[Ryôzanpaku Tiger]

Dear all,

I am solving a heat transfer multi-region problem: a hot air flow in a pipe with thick walls. The geometry is created in FreeCAD, imported to STEP, exported to SALOME for triangulation, imported to STL. The resulted STL file is a closed surface and has a nice triangulation (see attached figure).

The resulted mesh doesn't look good at the interface between air and pipe (see attached figures).

I tried to adjust the parameters, following various guidelines (including those found on this forum), but couldn't improve the mesh at the interface.

I would very appreciate if you could give me a hint on how I can improve my mesh. Which parameters should be variated? Should I try layers?

Here is a link to the case if needed: case files

Thanks!

[Yann]

Hello,

When meshing multiregion cases, non-orthogonality can be a problem when snapping on curved surface like your cylinder.

You might try to increase the maxNonOrtho parameter in meshQualityControls and see if it improves snapping on the cylinder surface.

Yann

[Ryôzanpaku Tiger]

Originally Posted by Yann

Hello,

When meshing multiregion cases, non-orthogonality can be a problem when snapping on curved surface like your cylinder.

You might try to increase the maxNonOrtho parameter in meshQualityControls and see if it improves snapping on the cylinder surface.

Yann

Dear Yann,

thank you very much! I increased maxNonOrtho and my mesh has improved!

I will check mesh quality parameters more thoroughly.

Thank you!

[Ryôzanpaku Tiger]

Originally Posted by Yann

Hello,

When meshing multiregion cases, non-orthogonality can be a problem when snapping on curved surface like your cylinder.

You might try to increase the maxNonOrtho parameter in meshQualityControls and see if it improves snapping on the cylinder surface.

Yann

Hello Yann,

I improved the mesh significantly, thank you very much!

The interface is now very smooth, but there are few spots, where the outer edges are not resolved properly. These are the tiny bits which are “cut out” of the mesh.

I read few topics here on the forum about 90 degrees edges and tried few things, but I couldn’t get read of these tiny bits. Could you please give a hint, what could it be? Here is snappyHexMeshDict:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2306                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

castellatedMesh true;
snap            true;
addLayers       false;

geometry
{
    pipe.stl   { type triSurfaceMesh; name pipe;   }
    inlet.stl  { type triSurfaceMesh; name inlet;  }
    outlet.stl { type triSurfaceMesh; name outlet; }
}

castellatedMeshControls
{
    maxGlobalCells 10000000;
    maxLocalCells 5000000;
    minRefinementCells 10;
    nCellsBetweenLevels 1;
    features
    (
        { file "pipe.extendedFeatureEdgeMesh";  level 3; }
    );

    refinementSurfaces
    {
        pipe   { level (2 3); }
        inlet  { level (2 3); patchInfo { type patch; } }
        outlet { level (2 3); patchInfo { type patch; } }
    }
    resolveFeatureAngle 30;
    refinementRegions
    {
    }
    locationsInMesh
    (
        ((0.0081 0.0001 0.0049) fluid )
        ((0.0139 0.0001 0.0001) pipe  )
    );
    allowFreeStandingZoneFaces false;
}

snapControls
{
    nSmoothPatch 3;
    tolerance 1.0;
    nSolveIter 100;
    nRelaxIter 5;
    nFeatureSnapIter 5;
    implicitFeatureSnap false;
    explicitFeatureSnap true;
    multiRegionFeatureSnap false;
}

addLayersControls
{
    relativeSizes false;
    layers
    {
        pipeInnerWall { nSurfaceLayers 8; }
    }
    expansionRatio 1.2;
    finalLayerThickness 0.001;
    minThickness 0.0005;
    nGrow 0;
    featureAngle 80;
    nRelaxIter 5;
    nSmoothSurfaceNormals 1;
    nSmoothNormals 3;
    nSmoothThickness 10;
    maxFaceThicknessRatio 0.5;
    maxThicknessToMedialRatio 0.3;
    minMedialAxisAngle 90;
    nBufferCellsNoExtrude 0;
    nLayerIter 50;
    nRelaxedIter 20;
}

