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snappyHexMesh Boundary Layer at Corner

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Old   September 24, 2013, 16:49
Default snappyHexMesh Boundary Layer at Corner
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Peng Zhong
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Hi OpenFoam users, I am using snappyHexMesh in OF2.2.1 to generate a mesh of a duct. Everything is fine except the layers at the corner of the duct always shrink in thickness. I have tried changing the featureAngle in addlayers section and it does seem to help a bit. But the best result I have obtained from changing featureAngle still has corner layers shrinking a bit. Below is my snappyHexMesh code. Please also see the attached picture generated by this code. Thank you very much


/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.2.1 |
| \\ / 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 true;

// 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
{
nozzle.stl
{
type triSurfaceMesh;

//tolerance 1E-5; // optional:non-default tolerance on intersections
//maxTreeDepth 10; // optional:depth of octree. Decrease only in case
// of memory limitations.

// Per region the patchname. If not provided will be <name>_<region>.
regions
{
}
}
};

// 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 1000000;

// 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 5000000;

// 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;

// 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 explicitly provided
// edges.
// This is a featureEdgeMesh, read from constant/triSurface for now.
// Specify 'levels' in the same way as the 'distance' mode in the
// refinementRegions (see below). The old specification
// level 2;
// is equivalent to
// levels ((0 2));

features
(
/*
{
// file "nozzle.eMesh";
// level 1;
// levels ((0.0 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
{
nozzle.stl
{
// Surface-wise min and max refinement level
level (0 0);

//- Optional angle to detect small-large cell situation
// perpendicular to the surface. Is the angle of face w.r.t.
// the local surface normal. Use on flat(ish) surfaces only.
// Otherwise leave out or set to negative number.
//perpendicularAngle 10;

//- Optional faceZone and (for closed surface) cellZone with
// how to select the cells that are in the cellZone
// (inside / outside / specified insidePoint)
//faceZone sphere;
//cellZone sphere;
//cellZoneInside inside; //outside/insidePoint

//- Optional specification of what to do with faceZone faces:
// internal : keep them as internal faces (default)
// baffle : create baffles from them. This gives more
// freedom in mesh motion
// boundary : create free-standing boundary faces (baffles
// but without the shared points)
//faceType baffle;
}
}

// Feature angle:
// - used if min and max refinement level of a surface differ
// - used if feature snapping (see snapControls below) is used
resolveFeatureAngle 60;


// 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
// increasing 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
{
nozzle.stl
{
mode distance;
levels ((0.002 2) (0.004 1));
// levels ((1.0 5) (2.0 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.
locationInMesh (0 0 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;//3;

// Maximum relative distance for points to be attracted by surface.
// True distance is this factor times local maximum edge length.
// Note: changed(corrected) w.r.t 17x! (17x used 2* tolerance)
tolerance 1.0;//2.0;

// Number of mesh displacement relaxation iterations.
nSolveIter 50;//200;//30;

// Maximum number of snapping relaxation iterations. Should stop
// before upon reaching a correct mesh.
nRelaxIter 5;//5;

// Feature snapping

// Number of feature edge snapping iterations.
// Leave out altogether to disable.
nFeatureSnapIter 10;//50;

// 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 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 false;

// Layer thickness specification. This can be specified in one of four ways
// - expansionRatio and finalLayerThickness (cell nearest internal mesh)
// - expansionRatio and firstLayerThickness (cell on surface)
// - overall thickness and firstLayerThickness
// - overall thickness and finalLayerThickness

// Expansion factor for layer mesh
expansionRatio 1.2;

// Wanted thickness of the layer furthest away from the wall.
// If relativeSizes this is relative to undistorted size of cell
// outside layer.
//finalLayerThickness 0.3;

// Wanted thickness of the layer next to the wall.
// If relativeSizes this is relative to undistorted size of cell
// outside layer.
firstLayerThickness 0.000089;

// Wanted overall thickness of layers.
// If relativeSizes this is relative to undistorted size of cell
// outside layer.
//thickness 0.5


// Minimum overall thickness of total layers. If for any reason layer
// cannot be above minThickness do not add layer.
// If relativeSizes this is relative to undistorted size of cell
// outside layer..
minThickness 0.000089;//0.5;


// Per final patch (so not geometry!) the layer information
// Note: This behaviour changed after 21x. Any non-mentioned patches
// now slide unless:
// - nSurfaceLayers is explicitly mentioned to be 0.
// - angle to nearest surface < slipFeatureAngle (see below)
// Disable any mesh shrinking and layer addition on any point of
// a patch by setting nSurfaceLayers to 0
layers
{
nozzle.stl_Combustor_ASCII
{
nSurfaceLayers 9;
// Per patch layer data
// expansionRatio 1.3;
// finalLayerThickness 0.3;
// 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 360;

// At non-patched sides allow mesh to slip if extrusion direction makes
// angle larger than slipFeatureAngle.
slipFeatureAngle 20;//20;

// Maximum number of snapping relaxation iterations. Should stop
// before upon reaching a correct mesh.
nRelaxIter 5;

// Number of smoothing iterations of surface normals
nSmoothSurfaceNormals 5;//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 1000;

// Angle used to pick up medial axis points
// Note: changed(corrected) w.r.t 17x! 90 degrees corresponds to 130 in 17x.
minMedianAxisAngle 300;

// 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 nRelaxedIter it uses the settings in
// meshQualityControls,
// after nRelaxedIter it uses the values in
// meshQualityControls::relaxed.
nRelaxedIter 20;

// Additional reporting: if there are just a few faces where there
// are mesh errors (after adding the layers) print their face centres.
// This helps in tracking down problematic mesh areas.
additionalReporting true;
}

// 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;//20;
maxInternalSkewness 4;//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;//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. 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-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;

// ************************************************** *********************** //
Attached Images
File Type: jpg Screenshot from 2013-09-24 14:47:45.jpg (41.1 KB, 232 views)
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Old   February 6, 2014, 12:42
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Cong
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Hello,

have you finally solved the problem?
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Old   February 11, 2014, 05:28
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Hi,

Have you took a look at:
http://www.openfoam.org/mantisbt/view.php?id=1038

I am not sure that it help you.

Regards,
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Old   October 27, 2014, 04:03
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Hi everybody,
I have same problem
have you finally solved the problem? please help me !!!!!
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Old   October 27, 2014, 05:31
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Yuanchuan Liu
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I have exactly the same problem as this one.
Does anyone have any ideas?
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