[Arzed23]

Hi all,

I've been trying to achieve a boundary layer mesh with first cell height of 5e-6 to give a y+ of 1. However, whatever I seem to do, no boundary layer mesh will form at all. Attached below is my snappyHexMeshDict and resulting mesh. Would anyone be able to steer me in the right direction? Cheers guys!

snappyHexMeshDict

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  3.0.1                                 |
|   \\  /    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       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
{
    final_validation.stl
    {
        type triSurfaceMesh;
        name final_validation;
    }

    /*refinementBox_1
    {
        type searchableBox;
        min (-0.09 -0.15 0.0); //1 cell to the left
        max (0.7 0.12 0.9144); //twice the chord length to the right
    }

    refinementBox_2
    {
        type searchableBox;
        min (-0.06 -0.12 0.0); //1 cell to the left
        max ( 0.65 0.09 0.9144); //twice the chord length to the right
    }

    refinementBox_3
    {
        type searchableBox;
        min (-0.03 -0.09 0.0); //1 cell to the left
        max ( 0.6 0.06 0.9144); //twice the chord length to the right
    }*/

    /*refinementBox_1
    {
        type searchableBox;
        min (-0.4572 -0.3048 0.0); //1 cell to the left
        max (0.762 0.3048 0.6096); //twice the chord length to the right
    }*/

    /*refinementBox_2
    {
        type searchableBox;
        min (-0.36 -0.24 0.0); //1 cell to the left
        max ( 0.74 0.24 0.6096); //twice the chord length to the right
    }*/

    /*refinementBox_3
    {
        type searchableBox;
        min (-0.24 -0.18 0.0); //1 cell to the left
        max ( 0.7 0.18 0.6096); //twice the chord length to the right
    }*/
};



// Settings for the castellatedMesph 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 10000000;

    // 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 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
    (
	{file "final_validation.eMesh"; level 10;}
    );



    // 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
    {
        final_validation
        {
            // Surface-wise min and max refinement level
            level (1 10);

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

    // Resolve sharp angles
    resolveFeatureAngle 80; //was 30 by default


    // 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_1
        {
            mode inside;
            levels ((1.0 1));
        }

        refinementBox_2
        {
            mode inside;
            levels ((1.0 2));
        }
	
        refinementBox_3
        {
            mode inside;
            levels ((1.0 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 (-0.001 -0.001 0.001);


    // 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 2.0;

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

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

    // 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 false;

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

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



// 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 false;

    // Per final patch (so not geometry!) the layer information
    layers
    {
	    final_validation
	    {
		nSurfaceLayers 2;
	    }
    }

    // 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 0.000006; //5 layers: 0.00015 * 1.2^4 = 0.000311
				  //10 layers: 1.2^9 = 0.000774
				  //12 layers: 1.2^11 = 0.001115
				  //13 layers: 1.2^12 = 0.0013375
				  //14 layers: 1.2^13 = 0.001605
				  //15 layers: 1.2^14 = 0.00193
				  //20 layers: 1.2^19 = 0.00479

    // 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 0.000005;

    // 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 0;

    // 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 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 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;
}



// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
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;


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

close up of mesh

http://imgur.com/Wal70CZ

[louisgag]

Hi,
I'm having the same problem, did you find a solution?
Regards,
-Louis

[rahulshivanathri]

Did you find an solution?I am having the same issue

[louisgag]

you could try this dirty trick in meshQualityControls:

minTetQuality   -1e+30;

[rahulshivanathri]

I am looking have a boundary layer for Y+ = 1 , can you suggest any add layer controls as well?

[klausb]

This (y+ <= 1 boundary layers) is a weak point in SHM. Many people have spent a lot of time trying to do, what you are trying to do.

You could try create/refine your boundary mesh based on thicker initial boundary layers created with SHM using RefineMesh see: https://openfoamwiki.net/index.php/RefineMesh

Or, use cfMesh instead of SHM.

Or, use EnGrid see post: Primsatic boundary layer creation fixed

Please post your progress/results as many people run into this problem.

[Fouch]

Hi all,


On my side, I totally give up with SHM. I never find a way to reach a correct Y+.


Now I use :
- Salome with tetrahedron and layers generated with SMESH (integrated NetGen of Salome).
- Or Tetrahedron with Salome but without layers then I export in UNV format and use ideasUnvToFoam -> polyDualMesh -> combinePatchFaces -> generateBoundaryLayers. Thanks to this thread : polyDualMesh + Layers + cfMesh/improveMeshQuality = usable polyMeshes witH BLayer



With the second solution (my favorite), I get polyhedral mesh with layers and a correct quality.