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Old   July 12, 2011, 11:07
Default non-smooth mesh
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Alexander
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I remade the flange case from tutorial and tried to make at its base mesh for ship.
But there is a problem - the resulting grid in some places is not smooth and does not coincide with stl-geometry:



Here is the text of my snappyHexMeshDict:

HTML Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.0.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      autoHexMeshDict;
}

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

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


// Geometry. Definition of all surfaces. All surfaces are of class
// searchableSurface.
// Surfaces are used
// - to specify refinement for any mesh cell intersecting it
// - to specify refinement for any mesh cell inside/outside/near
// - to 'snap' the mesh boundary to the surface
geometry
{
    ship.stl
    {
        type triSurfaceMesh;
        name ship;
    }

    //- Refine a bit extra around the small centre hole
    refinementBox
    {
        type searchableBox;
        min (-0.14 -0.050 -0.024);
        max ( 0.122  0.050 0.04);
    }
};



// Settings for the castellatedMesh generation.
castellatedMeshControls
{

    // Refinement parameters
    // ~~~~~~~~~~~~~~~~~~~~~

    // While refining maximum number of cells per processor. This is basically
    // the number of cells that fit on a processor. If you choose this too small
    // it will do just more refinement iterations to obtain a similar mesh.
    maxLocalCells 500000;

    // Overall cell limit (approximately). Refinement will stop immediately
    // upon reaching this number so a refinement level might not complete.
    // Note that this is the number of cells before removing the part which
    // is not 'visible' from the keepPoint. The final number of cells might
    // actually be a lot less.
    maxGlobalCells 500000;

    // The surface refinement loop might spend lots of iterations refining just a
    // few cells. This setting will cause refinement to stop if <= minimumRefine
    // are selected for refinement. Note: it will at least do one iteration
    // (unless the number of cells to refine is 0)
    minRefinementCells 0;

    // Number of buffer layers between different levels.
    // 1 means normal 2:1 refinement restriction, larger means slower
    // refinement.
    nCellsBetweenLevels 1;



    // Explicit feature edge refinement
    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    // Specifies a level for any cell intersected by its edges.
    // This is a featureEdgeMesh, read from constant/triSurface for now.
    features
    (
        {
            file "ship.eMesh";
            level 7;
        }
    );



    // Surface based refinement
    // ~~~~~~~~~~~~~~~~~~~~~~~~

    // Specifies two levels for every surface. The first is the minimum level,
    // every cell intersecting a surface gets refined up to the minimum level.
    // The second level is the maximum level. Cells that 'see' multiple
    // intersections where the intersections make an
    // angle > resolveFeatureAngle get refined up to the maximum level.

    refinementSurfaces
    {
        ship
        {
            // Surface-wise min and max refinement level
            level (2 2);
        }
    }

    resolveFeatureAngle 30;


    // Region-wise refinement
    // ~~~~~~~~~~~~~~~~~~~~~~

    // Specifies refinement level for cells in relation to a surface. One of
    // three modes
    // - distance. 'levels' specifies per distance to the surface the
    //   wanted refinement level. The distances need to be specified in
    //   descending order.
    // - inside. 'levels' is only one entry and only the level is used. All
    //   cells inside the surface get refined up to the level. The surface
    //   needs to be closed for this to be possible.
    // - outside. Same but cells outside.

    refinementRegions
    {
        refinementBox
        {
            mode inside;
            levels ((1E15 3));
        }
    }


    // Mesh selection
    // ~~~~~~~~~~~~~~

    // After refinement patches get added for all refinementSurfaces and
    // all cells intersecting the surfaces get put into these patches. The
    // section reachable from the locationInMesh is kept.
    // NOTE: This point should never be on a face, always inside a cell, even
    // after refinement.
    // This is an outside point locationInMesh (-0.033 -0.033 0.0033);
    locationInMesh (-9.23149e-05 -0.0025 -0.0025); // Inside point

    // Whether any faceZones (as specified in the refinementSurfaces)
    // are only on the boundary of corresponding cellZones or also allow
    // free-standing zone faces. Not used if there are no faceZones.
    allowFreeStandingZoneFaces true;
}



// Settings for the snapping.
snapControls
{
    //- Number of patch smoothing iterations before finding correspondence
    //  to surface
    nSmoothPatch 3;

    //- Relative distance for points to be attracted by surface feature point
    //  or edge. True distance is this factor times local
    //  maximum edge length.
    tolerance 1.0;

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 300;

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 5;

    //- Highly experimental and wip: number of feature edge snapping
    //  iterations. Leave out altogether to disable.
    nFeatureSnapIter 10;
}



// Settings for the layer addition.
addLayersControls
{
    // Are the thickness parameters below relative to the undistorted
    // size of the refined cell outside layer (true) or absolute sizes (false).
    relativeSizes true;

