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Old   January 30, 2013, 13:16
Default snappyHexMesh makes strange things
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Hi guys, I'm experiencing some troubles and I really don't know why this is happening!!! As the title says, I'm trying to mesh a wing with its box around (exported as .stl unique file) with sHM, but something really strange happens next to the wing, precisely in the patch that should become the symmetryPlane.

Actually the symmetry plane, once castellated and snapped is made up of hexa on different planes.

I upload an image to show better what I'm trying to explain.

I also upload my current setting of blockMesh and snappyHexMeshDict.
I really hope that someone can help me because I don't really know what to do.

Cheers

Quote:
convertToMeters 1000;

vertices
(
(-1.2 0.7 -0.7) //H'
(3.8 0.7 -0.7) //G'
(3.8 0.7 2.3) //C'
(-1.2 0.7 2.3) //D'
(-1.2 -1.3 -0.7) //E'
(3.8 -1.3 -0.7) //F'
(3.8 -1.3 2.3) //B'
(-1.2 -1.3 2.3) //A'
);

blocks
(
hex (0 1 2 3 4 5 6 7) (50 30 20) simpleGrading (1 1 1)
);

edges
(
);

boundary
(
symmetry
{
type symmetryPlane;
faces
(
(3 7 6 2)
(1 5 4 0)
(4 5 6 7)
(0 3 2 1)
);
}

inlet
{
type patch;
faces
(
(0 4 7 3)
);
}

outlet
{
type patch;
faces
(
(2 6 5 1)
);
}

);

mergePatchPairs
(
);
Quote:
// 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
{
WING.stl
{
type triSurfaceMesh;
regions
{ inlet
{ name inlet; }
outlet
{ name outlet; }
symmetry
{ name symmetry; }
RearWing
{ name RearWing; }
}
}

//refinementBox
//{
// type searchableBox;
// min (-1 -1 -1);
// max ( 5 1 1);
//}
};



// 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 4000000;

// 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 100;

// Number of buffer layers between different levels.
// 1 means normal 2:1 refinement restriction, larger means slower
// refinement.
nCellsBetweenLevels 3;



// 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 ();



// 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
{
WING.stl
{
// Surface-wise min and max refinement level
level (1 8); // almeno (6 6)
regions
{
RearWing
{level (8 10);}
}
}
}

// Resolve sharp angles on fridges
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 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 (-0.5 0 0);
locationInMesh (1500 -300 1400);


// 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 15;

//- 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 100;

//- Maximum number of snapping relaxation iterations. Should stop
// before upon reaching a correct mesh.
nRelaxIter 20;
}



// 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 false;

// Per final patch (so not geometry!) the layer information
layers
{
RearWing
{
nSurfaceLayers 4;
}
}

// 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.
// 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.
// 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.
// Note: changed(corrected) w.r.t 17x! (didn't do anything in 17x)
nGrow 3;

// Advanced settings

//- When not to extrude surface. 0 is flat surface, 90 is when two faces
// make straight angle.
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
// Note: changed(corrected) w.r.t 16x! 90 degrees corresponds to 130 in 16x.
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;

//- 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-30;

//- 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;


// Advanced

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


// 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;
Attached Images
File Type: jpg wing.jpg (84.9 KB, 123 views)
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Old   January 31, 2013, 09:15
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Any reply please? I thought it was a common issue
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Old   January 31, 2013, 15:32
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Is the symmetry plane a patch that comes from blockMesh?

One more thing, it looks like you don't look at the smoothed out version in OpenFOAM. Do you overwrite the steps at every meshing step or do you write out every step during meshing?
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Old   February 1, 2013, 03:35
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I have both a patch of symmetry in the .stl file ( I import the whole domain with the wing), and in the blockMesh file.

And for the second question I don't overwrite any step of the mesh, the one you see is the second folder, the one which already contains the snap phase
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Old   February 1, 2013, 04:24
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In order to get a good symmetry plane, I would suggest that you only use the wing (no other patches) as an import. Then make a box in blockMesh that has all the patches that you need, similar to the motorCycle tutorial in meshing tutorials.

The way the symmetry plane will not be affacted by snappyHexMesh
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Old   February 1, 2013, 04:28
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Ok I'll try maybe this was the issue . But, just a quick question..in the case I import only the wing, I have to provide an intersection between the patch and the wing or the coordinate in the stl file has to be such that there is just a serie of contact points between the wing and the patch and not an intersection?
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Old   February 1, 2013, 04:40
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The patch in blockMesh should be right where the wing geometry starts, you can let the patch intersect the wing profile exactly where you need the symmetry plane, like in the sketch that is attached
Attached Images
File Type: png sketch.png (47.3 KB, 53 views)
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Old   February 1, 2013, 07:07
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Ok thank you very much. So it's enough to provide a .stl of the solid body whose coordinate are (where needed) the same of those of the patch where it has to be attached.

I have another question if you please to answer..which parameters can I change to obtain the best smoothed surface once snapped? Some "unrealistic spot" still remain and I've been changing the settings a lot of time but I can't obtain a "perfect" surface..

Thanks

Simone
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Old   February 1, 2013, 09:01
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It really depends, I have never meshed a wing profile with snappyHexMesh. If you have problems at the edgdes especially, the you should take a look at how to extract an edge file for snappyHexMesh and switch on the meshing of edge profiles as well. This usually will give you some really good results http://www.hydroniumion.de/openfoam/...snappyhexmesh/ . there are also some more tutorials here http://openfoamwiki.net/index.php/SnappyHexMesh
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Old   February 5, 2013, 12:15
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Hi guys I've another question..
I'm trying to mesh a surface inside a background mesh, also adding a refinementRegion (in particular a refinementBox). If I do the castellated part with only the refinementBox, it appears when I see the mesh in paraView, but, if I add also the .stl file which contain the actual mesh, the refinement box suddenly disappears..anyone knows which could be the reason of this strange thing?
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