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[snappyHexMesh] Bad meshing result on corrugated metal sheet

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Old   August 18, 2016, 06:31
Default Bad meshing result on corrugated metal sheet
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Hi guys.

I've spent some hours on my current meshing problem but as nothing seems to solve the problem I'm in need of your help.
My target is to mesh the surface of a corrugated metal sheet as can be found in my STL-File below.

I can get a perfectly smooth surface when using relatively few large cells (~500.000). However, the following CFD will need at least ~3.000.000 cells to run smooth. If I try to increase the cell amount in the BlockMesh I cant get a smooth surface anymore.

I have attached my BlockMeshDict, SnappyHexMeshDict and the STL File and a screenshot of my resulting mesh.

My current version is 2.4.0 .


BlockMeshDict:
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.0.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

convertToMeters 1; // does nothing in this case, 0.1 would shrink it by 10x

vertices   // (x y z)   
(
    (-0.016 -0.0155 -0.0125)       //0
    (0.016 -0.0155 -0.0125)     //1
    (0.016 0.0365 -0.0125)     //2
    (-0.016 0.0365 -0.0125)   //3
    (-0.016 -0.0155 0.0125)       //4 
    (0.016 -0.0155 0.0125)     //5
    (0.016 0.0365 0.0125)     //6
    (-0.016 0.0365 0.0125)   //7
);

blocks          
(
    hex (0 1 2 3 4 5 6 7) (128 208 100) simpleGrading (1 1 1)
);

edges           
(
);

boundary         
(               
       top
    {type patch;
    faces
    (
        (2 3 7 6)
        
    );
    }
    sideL
    {type patch;
    faces
    (

        (0 4 7 3)
        
    );
    }
    sideR
    {type patch;
    faces
    (
        (1 2 6 5)

        
    );
    }
    bottom
    {type patch;
    faces
    (
        (0 1 5 4)
        
    );
    }
    front
    {type patch;
    faces
    (
        (0 3 2 1)
        
    );
    }
    back
    {type patch;
    faces
    (
        (4 5 6 7)
        
    );
    }

);

mergePatchPairs
(
);

// ************************************************************************* //
SnappyHexMeshDict:

NOTE: I'm only having problems with the "packing.stl", the other stl's just form a surrounding Box which is necessary for post processing.
Settings that I've already tried include the following:

nCellsBetweenLevels: 1; 2; 3
refinement level of packing.stl : 1 ; 2
nSmoothPatch: 1;5
tolerance : 0.5; 1; 2; 5; 8; 12; 15; 50
nSolveIter: 50;300
both implicit / explicit Feature Snap
nFaceSplitInterval: -1; 3;


Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.3.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     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. 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
{
 
packing.stl
    {
        type triSurfaceMesh;
        name packing;
    }

    xpos.stl
    {
        type triSurfaceMesh;
        name xpos;
    }
    xneg.stl
    {
        type triSurfaceMesh;
        name xneg;
    }
    zpos.stl
    {
        type triSurfaceMesh;
        name zpos;
    }
    zneg.stl
    {
        type triSurfaceMesh;
        name zneg;
    }
    yneg.stl
    {
        type triSurfaceMesh;
        name yneg;
    }
     
};



// Settings for the castellatedMesh generation.
castellatedMeshControls
{

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

    // If local number of cells is >= maxLocalCells on any processor
    // switches from from refinement followed by balancing
    // (current method) to (weighted) balancing before refinement.
    maxLocalCells 100000000;

    // 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 200000000;

    // 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 10;

    // Allow a certain level of imbalance during refining
    // (since balancing is quite expensive)
    // Expressed as fraction of perfect balance (= overall number of cells /
    // nProcs). 0=balance always.
    maxLoadUnbalance 0.10;


    // 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 "packing.eMesh";
            level 2;
        }

        {
            file "xpos.eMesh";
            level 0;
        }
        {
            file "xneg.eMesh";
            level 0;
        }
        {
            file "zpos.eMesh";
            level 0;
        }
        {
            file "zneg.eMesh";
            level 0;
        }
        {
            file "yneg.eMesh";
            level 0;
        }
                
    );