    // Per final patch (so not geometry!) the layer information
    layers
    {
        ship
        {
            nSurfaceLayers 1;
        }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.0;


    //- Wanted thickness of final added cell layer. If multiple layers
    //  is the thickness of the layer furthest away from the wall.
    //  See relativeSizes parameter.
    finalLayerThickness 0.3;

    //- Minimum thickness of cell layer. If for any reason layer
    //  cannot be above minThickness do not add layer.
    //  See relativeSizes parameter.
    minThickness 0.25;

    //- If points get not extruded do nGrow layers of connected faces that are
    //  also not grown. This helps convergence of the layer addition process
    //  close to features.
    nGrow 0;


    // Advanced settings

    //- When not to extrude surface. 0 is flat surface, 90 is when two faces
    //  make straight angle.
    featureAngle 30;

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 5;

    // Number of smoothing iterations of surface normals
    nSmoothSurfaceNormals 1;

    // Number of smoothing iterations of interior mesh movement direction
    nSmoothNormals 3;

    // Smooth layer thickness over surface patches
    nSmoothThickness 10;

    // Stop layer growth on highly warped cells
    maxFaceThicknessRatio 0.5;

    // Reduce layer growth where ratio thickness to medial
    // distance is large
    maxThicknessToMedialRatio 0.3;

    // Angle used to pick up medial axis points
    minMedianAxisAngle 90;

    // Create buffer region for new layer terminations
    nBufferCellsNoExtrude 0;


    // Overall max number of layer addition iterations. The mesher will exit
    // if it reaches this number of iterations; possibly with an illegal
    // mesh.
    nLayerIter 50;

    // Max number of iterations after which relaxed meshQuality controls
    // get used. Up to nRelaxIter it uses the settings in meshQualityControls,
    // after nRelaxIter it uses the values in meshQualityControls::relaxed.
    nRelaxedIter 20;
}



// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
meshQualityControls
{
    //- Maximum non-orthogonality allowed. Set to 180 to disable.
    maxNonOrtho 65;

    //- Max skewness allowed. Set to <0 to disable.
    maxBoundarySkewness 20;
    maxInternalSkewness 4;

    //- Max concaveness allowed. Is angle (in degrees) below which concavity
    //  is allowed. 0 is straight face, <0 would be convex face.
    //  Set to 180 to disable.
    maxConcave 80;

    //- Minimum pyramid volume. Is absolute volume of cell pyramid.
    //  Set to a sensible fraction of the smallest cell volume expected.
    //  Set to very negative number (e.g. -1E30) to disable.
    minVol 1e-13;

    //- Minimum quality of the tet formed by the face-centre
    //  and variable base point minimum decomposition triangles and
    //  the cell centre.  Set to very negative number (e.g. -1E30) to
    //  disable.
    //     <0 = inside out tet,
    //      0 = flat tet
    //      1 = regular tet
    minTetQuality 1e-30;

    //- Minimum face area. Set to <0 to disable.
    minArea -1;

    //- Minimum face twist. Set to <-1 to disable. dot product of face normal
    //- and face centre triangles normal
    minTwist 0.05;

    //- minimum normalised cell determinant
    //- 1 = hex, <= 0 = folded or flattened illegal cell
    minDeterminant 0.001;

    //- minFaceWeight (0 -> 0.5)
    minFaceWeight 0.05;

    //- minVolRatio (0 -> 1)
    minVolRatio 0.01;

    //must be >0 for Fluent compatibility
    minTriangleTwist -1;

    //- if >0 : preserve single cells with all points on the surface if the
    //  resulting volume after snapping (by approximation) is larger than
    //  minVolCollapseRatio times old volume (i.e. not collapsed to flat cell).
    //  If <0 : delete always.
    //minVolCollapseRatio 0.5;


    // Advanced

    //- Number of error distribution iterations
    nSmoothScale 4;
    //- amount to scale back displacement at error points
    errorReduction 0.75;



    // Optional : some meshing phases allow usage of relaxed rules.
    // See e.g. addLayersControls::nRelaxedIter.
    relaxed
    {
        //- Maximum non-orthogonality allowed. Set to 180 to disable.
        maxNonOrtho 75;
    }
}


// Advanced

// Flags for optional output
// 0 : only write final meshes
// 1 : write intermediate meshes
// 2 : write volScalarField with cellLevel for postprocessing
// 4 : write current intersections as .obj files
debug 0;


// Merge tolerance. Is fraction of overall bounding box of initial mesh.
// Note: the write tolerance needs to be higher than this.
mergeTolerance 1E-6;


// ************************************************************************* //
Here is the the project itself:

http://narod.ru/disk/18857088001/meshcfd.tar.gz.html
Thanks in advance.

