[jvinuales]

Hi everyone,

This is my new post, I am new to OpenFOAM but I have been using commercial CFD for a while. I have been trying to learn OF for a few weeks, looking at tutorials and so on, but now trying to work on my own cases, starting with meshing.

From my research, it seems like SnappyHexMesh could handle well what I am trying to do (single-phase internal flows both isothermal and with heat transfer), and especially looking at automation of the process. However, I cant get well-defined edges/corners in some parts.

I tried to follow the recommendations I found in the forum and other sources. I am using explicit features extraction with edge refinement, and I included high snapping iterations (nFeatureSnapIter) and tolerance, but does not seem to work.

I would appreciate some advice to get me started with snappyHexMesh, any comments are welcome. From what I have seen, I think snappyHexMesh could handle what I am trying to do, so I do not want to start looking at other meshing alternatives.

Cheers,

Javier





snappyHexMeshDict:
https://drive.google.com/file/d/1sFs...ew?usp=sharing

[jvinuales]

mesh1.JPG
mesh1.JPG
mesh3.JPG

[john myce]

Hi Javier,

Try to decrease the resolveFeatureAngle of the castellated phase to either 30 or 15 degrees . It would help to capture the sharp areas.

Cheers,

[jvinuales]

Yes! that worked much better!! Thanks!!

mesh_1.jpg

Still some issues around the square profile corners though, I will try to improve these.

mesh_2.jpg

[jcoelho5]

Hi guys.

I would like to have some tips on how to capture this complex feature. Even with significant refinement, there is no way to capture this correctly.
It is a sharp acute angle with curved surface.

I added the snappyHexMeshDict and some images that may help me understand my problem. I use a blockMesh mesh as a base.

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  9
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

// Which of the steps to run
castellatedMesh false;
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
{
    volume
    {
        type triSurfaceMesh;
        file "volume.stl";
    }
    
    Inlet
    {
        type triSurfaceMesh;
        file "Inlet.stl";
    }

    Outlet_In
    {
        type triSurfaceMesh;
        file "Outlet_In.stl";
    }
    
    Outlet_2nd_Ring
    {
        type triSurfaceMesh;
        file "Outlet_2nd_Ring.stl";
    }
    
    Outlet_3rd_Ring
    {
        type triSurfaceMesh;
        file "Outlet_3rd_Ring.stl";
    }
    
    Outlet_4th_Ring
    {
        type triSurfaceMesh;
        file "Outlet_4th_Ring.stl";
    }
    
    Outlet_Out
    {
        type triSurfaceMesh;
        file "Outlet_Out.stl";
    }
    
    
    Wall
    {
        type triSurfaceMesh;
        file "Wall.stl";
    }
    
    Refin1
    {
        type triSurfaceMesh;
        file "Refin1.stl";
    }
    
    Refin2
    {
        type triSurfaceMesh;
        file "Refin2.stl";
    }
    
    
    
    
    refinementBox
    {
        type searchableBox;
        min (-43 -100 -100);
        max (-38  100 100);
    }
    refinementBox2
    {
        type searchableBox;
        min (-77 25 -52);
        max (-72 32 -45);
        //	(-74.5 28.5 -48.5)
    }
};



// 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 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 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 2;



    // Explicit feature edge refinement
    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    // Specifies a level for any cell intersected by its edges.
    // This is a featureEdgeMesh, read from constant/geometry for now.
    features
    (
        {
            file "volume.eMesh";
            level 2;
        }
        
    );



    // 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
    {
        Inlet
        {
            // Surface-wise min and max refinement level
            level (2 3);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type patch;
            }
        }
        
        Outlet_In
        {
            // Surface-wise min and max refinement level
            level (2 3);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type patch;
                inGroups (Outlets_group);
            }
        }
        Outlet_2nd_Ring
        {
            // Surface-wise min and max refinement level
            level (2 3);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type patch;
                inGroups (Outlets_group);
            }
        }
        Outlet_3rd_Ring
        {
            // Surface-wise min and max refinement level
            level (2 3);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type patch;
                inGroups (Outlets_group);
            }
        }
        Outlet_4th_Ring
        {
            // Surface-wise min and max refinement level
            level (2 3);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type patch;
                inGroups (Outlets_group);
            }
        }
        Outlet_Out
        {
            // Surface-wise min and max refinement level
            level (2 3);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type patch;
                inGroups (Outlets_group);
            }
        }
        Wall
        {
            // Surface-wise min and max refinement level
            level (2 4);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type wall;
            }
        }
        
        
        Refin1
        {
            // Surface-wise min and max refinement level
            level (5 6);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type none;
            }
        }
        
        
        Refin2
        {
            // Surface-wise min and max refinement level
            level (6 8);

            // Optional specification of patch type (default is wall). No
            // constraint types (cyclic, symmetry) etc. are allowed.
            patchInfo
            {
                type none;
            }
        }
        
    }

    // Resolve sharp angles
    resolveFeatureAngle 10;


    // 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;
            level   4;
        }
        refinementBox2
        {
            mode    inside;
            level   10;
        }
    }


    // 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 (-83 86 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 0.8;

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 15;

    //- 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 true;

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

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



// 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
    {
        /*"(lowerWall|motorBike).*"
        {
            nSurfaceLayers 1;
        }*/
    }

    // 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.
    // See relativeSizes parameter.
    finalLayerThickness 0.3;

    // 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;

    // 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 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

// 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;


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