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-   -   Zero thickness surface using snappyhex (http://www.cfd-online.com/Forums/openfoam-meshing-snappyhexmesh/113684-zero-thickness-surface-using-snappyhex.html)

Balti February 25, 2013 05:42

Zero thickness surface using snappyhex
 
Hello,

Is it possible to mesh zero-thickness surface inside a car (for example) using snappyhex ?
Do I need to do lot of intermediate step to do that or it is possible to say to snappyhex the name of my selection zero-thickness-surface to mesh ?

Thanks

mgdenno March 8, 2013 14:01

Do you mean a 2D mesh vs a 3D mesh? Or do you mean a 3D mesh of the inside of a car? Maybe a picture would help?

Also see this: http://www.cfd-online.com/Forums/ope...-get-help.html

Balti March 12, 2013 07:01

3 Attachment(s)
I have a more simple case:
I have a box (geometry with triangle in cube.stl). Inside this box I have a half-sphere (geometry with triangle in sphereTriangulated.stl).
I want to mesh these geometries: i.e. I want to snap the cube but also the internal zero-thickness surface half-sphere inside the cube.

I don't understand how can I do that. I'm new with snappyHex.

I think I need to use createBaffles but I don't understand how to complete the createBafflesDict dictionnary.

In attachments my dictionnaries:
my snappyHexmeshDict
Code:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  2.2.0                                |
|  \\  /    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
{
    cube.stl
    {
        type triSurfaceMesh;
        name cube;
    }

    sphereTriangulated.stl
    {
        type triSurfaceMesh;
        name sphereTriangulated;
    }

};



// 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;

    // 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 "cube.eMesh";
            level 1;
        }
    {
            file "sphereTriangulated.eMesh";
            level 1;
        }
    );



    // 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
    {
        cube
        {
            // Surface-wise min and max refinement level
            level (4 5);
    //        // Optional specification of patch type (default is wall). No
    //        // constraint types (cyclic, symmetry) etc. are allowed.   
    //        patchInfo                                                   
    //        {                                                           
    //            type wall;                                             
    //            inGroups (motorBike);                                   
    //        } 
        }

        sphereTriangulated
        {
      level (1 1);
      }
    }

    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
    {
        cube
        {
          mode inside;
          levels ((1E15 4));
        }

        sphereTriangulated
        {
          mode distance;
          levels ((0.1 5) (0.4 4) (1 2));
        }

    }


    // 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 (0.51 0.51 0.51); // 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 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;

    //- 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) 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
    {
        cube
        {
            nSurfaceLayers 2;
        }

    sphereTriangulated
        {
            nSurfaceLayers 2;
        }
    }

    // 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.
    // 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 0.25;

    // 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 30;

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

    //- 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-9;

    //- 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;
    }
}


// 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;


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

and the createBaffleDict
Code:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                |                                                |
| \\      /  F ield        | OpenFOAM: The Open Source CFD Toolbox          |
|  \\    /  O peration    | Version:  2.2.0                                |
|  \\  /    A nd          | Web:      www.OpenFOAM.org                      |
|    \\/    M anipulation  |                                                |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version    2.0;
    format      ascii;
    class      dictionary;
    object      createBafflesDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
//From a forum
//  createBaffles nameOfFaceZone '(nameOfMasterPatch nameOfSlavePatch)' -overwrite
//  The patches have to exist already, for which createPatches can come in handy.



// Whether to convert internal faces only (so leave boundary faces intact).
// This is only relevant if your face selection type can pick up boundary
// faces.
internalFacesOnly true;


// Baffles to create.
baffles
{
    sphereTriangulated.stl
    {
        //- Use predefined faceZone to select faces and orientation.
        type    triSurfaceMesh;
        name    sphereTriangulated;

        //- Optional flip
        //flip        false;

        patches
        {
            master
            {
                //- Master side patch
                name            sphereTriangulated;
                type            baffle;  ///wall;

                patchFields
                {
                    epsilon
                    {
                        type            epsilonWallFunction;
                        Cmu            0.09;
                        kappa          0.41;
                        E              9.8;
                        value          uniform 0;
                    }
                    k
                    {
                        type            kqRWallFunction;
                        value          uniform 0;
                    }
                    nut
                    {
                        type            nutkWallFunction;
                        Cmu            0.09;
                        kappa          0.41;
                        E              9.8;
                        value          uniform 0;
                    }
                    nuTilda
                    {
                        type            zeroGradient;
                    }
                    p
                    {
                        type            zeroGradient;
                    }
                    U
                    {
                        type            fixedValue;
                        value          uniform (0 0 0);
                    }
                }
            }
            slave
            {
                // Reuse master data
                ${..master}
            }
        }
    }
}


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


mgdenno March 12, 2013 08:50

I haven't done it myself but it looks like you can create internal baffles directly using snappyHexMesh. An example of how to write it in the dictionary is included in the release notes:

http://www.openfoam.org/version2.2.0/snappyHexMesh.php

Let us know how you make out.

MD

Balti March 12, 2013 09:43

1 Attachment(s)
Yes !
It works (without call to createBaffles). In fact I use paraFoam to view and I need to change the value of "Time" to obtain the last mesh (because in the first mesh there is no creation of the internal baffles).
So, now I have selection of my half-sphere like a thin-surface.

But in my viewer (paraFoam) I see two selection:
- sphereTriangulated_allEdgeSel0 - patch
- sphereTriangulated_allEdgeSel0_slave - patch

what is it ....._slave - patch ?

And also on my snapshot you can see that the boundary of the half-sphere is not exactly snapped. It is possible to improve it or to exact capturing the boundary edges of the half-sphere ?

Thanks a lot

mgdenno March 12, 2013 10:24

Quote:

Originally Posted by Balti (Post 413433)
Yes !
It works (without call to createBaffles). In fact I use paraFoam to view and I need to change the value of "Time" to obtain the last mesh (because in the first mesh there is no creation of the internal baffles).

That is pretty standard unless you use the -overwrite flag


Quote:

Originally Posted by Balti (Post 413433)
But in my viewer (paraFoam) I see two selection:
- sphereTriangulated_allEdgeSel0 - patch
- sphereTriangulated_allEdgeSel0_slave - patch

what is it ....._slave - patch ?

I believe those are your co-located pairs of faces. From the release notes:

"baffle creates co-located pairs of faces from the faceZone, to represent a zero-thickness boundary, i.e. a baffle;"

Quote:

Originally Posted by Balti (Post 413433)
And also on my snapshot you can see that the boundary of the half-sphere is not exactly snapped. It is possible to improve it or to exact capturing the boundary edges of the half-sphere ?

Checkout feature edge snapping here:

http://www.cfd-online.com/Forums/ope...ture-edge.html


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