[costabarro95]

So apparently one of the parts of my model is to small for my blockMesh. Problem is that the domain is quite big, so decreasing the dimension of the blockMesh cells is not an option, I'm already running with too much cells because of the needed refinement on the surfaces of the model.
So basically snappy is non able to see the surface of the smallest parts so it is not able to perform a surface refinement.
Someone has some advices?


the problematic part is the one in the middle and as you can see there's no refinement on it

[AtoHM]

Would you share your snappyHexMeshDict and mark for us the regions names in the image. I suspect not that it is too small: the geometry is clearly captured, just the refinement is missing. If it was too small, the boundary would be gone.
My first guess: you maxGlobalCells-value was exceeded. Once this happens, snappy stops the refinement process and continues with the rest.

[costabarro95]

Don't think its a problem of maxGlobCell cause its set to 4 million and right now the mesh is less than two. Also if I increase the numbers of subdivisions in the blockmesh for some combinations of parameters I'm able to obtain the refinement, but do not understand how and why.
The problematic part is "hub"




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
{
shamal
{
type triSurfaceMesh;
file "shamal.stl";
}
hub
{
type triSurfaceMesh;
file "hub.stl";
}

refinementBox
{
type searchableBox;
min (-0.7 -0.3 -0.6);
max ( 0.7 0.3 0.6);
}
refinementBox2
{
type searchableBox;
min (0.7 -0.3 -0.6);
max ( 3 0.3 0.6);
}
cylinder
{
type searchableCylinder;
point1 (0 -0.15 0);
point2 (0 0.15 0);
radius 0.45;
}
};



// 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 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
(
);



// 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
{
shamal
{
// Surface-wise min and max refinement level
level (7 7);

// Optional specification of patch type (default is wall). No
// constraint types (cyclic, symmetry) etc. are allowed.
//patchInfo
//{
// type wall;
// inGroups (oggettoGroup);
//}
}
hub
{
// Surface-wise min and max refinement level
level (6 6);

// Optional specification of patch type (default is wall). No
// constraint types (cyclic, symmetry) etc. are allowed.
//patchInfo
//{
// type wall;
// inGroups (oggettoGroup);
//}
}
}

// 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 3));
}
refinementBox2
{
mode inside;
levels ((1E15 2));
}
cylinder
{
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 (-3.1099999999 -3.109999999999 -3.10999999999999);


// 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 2.0;

//- Number of mesh displacement relaxation iterations.
nSolveIter 30;

//- 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 true;

//- Use castellatedMeshControls::features (default = true)
explicitFeatureSnap false;

//- Detect points on multiple surfaces (only for explicitFeatureSnap)
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
{
"hub.*"
{
nSurfaceLayers 6;
}
"shamal.*"
{
nSurfaceLayers 6;
}

}

// Expansion factor for layer mesh
expansionRatio 1.3;

// 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 0.05;

// 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
// are perpendicular
featureAngle 90;

// At non-patched sides allow mesh to slip if extrusion direction makes
// angle larger than slipFeatureAngle.
slipFeatureAngle 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 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
{
#include "meshQualityDict"
}


// Advanced

// Write flags
writeFlags
(
scalarLevels
layerSets
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;

[AtoHM]

You are right, but I wanted to point this out, as I didn't know what you have set there. But maxLocalCells could cause this too obviously, if the cells exceed that 100,000 on one core. Have you tried increasing this?
Would be plausible, that is depending on the blockMesh then, cause it gets divided differently by the partitioner and might not run into the max value then.

[costabarro95]

Yeah I did increase it to 500'000 but still the same problem. As far as I understood its a problem deriving from the blockMesh, cause if I increase from (60 40 40) to (70 45 45) the number of subdivisions it works. But I dont understand why. I also tried to use a set of parameters so the "hub" geometry it's intersecated perfectly in the middle by the blockMesh but still no refinement.

[AtoHM]

Honestly, I am running out of ideas. The only difference to my cases is the setting for minRefinementCells. I usually have it at 1. No idea if that helps here.

[snak]

Hi,


You have geometry named "hub" based on "hub.stl".

You refined the surface and its neighborhood using a "refeinementSurfaces" entry.

refinementSurfaces{ hub{ level (6 6); }

How about refinementRegions? Does one of the refinementBox, fefinementBox2, and cylinder cover the hub?

refinementRegions
{
    refinementBox { mode inside; levels ((1E15 3)); }
    refinementBox2 { mode inside; levels ((1E15 2)); }
    cylinder { mode inside; levels ((1E15 4)); }
}

Levels of refinementRegions are much larger than surface refinements. Without refinementRegion with an adequate refinement level, you can not get finer mesh in a far away region from the surface.

[AtoHM]

If I understand correctly, he wonders why hub is not refined at the surface, not far away.
Something else I just notice: if the two big circles are corresponding to region "shamal.stl" with lvl 7 as prescribed, then hub seems to be at level 6 exactly as specified, or not?

[costabarro95]

Originally Posted by AtoHM

If I understand correctly, he wonders why hub is not refined at the surface, not far away.
Something else I just notice: if the two big circles are corresponding to region "shamal.stl" with lvl 7 as prescribed, then hub seems to be at level 6 exactly as specified, or not?

That's right

[costabarro95]

Originally Posted by AtoHM

Honestly, I am running out of ideas. The only difference to my cases is the setting for minRefinementCells. I usually have it at 1. No idea if that helps here.

Already tried but doesn't resolve the problem

[snak]

Originally Posted by costabarro95

That's right

Now I understand the problem. My misunderstanding was "one outer big circle is the shamal and one inner big circle is the hub". The outer and inner big circles are the shamal, and the inner tiny circle is the hub, right?

As AtoHM pointed out, it looks like that shamal is refined at level 7 and hub is refined at level 6 as you instruct with snappyHexDict.

What happens if you change the level for the hub surface?

hub { level (7 7); }

[costabarro95]

Don't know what happened but i rebuilt the case from zero and it was working properly. Have no idea why since from looking at them they are identical. Thanks everybody for your time