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[snappyHexMesh] snappyHexMesh and CfMesh: Surface is not smooth

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Old   October 10, 2023, 16:12
Default snappyHexMesh and CfMesh: Surface is not smooth
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I am trying to generate a mesh on a fluid domain the figure of which is attached. I have used both CfMesh and snappyHexMesh. Both result in a surface which is not smooth, and consequently, checkMesh gives a lot of errors (Failed 6 mesh checks). Increasing refinement settings does not fix it. Using the "surfaceCheck" command on the stl file shows the surface file is clean:
-Surface has no illegal triangles
-Surface is closed.
-All edges connected to two faces Number of unconnected parts : 1
-Number of zones (connected area with consistent normal) : 1

At first I thought the reason is snapping settings in snappyHexMesh, and tried different options, but it did not work. Getting the same problem (not a smooth surface) with CfMesh shows the problem is not the snapping settings in snappyHexMesh.

Any ideas on how to fix this? BTW, running in serial and parallel give same results. I have attached the resulting figures.

The rest of the post is my snappyHexMeshDict.

// 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
type triSurfaceMesh;
name walls;
name walls;

// Settings for the castellatedMesh generation.

// 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 6000000;

// 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 2;

// Explicit feature edge refinement
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

// Specifies a level for any cell intersected by its edges.
// This is a featureEdgeMesh, read from constant/triSurface for now.
file "walls.eMesh";
level 4;

// 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.

level (5 5);

// 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.


// 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 (29.3629 -11.4856 -0.35249);

// 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.
//- Number of patch smoothing iterations before finding correspondence
// to surface
nSmoothPatch 6; //**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; //**4.0;

//- Number of mesh displacement relaxation iterations.
nSolveIter 300;

//- Maximum number of snapping relaxation iterations. Should stop
// before upon reaching a correct mesh.
nRelaxIter 10; //**5;

// Feature snapping

// Number of feature edge snapping iterations.
// Leave out altogether to disable.
nFeatureSnapIter 20; //**10;

// 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 true;

// Settings for the layer addition.
// 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

// 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.
// 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 1.7.x! (didn't do anything in 1.7.x)
nGrow 0;

// Advanced settings

// When not to extrude surface. 0 is flat surface, 90 is when two faces
// are perpendicular
featureAngle 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 1.7.x! 90 degrees corresponds to 130
// in 1.7.x.
minMedialAxisAngle 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.
//- 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 -1; // 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 0.01;

//- 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;

// Optional : some meshing phases allow usage of relaxed rules.
// See e.g. addLayersControls::nRelaxedIter.
//- Maximum non-orthogonality allowed. Set to 180 to disable.
maxNonOrtho 75;

// 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: png 1.PNG (52.9 KB, 44 views)
File Type: png 2.png (151.8 KB, 49 views)
File Type: png 3.png (160.5 KB, 54 views)
mortezahdr is offline   Reply With Quote

Old   October 11, 2023, 03:21
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From the cfMesh picture, it looks like the cells are too coarse to properly fit the geometry (I assume it is hollow inside, with a thin annulus around?). I know you tried to refine but you might have to go a level finer to make sure you have 2-3 cells in small gaps. Also, do you use the STL file as an input for cfMesh, or do you generate a FMS file?

The other thing is the quality of the STL file used. The elongated triangles forming the cylinder suggest that you exported it directly from the CAD software. I have rarely experienced issues with this approach, but I usually generate a new STL file from the CAD file with Salome (plus, you can create your patches easily ). I remember reading somewhere a high quality STL will save you some trouble.

But it could also be something totally different hope it helps.
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Old   April 29, 2024, 21:59
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Nick Howlett
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Originally Posted by Alczem View Post
But it could also be something totally different hope it helps.
I had perhaps a similar scenario when using CFMesh. Trying to mesh a box around a surface and the surface was degraded pretty significantly, with holes that I needed patched (needed fully-closed curved surface). Had a look at CFMesh's user guide [1] and low-and-behold cfmesh's developer suggested to reduce global cell size (maxCellSize), which resolved my hole issue.


Hope this helps, Nick
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cfmesh, openfoam, snappyhexmesh, stl

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