meshQualityControls
{
    maxNonOrtho 180;
    maxBoundarySkewness 20;
    maxInternalSkewness 4;
    maxConcave 80;
    minFlatness 0.5;
    minVol 1e-13;
    minTetQuality 1e-30;
    minArea -1;
    minTwist 0.02;
    minDeterminant 0.001;
    minFaceWeight 0.02;
    minVolRatio 0.01;
    minTriangleTwist -1;
    nSmoothScale 4;
    errorReduction 0.75;
}

mergeTolerance 1e-6;

// ************************************************************************* //

I would also appreciate your thoughts about the STL files problem: I use 3 STL files – pipe, inlet, outlet. All 3 surfaces have nice triangulation, but inlet and outlet are not closed. If I combine all files together, then the resulted file is not closed too. Well, the mesh is generated, but I know that it is not the ideal situation. What could be a solution here? Or should I simply be happy that my mesh is generated quite nicely, despite of the not closed surfaces?

Thank you very much for your help!

[Yann]

Hello,

Having a closed STL file might help with snapping. Nothing certain about that, but it might be worth trying.
After concatenating your STL file, there are 2 utilities you can use to clean up the concatenated file : surfacePointMerge and surfaceHookUp.

Have a look at this post: Surface Check and Repair Utilities - Need Explanation

Another thing you can try is to activate this switch and see how it affects your mesh:

multiRegionFeatureSnap false;

Regards,
Yann

[Ryôzanpaku Tiger]

Originally Posted by Yann

Hello,

Having a closed STL file might help with snapping. Nothing certain about that, but it might be worth trying.
After concatenating your STL file, there are 2 utilities you can use to clean up the concatenated file : surfacePointMerge and surfaceHookUp.

Have a look at this post: Surface Check and Repair Utilities - Need Explanation

Another thing you can try is to activate this switch and see how it affects your mesh:

multiRegionFeatureSnap false;

Regards,
Yann

Hello Yann,

Thank you for the hints! I will check these utilities and try to repair the concatenated file.

Before I read your answer, I have solved the problem with the edge resolution by using

nFeatureSnapIter 10;

The "teeth" disappeared. However, I still tried to use multiRegionFeatureSnap and with a new value of nFeatureSnapIter the mesh didn't change (is this normal? ), so I returned back to multiRegionFeatureSnap set to false.

(Here is a link to the updated case: case files)

Thank you very much again for the help!

[Ryôzanpaku Tiger]

Hello Yann,

May I bother you with another question?

Now, when the mesh in the pipe is fine, I would like to add a thin plate with small holes inside the pipe.

Here is snappyHexMeshDict:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2306                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

// Which of the steps to run
castellatedMesh true;
snap            true;
addLayers       false;

geometry
{
    pipe.stl   { type triSurfaceMesh; name pipe;}
    inlet.stl  { type triSurfaceMesh; name inlet;  }
    outlet.stl { type triSurfaceMesh; name outlet; }
    plane.stl { type triSurfaceMesh; name plane;
        regions
        {
            planeBackFace {name planeBackFace;}
            planeBottomFace {name planeBottomFace;}
            planeFrontFace {name planeFrontFace;}
            planeHole1 {name planeHole1;}
            planeHole2 {name planeHole2;}
            planeHole3 {name planeHole3;}
            planeHole4 {name planeHole4;}
            planeHole5 {name planeHole5;}
            planeHole6 {name planeHole6;}
            planeLeftFace {name planeLeftFace;}
            planeRightFace {name planeRightFace;}
            planeTopFace {name planeTopFace;}
        }
    }
}


