oryx |
August 28, 2021 06:48 |
Layer coverage at 98% with collapses at corners
4 Attachment(s)
Hello,
I am trying to understand different entries in snappyHexMeshDict "addLayersControls" section.
So, I created a geometry with sharp corners (90 degree), and to increase the boundary layer coverage I did the following: - changed featureAngle to be larger than 90
- reduced minThickness to be a very small value
- edited meshQualityDict's minVol to be 1e-30
- increased some iterations entries such as nRelaxIter, nLayerIter (honestly, not knowing what they do exactly).
I've managed to get a 98.1% coverage, due to some collapses in corners. while the layers were added in some corners successfully, other corners are collapsed completely, as shown in the attachments.
So why did snappyHexMesh failed to add layers in some corners when succeeded in adding layers for corners with same angle?
And one last question, when I try to increase the layers number nSurfaceLayers, the coverage percentage is reduced drastically (~ 70%), so my question is for OpenFOAM professionals who work with cases with y+ ~ 1. Is mesh preparation for such small y+ values possible using snappyHexMesh, and how to achieve it?
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 4.x |
| \\ / 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 false;
snap false;
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
{
regionsSTL.stl
{
type triSurfaceMesh;
regions
{
inlet { name inlet; }
outlet { name outlet; }
walls { name walls; }
}
}
};
// 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 200000000;
// 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 100000000;
// 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)
// if you have less than 'minRefinementCells' cells to refine, stop refinement
minRefinementCells 5;
// 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 "regionsSTL.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
{
regionsSTL.stl
{
// Surface-wise min and max refinement level
level (0 0);
regions
{
inlet { level (0 0); patchInfo { type patch; } }
outlet { level (0 0); patchInfo { type patch; } }
walls { level (3 3); patchInfo { type walls; } }
}
}
}
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
{
}
// 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 (0.291667 0.291667 0.291667); // 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 1.1;
//- Number of mesh displacement relaxation iterations.
nSolveIter 100;
//- 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 8;
//- 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
{
walls
{
nSurfaceLayers 3;
}
}
// Expansion factor for layer mesh
expansionRatio 1.4;
// Wanted thickness of final added cell layer. If multiple layers
// is the thickness of the layer furthest away from the walls.
// 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 0.00000001;
// 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 1;
// Advanced settings
// When not to extrude surface. 0 is flat surface, 90 is when two faces
// are perpendicular
featureAngle 95;
// Maximum number of snapping relaxation iterations. Should stop
// before upon reaching a correct mesh.
nRelaxIter 50;
// Number of smoothing iterations of surface normals
nSmoothSurfaceNormals 5;
// Number of smoothing iterations of interior mesh movement direction
nSmoothNormals 10;
// Smooth layer thickness over surface patches
nSmoothThickness 5;
// 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 200;
// 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 60;
}
// 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 65;
}
// 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-6;
// ************************************************************************* //
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