Last edited by Svensson; July 13, 2011 at 16:31.
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Old   July 13, 2011, 08:30
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Alexander
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And in the case of 2D profile of ship, mesh is smooth:

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Old   July 14, 2011, 05:48
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stephane sanchi
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Pictures of the mesh done with sHM.

Regards,

Stephane.
Attached Images
File Type: jpg pict1.jpg (98.3 KB, 188 views)
File Type: jpg pict2.jpg (95.7 KB, 193 views)
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Old   July 14, 2011, 07:19
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Alexander
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Please give a link if it is possible for your project =)

Or explain what parameters need to be varied to obtain a smooth boundary surface after snapping.
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Old   July 14, 2011, 07:22
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stephane sanchi
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http://www.cfse.ch/cfse/site/openfoam.php

Regards,

Stephane.
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Old   November 7, 2011, 08:04
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Tomas Hofer
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Hey guys.

I ran into the same trouble. Your posts look great! But, how do you implement the snapEdge into the sHM algorithm...? I understood, you run sHM and than the snapEdge, but there must be some trick... I must be missing something, since the result is very poor. Could you give me a hint?
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Old   November 7, 2011, 09:36
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Tibor Nyers
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Hi Thomas,

I'm not an expert in sHM, but here's what I found about the issue:
* snapEdge wiki page - never tried, maybe not compatible with OF2.0
* feature edge refinement can be enabled in snappyHexMeshDict, looks similar to snapEdge

Check out the tutorials - and the Allrun scripts:
Code:
find $FOAM_TUTORIALS -name snappyHexMeshDict | xargs grep 'featureEdge' -sl
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Old   November 7, 2011, 10:06
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Hi Tibor.

Thanks a lot for your reply. I have seen the new feature in sHM and I have installed this new version of OF. I have modified my sHMdict according to the tutorial for flange (as recommended in the OF web) but I am not lucky with my geometry... Somehow, the edges are still not sharp... It is better though. As for the featureEdge I have not find much till now. There is a note in the sHMdict files of some tutorials about featureEdgeMesh but that is all... Do you have any idea about that? It says it is going to read it from triSurface for now, but that is it
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Old   November 7, 2011, 10:33
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Tibor Nyers
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Hi,

Have you run surfaceFeatureExtract before snappyHexMesh? This creates the info sMH needs for feature edge refinement - the eMesh file.
Code:
surfaceFeatureExtract -includedAngle 150 constant/triSurface/myTriSurf.stl myTriSurf
This outputs the file 'myTriSurf.eMesh'. Check out the Allrun script for sMH tuts!

Unfortunatelly I couldn't generate for my case a perfectly conforming mesh as you can see in the attached picture below, it has some wired elements unfortunately.
Attached Images
File Type: png heatsink.png (12.0 KB, 86 views)
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Old   November 7, 2011, 14:46
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Hi Thomas,

You can use the snapEdge tool until OF-1.7.

OF-2.0.x has introduced a feature-edge handling into snappyHexMesh.

http://www.openfoam.com/news/snappyH...ature-edge.php

Try to make the flange tutorial.

Best regards,

Stéphane.
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Old   November 8, 2011, 05:44
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Thanks a lot both of you! Shame on me, I have not checked the Allrun of the flange tutorial. However, even if I do everything with respect to this tutorial and set the parameters in sHMdict accordingly I get very poor results, though the geometry itself is not that complex. The difference is in size and that is what matters too, I guess. If I make the original blockMesh sufficiently fine and set the refinementSurface levels high enough, most of the edges are snapped. Is it possible that this can be adjusted by some parameter? I wonder what is the precise definition of the "tolerance" in snapControls. This might be a stupid question... I am still fresh in OF.

Last edited by trautenberg; November 8, 2011 at 07:54.
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Old   January 18, 2012, 10:13
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Aleksander Dubas
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Quote:
Originally Posted by trautenberg View Post
Thanks a lot both of you! Shame on me, I have not checked the Allrun of the flange tutorial. However, even if I do everything with respect to this tutorial and set the parameters in sHMdict accordingly I get very poor results, though the geometry itself is not that complex. The difference is in size and that is what matters too, I guess. If I make the original blockMesh sufficiently fine and set the refinementSurface levels high enough, most of the edges are snapped. Is it possible that this can be adjusted by some parameter? I wonder what is the precise definition of the "tolerance" in snapControls. This might be a stupid question... I am still fresh in OF.
Hi, I am having similar problems with snappyHexMesh not snapping to faces correctly and I have found that adjusting the tolerance parameter does help with this and a higher tolerance (rather than lower) appears to get a better snapping, although too high creates some interesting anomalies.

The "tolerance" in snapControls is apparently defined by:
Ratio of distance for points to be attracted by surface feature point or edge, to local maximum edge length

Hope this helps,

Aleksander
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