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Old   May 11, 2017, 09:52
Default Help with snappyHexMesh: impeller
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Hello,

I designed this impeller in order to be as simple as possible but I am having so much trouble meshing it I'm worried what I'll do with complicated ones...

After countless experiments with snappyHexMesh settings I managed to get a good enough surface fit and mesh works with AMI but the simulation fails because there is one edge that the mesh can't snap to. A case without turbulence modelling somehow works, but with k-Epsilon the epsilon goes berserk.

I know the STL are OK because I made a 2D mesh on a STEP model first, using Salome and Netgen 2D. I got a set of watertight surfaces with very little deflection from the original model...

Here is my snappyHexMeshDict:
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.1                                   |
|   \\  /    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       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
{
    impeller-inlet-ami.stl
    {
        type triSurfaceMesh;
        name impeller_inlet_ami;
    }
    impeller-walls.stl
    {
        type triSurfaceMesh;
        name impeller_walls;
    }
    impeller-outlet-ami.stl
    {
        type triSurfaceMesh;
        name impeller_outlet_ami;
    }

    impellerZone
    {
    type    searchableSphere;
    centre (0 0 0);
    radius 0.05;
    
    }

    //- Refine a bit extra around the small centre hole
    //refineHole
    //{
    //    type searchableSphere;
    //    centre (0 0 -0.012);
    //    radius 0.003;
    //}
};



// 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 5000000;

    // 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 15000000;

    // 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;

    // Number of buffer layers between different levels.
    // 1 means normal 2:1 refinement restriction, larger means slower
    // refinement.
    nCellsBetweenLevels 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 "impeller-inlet-ami.extendedFeatureEdgeMesh";
            level 0;
        }
        {
            file "impeller-walls.extendedFeatureEdgeMesh";
            level 1;
        }
        {
            file "impeller-outlet-ami.extendedFeatureEdgeMesh";
            level 0;
        }
    );



    // 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
    {
        impeller_inlet_ami
        {
            // Surface-wise min and max refinement level
            level (0 1);
            patchInfo { type patch; }
        }
        impeller_walls
        {
            // Surface-wise min and max refinement level
            level (1 2);
            patchInfo { type wall; }
        }
        impeller_outlet_ami
        {
            // Surface-wise min and max refinement level
            level (0 1);
            patchInfo { type patch; }
        }
        impellerZone
        {
            level (0 0);
           cellZone impeller;
            faceZone impeller;
            cellZoneInside inside;
        }
    }

    resolveFeatureAngle 1;


    // 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
    {
        //refineHole
        //{
        //    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.
    // This is an outside point locationInMesh (-0.033 -0.033 0.0033);
    locationInMesh (-0.02 0 0); // Inside point

    // 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 5;

    //- 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 3;

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 3;

    // Feature snapping

    //- Number of feature edge snapping iterations.
    //  Leave out altogether to disable.
    nFeatureSnapIter 30;

    //- Detect (geometric) features by sampling the surface
    implicitFeatureSnap false;

    //- Use castellatedMeshControls::features
    explicitFeatureSnap true;

    //- 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
    {
        impeller_walls
        {
            nSurfaceLayers 3;
        }
    }

    // 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 0.5;

    // Minimum thickness of cell layer. If for any reason layer
    // cannot be above minThickness do not add layer.
    // See relativeSizes parameter.
    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.
    nGrow 0;


    // Advanced settings

    // When not to extrude surface. 0 is flat surface, 90 is when two faces
    // are perpendicular
    featureAngle 180;

    // 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 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
    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;

    // 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"

    // 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 180;
    }

    // Advanced

    //- Number of error distribution iterations
    nSmoothScale 4;
    //- Amount to scale back displacement at error points
    errorReduction 0.75;
}


// Advanced

// Write flags
writeFlags
(
    scalarLevels    // write volScalarField with cellLevel for postprocessing
    layerSets       // write cellSets, faceSets of faces in layer
    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-7;


// ************************************************************************* //
I guess I'm missing something from all this because I don't think this is a very complicated geometry and I'm having too much trouble with it.

I have searched all over this forum and the internet and I can't get a good mesh. I haven't even started with adding layers; does adding layers refer to the original STL surface or to the existing (bad) mesh?

Do you have any advice for me?

Thanks!
Attached Images
File Type: jpg snappyhexmesh-impeller-whole.jpg (208.5 KB, 40 views)
File Type: jpg impeller-snappyhexmesh-stl.jpg (199.9 KB, 32 views)
File Type: jpg impeller-shappyhexmesh-detail.jpg (204.8 KB, 33 views)
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Old   May 15, 2017, 08:46
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Try larger values for nSmoothPatch, nSolveIter and nRelaxIter
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Old   May 15, 2017, 16:34
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Hello,

thanks for your suggestion. i tried with

nSmoothPatch 30;
nSolveIter 30;
nRelaxIter 30;
nFeatureSnapIter 30;

(that took quite some time )

but the results aren't better, a bit worse actually. see the attached mesh.
increasing refinement level should produce better meshes, but in my case it's not snapping well for any refinement level at all, it just makes the edges more finely chipped.

i'm asking for help here for two reasons, first because i'm running out of things to try and second, because i'm not very fond of my trial-and-error approach. i have plans for the future to automate some things (just like that's already done in commercial packages) and it won't go with my current "workflow"...
Attached Images
File Type: jpg mesh-impeller-iterations.jpg (189.4 KB, 27 views)
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Old   May 16, 2017, 01:34
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Why do you use these values: resolveFeatureAngle 1 and featureAngle 180?

