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-   -   convergence problem in using incompressible transient solvers. (https://www.cfd-online.com/Forums/openfoam-solving/94642-convergence-problem-using-incompressible-transient-solvers.html)

Geon-Hong November 22, 2011 01:16

convergence problem in using incompressible transient solvers.
 
Dear Foarmers,

Hello, I'd like to simulate the 3D half-wing which is mounted on the symmetry plane.

Recently, I applied pisoFoam and pimpleFoam to get the unsteady solutions but the solutions have been diverged.

I tried to apply those solvers with lower tolerance criteria, various solver types and schemes but they generally weren't helpful at all.

Please give me an advice on using the transient solvers.

Here I attached the control, fvSolution and fvSchemes files recently used.

controlDict :
Code:

  1 /*--------------------------------*- C++ -*----------------------------------*\
  2 | =========                |                                                |
  3 | \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
  4 |  \\    /  O peration    | Version:  1.7.1                                |
  5 |  \\  /    A nd          | Web:      www.OpenFOAM.com                      |
  6 |    \\/    M anipulation  |                                                |
  7 \*---------------------------------------------------------------------------*/
  8 FoamFile
  9 {
 10    version    2.0;
 11    format      ascii;
 12    class      dictionary;
 13    location    "system";
 14    object      controlDict;
 15 }
 16 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 17
 18 //application    simpleFoam;
 19  application    pimpleFoam;
 20 //application    pisoFoam;
 21
 22 startFrom      latestTime;
 23
 24 startTime      0;
 25
 26 stopAt          endTime;
 27
 28 endTime        200;
 29
 30 deltaT          1e-5;
 31
 32 writeControl    timeStep;
 33
 34 writeInterval  100;
 35
 36 purgeWrite      0;
 37
 38 writeFormat    ascii;
 39
 40 writePrecision  6;
 41
 42 writeCompression uncompressed;
 43
 44 timeFormat      general;
 45
 46 timePrecision  6;
 47
 48 runTimeModifiable yes;
 49
 50 adjustTimeStep yes;
 51
 52 maxCo 5;
 53
 54 functions
 55 {
 56        forces
 57        {
 58                type            forceCoeffs;
 59                functionObjectLibs      ( "libforces.so" );
 60                outputControl          timeStep;
 61                outputInterval          1;
 62                patches        ( wing flap );
 63                rhoName        rhoInf;
 64                log            true;
 65                rhoInf          1.225;
 66                CofR            ( 45.89 0 0 );
 67                liftDir        ( 0 1 0 );
 68                dragDir        ( 1 0 0 );
 69                pitchAxis      ( 0 0 1 );      // PITCH
 70                magUInf        2;
 71                lRef            53.97;
 72                Aref            3006.1;
 73        }
 74 }
 75
 76
 77 // ************************************************************************* //

fvSolution :
Code:

