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Poiseuille flow: strange behaviour

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Old   February 15, 2013, 08:23
Default Poiseuille flow: strange behaviour
  #1
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Samuele Z
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Dear all,

I am trying to simulate a Poiseuille-flow. It is a very simple problem, but I notice a strange behaviour. Everything seems to be ok. Actually, if you analyze the attached picture, you can see that Uz presents a strange `red cell' (i.e. a value pretty high) in the cell between the upper wall and the inlet. As far as the BC are concerned, here are the files for P and U:

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       volScalarField;
    object      p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 2 -2 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    wall
    {
        type            zeroGradient;
        
    }

    xwall													// facce perpendicolari a x
    {
        type			empty;
    }

    inlet1
    {
        type            zeroGradient;
    }
    
    outlet
    {
        type            fixedValue;
        value			uniform 0;
    }

}

// ************************************************************************* //
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       volVectorField;
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 1 -1 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{
    wall
    {
        type            	fixedValue;
        value			uniform (0 0 0);
    }

    inlet1

    {
	type			groovyBC;
	value			uniform (0 1 0);
	variables		"Vm=0.5;xp=pos().x;zp=pos().z;hx=max(xp);hz=max(zp);a=-8*Vm/(hz*hz);b=2*Vm;R=hz/2;";
	fractionExpression	"1"; 
	valueExpression		"vector (0, a*((zp-R)*(zp-R))+b, 0)";
        
    }

    outlet
    {
    type		zeroGradient;
    }

    xwall													// facce perpendicolari a x
    {
        type			empty;
    }
}



// ************************************************************************* //
Also, I am running simpleFoam.

Why this happens? Could this cell be `dangerous' for the whole solution?

Any idea?

Thanks a lot,
Samuele
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File Type: jpg clip.jpg (97.2 KB, 22 views)
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Old   February 15, 2013, 11:39
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Bernhard
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This is completely wrong.

What solver are you using?

Do you by any chance have a pRef defined in system/fvSolution?
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Old   February 15, 2013, 11:45
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Samuele Z
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I know that it is completely wrong.

I am using simpleFoam and I do not have pRefCell defined in the fvSolution file.

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-5;
        relTol           0.1;
        smoother         GaussSeidel;
        nPreSweeps       0;
        nPostSweeps      2;
        cacheAgglomeration on;
        agglomerator     faceAreaPair;
        nCellsInCoarsestLevel 10;
        mergeLevels      1;
    }

    U
    {
        solver           smoothSolver;
        smoother         GaussSeidel;
        tolerance        1e-5;
        relTol           0.1;
        nSweeps          1;
    }
}

SIMPLE
{
    nNonOrthogonalCorrectors 0;

	residualControl
    {
        p           1e-5;
        U           1e-5;
    }
}

potentialFlow
{
    nNonOrthogonalCorrectors 10;
}

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

    }
}

cache
{
    grad(U);
}

// ************************************************************************* //
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Old   February 15, 2013, 11:53
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Bernhard
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You are looking at a unconverged solution. You now look at the second iteration. Note that deltaT does not mean time in the simple*Foam solvers. Set deltaT to 1, endTime to 1000 (thus 1000 iterations), and look at the last saved step to see what has changed. Check out the simpleFoam source code to see what is going on (loop-wise).

Did your log-file end with a statement "Solution has converged", or something like that?
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Old   February 15, 2013, 12:02
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Samuele Z
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That was not a converged solution (but the same happens for the converged one) and my DeltaT is already set to 1. You see Time 2000 and `snapshot' 2 because I save the solution each 1000 steps.

Also, adding a pRef in 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-5;
        relTol           0.1;
        smoother         GaussSeidel;
        nPreSweeps       0;
        nPostSweeps      2;
        cacheAgglomeration on;
        agglomerator     faceAreaPair;
        nCellsInCoarsestLevel 10;
        mergeLevels      1;
    }

    U
    {
        solver           smoothSolver;
        smoother         GaussSeidel;
        tolerance        1e-5;
        relTol           0.1;
        nSweeps          1;
    }
}

SIMPLE
{
    nNonOrthogonalCorrectors 0;

	pRefCell 	14436;
	pRefValue	0;

	residualControl
    {
        p           1e-5;
        U           1e-5;
    }
}

potentialFlow
{
    nNonOrthogonalCorrectors 10;
}

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

    }
}

cache
{
    grad(U);
}

// ************************************************************************* //
I get the same behaviour (see the picture attached).

The picture shows the solution after 100 iterations, but the same happens for the converged solution (I'll post it soon).

Ideas?
Attached Images
File Type: jpg clip.jpg (92.1 KB, 12 views)
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Old   February 15, 2013, 12:05
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Bernhard
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You definitely don't need the pRefCell (it will also not be used hopefully). Can you show a snippet of your logfile? I think a lot can be gained in your fvSolutions.
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Old   February 17, 2013, 04:31
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dear samuele,

could you please attach your entire case, i faced same problems before but i simulated this case in tube and used icoFoam. i think the problem doesn't relate to refP but to boundary conditions. you may try make outlet BCs as inletOutlet.

thanks and good luck
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