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[snappyHexMesh] Propeller mesh not smooth

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Old   May 16, 2014, 03:26
Default Propeller mesh not smooth
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Nor Tanapura
Join Date: May 2014
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Hi,

I am hoping someone could help me with this problem. I'm new to snappyHexMesh and I've seen that I'm not the only one in this forum that has this problem. I've tried the solutions presented in other threads but it doesn't seem to work.

I'm trying to mesh a propeller according to the Propeller Tutorial but instead of a marine propeller, I'm using an aircraft propeller (RC). The propeller .stl file has the highest quality that could be set in solidworks. I've attached some figures of the .stl geometry, the mesh I got from snappy, as well as, blockMeshDict, snappyMeshDict, and a checkMesh.log.

Some parameters that seem to affect the smoothness of the mesh are the nSmoothPatch and minTetQuality. Currently I have the nSmoothPatch = 10 and minTetQuality = -1. Is it reasonable to increase nSmoothPatch to perhaps 20 or even 30? I don't really have a feeling for these numbers.

Any assistance would be appreciated.

Nor

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.3.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}

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

convertToMeters 1;

vertices
(
    //(-10 -20 -10)
    //( 10 -20 -10)
    //( 10  20 -10)
    //(-10  20 -10)
    //(-10 -20  10)
    //( 10 -20  10)
    //( 10  20  10)
    //(-10  20  10)
    (0  0 0) //vertex 0
    (8  0 0) //vertex 1
    (8 41 0) //vertex 2
    (0 41 0) //vertex 3
    (0  0 8) //vertex 4
    (8  0 8) //vertex 5
    (8 41 8) //vertex 6
    (0 41 8) //vertex 7
);

blocks
(
    hex (0 1 2 3 4 5 6 7) (100 200 100) simpleGrading (1 1 1)
);

edges
(
);

boundary
(
    walls
    {
        type wall;
        faces
        (
            (2 6 5 1)
            (0 3 2 1)
            (0 4 7 3)
            (4 5 6 7)
        );
    }
    inlet
    {
        type patch;
        faces
        (
            (3 7 6 2)
        );
    }
    outlet
    {
        type patch;
        faces
        (
            (1 5 4 0)
        );
    }
);

// ************************************************************************* //
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.3.0                                 |
|   \\  /    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       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
{
    innerCylinder.stl
    {
        type        triSurfaceMesh;
        name        innerCylinder;
        regions
        {
            innerCylinder
            {
                 name       innerCylinder;

            }
	    
        }
    }
    innerCylinderSmall.stl
    {
        type        triSurfaceMesh;
        name        innerCylinderSmall;
        regions
        {
            innerCylinderSmall
            {
                 name       innerCylinderSmall;

            }
        }
    }
    
    outerCylinder.stl
    {
        type        triSurfaceMesh;
        name        outerCylinder;
        regions
        {
            outerCylinder
            {
                 name       outerCylinder;

            }
        }
    }
    
    propellerACP.stl
    {
        type        triSurfaceMesh;
        name        propellerACP;
        regions
        {
            propellerACP
            {
                 name       propellerACP;

            }
        }
    }

   
 };



// 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 2500000;

    // 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 50000000;

    // 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; // default  = 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 5;



    // 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        "innerCylinderSmall.eMesh";
            level       4;
        }
        {
            file        "outerCylinder.eMesh";
            level       0;
        }
        {
            file        "propellerACP.eMesh";
            level       10; //default was 5;
        }

    );



    // 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
    {
        innerCylinderSmall
        {
            level       (5 5);

            faceType    boundary;
            cellZone    innerCylinderSmall;
            faceZone    innerCylinderSmall;
            cellZoneInside  inside;
        }
        outerCylinder
        {
            level       (0 0);
        }
        propellerACP
        {
            level       (5 10);
        }
       
    }

    // Resolve sharp angles
    resolveFeatureAngle 5;


    // 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
    {
        innerCylinder
        {
            mode        inside;
            levels      ((1E15 3));
        }
        innerCylinderSmall
        {
            mode        inside;
            levels      ((1E15 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 (0.01 -0.5 0.01);
    locationInMesh (3.9 20 3.9);

    // 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 false;
}



// Settings for the snapping.
snapControls
{
    //- Number of patch smoothing iterations before finding correspondence
    //  to surface
    nSmoothPatch 10; // default 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 1.0; // 1.0;

    //- Number of mesh displacement relaxation iterations.
    nSolveIter 300;

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 10;

    // Feature snapping

        // Number of feature edge snapping iterations.
        // Leave out altogether to disable.
        nFeatureSnapIter 25; //default was 10

        // Detect (geometric only) features by sampling the surface
        // (default=false).
        implicitFeatureSnap true;

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

        // Detect features between multiple surfaces
        // (only for explicitFeatureSnap, default = false)
        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
    {
    }

    // Expansion factor for layer mesh
    expansionRatio 1.1;

    // 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.
    // is the thickness of the layer furthest away from the wall.
    // See relativeSizes parameter.
    finalLayerThickness 0.7;

    // 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
{
    //- Maximum non-orthogonality allowed. Set to 180 to disable.
    maxNonOrtho 65; //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; //1e-13 default value

    //- Minimum quality of the tet formed by the face-centre
    //  and variable base point minimum decomposition triangles and
    //  the cell centre. This has to be a positive number for tracking
    //  to work. Set to very negative number (e.g. -1E30) to
    //  disable.
    //     <0 = inside out tet,
    //      0 = flat tet
    //      1 = regular tet
    
    minTetQuality -1; //-1E30;

    //- 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 0.01;//0.01

    //- minimum normalised cell determinant
    //- 1 = hex, <= 0 = folded or flattened illegal cell
    minDeterminant 0.001; //0.001

    //- minFaceWeight (0 -> 0.5)
    minFaceWeight 0.01; //0.02

    //- minVolRatio (0 -> 1)
    minVolRatio 0.01; //0.01

    //must be >0 for Fluent compatibility
    minTriangleTwist -1;


    // 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;
    }
}



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


// ************************************************************************* //
Attached Images
File Type: jpg PropSTLSurfaceView.jpg (12.6 KB, 140 views)
File Type: jpg PropSTLWireframeView.jpg (24.1 KB, 130 views)
File Type: jpg PropSurfaceMesh1.jpg (23.9 KB, 162 views)
File Type: jpg PropSurfaceMesh2.jpg (22.9 KB, 130 views)
Attached Files
File Type: txt checkMesh.log.txt (3.5 KB, 16 views)
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