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[snappyHexMesh] Shell Refinement running for 99 iterations

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Old   February 28, 2014, 14:34
Default Shell Refinement running for 99 iterations
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Paul McClafferty
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Sorry if this is a stupid question, but I was wondering if anyone could tell me what part of my snappyHexMeshDict is causing this to run for so long? I have tried changing some of it but have no luck reducing the time snappyHexMesh takes to run. Thanks very much
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Old   February 28, 2014, 15:05
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it also took just over an hour simply to run snappyHexMesh, any advice for a newcomer would be greatly appreciated
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Old   March 1, 2014, 11:04
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Dear Paul,
would be helpful to see your SnappyDict and the log as

Code:
Code.
Anyhow, I guess it is

minRefinementCells

you are looking for.

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Old   March 1, 2014, 13:49
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Thanks for the response. I have tried changing that to 5, and then 0 but it still runs for a long time. here is the snappyHexMeshDict file

Quote:
/*--------------------------------*- 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 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
{
s1223_2.stl
{
type triSurfaceMesh;
name s1223;
}

refinementBox
{
type searchableBox;
min (-0.1 -0.1 0.5);
max ( 0.5 0.1 0.5);
}
};



// 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 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 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
(
{
file "s1223_2.eMesh";
level 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
{
s1223
{
// Surface-wise min and max refinement level
level (7 8);

// Optional specification of patch type (default is wall). No
// constraint types (cyclic, symmetry) etc. are allowed.
patchInfo
{
type wall;
inGroups (s1223Group);
}
}
}

// Resolve sharp angles
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 8));
}
}


// 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 (2 0.1 0.1);


// 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;

//- 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 5;

// Feature snapping

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

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

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

//- Detect points on multiple surfaces (only for explicitFeatureSnap)
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
{
"(lowerWall|s1223_2).*"
{
nSurfaceLayers 1;
}
}

// 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.
// Relative to undistorted size of cell outside layer.
// 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.
// 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;

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

//- 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. 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-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 0.02;

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

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

//- 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;


// ************************************************** *********************** //
I don't know if thats the correct way of posting code but it's the only way i could find.

Thanks for your help
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Old   March 1, 2014, 14:12
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with that shmd i am getting the following error

Quote:
--> FOAM FATAL IO ERROR:
keyword minTetQuality is undefined in dictionary "/home/paul/OpenFOAM/paul-2.2.2/run/s1223_2/system/snappyHexMeshDict.meshQualityControls"

file: /home/paul/OpenFOAM/paul-2.2.2/run/s1223_2/system/snappyHexMeshDict.meshQualityControls from line 301 to line 353.

From function dictionary::lookupEntry(const word&, bool, bool) const
in file db/dictionary/dictionary.C at line 402.

FOAM exiting
minTetQuality is set to 1e-30. Any ideas on this?

Thanks again
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Old   March 1, 2014, 14:23
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Dear Paul,
concerning faster snapping: Try increasing minRefinementCells. Snappy tries to refine the mesh, until the defined number of cells still to refine is reached. Thus it stops earlier, if you increase the value.


Quote:
Originally Posted by paulmcclafferty View Post

minTetQuality is set to 1e-30.
The other thing: Check the error message:
Quote:
file: /home/paul/OpenFOAM/paul-2.2.2/run/s1223_2/system/snappyHexMeshDict.meshQualityControls from line 301 to line 353.
If you look closely you see, that a semicolon is missing

Best regards
Julian
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Old   March 20, 2015, 07:08
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Is this topic solved?
Otherwise I will give some suggestions.
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Old   March 20, 2015, 11:05
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Yes it is solved thanks anyway
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Old   August 13, 2017, 19:26
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Yes Tobi please give suggestions. I am dealing with a similar issue.

Thank you!
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Old   December 22, 2017, 06:49
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Quote:
Originally Posted by Tobi View Post
Is this topic solved?
Otherwise I will give some suggestions.
Hi guys,
I also experienced this recently.
Sometimes it takes just 2 iterations for shell refinement of 2 Million cells and sometimes 90 iterations for the same number cells to refine.

Could anyone enlighten the reason behind this/ significance of this?

Regards,
Rajan
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Old   February 25, 2020, 15:19
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It is based on the refinement level one sets up. E.g.;


Code:
level (1 3)

This will refine your patch by level one in any case. However, based on the featureAngle you will refine e.g., some cells. Based on the new refinements it might occur that new cells are now selected to be refined (based on the refinement done previously). Thus, one can run into a refinement loop that does not stop.

One example in which I observed that stuff. I did a refinement of (1 2) on a roof. The roof was aligned in such a way in accordance with the featureAngle that only a few cells gets selected for each surface refinement. After the refinement was done, the neighbor cells got selected to be refined until the whole roof was refined by level two. To overcome this there are more options:


  • Select a good featureAngle
  • Use minRefinementCells (to stop before only a few cells get refined)
  • use levels such as (2 2) or (1 1)


Furthermore, as also mentioned by the CEO from cfMesh, the surface triangulation is important; c.f. Creating waterproof STL using snappyHexMesh or salome
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