[Nafiz375]

Hello Everyone,
Please accept my warm greetings.
I've made a simple mesh using the searchableCylinder in snappyHexMeshDict, as shown in Figure 1.
1.PNG
But, while I tried to tilt the entire mesh, the edges were distorted and not uniform, as shown in figure 2.
2.jpg

How do I solve this issue?

snappyHexMeshDict:

/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 3.0.1 |
| \\ / A nd | Web: 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 false;


geometry
{
windTurbine1
{
type searchableCylinder; //modificado para tentar buscar uma regiao cilindrica - o original era searchableBox
point1 (618 535.5 535.5);
point2 (628 535.5 535.5);
radius 70;
}

};

// Settings for the castellatedMesh generation.
castellatedMeshControls
{

maxLocalCells 100000;

maxGlobalCells 2000000;

minRefinementCells 0;

maxLoadUnbalance 0.10;
.
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
(
//{
// file "someLine.eMesh";
// level 2;
//}
);



// Surface based refinement
// ~~~~~~~~~~~~~~~~~~~~~~~~

refinementSurfaces
{
/*terrain
{
// Surface-wise min and max refinement level
level (2 2);

// Optional region-wise level specification
/*
regions
{
windTurbine
{
level (3 3);
}
}

//- Optional angle to detect small-large cell situation
// perpendicular to the surface. Is the angle of face w.r.t.
// the local surface normal. Use on flat(ish) surfaces only.
// Otherwise leave out or set to negative number.
//perpendicularAngle 10;


//- Optional faceZone and (for closed surface) cellZone with
// how to select the cells that are in the cellZone
// (inside / outside / specified insidePoint)
//faceZone sphere;
//cellZone sphere;
//cellZoneInside inside; //outside/insidePoint
}*/
}

resolveFeatureAngle 30;


// Region-wise refinement
// ~~~~~~~~~~~~~~~~~~~~~~


refinementRegions
{

windTurbine1
{
mode inside;
levels ((4 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 (530 500 500);


// 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 4.0;

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

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



// 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
{
}

// 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.
// 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.
// 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 60;

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

// 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 16x! 90 degrees corresponds to 130 in 16x.
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

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


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

Regards,
Nafiz

[Yann]

Hello,

What you see on the screenshot is the expected result if you just changed the orientation of your searchableCylinder.

In paraView if you want to see the actual mesh your can toggle the advanced properties and untick "Decompose polyhedra".
In addition if you used the slice filter you can untick "Triangulate the slice".

Regards,
Yann

[Nafiz375]

Hello Yann,
Thanks for your reply.
I've unchecked "Decompose polyhedra" and also unticked "Triangulate the slice." The meshing looks like below now.

3.PNG

Is there any other way to improve the mesh quality at the edges?

Regards,
Nafiz

[Yann]

There is probably a misunderstanding about what you are trying to do.

In your snappyHexMeshDict file, everything inside the refinementSurfaces is commented.
Your windTurbine1 geometry is only used in the refinementRegions section to refine mesh inside the geometry to level 4.

This is the result you get: the mesh is not snapped on your geometry, and the mesh inside or outside the geometry is not removed, but the region inside the geometry is refined as specified.

If you actually want to mesh the windTurbine1 geometry, you need to define it in the refinementSurfaces section. In this case the mesh will be snapped on windTurbine1 and the cells inside or outside (depending on the locationInMesh parameter) will be removed from the mesh.

Regards,
Yann

[Nafiz375]

Hello Yann,
I hope you are doing well.
Thanks for the clarifications. I actually didn't use any STL file. I just used searchableCylinder in snappyHexMeshDict. That's why I couldn't use the refinementSurfaces for the surface and edges.

Do I need to do the geometry with any other CAD software and then import it for the meshing with snappyHexMeshDict to imporve the mesh quality at the edges?

Regards and thanks,
Nafiz

[Yann]

Hello Nafiz,

Originally Posted by Nafiz375

I actually didn't use any STL file. I just used searchableCylinder in snappyHexMeshDict. That's why I couldn't use the refinementSurfaces for the surface and edges.

You can use your searchableCylinder in refinementSurfaces just as you would do with a STL.

Regards,
Yann

[Nafiz375]

Dear Yann,
Thank you so much for your suggestions. I have followed and done my meshing by importing a file.
Capture.jpg
I've attached the new mesh for your reference. Is there any way to improve more at the edges of the circle?