    // 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
    {
       packing
        {
            // Surface-wise min and max refinement level
            level (1 2);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type wall;
            }
        }

    xpos 
        {
            // Surface-wise min and max refinement level
            level (0 0);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type patch;
            }
        }
    xneg 
        {
            // Surface-wise min and max refinement level
            level (0 0);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type patch;
            }
        }
    zpos 
        {
            // Surface-wise min and max refinement level
            level (0 0);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type patch;
            }
        }
    zneg
        {
            // Surface-wise min and max refinement level
            level (0 0);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type patch;
            }
        }
    yneg 
        {
            // Surface-wise min and max refinement level
            level (0 0);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type patch;
            }
        }
        
    

    }

    // Resolve sharp angles
    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
    {


    }


    // 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.
    locationInMesh (0 0.0175 0);


    // 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 false;
}



// Settings for the snapping.
snapControls
{
    //- Number of patch smoothing iterations before finding correspondence
    //  to surface
    nSmoothPatch 5;

    // Number of smoothing of internal points on refinement interfaces 
    nSmoothInternal     $nSmoothPatch; 

    //- Relative distance for points to be attracted by surface feature point
    //  or edge. True distance is this factor times local
    //  maximum edge length.
    tolerance 5;

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 50;

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 8;

    // Feature snapping

        //- Number of feature edge snapping iterations.
        //  Leave out altogether to disable.
        nFeatureSnapIter 10;

        //- Detect (geometric only) features by sampling the surface
        //  (default=false).
        implicitFeatureSnap true;

        //- Use castellatedMeshControls::features (default = true)
        explicitFeatureSnap false;

        //- Detect points on multiple surfaces (only for explicitFeatureSnap)
        multiRegionFeatureSnap true;

    // When to run face splitting (never at first iteration, always 
    // at last iteration). Is interval. Default -1 (disabled) 
    // Recommendation: set to half the number of feature snap iterations 

    nFaceSplitInterval  3;
}



// 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
    {
        packing       
        {
            nSurfaceLayers 3;
        }
    }

    // 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.
    // Relative to undistorted size of cell outside layer.
    // is the thickness of the layer furthest away from the wall.
    // See relativeSizes parameter.
    finalLayerThickness 1;

    // Minimum thickness of cell layer. If for any reason layer
    // cannot be above minThickness do not add layer.
    // Relative to undistorted size of cell outside layer.
    minThickness 0.5;

    // 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.
    // Note: changed(corrected) w.r.t 17x! (didn't do anything in 17x)
    nGrow 0;

    // Advanced settings

    // When not to extrude surface. 0 is flat surface, 90 is when two faces
    // are perpendicular
    featureAngle 60;

    // At non-patched sides allow mesh to slip if extrusion direction makes
    // angle larger than slipFeatureAngle.
    slipFeatureAngle 30;

    // Maximum number of snapping relaxation iterations. Should stop
    // before upon reaching a correct mesh.
    nRelaxIter 3;

    // Number of smoothing iterations of surface normals
    nSmoothSurfaceNormals 10;

    // 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
    // Note: changed(corrected) w.r.t 17x! 90 degrees corresponds to 130 in 17x.
    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;

}



meshQualityControls
{
    #include "meshQualityDict"


    // Advanced

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


// Advanced

// Write flags
writeFlags
(
    scalarLevels
    layerSets
    layerFields     // write volScalarField for layer coverage
);


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


// ************************************************************************* //;
STL-File:
https://www.dropbox.com/s/f6qlrrs6va...cking.stl?dl=0


Results:
See "BadMesh" in Attachments.
The regions with larger cells look perfectly fine to me. What bothers me are the parallel "stripes" of smaller cells that are present despite the fact of the stl's surface being smooth.


Thanks for your help!
Attached Images
File Type: jpg BadMesh.jpg (189.6 KB, 100 views)

Last edited by UebertreibeR; August 18, 2016 at 07:41.
UebertreibeR is offline   Reply With Quote

Old   August 19, 2016, 05:55
Default
  #2
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Pascal Balz
Join Date: Feb 2015
Location: Germany
Posts: 44
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Hi,

this looks like too many feature edges were detected.
You should check your feature edge mesh for the packing geometry (packing_edgeMesh.obj), if there are edges on the flat surface try to decrease the feature detection angle in your surfaceFeatureExtractDict.

Additionally or if the above doesn't help you could try to change the surface refinement value in your snappyHexMeshDict to the following:
Code:
packing
        {
            level (1 1);
            patchInfo
            {
                type wall;
            }
        }
This generally avoids level 2 refinements on surfaces with high curvature so that only your feature edges should have level 2 cells and the metal sheet should be completely level 1.
__________________
Regards,
Pascal
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