[aeroarunkampi]

Hi All,

I am trying to generate a mesh in a 3D obstacle with snappyhexmesh. when I execute the checkMesh, the following errors are showing up. It would be great somebody could help me to sort out this issue. I have attached the snapshot of the SnappyHexMeshdict and checkMesh error along with this. Please go through it. Is there any way to improve the mesh quality such that following error are not going to showing up (example : under-determined cells and warped faces) Thanks in advance.

CheckMesh-error

Time = 0.0002

Mesh stats
points: 4108131
faces: 10402095
internal faces: 9552505
cells: 3130473
faces per cell: 6.37431
boundary patches: 8
point zones: 0
face zones: 0
cell zones: 0

Overall number of cells of each type:
hexahedra: 2509047
prisms: 203407
wedges: 0
pyramids: 0
tet wedges: 16
tetrahedra: 0
polyhedra: 418003
Breakdown of polyhedra by number of faces:
faces number of cells
4 84
5 1957
6 117626
9 166722
12 109432
14 45
15 15150
17 10
18 6969
21 8

Checking topology...
Boundary definition OK.
Cell to face addressing OK.
Point usage OK.
Upper triangular ordering OK.
Face vertices OK.
Topological cell zip-up check OK.
Face-face connectivity OK.
Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
Patch Faces Points Surface topology Bounding box
IN 6660 6825 ok (non-closed singly connected) (-0.25 -2.7 -0.5) (-0.25 1 4)
outer 6660 6825 ok (non-closed singly connected) (7.75 -2.7 -0.5) (7.75 1 4)
outer1 19281 20146 ok (non-closed singly connected) (-0.25 -2.7 -0.5) (7.75 -2.7 4)
outer2 14400 14651 ok (non-closed singly connected) (-0.25 1 -0.5) (7.75 1 4)
outer3 11840 12075 ok (non-closed singly connected) (-0.25 -2.7 4) (7.75 1 4)
seaGround 11840 12075 ok (non-closed singly connected) (-0.25 -2.7 -0.5) (7.75 1 -0.5)
wallFaces 0 0 ok (empty)
cadGeometry.stl 778909 780741 ok (closed singly connected) (-0.00199431 -2.65432 -7.8234e-05) (2.65203 9.23468e-05 2.64352)

Checking faceZone topology for multiply connected surfaces...
No faceZones found.

Checking basic cellZone addressing...
No cellZones found.

Checking geometry...
Overall domain bounding box (-0.25 -2.7 -0.5) (7.75 1 4)
Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
Mesh has 3 solution (non-empty) directions (1 1 1)
Boundary openness (-2.24681e-15 -7.2359e-16 1.252e-14) OK.
Max cell openness = 4.40581e-16 OK.
Max aspect ratio = 3.49541 OK.
Minimum face area = 6.75435e-06. Maximum face area = 0.00271589. Face area magnitudes OK.
Min volume = 4.4919e-07. Max volume = 0.000135581. Total volume = 130.978. Cell volumes OK.
Mesh non-orthogonality Max: 57.9935 average: 11.5765
Non-orthogonality check OK.
Face pyramids OK.
Max skewness = 3.59222 OK.
Coupled point location match (average 0) OK.
Face tets OK.
Min/max edge length = 0.000525096 0.0531124 OK.
*There are 15671 faces with concave angles between consecutive edges. Max concave angle = 68.6977 degrees.
<<Writing 15671 faces with concave angles to set concaveFaces
Face flatness (1 = flat, 0 = butterfly) : min = 0.778355 average = 0.999953
*There are 1 faces with ratio between projected and actual area < 0.8
Minimum ratio (minimum flatness, maximum warpage) = 0.778355
<<Writing 1 warped faces to set warpedFaces
Cell determinant (wellposedness) : minimum: 0.06774 average: 2.35047
Cell determinant check OK.
***Concave cells (using face planes) found, number of cells: 219281
<<Writing 219281 concave cells to set concaveCells
Face interpolation weight : minimum: 0.122882 average: 0.460917
Face interpolation weight check OK.
Face volume ratio : minimum: 0.0242229 average: 0.793322
Face volume ratio check OK.

Failed 1 mesh checks.

