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Old   December 24, 2014, 12:16
Default Validation SimpleFoam sphere Cd
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H25E
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Hello,

I was trying to simulate a body car in simpleFoam but i had a output Cd slightly high so I tried to validate my "virtual wind tunnel" simulating an smooth sphere that its Cd is known and tabulated depending on the number of Reynolds, something like that:

But, unfortunately I get that:



I have compressed the simulation directory of 20m/s and I uploaded to dropbox, I erased the 100 200 300 and 400 intermediate folders and the stl file to save space. If u need them i can upload them.

I tried with a more accurate mesh (with 2.8 million cells) and with pisoFoam but I get the same results and I'm stucked here, I need some help.

Thanks for your time, greetings.
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Old   December 25, 2014, 17:06
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I'm working really hard but I'm totally stucked here. It's an smooth sphere with 5cm of diameter and I tried everything I could tried but I always get the same Cd, so low. What could be failling? I'm using the kOmegaSST turbulent model.

Also I can upload the simulation files in another server if it's necessary.

Thanks again.
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Old   December 25, 2014, 17:56
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Could it be that the value of the first graph uses Radius as the dimensional parameter instead of Diameter?

Which relaxationFactors are you using? Relaxing help a lot convergence, but can give some garbage when we are talking about fine tuning.

Have you tried running a transient solver?

Mesh photo?
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Old   December 25, 2014, 22:33
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Hello ssss thanks for the answer,

To the first question, I have found in several places and Re seems to be calculated everywhere with the diameter. Anyway, this only would displace the graph in the horizontal axis where it's well placed approximately. Where I have a big error is in the vertical axis with the Cd values.

To the second one, I have used 0.3 for pressure and 0.7 for U, k, and omega.

To the third one, I tried running pisoFoam at the simulation of 20m/s and i have practically the same Cd (0.114345 with simpleFoam VS 0.108134 with pisoFoam)

And to the last one I have simulated at all the speeds with the following mesh, that has 250.000 cells:


Then, and only for the case of 20m/s I have tried with a more accurate mesh with 2.800.000 cells:



But I get the same error, Cd of 0.124017 for the accurate case VS the 0.114345 of the coarse case.

(The 20m/s is supposed to to give a Cd between 0.45 and 0.5)

Thanks for your time.
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Old   December 26, 2014, 08:22
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I'm sure your problem is related to your mesh. Why don't you try to mesh a 2D case and see what happens? What does checkMesh say about your mesh?

Did you try to run it without relaxationFactors? Did you try a transient solver like pimpleFoam icoFoam, pisoFoam,etc?

Could you also post your fvSchemes?
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Old   December 26, 2014, 08:56
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Hello ssss,

Yes, I have written in the previous post over the mesh pictures info about the relaxation factors and pisoFoam.

I give you the chechMesh logs for the coarse case:
Code:
/*---------------------------------------------------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
Build  : 2.1.1-221db2718bbb
Exec   : checkMesh
Date   : Dec 26 2014
Time   : 13:48:00
Host   : "pc"
PID    : 2703
Case   : /home/hector/Desktop/TFG/corr/20
nProcs : 1
sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE).
fileModificationChecking : Monitoring run-time modified files using timeStampMaster
allowSystemOperations : Disallowing user-supplied system call operations

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Create time

Create polyMesh for time = 0

Time = 0

Mesh stats
    points:           271861
    faces:            772008
    internal faces:   745264
    cells:            250752
    boundary patches: 6
    point zones:      0
    face zones:       0
    cell zones:       0

Overall number of cells of each type:
    hexahedra:     242170
    prisms:        888
    wedges:        0
    pyramids:      0
    tet wedges:    4
    tetrahedra:    0
    polyhedra:     7690

Checking topology...
    Boundary definition OK.
    Cell to face addressing 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                  
    frontAndBack        9600     9922     ok (non-closed singly connected)  
    outlet              1600     1681     ok (non-closed singly connected)  
    inlet               1600     1681     ok (non-closed singly connected)  
    lowerWall           4800     4961     ok (non-closed singly connected)  
    upperWall           4800     4961     ok (non-closed singly connected)  
    PTri_esfera         4344     5486     ok (closed singly connected)      

Checking geometry...
    Overall domain bounding box (-0.1 -0.1 -0.1) (0.1 0.1 0.5)
    Mesh (non-empty, non-wedge) directions (1 1 1)
    Mesh (non-empty) directions (1 1 1)
    Boundary openness (3.44367e-17 -5.83811e-17 8.31597e-19) OK.
    Max cell openness = 2.70701e-16 OK.
    Max aspect ratio = 10 OK.
    Minumum face area = 2.48988e-07. Maximum face area = 2.57476e-05.  Face area magnitudes OK.
    Min volume = 3.76546e-10. Max volume = 1.26596e-07.  Total volume = 0.0239346.  Cell volumes OK.
    Mesh non-orthogonality Max: 30.9833 average: 4.52373
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 0.488181 OK.
    Coupled point location match (average 0) OK.

