.Mesh4. Solve5. Reports6.Post processing
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CFD Process
ViscousModel
BoundaryConditions
InitialConditions
ConvergentLimit
Contours
Precisions(single/double)
NumericalScheme
Vectors
StreamlinesVerification
Geometry
SelectGeometry
GeometryParameters
Physics Mesh Solve Post-Processing
CompressibleON/OFF
Flowproperties
Unstructured(automatic/
manual)
Steady/Unsteady
Forces Report(lift/drag,shear stress,etc)
XY Plot
DomainShape and
Size
HeatTransferON/OFF
Structured(automatic/
manual)
Iterations/Steps
Validation
Reports
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Geometry
Selectionofanappropriatecoordinate Determine thedomainsizeandshape Anysimplificationsneeded? What kindsofshapesneeded tobeused tobest
resolve thegeometry? (lines,circular,ovals,etc.) Forcommercialcode,geometryisusuallycreated
usingcommercialsoftware (eitherseparatedfrom thecommercialcodeitself,like Gambit,orcombinedtogether,like FlowLab)
Forresearch code,commercialsoftware (e.g. Gridgen)isused.
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Physics
Flow conditionsandfluid properties1.Flow conditions:inviscid,viscous,laminar,orturbulent,etc.
2.Fluid properties:density,viscosity,andthermalconductivity,etc.
3. Flow conditionsand propertiesusually presentedindimensionalforminindustrialcommercial CFDsoftware, whereasinnon-dimensionalvariablesforresearch codes.
Selectionofmodels:different modelsusuallyfixedbycodes,optionsforuser tochoose Initialand Boundary Conditions:not fixedbycodes,userneedsspecify themfordifferentapplications.
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Mesh
Meshesshouldbe welldesigned toresolveimportantflow features which aredependent uponflowcondition parameters (e.g., Re),such as thegridrefinement inside the wallboundarylayer
M
esh canbegenerated byeithercommercialcodes(Gridgen, Gambit,etc.)orresearch code (usingalgebraicvs.PDE based,conformalmapping,etc.)
Themesh, together with theboundaryconditionsneed tobeexportedfromcommercialsoftwareinacertainformat that canberecognizedby theresearch CFD codeorothercommercial CFDsoftware.
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Solve
Setup appropriatenumerical parameters Chooseappropriate Solvers Solution procedure (e.g.incompressibleflows)
Solve themomentum, pressurePoissonequationsandget flow field quantities,such asvelocity,turbulenceintensity, pressureand integralquantities (lift,dragforces)
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Reports
Reportssaved the time historyof theresidualsofthevelocity, pressureand temperature,etc. Report theintegral quantities,such as total
pressuredrop,frictionfactor (pipeflow),lift anddragcoefficients (airfoilflow),etc.
XY plotscould present thecenterlinevelocity/pressuredistribution,frictionfactordistribution (pipeflow), pressurecoefficientdistribution (airfoilflow).
AFD or EFD datacanbeimportedand put on top ofthe XY plotsforvalidation
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Post-processing
Analysisandvisualization Calculationofderivedvariables
Vorticity Wallshearstress
Calculationofintegral parameters:forces,moments
Visualization (usually with commercialsoftware) Simple2D contours 3D contourisosurface plots Vector plotsandstreamlines
(streamlinesare thelines whosetangent directionis thesameas thevelocityvectors)
Animations
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Post-processing (Uncertainty Assessment)
Simulationerror:thedifferencebetweenasimulationresult S and the truth T (objectivereality),assumedcomposedofadditivemodeling SM andnumerical SN errors:
Verification:processforassessingsimulationnumericaluncertainties U
SNand, whenconditions permit,estimating the
signandmagnitude Delta *SN of thesimulationnumericalerroritselfand theuncertaintiesin that errorestimate UScN
Validation:processforassessingsimulationmodelinguncertainty USM byusingbenchmarkexperimentaldataand,whenconditions permit,estimating thesignandmagnitudeofthemodelingerror SM itself.
SNSMSTS HHH !!
222
SNSMSUUU !
!
!!J
j
jIPTGISN
1
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)(SNSMDSDE HHH !!
222
SNDV UUU !
VUE Validation achieved
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Post-processing (UA, Verification)
Convergencestudies: Convergencestudiesrequireaminimumof
m=3 solutions toevaluateconvergence with respective toinputparameters. Consider thesolutionscorresponding tofine ,medium ,andcoarsemeshes 1k
S
2kS
3kS
(i). Monotonic convergence: 0
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Post-processing (UA, Verification,contd)
Monotonic Convergence: Generalized Richardson Extrapolation
? A ? A
!
*
1
2
*
1
1.014.2
11
kREk
kREk
C
CkcU
H
H
Oscillatory Convergence: Uncertainties can be estimated, butwithoutsigns and magnitudes of the errors. Divergence
LUk SSU ! 21
1. Correctionfactors
2. GCI approach *1kREsk
FU H! *1
1kREskc
FU H!
32 21ln
ln
k k
k
k
pr
I I! 11
k
kest
p
kk p
k
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!
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* 21
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k
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kr
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9.6 1 1.1
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k
k
k RE
k
k RE
CU
C
H
H
- ! -
1 0.125kC
1 0.125kC u
1 0.25kC
25.0|1| u kC|||]1[|*
1kREkC H
In this course, only grid uncertainties studied. So, all thevariables with
subscribe symbol k will be replaced by g, such as Uk will be U
g
estk
p is the theoretical order of accuracy,2 for 2nd order and 1 for 1st orderschemes
kU is the uncertainties based on finemesh solution, is theuncertainties based on numericalbenchmark SC
kcUis the correctionfactor
kC
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Asymptotic Range: For sufficiently small (xk, thesolutions are in the asymptotic range such thathigher-order terms are negligible and theassumption that and are independent of (xkis valid.
When Asymptotic Range reached, will be close tothe theoretical value , and the correctionfactor
will be close to 1.
To achieve the asymptotic range for practicalgeometry and conditions is usually not possible andm>3 is undesirable from a resources point of view
Post-processing (Verification, AsymptoticRange)
ik
p ik
g
estkp
kp
kC
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Exampleof CFD Processusingeducationalinterface (Geometry)
Turbulent flows (Re=143K)around Clarky airfoil withangleofattack6degreeissimulated. C shapedomainisapplied Theradiusof thedomain Rc anddownstreamlength Lo
shouldbespecifiedinsuch a way that thedomainsizewillnot affect thesimulationresults
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Exampleof CFD Process (Physics)No heattransfer
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Exampleof CFD Process (Mesh)
Grid need to be refined nearthe foil surface to resolvethe boundary layer
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Exampleof CFD Process (Solve)
Residuals vs. iteration
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Exampleof CFD Process (Reports)
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Exampleof CFD Process (Post-processing)
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Produtos Ansys
Lista Completa de Produtos
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ANSYS LS-DYNA
ANSYS EKM
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