I have had quite good meshes with values: resolveFeatureAngle 30 and featureAngle 60.
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Old   May 16, 2017, 03:15
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those values were a part of one of a million experiments. i changed them back to what you suggested. here's the whole snappyHexMeshDict if you find any clues.

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.1                                   |
|   \\  /    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       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
{
    impeller-inlet-ami.stl
    {
        type triSurfaceMesh;
        name impeller_inlet_ami;
    }
    impeller-walls.stl
    {
        type triSurfaceMesh;
        name impeller_walls;
    }
    impeller-outlet-ami.stl
    {
        type triSurfaceMesh;
        name impeller_outlet_ami;
    }

    impellerZone
    {
    type    searchableSphere;
    centre (0 0 0);
    radius 0.05;
    
    }

    //- Refine a bit extra around the small centre hole
    //refineHole
    //{
    //    type searchableSphere;
    //    centre (0 0 -0.012);
    //    radius 0.003;
    //}
};



// 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 200000;

    // 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;

    // 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/triSurface for now.
    features
    (
        {
            file "impeller-inlet-ami.extendedFeatureEdgeMesh";
            level 2;
        }
        {
            file "impeller-walls.extendedFeatureEdgeMesh";
            level 4;
        }
        {
            file "impeller-outlet-ami.extendedFeatureEdgeMesh";
            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
    {
        impeller_inlet_ami
        {
            // Surface-wise min and max refinement level
            level (1 2);
            patchInfo { type patch; }
        }
        impeller_walls
        {
            // Surface-wise min and max refinement level
            level (2 3);
            patchInfo { type wall; }
        }
        impeller_outlet_ami
        {
            // Surface-wise min and max refinement level
            level (1 2);
            patchInfo { type patch; }
        }
        impellerZone
        {
            level (0 0);
           cellZone impeller;
            faceZone impeller;
            cellZoneInside inside;
        }
    }

    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
    {
        //refineHole
        //{
        //    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.
    // This is an outside point locationInMesh (-0.033 -0.033 0.0033);
    locationInMesh (-0.02 0 0); // Inside point

    // 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 30;

    //- 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 30;

    // Feature snapping

    //- Number of feature edge snapping iterations.
    //  Leave out altogether to disable.
    nFeatureSnapIter 5;

    //- Detect (geometric) features by sampling the surface
    implicitFeatureSnap false;

    //- Use castellatedMeshControls::features
    explicitFeatureSnap true;

    //- Detect features between multiple surfaces
    //  (only for explicitFeatureSnap, default = false)
    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
    layers
    {
        impeller_walls
        {
            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 0.3;

    // Minimum thickness of cell layer. If for any reason layer
    // cannot be above minThickness do not add layer.
    // See relativeSizes parameter.
    minThickness 0.05;

    // 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.
    nGrow 0;


    // Advanced settings

    // When not to extrude surface. 0 is flat surface, 90 is when two faces
    // are perpendicular
    featureAngle 60;

    // 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 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
    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;

    // 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"

    // 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 180;
    }

    // Advanced

    //- Number of error distribution iterations
    nSmoothScale 4;
    //- Amount to scale back displacement at error points
    errorReduction 0.75;
}


// Advanced

// Write flags
writeFlags
(
    scalarLevels    // write volScalarField with cellLevel for postprocessing
    layerSets       // write cellSets, faceSets of faces in layer
    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-7;


// ************************************************************************* //
and meshQualityDict:
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      meshQualityDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

//- Maximum non-orthogonality allowed. Set to 180 to disable.
maxNonOrtho 65;
//maxNonOrtho 180;

//- Max skewness allowed. Set to <0 to disable.
maxBoundarySkewness 20;
maxInternalSkewness 5;
//maxBoundarySkewness -1;
//maxInternalSkewness -1;

//- 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;
//maxConcave 180;

//- 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 5e-15;
minVol -1e30;

//- 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 1e-15;
//minTetQuality -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 0.02;
//minTwist -10;

//- Minimum normalised cell determinant. This is the determinant of all
//  the areas of internal faces. It is a measure of how much of the
//  outside area of the cell is to other cells. The idea is that if all
//  outside faces of the cell are 'floating' (zeroGradient) the
//  'fixedness' of the cell is determined by the area of the internal faces.
//  1 = hex, <= 0 = folded or flattened illegal cell
minDeterminant 0.001;
//minDeterminant -1e30;