// Settings for the castellatedMesh generation.
castellatedMeshControls
{
    maxGlobalCells 10000000;
    maxLocalCells 5000000;
    minRefinementCells 10;
    nCellsBetweenLevels 1;

    features
    (
        { file "pipe.extendedFeatureEdgeMesh";   level 2; }
        { file "plane.extendedFeatureEdgeMesh";  level 2; }
    );

    refinementSurfaces
    {
        pipe
        {
            level (2 2);
        }
        inlet
        {
            level (2 2);
            patchInfo { type patch; }
        }
        outlet
        {
            level (2 2);
            patchInfo { type patch; }
        }
        plane
        {
            level (2 2);
            patchInfo { type wall; }
            regions
            {
                planeHole1 { level (4 5); }
                planeHole2 { level (4 5); }
                planeHole3 { level (4 5); }
                planeHole4 { level (4 5); }
                planeHole5 { level (4 5); }
                planeHole6 { level (4 5); }
            }
        }
    }

    resolveFeatureAngle 30;

    refinementRegions
    {
    }

    locationsInMesh
    (
        ((0.0081 0.0001 0.0049) fluid )
        ((0.0139 0.0001 0.0001) pipe  )
        ((0.0001 0.0001 0.0001) plane)
    );

    allowFreeStandingZoneFaces false;
}


// Settings for the snapping.
snapControls
{
    nSmoothPatch 3;
    tolerance 1.0;
    nSolveIter 100;
    nRelaxIter 5;
    nFeatureSnapIter 10;
    implicitFeatureSnap false;
    explicitFeatureSnap true;
    multiRegionFeatureSnap false;
}

addLayersControls
{
    relativeSizes false;

    layers
    {
        pipeInnerWall { nSurfaceLayers 8; }
    }

    expansionRatio 1.2;
    finalLayerThickness 0.001;
    minThickness 0.0005;
    nGrow 0;
    featureAngle 80;
    nRelaxIter 5;
    nSmoothSurfaceNormals 1;
    nSmoothNormals 3;
    nSmoothThickness 10;
    maxFaceThicknessRatio 0.5;
    maxThicknessToMedialRatio 0.3;

    minMedialAxisAngle 90;
    nBufferCellsNoExtrude 0;
    nLayerIter 50;
    nRelaxedIter 20;
}

meshQualityControls
{
    maxNonOrtho 180;
    maxBoundarySkewness 20;
    maxInternalSkewness 4;
    maxConcave 80;
    minFlatness 0.5;
    minVol 1e-13;
    minTetQuality 1e-30;
    minArea -1;
    minTwist 0.02;
    minDeterminant 0.001;
    minFaceWeight 0.02;
    minVolRatio 0.01;
    minTriangleTwist -1;
    nSmoothScale 4;
    errorReduction 0.75;
}

mergeTolerance 1e-6;

// ************************************************************************* //

The mesh doesn't look bad (see figure 1), but I would like to achieve better smoothness of the inner surfaces of the holes. However, if, for example, for

planeHole1 { level (5 6); }

the mesh does not improve.

I tried changing snapping parameters, but it didn't give any improvement. I tried using refinementRegions (see figure 2), but it only made the mesh worse, and the edges were not resolved correctly.

Could you please give a hint: what else could be adjusted here?

And another problem with the current mesh: the mesh quality complains about

Checking faces in error :
    faces with face pyramid volume < 1e-13                 : 128128

Of course, I could remove this warning by adjusting

minVol

parameter, but I am not sure whether I should do it.

I would very appreciate any hints! Thank you!

[Yann]

Hello,

Hard to say, how does the holes look in your STL file ?

[Ryôzanpaku Tiger]

Originally Posted by Yann

Hello,

Hard to say, how does the holes look in your STL file ?

Please see attached the part of the STL model with the hole. Thanks!

[Ryôzanpaku Tiger]

UPDATE: I refined the STL model in SALOME reducing the local length form 0.0003 to 0.0001. The STL model has now a very big size, but the resulted surface is smooth.

However, the mesh quality still complains about the faces:

Checking faces in error :
    faces with face pyramid volume < 1e-13                 : 268112

I assume that because of the length values the volume can be that small. But to which extent can this warning be ignored?

I would very appreciate any hints! Thank you!