--> FOAM Warning :
From function virtual Foam:olyMesh::readUpdateState Foam:olyMesh::readUpdate()
in file meshes/polyMesh/polyMeshIO.C at line 217
Number of patches has changed. This may have unexpected consequences. Proceed with care.
Time = 0.0003

Mesh stats
points: 7154000
faces: 19514485
internal faces: 18664896
cells: 6163626
faces per cell: 6.19431
boundary patches: 7
point zones: 0
face zones: 0
cell zones: 0

Overall number of cells of each type:
hexahedra: 5512580
prisms: 203408
wedges: 0
pyramids: 0
tet wedges: 16
tetrahedra: 0
polyhedra: 447622
Breakdown of polyhedra by number of faces:
faces number of cells
4 3081
5 1957
6 120157
7 16677
8 7209
9 166803
10 104
11 16
12 109436
14 45
15 15150
17 10
18 6969
21 8

Checking topology...
Boundary definition OK.
Cell to face addressing OK.
Point usage OK.
Upper triangular ordering OK.
Face vertices OK.
Topological cell zip-up check OK.
Face-face connectivity OK.
Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
Patch Faces Points Surface topology Bounding box
IN 6660 6825 ok (non-closed singly connected) (-0.25 -2.7 -0.5) (-0.25 1 4)
outer 6660 6825 ok (non-closed singly connected) (7.75 -2.7 -0.5) (7.75 1 4)
outer1 19281 20146 ok (non-closed singly connected) (-0.25 -2.7 -0.5) (7.75 -2.7 4)
outer2 14400 14651 ok (non-closed singly connected) (-0.25 1 -0.5) (7.75 1 4)
outer3 11840 12075 ok (non-closed singly connected) (-0.25 -2.7 4) (7.75 1 4)
seaGround 11840 12075 ok (non-closed singly connected) (-0.25 -2.7 -0.5) (7.75 1 -0.5)
cadGeometry.stl 778908 780740 ok (closed singly connected) (-0.00199431 -2.65432 -7.8234e-05) (2.65203 9.23468e-05 2.64352)

Checking faceZone topology for multiply connected surfaces...
No faceZones found.

Checking basic cellZone addressing...
No cellZones found.

Checking geometry...
Overall domain bounding box (-0.25 -2.7 -0.5) (7.75 1 4)
Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
Mesh has 3 solution (non-empty) directions (1 1 1)
Boundary openness (-2.09731e-15 -4.81978e-15 1.45547e-14) OK.
Max cell openness = 5.47524e-16 OK.
Max aspect ratio = 21.3319 OK.
Minimum face area = 4.54779e-06. Maximum face area = 0.00315622. Face area magnitudes OK.
Min volume = 1.76524e-08. Max volume = 0.000155875. Total volume = 130.978. Cell volumes OK.
Mesh non-orthogonality Max: 64.5845 average: 11.191
Non-orthogonality check OK.
Face pyramids OK.
Max skewness = 3.59222 OK.
Coupled point location match (average 0) OK.
Face tets OK.
Min/max edge length = 0.000525096 0.0564325 OK.
*There are 67132 faces with concave angles between consecutive edges. Max concave angle = 68.6977 degrees.
<<Writing 67132 faces with concave angles to set concaveFaces
Face flatness (1 = flat, 0 = butterfly) : min = 0.778355 average = 0.999883
*There are 8 faces with ratio between projected and actual area < 0.8
Minimum ratio (minimum flatness, maximum warpage) = 0.778355
<<Writing 8 warped faces to set warpedFaces
Cell determinant (wellposedness) : minimum: 0.000307997 average: 1.31362
***Cells with small determinant (< 0.001) found, number of cells: 5
<<Writing 5 under-determined cells to set underdeterminedCells
***Concave cells (using face planes) found, number of cells: 305167
<<Writing 305167 concave cells to set concaveCells
Face interpolation weight : minimum: 0.0275114 average: 0.457697
***Faces with small interpolation weight (< 0.05) found, number of faces: 57
<<Writing 57 faces with low interpolation weights to set lowWeightFaces
Face volume ratio : minimum: 0.0116442 average: 0.813441
Face volume ratio check OK.

Failed 3 mesh checks.

End

SnappyHexMeshDict

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
{
cadGeometry.stl
{
type triSurfaceMesh;
//name cadGeometry;
}



};



// 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 10;

// 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 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 "cadGeometry.eMesh";
level 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
{
cadGeometry.stl
{
// Surface-wise min and max refinement level
level (3 3);
}
}

// 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 4));
}
}


// 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 0.0 0.50 );
locationInMesh (-0 -2.5 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 1.0;

//- Number of mesh displacement relaxation iterations.
nSolveIter 10;

//- 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 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
{
"(cadGeometry).*"
{
nSurfaceLayers 4;
}

}

// 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
// make straight angle.
featureAngle 180;

//- 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 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. 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-16;

//- 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 0.02;

//- 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; //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;


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


Regards
Arun