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-   -   snappyHexMesh - Refinement Level (http://www.cfd-online.com/Forums/openfoam/72384-snappyhexmesh-refinement-level.html)

andrea.pasquali February 3, 2010 17:27

snappyHexMesh - Refinement Level
 
1 Attachment(s)
Hello,
I'm using OF 1.6.x.
I would like to obtain a car into tunnel mesh with 3 different refinement boxes (refinement mode inside).
See the attached picture.

First, I tried:
level 1 for "refinementBox_1", level 2 for "refinementBox_2", level 3 for "refinementBox_3", level min 4 max 5 for car surface.
I obtain the mesh but, for my case, it's not very thick.

Second, I tried:
level 3 for "refinementBox_1", level 4 for "refinementBox_2", level 5 for "refinementBox_3", level min 6 max 7 for car surface.
I obtain the mesh but I lost the refinement boxes! I have a thick mesh only around the car surface, but not into the 3 refinement boxes!

So, what am I wronging? Is there a bug in 1.6.x?
Is there a maximum global level for refine mesh?
Can it depend from the cells dimension in blockMesh?
How can I obtain my desired mesh?

Thanks for any reply

Andrea

Wolle February 4, 2010 07:56

Hi Andrea,

I'm not an expert at all, but t first reading, this comes to my mind:

Code:

    object      snappyHexMeshDict;

....

// 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
{
    bla
    {
        type triSurfaceMesh;
        name blabla;
    }
  refinementBox
  {
  type searchableBox;
  min ( 0.0  0.0  0.0);
  max ( 1.0 1.0 1.0);
  }
};

// Settings for the castellatedMesh generation.
castellatedMeshControls
{
// 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
    {
        blabla
        {
            // Surface-wise min and max refinement level
            level (4 5);
        }
    }

// 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));
        }
    }

Maybe your version doesn't work, because it's not related to the surface of your object?

If I were you, I would set up the surrounding mesh using blockMesh manually and then using snappy to add your special geometry.

Cheers
Wolle

andrea.pasquali February 4, 2010 09:21

Hi Wolle,
thank you very much for you reply!

I have a car surface in .stl file (average tria dimension 6 mm).
The tunnel is in X 55 m, in Y 13 m, in Z 10 m. My car is in X 4.8 m, in Y 2 m, in Z 1.5 m. The cell dimension of blockMesh is 350 mm. So:
refinementBox1 -> level 1 -> 1/2 -> cell dimension = 175 mm
refinementBox2 -> level 2 -> 1/4 -> cell dimension = 87.5 mm
refinementBox3 -> level 3 -> 1/8 -> cell dimension = 43.75 mm
refinementSurfaces level (4 5)
This mesh is whit 3 boxes but is not thick.

If I try with
refinementBox1 -> level 2
refinementBox2 -> level 3
refinementBox3 -> level 4
refinementSurfaces level (5 6)
I lost my 3 boxes!!!

Why?
Do you think I have to refine my blockMesh?

Thanks

Wolle February 4, 2010 11:43

Hi Andrea,

I have to repeat, that I'm not experienced at all!

I just think, that the refinement areas and levels refer to the STL surface and intersections with it. As your boxes are far from the STL surface, they wont be refined. You give the level, alright, but for snappy there's no reason to refine as there's no related surface. That's how I understood snappy. However I'm curious why the first attempt seems to work.

On the other hand I remember somebody mentioning wake areas to be refined with refinement boxes (which would be an antithesis to what I wrote).

Anyway: To me it would be an easy workaround (or maybe just to give it a try), to mesh your boxes manually with blockMesh, i.e. meshing the boxes as described in the damBreak tutorial, each of them with a different grading.

Cheers
Wolle

P.S.: If you like... give your first version a try with refinementSurfaces level (5 6) and your second version a try with refinementSurfaces level (5 5), leaving the other settings as they are. I'm just curious what would happen...

fra76 February 7, 2010 06:28

Hi Andrea,
what you describe in your first email is really strange...
It would be useful if you could post here your snappyHexMeshDict and maybe try with a more recent 1.6.x version.

Francesco

andrea.pasquali February 8, 2010 04:08

Hello everybody,
I tried the first Wolle's advice: level 1,2,3 for the boxes and level (5 6) for the car surface: I got 3 boxes! Now I would like to increase the refinement level.
Before this test I tried whit OF 1.6 but whit same result...
This is my snappy dictionary:
Quote:

/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 1.6 |
| \\ / A nd | Web: http://www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
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
{
car.stl
{
type triSurfaceMesh;
name car;
}

refinementBox_int
{
type searchableBox;
min (-2.5 -1.55 -0.495);
max (10.0 1.55 1.975);
}
refinementBox_med
{
type searchableBox;
min (-4.0 -2.1 -0.495);
max (12.5 2.1 2.975);
}
refinementBox_est
{
type searchableBox;
min (-5.5 -2.6 -0.495);
max (15.0 2.6 4.975);
}
/*
refinementBox_spoiler
{
type searchableBox;
min (0.5 -0.9 0.9);
max (1.3 0.9 1.5);
}
*/
};

// Settings for the castellatedMesh generation.
castellatedMeshControls
{

// Refinement parameters
// ~~~~~~~~~~~~~~~~~~~~~

// While refining maximum number of cells per processor. 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 10000000;
//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 20000000;
//maxGlobalCells 2000000;

// 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;

// 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
(
//{
// file "someLine.eMesh";
// level 2;
//}
);

// 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
{
car
{
// Surface-wise min and max refinement level
level (5 6);
}
}

// 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
{
/*
car
{
mode distance;
levels ((0.05 4));
}
*/
refinementBox_est
{
mode inside;
levels ((1E15 1));
}

refinementBox_med
{
mode inside;
levels ((1E15 2));
}

refinementBox_int
{
mode inside;
levels ((1E15 3));
}
/*
refinementBox_spoiler
{
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 (-4 0 1);
}

// Settings for the snapping.
snapControls
{
//- Number of patch smoothing iterations before finding correspondence
// to surface
nSmoothPatch 10;

//- Relative distance for points to be attracted by surface feature point
// or edge. True distance is this factor times local
// maximum edge length.
tolerance 4.0;

//- Number of mesh displacement relaxation iterations.
nSolveIter 50;

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

// Settings for the layer addition.
addLayersControls
{
relativeSizes true;

// Per final patch (so not geometry!) the layer information
layers
{
car_wheels
{
nSurfaceLayers 3;
}
car_body
{
nSurfaceLayers 3;
}
car_spoiler-middle
{
nSurfaceLayers 3;
}
car_spoiler-bottom
{
nSurfaceLayers 3;
}
car_spoiler-side
{
nSurfaceLayers 3;
}
car_spoiler-top
{
nSurfaceLayers 3;
}
}

// Expansion factor for layer mesh
expansionRatio 1.5;

//- 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.
finalLayerThickness 0.6;

//- 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.
nGrow 1;

// Advanced settings

//- When not to extrude surface. 0 is flat surface, 90 is when two faces
// make straight angle.
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
minMedianAxisAngle 130;

// Create buffer region for new layer terminations
nBufferCellsNoExtrude 0;

// Overall max number of layer addition iterations
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 projected area v.s. actual area. Set to -1 to disable.
minFlatness 0.5;

//- Minimum pyramid volume. Is absolute volume of cell pyramid.
// Set to very negative number (e.g. -1E30) to disable.
minVol 1e-13;

//- 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 0.02;

//- 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;

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

// ************************************************** *********************** //
Now I'm trying the second Wolle's advice and whit finer blockMesh size.

Thanks

Andrea


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