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Old   January 3, 2013, 12:02
Default Hex and Tet meshes - simplefoam comparison
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Daniele Vicario
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Hi everybody,

Since I need to use the dynamicTopoFvMesh class I'm trying to understand how my sims (already run with hex meshes) performs using Tet meshes.

I started studying the flow rate of a custom valve.

Enclosed you can see the same geometry meshed with SHM and with Netgen, togheter with their checkmesh results.

Some notes:
- Same pressure driven BC for both the cases.
- Same turbolence model setup (komegaSST).
- Same fvscheme and fvsolution.

This is the log file of the flow rate using hex meshes:

Code:
#          Time         inlet        outlet
             1     -5.700601      5.700579
             2      -3.79034      3.790358
             3     -4.903445      4.903438
             4     -5.377026      5.377034
             5     -5.704897      5.704898
             6     -5.944335      5.944333
             7     -6.119664      6.119663
             8     -6.220943      6.220941
             9     -6.256514      6.256513
            10     -6.250962      6.250963
            11     -6.218107      6.218109
            12     -6.161133      6.161132
            13     -6.088814      6.088814
            14      -6.00451      6.004513
            15     -5.912531      5.912534
            16     -5.815976      5.815976
            17     -5.716687      5.716688
            18       -5.6167        5.6167
            19     -5.517849       5.51785
            20     -5.422765      5.422765
            21     -5.333158      5.333159
            22     -5.250153      5.250153
            23     -5.173377      5.173377
            24     -5.102728      5.102728
            25     -5.037989      5.037989
            26     -4.978615      4.978616
            27     -4.924211      4.924208
            28     -4.874001         4.874
            29     -4.827512      4.827511
            30     -4.784434      4.784432
            31     -4.744479      4.744479
            32     -4.708216      4.708216
            33     -4.675977      4.675978
            34     -4.647641      4.647642
            35     -4.622598      4.622598
            36     -4.600458      4.600458
            37     -4.580474      4.580475
            38     -4.562082      4.562083
            39     -4.544581       4.54458
            40     -4.527438       4.52744
            41     -4.510434      4.510434
            42     -4.493563      4.493563
            43     -4.476761      4.476761
            44     -4.460181      4.460181
            45     -4.443663      4.443663
            46     -4.426965      4.426965
            47     -4.410062      4.410062
            48      -4.39336       4.39336
            49     -4.376991      4.376991
....
 
          2627      -4.27264      4.270662
          2628     -4.272626       4.27058
          2629     -4.272489      4.270488
          2630     -4.272234      4.270317
          2631     -4.271874      4.270348
          2632     -4.271439      4.270732
          2633      -4.27099      4.271581
          2634     -4.270574      4.272077
          2635     -4.270118      4.271661
This is the same file using the tet mesh:

Code:
#          Time         inlet        outlet
             1     -172.0041      21.71271
             2     -29.95186      1.310735
             3     -57.05148      23.14831
             4     -50.40974      30.45741
             5      -42.4858      35.18858
             6     -10.63777      21.76366
             7     -14.34278      7.928121
             8     -19.71502      2.089407
             9     -20.83303      9.133087
            10     -12.97812         21.45
            11     -15.64673       24.9519
            12     -18.66453      22.21093
            13     -20.08745      5.498206
            14     -10.07858     0.3255894
            15     -8.387256       3.84717
            16      -2.87546      10.25943
            17     -4.728935       13.8913
            18     -1.959338      13.97463
            19     -9.568247      5.668299
            20     -7.270501       3.24852
            21     -6.705826      5.219731
            22     -4.514791      7.281349
            23     -6.902184      8.871348
            24     -5.602596      7.402167
            25     -8.852266      4.557721
            26     -6.024786       9.46239
            27     -5.604201      6.377496
            28     -3.582253      7.762387
            29     -4.761439      7.010344
            30      -5.44479      5.005502
            31     -6.136755       4.06424
            32     -4.496904      4.433816
            33     -4.910032      5.501521
            34     -5.057533      5.617444
            35     -5.251011      5.396437
            36     -3.316635      4.600008
            37     -5.171873      4.411376
            38     -5.573408      4.209184
            39       -4.3302      4.705713
            40     -4.163483      4.676618
            41     -2.761881      4.432551
            42     -3.619489      3.871926
            43     -4.572584      3.528158
            44      -3.58269      3.626229
            45     -3.723246       3.58942
            46      -3.62441      3.603673
            47     -3.386411      3.532791
            48     -3.105231      3.406442
            49     -2.682748      3.179411
            50     -2.841984      2.989721
            51     -2.799791      2.711039
            52     -2.827969      2.447284
            53     -2.735649      2.314938
            54     -2.527999      2.362736
            55     -2.452254      2.342192
            56     -2.333465      2.369357
            57     -2.174707      2.385321
            58     -2.045561       2.35066
            59     -2.002889       2.26217
            60     -1.992403      2.144051
            61     -1.961413      2.003014
            62     -1.941028      1.842707
            63     -1.969635      1.706767
            64     -1.976639      1.629558
            65     -1.983343      1.656954
            66     -2.024261      1.763623
            67     -2.057045      1.869765
            68      -2.08375      1.999511
            69     -2.208145       2.08836
            70     -1.955683      2.163669
            71     -2.044087        2.2096
            72     -2.084881      2.236304
            73     -2.368958      2.315571
            74     -2.469138      2.371803
            75      -2.61045      2.457574
            76     -2.798879      2.556751
            77     -3.012586      2.678196
            78     -3.194521      2.811004
            79     -3.356413      2.947216
            80      -3.43074      3.077914
            81     -3.443503      3.194778
            82     -3.465136      3.298215
            83     -3.524945      3.395603
            84       -3.6387      3.500317
            85     -3.807765      3.615832
....            
           563     -4.316257       4.31451
           564     -4.315974       4.31457
           565     -4.316129        4.3145
           566     -4.316908       4.31411
           567     -4.316524      4.314286
           568     -4.316411      4.314671
           569     -4.316268      4.314813
           570     -4.316568      4.314814
           571     -4.316601      4.314811
           572     -4.317013      4.314497