  1 /*--------------------------------*- C++ -*----------------------------------*\
  2 | =========                |                                                |
  3 | \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
  4 |  \\    /  O peration    | Version:  1.7.1                                |
  5 |  \\  /    A nd          | Web:      www.OpenFOAM.com                      |
  6 |    \\/    M anipulation  |                                                |
  7 \*---------------------------------------------------------------------------*/
  8 FoamFile
  9 {
 10    version    2.0;
 11    format      ascii;
 12    class      dictionary;
 13    object      fvSolution;
 14 }
 15 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 16
 17 solvers
 18 {
 19    p
 20    {
 21                solver                  GAMG;
 22                tolerance              1e-07;
 23                relTol                  0.001;
 24                smoother                GaussSeidel;
 25                cacheAgglomeration      true;
 26                nCellsInCoarsestLevel  10;
 27                agglomerator            faceAreaPair;
 28                mergeLevels            1;
 29    };
 30
 31    pFinal
 32    {
 33                solver                  GAMG;
 34                tolerance              1e-07;
 35                relTol                  0.001;
 36                smoother                GaussSeidel;
 37                cacheAgglomeration      true;
 38                nCellsInCoarsestLevel  10;
 39                agglomerator            faceAreaPair;
 40                mergeLevels            1;
 41    };
 42
 43    U
 44    {
 45                solver                  PBiCG;
 46                preconditioner  DILU;
 47                tolerance              1e-08;
 48                relTol                  0;
 49    };
 50
 51    UFinal
 52    {
 53                solver                  PBiCG;
 54                preconditioner  DILU;
 55                tolerance              1e-08;
 56                relTol                  0;
 57    };
 58
 59    k
 60    {
 61                solver                  PBiCG;
 62                preconditioner  DILU;
 63                tolerance              1e-08;
 64                relTol                  0;
 65    };
 66
 67    omega
 68    {
 69                solver                  PBiCG;
 70                preconditioner  DILU;
 71                tolerance              1e-08;
 72                relTol                  0;
 73    };
 74 }
 75
 76 SIMPLE
 77 {
 78    nNonOrthogonalCorrectors 1;
 79    pRefCell        0;
 80    pRefValue      0;
 81 }
 82
 83 PISO
 84 {
 85        nCorrectors 5;
 86    nNonOrthogonalCorrectors 0;
 87    pRefCell        0;
 88    pRefValue      0;
 89 }
 90
 91 PIMPLE
 92 {
 93        nOuterCorrectors 10;
 94        nCorrectors 2;
 95        nNonOrthogonalCorrectors 0;
 96    pRefCell        0;
 97    pRefValue      0;
 98 }
 99
100 relaxationFactors
101 {
102    U          1;
103    k          1;
104    omega      1;
105 //  p  0.01;
106 //  U  0.1;
107 //  k  0.1;
108 //  omega 0.1;
109 }
110
111 // ************************************************************************* //

fvSchemes :
Code:

  1 /*--------------------------------*- C++ -*----------------------------------*\
  2 | =========                |                                                |
  3 | \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
  4 |  \\    /  O peration    | Version:  1.7.1                                |
  5 |  \\  /    A nd          | Web:      www.OpenFOAM.com                      |
  6 |    \\/    M anipulation  |                                                |
  7 \*---------------------------------------------------------------------------*/
  8 FoamFile
  9 {
 10    version    2.0;
 11    format      ascii;
 12    class      dictionary;
 13    object      fvSchemes;
 14 }
 15 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 16
 17 ddtSchemes
 18 {
 19 //      default steadyState;
 20 //      default        CrankNicholson 0.5;
 21 //      default backward;
 22        default Euler;
 23 }
 24
 25 gradSchemes
 26 {
 27    default        Gauss linear;
 28    grad(p)        Gauss linear;
 29    grad(U)        Gauss linear;
 30 //    grad(U)        cellLimited Gauss linear 1;
 31 }
 32
 33 divSchemes
 34 {
 35    default        Gauss upwind;
 36 //  div(phi,U)      Gauss linearUpwindV Gauss linear;
 37 //  div(phi,k)      Gauss linearUpwind Gauss linear;
 38 //  div(phi,omega)  Gauss linearUpwind Gauss linear;
 39    div((nuEff*dev(grad(U).T()))) Gauss linear;
 40 }
 41
 42 laplacianSchemes
 43 {
 44    default        Gauss linear corrected;
 45 //  default        Gauss linear limited 0.5;
 46 //  default        Gauss linear limited 0.333;
 47 }
 48
 49 interpolationSchemes
 50 {
 51    default        linear;
 52    interpolate(U)  linear;
 53 }
 54
 55 snGradSchemes
 56 {
 57    default        corrected;
 58 }
 59
 60 fluxRequired
 61 {
 62    default        no;
 63    p;
 64 }
 65
 66 // ************************************************************************* //

Regards,
Geon-Hong.

alberto November 22, 2011 04:03

Your maximum Courant number is 5, too high. Set it to something less than 1.

Geon-Hong November 22, 2011 05:12

Courant number
 
Dear alberto,

Thanks for your reply.

