[Snacker]

Hey everyone,

I've been tinkering around with SnappyHexMesh in OpenFOAM, attempting to add boundary layers to the outside of an airfoil and inside a hole. However, despite trying multiple approaches and tweaking various parameters, I'm still struggling to achieve the desired results.

I've experimented with different values for parameters such as expansionRatio, layers, thickness, and others, but I haven't been able to get the layer addition to work as expected.

Most of the time there is no layer addition at all.

Has anyone successfully implemented boundary layers using SnappyHexMesh for a similar application? If so, could you please share your approach or provide some insights on which parameters are crucial and how to adjust them effectively?

Additionally, if you have any tips or tricks for dealing with complex geometries like airfoils and holes, I'd greatly appreciate any guidance you can offer.

I tried this handy overview as well: [URL="https://www.cfd-online.com/Forums/openfoam-meshing/254447-snappyhexmesh-parametric-sweep-study.html"]

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  8
     \\/     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
{
    motorBike
    {
        type triSurfaceMesh;
        file "motorBike.stl";
    }

    refinementBox
    {
        type searchableBox;
        min (-0.1 -0.005 -0.2);
        max ( 0.5  0.005  0.2);
    }

// refinementBox2
//    {
//        type searchableBox;
//        min ( 0.0578 -0.005 -0.003); //0,002 - 0,0077
//        max ( 0.066  0.005  0.009);
//    }

 


};

     //    min (-1.0 -0.7 0.0);
      //   max ( 8.0  0.7 2.5);

// 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 20000000; // 10 000 000

    // 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 2000000000; // 200 000 000

    // 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 6; // 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 "motorBike.eMesh";
            level 8; // 6
        }
    );



    // 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
    {
        motorBike
        {
            // Surface-wise min and max refinement level
            level (8 8); // 6 6

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

    // 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
    {
        refinementBox
        {
            mode inside;
            levels ((1E15 3)); // 4
        }

	//refinementBox2
        //{
        //    mode inside;
        //   levels ((1E15 8)); // 4
        //}
	
	//motorBike
        //{
        //    mode inside;
        //    levels ((1E15 7)); // 4
        //}




    }


    // 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 (-0.2 0.0 0.0);


    // 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 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 true;

    // Per final patch (so not geometry!) the layer information  //14
    layers  
    {
        "(motorBike).*"
        {
            nSurfaceLayers 8;   
        }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.0; //1.09

    // 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 0.47; //0.3 mot // 0.46 naca

    // 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 0.1; //0.15

    // 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 160;

    // 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;

	// Add layers in multiple passes - gives layers a chance to rotate
	nOuterIter      4;

}



// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
meshQualityControls
{
    #include "meshQualityDict"
}


// 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;


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

Looking forward to hearing your suggestions and experiences!

Thanks in advance.

Here is a link to pictures: https://imgur.com/a/y5EQz0O

[Alczem]

Hey!


Adding layers is always tricky with snappy



I find that minThickness has to be set relatively low, don't use nRelaxedIter, and if you are using the ESI version, set nOuterIter to the number of layers you want, it really helps!


That being said, here is a subdict that I used recently with good coverage. Most of the settings come from someone else that tested snappy extensively:

https://github.com/tkeskita/snappyLayerTests/tree/main

addLayersControls
{
    layers
    {
        "walls.*"
        {
            nSurfaceLayers          3;
            expansionRatio          1.0;
            thickness                 0.5;
            minThickness            0.1;
        }
        
    }

    // Layer sizing
    relativeSizes   true;
    expansionRatio  1.2; // 1.3;
    finalLayerThickness 0.3;
    minThickness    0.01; // 0.2; // 0.001;
    nGrow           0;

    // Mesh dependencies
    featureAngle    130;
    mergePatchFacesAngle 45; // OpenFOAM.com option
    layerTerminationAngle -180; // OpenFOAM.com option
    maxFaceThicknessRatio 0.5;
    disableWallEdges false;

    // Mesh displacement iterations
    nSmoothSurfaceNormals 8; // 12; // 3 * max number of layers seems good
    nSmoothThickness 2; // 4; // 10;
    nSmoothNormals  0;
    nSmoothDisplacement 12; // 100;
    nMedialAxisIter 1000;

    // Medial axis analysis
    minMedialAxisAngle 90;
    maxThicknessToMedialRatio 0.5; // 0.9;
    slipFeatureAngle 30; // 90; // 30;
    nRelaxIter      4; // 1;

    // OpenFOAM.com displacement motion solver
    // meshShrinker displacementMotionSolver;
    // solver displacementLaplacian;
    // displacementLaplacianCoeffs { diffusivity quadratic inverseDistance ("wall.*"); }

    // Mesh shrinking overall settings
    nBufferCellsNoExtrude 0;
    nLayerIter      8; // 50;
    //nRelaxedIter    0;
    nOuterIter      3; // 2; // OpenFOAM.com option, half of max number of layers seems good
    additionalReporting true;
    
}

The snap settings are also quite efficient, but you will still need to experiment until you are satisfied. From your pictures, I am pretty sure it should be achievable good luck!