SnappyHexMesh AddLayer Control
 

Hi!
my problem is to create a boundary layer with snappyHexMesh. How to control the number and thickness of the layers? I'm playing wth nSurfaceLayers and thickness but things change in a unknown way..
In attach you find the blockMeshDict and the SnappyHexMeshDict.
The geometry is a big cube with a cubic hole inside, around which I'd like to create a BL.

Thank You All ! :)

Roberto

/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.0.0 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

convertToMeters 1;

vertices
(
( -0.03 -0.03 -0.03)
( 0.03 -0.03 -0.03)
( 0.03 0.03 -0.03)
( -0.03 0.03 -0.03)
( -0.03 -0.03 0.01)
( 0.03 -0.03 0.01)
( 0.03 0.03 0.01)
( -0.03 0.03 0.01)
);

blocks
(
hex (0 1 2 3 4 5 6 7) (20 16 12) simpleGrading (1 1 1)
);

edges
(
);

boundary
(
allBoundary
{
type patch;
faces
(
(3 7 6 2)
(0 4 7 3)
(2 6 5 1)
(1 5 4 0)
(0 3 2 1)
(4 5 6 7)
);
}
);

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


/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.0.0 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object autoHexMeshDict;
}

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

// 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
{
refinementBox
{
type searchableBox;
min (-0.01 -0.01 -0.01);
max ( 0.01 0.01 0.01);
}
};



// 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 0;

// 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
(
);



// 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
{
refinementBox
{
// Surface-wise min and max refinement level
level (3 3);
}
}

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 (-0.025121221 0.025 0); // 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 true;
}



// 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 10.0;

//- Number of mesh displacement relaxation iterations.
nSolveIter 20;

//- Maximum number of snapping relaxation iterations. Should stop
// before upon reaching a correct mesh.
nRelaxIter 20;

//- Highly experimental and wip: number of feature edge snapping
// iterations. Leave out altogether to disable.
nFeatureSnapIter 10;
}




// 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
{
"refinementBox.*"
{
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.
// See relativeSizes parameter.
finalLayerThickness 0.8;

//- Minimum thickness of cell layer. If for any reason layer
// cannot be above minThickness do not add layer.
// See relativeSizes parameter.
minThickness 0.25;

//- 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;


// Advanced settings

//- When not to extrude surface. 0 is flat surface, 90 is when two faces
// make straight angle.
featureAngle 90;

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

// Number of smoothing iterations of surface normals
nSmoothSurfaceNormals 0;

// Number of smoothing iterations of interior mesh movement direction
nSmoothNormals 0;

// Smooth layer thickness over surface patches
nSmoothThickness 0;

// 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 50;

// 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 20;
}



// 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. 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 0.05;

//- 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;

//- if >0 : preserve single cells with all points on the surface if the
// resulting volume after snapping (by approximation) is larger than
// minVolCollapseRatio times old volume (i.e. not collapsed to flat cell).
// If <0 : delete always.
//minVolCollapseRatio 0.5;


// 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.
relaxed
{
//- Maximum non-orthogonality allowed. Set to 180 to disable.
maxNonOrtho 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;


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

Hi Roberto;
I was playing with layers as well a few days ago and I kept getting strange results… eventually I found out the problem:
If you run sHM with the layers and then you're not happy with them you have to delete every file before running sHM with new layers parameters, otherwise the new layers will, somehow, be built upon the old ones… I think this is a sort of a bug…
i.e. if you set 5 layers then run sHM then set 6 layers and run sHM again without deleting anything, at the end you'll have 11 layers instead of 6.

Well, thanks for the help but unfortunately the problem is not there..
I'm playing with "ExpantionRatio" or "maxThicknessToMedialRatio" to obtain a regular Bounday Layer all along the cubic hole, for the moment I can't find the solution. Do you know how to set the addLayer parameters?
Thanks
Roberto

Please, have a look at the attached picture. What I set is the following parameters for the layers:

At the top of the snappyHexMeshDict you need to have:
and in the layer section (in red what I changed from the motorbike tutorial settings):

what's the problem you're having exactly? Have you tried looking in the snappyHexMesh log for the word "layer" to find any possible problem with them?

Hi lore!,
thx for the reply.. Actually, thing are going better
I use the same setting as yours and you can find in attach what are the results. Starting from the snapped mesh I do obtain 3 layers ( :D ) but not regular near the boundaries as you can see from the section also...:confused:
I'd like to have layers all along the faces
Thx
Roberto

I can barely understand what you're doing… these chamfers I see, are they from the original surface or are they due to the snappy?

the chamfers in the first picture are due to the snappy phase ( this is not a problem, I resolved this with feature edge). Then in the second picture is just the addLayers phase that do them...and I can't explain why

addLayersControls
 

Hello,

I'm new here and new on OpenFoam. I'm in an internship and I have some difficulties find a solution to a problem.
I want to add a layer, so I use addLayersControls (which works perfectly excepty for one region [you can see in the pic]
and I don't know what range I can use for expansionRatio and finalLayerThickness

Can some one help me please ?

Thank you :)

how does the mesh look like if you turn the add layer option to false?
it looks like the mesh is not good enough for layer addition.

the mesh looks like this, and the layer is good everywhere else but here

hi

is this hole wanted? maybe you should increase the refirement there and/or increase the featureangle which says which layers will collapsed

see:http://openfoamwiki.net/images/f/f0/...SlidesOFW7.pdf

best wishes

Andy

Yes this hole is wanted. I'll try something now and see if it works (try to increase the featureAngle) cuz when I try to increase refinement it creats problems

Thank you for the pdf

Best

Mehdi

Thank you again it's working now :)

I might need another help later on. about turbulence and stuff, it's okay for you ?

Best

Problem with residual
 

Hello everybody,

I'm using OF 2.1.1 for a simulation of a turbine in a dam and I have to see the pressure and the velocity at certain points.
but the problem is that my simulation doesn't last too long before it stops with an error (the file is attached)

I heard I have to change something about k-epsilon turbulence model


Please can someone help me ?

best

Mehdi

I think this was not working :-)

Yeah sorry I forgot to do it :)
here it is

hi Mehdi,

i just had a lokk at your log file.

i think your boundary file for epsilon is not ok.

"bounding epsilon, min: -2.33438e+50 max: 6.65946e+66 average: 4.9693e+62"

maybee ou will post your boundary file

best wishes

Andy

hi,

Thanks for looking at my file.
I think the problem is that I really don't know how to fix that

here's my boundary file (epsilon, k, nut, U, p and boundary)

Best

Mehdi

This one is better ;)

hi,

I've never worked with k-epsilon. sorry. but have you turned off the turbulence and tried a laminar simulation?

Andy