Of course I tried lower value of Courant number, which was set to be 0.5 or unity. However the problem was the same.

Regards,
Geon-Hong.

darai November 22, 2011 08:41

One possibility
 
One possible reason is a mistake in BC's,

To investigate this, I am looking for examples and using their BC's... Or I am looking in the log file what is the diverging variable and trying to find the error there.

Another approach is to try to simplify the case. Start from a working example and add feature after feature... and when you will know the step which is causing your troubles you can deal with it.

Martin.

Geon-Hong November 23, 2011 22:06

dear Holecek
 
Dear Holecek,

Thank you for your reply.

I tried other boundary conditions which are used in tutorial cases.

Still it doesn't work at all.

By the way, what log file should I check?

Thank you very much.

Regards,
Geon-Hong.

alberto November 23, 2011 23:17

Did you check the mesh for problems (checkMesh)?

Geon-Hong November 24, 2011 01:21

checkMesh
 
Dear Alberto,

Of course I did.

However, there is a awkward thing that,

High aspect ratio cells found

and

Failed 1 mesh checks.

But I can't find what is problem. The checkMesh result is as below,

Code:

/*---------------------------------------------------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  1.7.1                                |
|  \\  /    A nd          | Web:      www.OpenFOAM.com                      |
|    \\/    M anipulation  |                                                |
\*---------------------------------------------------------------------------*/
Build  : 1.7.1-03e7e056c215
Exec  : checkMesh
Date  : Nov 24 2011
Time  : 14:03:27
Host  : adlc11
PID    : 10513
Case  : /home/users/kgb/Work/flap/ucav1303/flap10c10b975p
nProcs : 1
SigFpe : Enabling floating point exception trapping (FOAM_SIGFPE).
 
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Create time
 
Create polyMesh for time = 0
 
Time = 0
 
Mesh stats
    points:          6933336
    faces:            20575358
    internal faces:  20351362
    cells:            6821120
    boundary patches: 8
    point zones:      0
    face zones:      1
    cell zones:      1
 
Overall number of cells of each type:
    hexahedra:    6821120
    prisms:        0
    wedges:        0
    pyramids:      0
    tet wedges:    0
    tetrahedra:    0
    polyhedra:    0
 
Checking topology...
    Boundary definition OK.
    Point usage OK.
    Upper triangular ordering OK.
    Face vertices OK.
    Number of regions: 1 (OK).
 
Checking patch topology for multiply connected surfaces ...
    Patch              Faces    Points  Surface topology                 
    flap                6172    6174    ok (closed singly connected)     
    bottom              20060    20382    ok (non-closed singly connected) 
    top                20060    20382    ok (non-closed singly connected) 
    side                29244    29508    ok (non-closed singly connected) 
    outflow            36816    37209    ok (non-closed singly connected) 
    inflow              47200    47637    ok (non-closed singly connected) 
    symPlane            28860    29223    ok (non-closed singly connected) 
    wing                35584    35685    ok (non-closed singly connected) 
 
Checking geometry...
    Overall domain bounding box (-247.502 -270 0) (900 270 358.535)
    Mesh (non-empty, non-wedge) directions (1 1 1)
    Mesh (non-empty) directions (1 1 1)
    Boundary openness (-1.03703e-16 1.14641e-15 -2.58996e-15) OK.
 ***High aspect ratio cells found, Max aspect ratio: 13183.7, number of cells 260418
  <<Writing 260418 cells with high aspect ratio to set highAspectRatioCells
    Minumum face area = 5.15158e-07. Maximum face area = 2886.02.  Face area magnitudes OK.
    Min volume = 4.04901e-08. Max volume = 71036.  Total volume = 1.94209e+08.  Cell volumes OK.
    Mesh non-orthogonality Max: 89.7243 average: 28.048
  *Number of severely non-orthogonal faces: 767159.
    Non-orthogonality check OK.
  <<Writing 767159 non-orthogonal faces to set nonOrthoFaces
    Face pyramids OK.
    Max skewness = 2.69123 OK.
 