//- Relative position of face in relation to cell centres
// (0.5 for orthogonal mesh) (0 -> 0.5)
minFaceWeight 0.05;

//- Volume ratio of neighbouring cells (0 -> 1)
minVolRatio 0.05;
//minVolRatio 0;


//- Per triangle normal compared to average normal. Like face twist
//  but now per (face-centre decomposition) triangle. Must be >0 for Fluent
//  compatibility
minTriangleTwist -1;


//- if >0 : preserve 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 0.05;


// ************************************************************************* //
i have two options for background mesh:
- one block / a cube
- five blocks that form a cylinder
with this 'try' i used the first mesh. see the attached result.
Attached Images
File Type: jpg mesh-impeller-3.jpg (198.8 KB, 31 views)
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Old   May 19, 2017, 10:56
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I would lower back patch smoothing and whatnot to what you had before. These settings rarely help getting better feature capturing for me.

To me it looks like that airfoil approaches the wall at a sharp angle, is that true? If so, the problem is probably that you need to refine that edge more than other edges. I would make sure to have just an edge available for the airfoils and refine those a bit.

Otherwise your only choice is to lower the size on the airfoil itself.
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Old   May 23, 2017, 14:37
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hello,

thanks for your suggestions. I have split the geometry of the impeller walls into blades and shroud so I could increase refinement on the blades only. And I changed back snapping iteration settings back to values from the flange tutorial.

Although the mesh conforms a little better now it's still not good. The very important AMI patches don't snap to geometry at all and the blade corners are still chipped but on a smaller scale.

I think I'll be giving up on snappyHexMesh very soon since I've been wrestling with this for the past few weeks and I obviously haven't found the very magic combination of all the parameters for a good mesh on a relatively simple geometry. I used Netgen with viscous layers in Salome and a got a perfect (though tetrahedral) mesh within a minute.

Interestingly, I was explaining to my brother what I've been doing and his question was why is it so hard to divide a model into small pieces? I couldn't answer him.
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Old   May 23, 2017, 19:06
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Quote:
Originally Posted by kandelabr View Post

Interestingly, I was explaining to my brother what I've been doing and his question was why is it so hard to divide a model into small pieces? I couldn't answer him.

I think the answer to this is obvious. It is easy to divide a model into small pieces, but it's hard to do so well.

If one were to rank the popular meshers out today I'm sure snappy would be somewhere near the bottom, but it's free and has few developers. This geometry is very easy to mesh in commercial meshers, but you have to pay for them.

CAD packages and meshers are two types of programs where there aren't really any good generalized open-source packages available.
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Old   May 24, 2017, 02:27
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I tried using exactly the same settings as in flange tutorial and it seems that refinement messes things up, particularly on edges. I simply made an insanely fine blockMesh and no refinement and it's all good now. My simulation runs now.

I wonder why there's nothing going on with foamyHexMesh, it looks much more promising than snappy. If I was to write a mesher I'd start from the surface, like foamy, not from the inside, like snappy.

Anyway, thanks for your help. You know, every beginner is difficult.
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Old   June 8, 2017, 13:35
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I tried cfMesh.
I wasted a month on snappy and couldn't get it to make a mesh that works in my MRF case but after a few hours with cfMesh, voila, everything runs with no problems.

Just sayin'.
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Old   June 9, 2017, 02:51
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Hi,
cool Avatar, reminds me of me.

Is it possible to have cfMesh directly create the cellZones needed for the MRF or how did you do it? I think it is possible with some OF utility to use cylinders and so one. I am asking because I have a case with a cylinder-conus shape MRF region and was wondering about using cfMesh.

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Old   June 9, 2017, 04:23
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hello,

i create zones with topoSet. in my case impeller itself is meshed separately and the whole mesh is in its own zone so i just have to use a box big enough to fit everything inside.

here's my topoSetDict:
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.2.2                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      topoSetDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //


actions
(
    // Example:pick up internal faces on outside of cellSet
    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    // Load initial cellSet
    {
        name    impeller;
        type    cellSet;
        action  new;
        source  boxToCell;
        sourceInfo
        {
            box (-0.1 -0.1 -0.1) (0.1 0.1 0.1);
        }
    }
);

// ************************************************************************* //
you have to run:

topoSet
setsToZones

and that's it.

[as for the avatar, Kudos go to xkcd.com ]
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Old   June 9, 2017, 04:38
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I see,
as you were using AMI together with MRF I guess meshing two meshes can work.
I think it would be nicer to mesh in one go as the mesh would then be on piece, just different cells would have a certain zone.
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Old   June 9, 2017, 06:18
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if you can describe your mrf zone with simple geometry like cylinder, that would certainly be better.

one of my problems was memory consumption - with finer background meshes, blockMesh ate my RAM very happily. so by dividing mesh into smaller regions I could achieve better resolution with the same amount of RAM. sort of.
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