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SnappyHex generates uneven mesh

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Old   January 12, 2016, 19:40
Default SnappyHex generates uneven mesh
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Jade Chantrell
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Hi Foamers,

I am trying to implement SnappyHexMesh over a farfield for an atmospheric boundary condition. It consists of a massive discretised block, the layers help to bring the yPlus value down.

It is meshing really ugly though (refer to photo of slice through the y-plane). In my mind there is no reason why it should be doing this. Surely the mesh should be even through the entire block.

Code:
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}

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

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


// 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
{
    refinementBox1
    {
        type searchableBox;
        min ( -10.0 -10.0 -10.0);
        max ( 10.0 10.0 -9.5);
    } 
    
   refinementBox2
    {
        type searchableBox;
        min ( -10.0 -10.0 -10.0);
        max ( 10.0 10.0 -9.9);
    }
    
    refinementBox3
    {
        type searchableBox;
        min ( -10.0 -10.0 -10.0);
        max ( 10.0 10.0 -9.95);
    }
};



// 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 100000;

    // 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 2000000;

    // 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;

    // 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 3;



    // 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
    (
    );



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

    refinementSurfaces
    {
	
	ground
	{
	level (5 5);
        

	
        }
    }

    resolveFeatureAngle 30;


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

    refinementRegions
    {

		refinementBox1
        {
            mode inside;
            levels ((2 2));
		}
		
		refinementBox2
        {
            mode inside;
            levels ((2 2));
		}
        
        refinementBox3
        {
            mode inside;
            levels ((3 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.
    locationInMesh (9 9 9);


    // 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 4.0;

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 30;

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



// 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
    {
		//helio
			//{nSurfaceLayers 4;}
	
		ground
			{nSurfaceLayers 7;}
		
    }

    // Expansion factor for layer mesh
    expansionRatio 1.2;


    // 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.
    // See relativeSizes parameter.
    finalLayerThickness 0.9;

    // Minimum thickness of cell layer. If for any reason layer
    // cannot be above minThickness do not add layer.
    // See relativeSizes parameter.
    minThickness 0.000001;

    // 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;

    // 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
    // Note: changed(corrected) w.r.t 16x! 90 degrees corresponds to 130 in 16x.
    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

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


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

Any help would be much appreciated.
Attached Images
File Type: jpg Farfield.JPG (165.0 KB, 39 views)
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Old   January 18, 2016, 10:46
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DanielP
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Hello Jade,

If you haven't yet, try running your case in steps and run checkMesh to validate the mesh quality. Try this:

1.Set snap and addLayers to false:

castellatedMesh true;
snap false:
addLayers false;

2. run checkMesh
[case]$ checkMesh -allTopology -allGeometry

3. Verify the checkMesh report.

4. Rerun the snap step by setting it to true.

6. Repeat step 2 and 3.

7. Set the addLayers to true and repeat step 2 and 3.

Daniel
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Old   February 11, 2016, 00:11
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Jade Chantrell
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Hi Daniel, thanks for your quick reply. I've been juggling a few things so I didn't get around to mesh refinement until a couple of days ago.

I did all what you suggested and all of my checkMesh results seem fine with "Mesh is OK" at the end. There are no failed mesh checks with snap controls on and with layers on. I even added a complex geometry into the farfield and it did not fail any mesh checks.

Could it be that I am making a slice in Paraview (/Foam) that it is exhibiting these features? As in, I am slicing through cell bricks?
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Old   February 16, 2016, 15:31
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DanielP
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Hello Jade,

Yes, that is a problem with Paraview. The rendering is something just plain weird.... If your numerical simulation works, then your mesh is ok. Not sure the version of Paraview that you are using, but when you cut your geometry (cut or clip) something the elements are not aligned with the cutting plain and Paraview is unable to represent the volume in three dimensions.

Also, I recommend that you try out this free mesh software. They have a free version.

http://cfmesh.com/#

Good luck,

Daniel
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Old   February 17, 2016, 05:56
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Quote:
Originally Posted by Chanterz View Post
Hi Daniel, thanks for your quick reply. I've been juggling a few things so I didn't get around to mesh refinement until a couple of days ago.

I did all what you suggested and all of my checkMesh results seem fine with "Mesh is OK" at the end. There are no failed mesh checks with snap controls on and with layers on. I even added a complex geometry into the farfield and it did not fail any mesh checks.

Could it be that I am making a slice in Paraview (/Foam) that it is exhibiting these features? As in, I am slicing through cell bricks?
When you open the case in Paraview, deactivate "Decompose Polyhedra", and when you make a clip activate "crinkle clip" to get a faithful representation of your cells.
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