[TheGenius]

Hi to everyone,
I've generated a mesh of a racing car with snappyHexMesh. However, on the body I can see that I have randon regions in which SHM has refined more than in all other parts as shown in picture. Does anybody know why this is happening?

https://ibb.co/f81DQ5v[/IMG]

[Fouch]

Originally Posted by TheGenius

Hi to everyone,
I've generated a mesh of a racing car with snappyHexMesh. However, on the body I can see that I have randon regions in which SHM has refined more than in all other parts as shown in picture. Does anybody know why this is happening?

https://ibb.co/f81DQ5v[/IMG]

Hi,

Could you share your snappyhexmeshdict ?

BR

[TheGenius]

I'm sending it below.

/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 6 |
| \\ / A nd | Web: www.OpenFOAM.com |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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
{
body.stl
{
type triSurfaceMesh;
name body;
}

frontWheel.stl
{
type triSurfaceMesh;
name frontWheel;
}

rearWheel.stl
{
type triSurfaceMesh;
name rearWheel;
}


};

// Settings for the castellatedMesh generation.
castellatedMeshControls
{

// If local number of cells is >= maxLocalCells on any processor
// switches from from refinement followed by balancing
// (current method) to (weighted) balancing before refinement.
// This is basically the number of cells that fit on a processor. If you choose this too small
// it will do just more refinement iterations to obtain a similar mesh.
maxLocalCells 99999999;

// 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 99999999;

// 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 0.1;

// Number of buffer layers between different levels.
// 1 means normal 2:1 refinement restriction, larger means slower
// refinement.
nCellsBetweenLevels 6;

// Specifies a level for any cell intersected by its edges.
// This is a featureEdgeMesh, read from constant/triSurface for now.
features
(
{
file "body.eMesh";
level 8;
}
{
file "frontWheel.eMesh";
level 8;
}
{
file "rearWheel.eMesh";
level 8;
}


);

// 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
{
body
{
level (6 6);
}
frontWheel
{
level (6 6);
}
rearWheel
{
level (6 6);
}
}

// Resolve sharp angles
// Applies maximum level of refinement to cells that can see intersections whose
// angle exceeds this
resolveFeatureAngle 10;

planarAngle 10;
gapLevelIncrement 2;

// 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.
// This point should never be on a face, always inside a cell, even
// after refinement.
locationInMesh (0 -1 10);

allowFreeStandingZoneFaces true;
}

// Settings for the snapping.
snapControls
{
// Number of patch smoothing iterations before finding correspondence
// to surface. This option changes how many times the exterior (the boundary
// wall) should be iterated. The more iterations, the smoother the mesh
// will become.
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.
// This option changes how long distance the program should look for a point
// to snap, the distance is the number put in "tolerance"*"size of the mesh".
tolerance 2.0;

// Number of mesh displacement relaxation iterations. This option changes
// how many times the "snapping" part of snappyHexMesh should be run:
// - the higher this number is the better mesh quality will be gained
// - the higher this number is the more equidistant mesh will be created
// when it comes to the boundary
// - the higher this number is the longer tha snapping will take
nSolveIter 30;

// Maximum number of snapping relaxation iterations. Should stop
// before upon reaching a correct mesh. This option changes how many times
// the mesh will run a relaxing script that removes some bad mesh point,
// however snappyHexMesh should stop before, when the correct mesh is created
nRelaxIter 5;

// Number of feature edge snapping iterations.
// Leave out altogether to disable. Do not use here since mesh resolution
// too low and baffles present. Controls the number of morph iterations
// within main snapping iterative process, to attract mesh points to
// surface and avoid sharp edges problems.
nFeatureSnapIter 15;

// True of false to enable implicit or explcit method for feature snap
// process. Implicit uses geometric only features by sampling the surface,
// while explcit uses castellatedMeshControls.
implicitFeatureSnap false;
explicitFeatureSnap true;

// True of false to enable the detection of the points on multiple surfaces
// (only for explicitFeatureSnap)
multiRegionFeatureSnap false;
}

// Settings for the layer addition.
addLayersControls
{

// True of false to changes the way to read the parameters given next.
// - true: next parameters define the layers' dimensions as
// "parameters" * "undistorted cell size outside layers"
// - false: next parameters define directly the layers' dimensions (absolute)
relativeSizes true;

// Layer thickness specification, can be specified in one of following ways:
// - expansionRatio and finalLayerThickness or firstLayerThickness.
// Where expansionRatio set the grow factor of the layers (ratio between two
// subsequent layers), while finalLayerThickness set the thickness of the last
// layer. 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.
// - overall thickness and firstLayerThickness
// - overall thickness and finalLayerThickness
// - overall thickness and expansionRatio
expansionRatio 1.5;
finalLayerThickness 0.4;

// 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 0.005;

// Per final patch (so not geometry!) the layer information
layers
{
body
{
nSurfaceLayers 5;
}
frontWheel
{
nSurfaceLayers 5;
}
rearWheel
{
nSurfaceLayers 5;
}
}

// 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 0;

// Angle above which surface is not extruded. One of the first parameters on
// which play when layer addition fails. 0 is flat surface,
// 90 is when two faces make straight angle.
featureAngle 330;

// Maximum number of snapping relaxation iterations. Should stop
// before upon reaching a correct mesh. This option changes how many times
// the mesh will run a relaxing script.
nRelaxIter 10;

// 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.
// This option changes the maximum allowable value of aspect ratio.
// Sometimes (expecially in corners) the layers must be generated over 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.
minMedialAxisAngle 130;

// 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;

// Number of smoothing iterations of interior mesh movement direction
nSmoothNormals 10;

slipFeatureAngle 30;

nRelaxedIter 20;

additionalReporting true;
}

// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
meshQualityControls
{
#include "meshQualityDict"
nSmoothScale 4;
relaxed
{
}
errorReduction 0.75;
}

debugFlags
(
);

writeFlags
(
// write volScalarField with cellLevel for postprocessing
scalarLevels

// write cellSets, faceSets of faces in layer
layerSets

// write volScalarField for layer coverage
layerFields
);

// Merge tolerance. Is fraction of overall bounding box of initial mesh. The
// write tolerance needs to be higher than this.
mergeTolerance 1e-6;

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

[Fouch]

I think you have to play with "resolveFeatureAngle 10;". Try to increase the value to 20 and post the result.


BR

[AtoHM]

Please check the recommendations here: Partial coarse mesh


Especially look for your cell count and if it reaches the max. You define alot of high levels so you probably run into the max count.

[TheGenius]

Reply to Fouch:
I thought that in this case the ResolveFeatureAngle is irrelevant as I'm using the same values for max and min refinement level.

Reply to AtyoHM:
I'm pretty sure I'm not reaching the Max Cells number imposed in the dictionary. I'm actually obtaining around 20 Milions of cells and maximum is set to 99.999 Millions.

[AtoHM]

Add a log of the run and pictures would help. Attach them directly to the forum, I don't click on random links.

[TheGenius]

Here it is.
Tell me which part of the log file you want as it's too large to be uploaded.

[AtoHM]

The refinement phase including the end of it.

[TheGenius]

Here are the first 5 parts.

[TheGenius]

And here the last two parts