Even if the converging result seems good I've got some questions:

- The hex mesh case is converging not only faster... but better. Is it possible that mass conservation law is not respected in the first iterations of the tet mesh case ? See differences in the inlet/outlet flow rates...

- I used the same fvschemes and fvsolution for both the meshes. I tried other settings (see http://www.cfd-online.com/Forums/ope...tml#post257294) but with more instabilities during the first iterations. Below are my settings:

fvSchemes:

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.0.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      fvSchemes;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
ddtSchemes
{
    //default         Euler;
    default         steadyState;
}
gradSchemes
{
    default         Gauss linear;
 
}
divSchemes
{
    default         none;
 
    div(phi,U)      Gauss linearUpwindV grad(U);
 
    div(phi,k)      Gauss upwind;
    div(phi,omega)  Gauss upwind;
    div((nuEff*dev(T(grad(U))))) Gauss linear;
 
}
laplacianSchemes
{
    default         Gauss linear corrected;
 
}
interpolationSchemes
{
    default         linear;
}
snGradSchemes
{
    default         corrected;
 
}
fluxRequired
{
    default         no;
    p;
}
 
// ************************************************************************* //
fVSolution:

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.0.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
solvers
{
    p
    {
        solver           GAMG;
        tolerance        1e-8;
        relTol           0.01;
        smoother         GaussSeidel;
        nPreSweeps       0;
        nPostSweeps      2;
        cacheAgglomeration on;
        agglomerator     faceAreaPair;
        nCellsInCoarsestLevel 20;
        mergeLevels      2;
    }
    pFinal
    {
        solver           GAMG;
        tolerance        1e-8;
        relTol           0;
        smoother         GaussSeidel;
        nPreSweeps       0;
        nPostSweeps      2;
        cacheAgglomeration on;
        agglomerator     faceAreaPair;
        nCellsInCoarsestLevel 10;
        mergeLevels      1;
    }
 
    U
    {
        solver           smoothSolver;
        smoother         GaussSeidel;
        tolerance        1e-8;
        relTol           0.05;
        nSweeps          1;
    }
    k
    {
        solver           smoothSolver;
        smoother         GaussSeidel;
        tolerance        1e-9;
        relTol           0.01;
        nSweeps          1;
    }
    omega
    {
        solver           smoothSolver;
        smoother         GaussSeidel;
        tolerance        1e-9;
        relTol           0.01;
        nSweeps          1;
    }
}
SIMPLE
{
    nNonOrthogonalCorrectors 1;
    pRefCell        0;
    pRefValue       0;
 
    residualControl
    {
        p               1e-5;
        U               1e-5;
        nuTilda         1e-5;
    }
}
potentialFlow
{
    nNonOrthogonalCorrectors 10;
}
relaxationFactors
{
    p               0.5;
    U               0.7;
    k               0.6;
    omega           0.6;
}
PISO
{
    nCorrectors     2;
    nNonOrthogonalCorrectors 0;
    pRefCell        0;
    pRefValue       0;
}
cache
{
    grad(U);
}
// ************************************************************************* //
Is there any better (general) setup ?

Personally I'm not unsatisfied of the comparison. There is about 1 % difference in the flow rates results and the tet sim is still running.

