[vava10]

Hey,

I am trying to generate layers around my geometry. I had a lot of issues in the beginning but I solved a lot, but yet I am not getting layering over the entire geometry. I have tried changing each main parameters in the snappyHexMesh and generated mesh and compared them. I have been at it for a few days now. I have asked it before in this forum, but I have improved the mesh a lot when compared to before(made the layers uniform, at places where the mesh are generated). I just am unable to create the layers at the bow and stern of the ship. Please help

This is my snappyHexMeshDict

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2012                                 |
|   \\  /    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       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
{
    kayak.stl
    {
        type triSurfaceMesh;
        name hull;

        patchInfo
        {
            type wall;
        }
    }
    refinementBox
	{
		type searchableBox;
		min (-0.8 0 -0.6);
		max (5.8 0.41 0.6);
		
	}
}


// 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 1000000;

    // 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 8000000;

    // 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 5; //3 means gradual transition



    // 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 "kayak.eMesh";
             level 4;
         }
    );



    // 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
    {
        hull
        {
            // Surface-wise min and max refinement level
            level (4 4);
        }
    }

    resolveFeatureAngle 30; //resolve sharp angles //default 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.
    locationInMesh (-0.7 0 0); // to decide which side of mesh to keep


    // 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;//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;
    tolerance 0.85;

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

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

    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
    {
        hull
        {
            nSurfaceLayers 5;

        }
    }

    // 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 0.3; //thickness of the layer which is far away from the boundary

    // Minimum thickness of cell layer. If for any reason layer
    // cannot be above minThickness do not add layer.
    // See relativeSizes parameter.
    minThickness 0.00025; // none of the layers can be less than this

    // 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 1.7.x! (didn't do anything in 1.7.x)
    nGrow 0;


    // Advanced settings

    // When not to extrude surface. 0 is flat surface, 90 is when two faces
    // are perpendicular
    featureAngle 160; //80
	
	mergePatchFacesAngle    45;

    // 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 30;

    // Stop layer growth on highly warped cells
    maxFaceThicknessRatio 0.1;

    // 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 1.7.x! 90 degrees corresponds to 130
    // in 1.7.x.
    minMedialAxisAngle 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
{
    #include "meshQualityDict"
}


// 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;


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

mesh quality

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2012                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      meshQualityDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

//- Maximum non-orthogonality allowed. Set to 180 to disable.
maxNonOrtho 70;

//- 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;
}


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

I tried increasing the featureEdge and surfaceFeature refinement to 5 but still had regions where layers are not created and regions where layers brake.
My best guess is that it is caused due to high non-orthogonal cell formed as a result of snapping. I tried changing the parameters within the snapping and the values in the above code is the best I could come up with.

If you need further information please feel free to ask

Thanks in advance!!

[HPE]

may the new runtime geometry selection models help? https://t.co/Cif9YjOgW5?amp=1

otherwise, adding layers is a difficult task requiring settings unique to the case at hand (i'm afraid)- you need to try to find out the best possible settings.

Having said that, first, I would turn off (or relax extremely) all the mesh quality metrics and run the layer addition step for a single cell.

Lastly, please no offense: "Layering Issue!!!! Help!!!!" is a very low-quality problem description/title. Please see one of the links below about MWE to improve the quality of questions you may ask in the future.

Many thanks.

[vava10]

Dear HPE,

thank you for the quick reply. I will try what you suggested and will check out the link you gave.


PS: no offence taken. I will pay attention to the tile from now own. Thank you for the suggestion

Kind regards
vava10

[vava10]

Hey HPE,

I am working with OpenFOAM version 2006 and I tried relaxing mesh quality and then run it and I still have the same issue
I am really stuck with this thesis work. Can you help?

Sincerely
vava10

[AtoHM]

Unfortunately, as HPE pointed out already, adding layers can be quite annoying with snappyHexMesh.


Another thing you could try is to use a recent ESI OpenFoam-Release. There is an option to use a non-default meshShrinker: displacementMotionSolver. You will find how to use it in the documentation.

For me, it sometimes gave better results than the default one, but its not "overall better". Its worth a try, though.