Failed 1 mesh checks.
 
End


alberto November 24, 2011 01:27

It seems you have 260418 cells with high aspect ratio, and at least one of them has an aspect ratio of 13183.7, which is extremely high.

Code:

***High aspect ratio cells found, Max aspect ratio: 13183.7, number of cells 260418  <<Writing 260418 cells with high aspect ratio to set highAspectRatioCells
The checkMesh utility should write a file listing the affected cells.

Geon-Hong November 24, 2011 02:27

checkMesh results
 
Dear Alberto,

Yes, I checked the high aspect ratio cells from the file highAspectRatioCells in constant/polyMesh/sets.

What can I do with this cell-set? Is there any way to improve this problem?

I guess those high-aspect-ratio cells are generated from the cells adjacent to the wall.

The wing I want to simulate is tapered, which means that the wing tip has smaller chord length than the root. Furthermore, I generated the structured grid over the wing. Thus the wing has the same number of grid points along the chord at both wing tip and root. This might cause such problems, I guess.

Regeneration of the grid is only hope? ToT

robbirobocop November 24, 2011 04:29

Quote:

Originally Posted by Geon-Hong (Post 333371)

Regeneration of the grid is only hope? ToT

What program did you use to generate the geometry as well as the grid?
It seems that you would have to regenerate the grid. If you have done it with sHM - it will not take you too long ^^

Geon-Hong November 24, 2011 06:16

about grid problems
 
I've noticed just before that the problems have been occurred far from the wing, not adjacent to the wing.

The problem is that the grid has been generated in structured manner and the grid size along in the downstream was stretched while thin layer of grid for resolving the boundary layer near the wall is maintained.

Now I am considering a hybrid grid arrangement : structured near the wall and unstructured at far field.


Rob / I used gambit to generate the grid. Now I'd like to try Ansys instead. I have no idea about sHM. What is it?

Geon-Hong November 24, 2011 06:24

Fluent
 
One more thing that I'd like to mention is that I ran the same case(mesh) with the FLUENT but it provided converged transient solution. At least, FLUENT may provide a transient solution though the mesh have extremely high aspect ratio cells which have order of 1e4 as my case.

From following thread, Mads Reck also referred that

aspect ratios of more than 1000 is by no means insane - we have it in airfoil aerodynamics all the time - and on wind turbine blades.

Aspect ratios of 1e5 or even higher is usual business when you want to have a first cell height of around 1e-6 chordLengths.

http://www.cfd-online.com/Forums/ope...e-comment.html

I hope to resolve this problem as soon as possible.

arjun November 24, 2011 06:39

Quote:

Originally Posted by Geon-Hong (Post 333403)
One more thing that I'd like to mention is that I ran the same case(mesh) with the FLUENT but it provided converged transient solution. At least, FLUENT may provide a transient solution though the mesh have extremely high aspect ratio cells which have order of 1e4 as my case.

From following thread, Mads Reck also referred that

aspect ratios of more than 1000 is by no means insane - we have it in airfoil aerodynamics all the time - and on wind turbine blades.

Aspect ratios of 1e5 or even higher is usual business when you want to have a first cell height of around 1e-6 chordLengths.

http://www.cfd-online.com/Forums/ope...e-comment.html

I hope to resolve this problem as soon as possible.


Fluent starccm+ etc apply lots of tricks (trade secrets) to keep solver stable. The main difficulty with high respect ratio is that it makes pressure correction equation very difficult to converge, which could make solver unstable.
OpenFOAM does not apply such tricks so things should be more difficult with it. For this reason you would need good quality meshes with openFOAM.

Aurelien Thinat November 24, 2011 06:48

Hi,

You have some cells with high nonorthogonality (90 starts to be a high value imo). You should used "limited 0,333" schemes and not "corrected" schemes (snGrad).
I don't know if it would solve your problem, but it should be a start.

Aurelien


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