I'm just worrying because I need to use tet meshes with dynamicTopoFvMesh in order to simulate the equivalent of a check valve: an object immersed in a pushing fluid is going to close the outlet... it's one of the worst setup I think...
Above all, here http://www.google.it/url?sa=t&rct=j&...55534169,d.bGE it says that "Crushing cell is not supported (Example: complete valve closure for cylinder simulations)" but it was one and half years ago...what about now ?

More: Looking in the forum it seems that tet meshes are a little delicate to work with... is there anyone who use dynamicTopoFvMesh on real cases ?

I'm sorry for the long thread. Any comment ?
Attached Images
File Type: jpg mesh1.jpg (51.9 KB, 56 views)
File Type: jpg sh1.jpg (86.0 KB, 59 views)
Attached Files
File Type: txt Tetchkmesh.txt (3.2 KB, 12 views)
File Type: txt Hexchkmesh.txt (3.3 KB, 1 views)
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Old   January 3, 2013, 12:36
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Sandeep Menon
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Daniele,

I'm not surprised to see that Hex meshes gives better results - they inherently have less non-orthogonality. You'll have to play with the nNonOrthogonalCorrectors with tets and see if that helps. Also, you have a gradient scheme that isn't really the best - try leastSquares. You could also try Hrv's reconCentral schemes from 1.6-ext and see if that changes things.
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Old   January 3, 2013, 13:01
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Daniele Vicario
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Thanks for the suggestions. Unfortunately I cannot use the ext version, but I'll try others schemes. Any news regarding the crushing cells limitation ?
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Old   January 3, 2013, 13:08
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Sandeep Menon
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Well, I suppose you could compile the sources for reconCentral in 2.1.x - I don't think there's anything in there specific to 1.6-ext.

The closest thing I can see in OpenFOAM to complete valve closure is either by changing a GGI / AMI patch at run-time to wall - which probably has adverse stability issues where you'll have to either under-relax or reduce the time-step. Or possibly by switching cells off by marking cells in a specific zone and deactivating it. You could try playing around with porous zones too...
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Old   January 3, 2013, 13:10
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Hello,

I use Tetrahedral meshes quite often (well... almost always), and as you have seen, the convergence of a simulation when using Tet meshes is in most cases not as good as for the same system with a hex dominant mesh.

However, I would suggest that you try the following in addition to the suggestions by Sandeep:

1. As Sandeep mentioned.... for Tet meshes, non-orthogonal correctors are almost always a requirement. I use 2 non-orthogonal correctors, and it works quite well.

2. I noticed that your relaxation factors are quite high.... You could try with lower values to start of with (say for the first 250 - 300 iterations). Again... a suggested value is: 0.3 for all the variables.


The results from Checkmesh seem to be quite ok for the Tet mesh, and I dont think you can get too much better than that.... as far as non-orthogonality goes.

You could try one more thing..... In netgen, in the meshing options, you can choose the number of optimisation steps for the surface and volume mesh generation......

Set the Surface Mesh optimisation to around 15 steps, and the volume mesh optimisation to around 12..... The mesh generation might take a little longer, but the quality of the resulting meshes should become a little better.


A great evening ahead!

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Old   January 3, 2013, 15:32
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@Sandeep:
I'll try to use reconCentral. I don't think I'm going to find particular problems compiling the source. Actually I'm more worried about your next suggestion...I just want to be sure: is it possible with the current release of dynamictopofvmesh class to manage crushing cells (i.e. the closing valve example) ? If not, it seems to me that the piston engine video shows this functionality... am I so wrong ?

@philippose:
Many thanks ! I'm going to play a little with those parameters and see what happens...
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Old   January 3, 2013, 15:36
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You're right. Valve closure is not supported at the moment.
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Old   January 3, 2013, 16:18
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Well, that's....interesting. I'm not enought prepare to fully understand why it can't work but I'm still however interested in testing my cases.

Even because your class is the most automatic system you can have in OF that can manage dynamic meshes... is it ? I read a lot regarding GGI and AMI (but never try none of them) but, as you said, they seem too...delicate for my application.

However easy question: is there any plan to support crushing cells in the near future ?
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Old   January 3, 2013, 16:25
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Valve closure: Can't guarantee it. Mainly because I haven't found a need to do it till now. I could look into it if there's significant interest, but that largely depends on whether I have the time on my hands.
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Old   January 3, 2013, 16:58
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Ok. Thanks anyway for your work.
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Old   January 4, 2013, 02:18
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Just a small note:

In fvSolution, using relTol=0 brings the inlet flow rate to be equal to the outlet one since the very first step.

This however increase each iteration time. Still don't know which value is better (from the speed point of view). Probably it depends on the case.
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