Mesh OK.

End
And for the accurate one too:
Code:
/*---------------------------------------------------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
Build  : 2.1.1-221db2718bbb
Exec   : checkMesh
Date   : Dec 26 2014
Time   : 13:48:22
Host   : "pc"
PID    : 2710
Case   : /home/hector/Desktop/TFG/corr/20MallaFina
nProcs : 1
sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE).
fileModificationChecking : Monitoring run-time modified files using timeStampMaster
allowSystemOperations : Disallowing user-supplied system call operations

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Create time

Create polyMesh for time = 0

Time = 0

Mesh stats
    points:           2978827
    faces:            8569572
    internal faces:   8496572
    cells:            2804124
    boundary patches: 6
    point zones:      0
    face zones:       0
    cell zones:       0

Overall number of cells of each type:
    hexahedra:     2657316
    prisms:        10760
    wedges:        0
    pyramids:      0
    tet wedges:    4
    tetrahedra:    0
    polyhedra:     136044

Checking topology...
    Boundary definition OK.
    Cell to face addressing 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                  
    frontAndBack        2400     2562     ok (non-closed singly connected)  
    outlet              400      441      ok (non-closed singly connected)  
    inlet               400      441      ok (non-closed singly connected)  
    lowerWall           1200     1281     ok (non-closed singly connected)  
    upperWall           1200     1281     ok (non-closed singly connected)  
    PTri_esfera         67400    84426    ok (closed singly connected)      

Checking geometry...
    Overall domain bounding box (-0.1 -0.1 -0.1) (0.1 0.1 0.5)
    Mesh (non-empty, non-wedge) directions (1 1 1)
    Mesh (non-empty) directions (1 1 1)
    Boundary openness (2.02228e-17 -2.26277e-17 2.10272e-18) OK.
    Max cell openness = 3.13147e-16 OK.
    Max aspect ratio = 9.04486 OK.
    Minumum face area = 1.43276e-08. Maximum face area = 0.00010299.  Face area magnitudes OK.
    Min volume = 5.70459e-12. Max volume = 1.00377e-06.  Total volume = 0.0239346.  Cell volumes OK.
    Mesh non-orthogonality Max: 48.6495 average: 5.63905
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 0.542972 OK.
    Coupled point location match (average 0) OK.

Mesh OK.

End
And the fvsolution:
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

solvers
{
    p
    {
        solver           GAMG;
        tolerance        1e-7;
        relTol           0.1;
        smoother         GaussSeidel;
        nPreSweeps       0;
        nPostSweeps      2;
        cacheAgglomeration on;
        agglomerator     faceAreaPair;
        nCellsInCoarsestLevel 10;
        mergeLevels      1;
    }

    U
    {
        solver           smoothSolver;
        smoother         GaussSeidel;
        tolerance        1e-8;
        relTol           0.1;
        nSweeps          1;
    }

    k
    {
        solver           smoothSolver;
        smoother         GaussSeidel;
        tolerance        1e-8;
        relTol           0.1;
        nSweeps          1;
    }

    omega
    {
        solver           smoothSolver;
        smoother         GaussSeidel;
        tolerance        1e-8;
        relTol           0.1;
        nSweeps          1;
    }
}

SIMPLE
{
    nNonOrthogonalCorrectors 0;
}

potentialFlow
{
    nNonOrthogonalCorrectors 10;
}

relaxationFactors
{
    fields
    {
        p               0.3;
    }
    equations
    {
        U               0.7;
        k               0.7;
        omega           0.7;
    }
}

cache
{
    grad(U);
}

// ************************************************************************* //
And the fvSchems
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      fvSchemes;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

ddtSchemes
{
    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;
}

// ************************************************************************* //
What I should do if the problem is in the mesh?

UPDATE: I tried to simulate with the relax factors =1 but the continuity diverges completely.
UPDATE2: With the relaxing factors at 0.1 I get the same Cd, so maybe the error is in the mesh but I don't know where...

Last edited by H25E; December 26, 2014 at 22:06.
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