The snappyHexmesh code is:

/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 3.0.1 |
| \\ / A nd | Web: 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
{
cylinder.obj
{
type triSurfaceMesh;
name cylinder.obj; //patch name

}

};

// Settings for the castellatedMesh generation.
castellatedMeshControls
{

maxLocalCells 100000;

maxGlobalCells 2000000;

minRefinementCells 0;

maxLoadUnbalance 0.10;

nCellsBetweenLevels 1;


// This is a featureEdgeMesh, read from constant/triSurface for now.
features
(
{
file "cylinder.eMesh";
level 6;
}
);

// Surface based refinement
// ~~~~~~~~~~~~~~~~~~~~~~~~

refinementSurfaces

{
cylinder.obj
{
// Surface-wise min and max refinement level
level (2 5);
}
}

resolveFeatureAngle 30;


// Region-wise refinement
// ~~~~~~~~~~~~~~~~~~~~~~

refinementRegions
{

cylinder.obj
{
mode inside;
levels ((4 4));
}

}

// Mesh selection
// ~~~~~~~~~~~~~~
locationInMesh (495 535.5 535.5);


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 4.0;

//- 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
{
}
// Expansion factor for layer mesh
expansionRatio 1.0;

finalLayerThickness 0.3;

minThickness 0.25;

nGrow 0;


// Advanced settings

featureAngle 60;

nRelaxIter 5;

nSmoothSurfaceNormals 1;

nSmoothNormals 3;

nSmoothThickness 10;

maxFaceThicknessRatio 0.5;

maxThicknessToMedialRatio 0.3;

// Angle used to pick up medial axis points
// Note: changed(corrected) w.r.t 16x! 90 degrees corresponds to 130 in 16x.
minMedianAxisAngle 90;

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

nLayerIter 50;

nRelaxedIter 20;
}

meshQualityControls
{
//- Maximum non-orthogonality allowed. Set to 180 to disable.
maxNonOrtho 65;

//- Max skewness allowed. Set to <0 to disable.
maxBoundarySkewness 20;
maxInternalSkewness 4;

maxConcave 80;

minVol 1e-13;

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 : 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

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


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

[Yann]

Hello Nafiz,

It is a bit hard to tell as I cannot really see the cylinder edge on the screenshot you posted.

Could you elaborate on the type of simulation you want to do with this geometry?

Yann

[Nafiz375]

Hi Yann,
I hope you are doing well.
I'm trying to do a 3D actuator disk RANS simulation. I've imported the geometry from outside and used SnappyHexMesh for meshing. I'm just trying to find out the wake velocity behind the AD model.
My issue is that around the edge of the circular mesh attached, the mesh isn't uniformly discretized. How can I improve the mesh at that circumference? The code for the SnappyHexMesh is attached above.

Capture.jpg
Capture.jpg
1.jpg
Best wishes
Nafiz

[Yann]

Hello Nafiz,

Thanks for the additional details.
I am not sure to understand what I'm looking at. On your first 2 pictures the mesh does not seem to be snapped on your cylinder.

On the last pic it kinda looks like it's snapped. How did you get the picture?

About your STL file, is it a closed volume?

If you intend to use actuation disk, your cylinder should be defined as a cellZone:

refinementSurfaces
{
	cylinder.obj
	{
		level (2 5);
		faceZone        ActDiskZone;
		cellZone        ActDiskZone;
		cellZoneInside  insidePoint;
	}
}

So you can directly use the cellZone in your actuationDisk definition.

Regards,
Yann

[Nafiz375]

Hello Yann,
Thanks for your comments. The last pic I zoomed in on in Paraview, and yes, volume is closed.

I'm trying to refine the meshing by adding an extra inner zone, like in the picture below.
1.jpg

But while I gave the snappyHexMesh overwrite command, the outer domain vanished, and only the inner zone remained. I didn't change anything in the snappyHexMesh code.
2.PNG

blockMesh code:

convertToMeters 1;

vertices
(
( 0 0 0)
( 4788 0 0)
( 4788 1071 0)
( 0 1071 0)
( 0 0 1071)
( 4788 0 1071)
( 4788 1071 1071)
( 0 1071 1071)

(378 409.5 661.5)
(4788 409.5 661.5)
(4788 661.5 661.5)
(378 661.5 661.5)
(378 409.5 409.5)
(4788 409.5 409.5)
(4788 661.5 409.5)
(378 661.5 409.5)
);

blocks
(
hex (0 4 5 1 3 7 6 2) (150 64 64) simpleGrading (1 1 1)
hex (8 9 13 12 11 10 14 15) (256 64 64) simpleGrading (1 1 1)

);

edges
(
);

boundary
(
outlet
{
type patch;
faces
(
(2 6 5 1)
(10 14 13 9)
);
}
sides
{
type patch;
faces
(
(1 5 4 0)
(3 7 6 2)
(4 5 6 7)
(0 3 2 1)
(9 13 12 8)
(11 15 14 10)
(12 13 14 15)
(8 11 10 9)
);
}
inlet
{
type patch;
faces
(
(0 4 7 3)
(8 12 15 11)
);
}
);

mergePatchPairs
(
);

How can I solve this issue ?

Regards,
Nafiz

[Yann]

Hello Nafiz,

If you want to refine a zone inside your mesh you can do it directly in snappyHexMesh: define a box in the geometry section and then use it refinement regions.

geometry
{
    a_box_for_refinement
    {
        type box;
        min  (1.5 1 -0.5);
        max  (3.5 2 0.5);
    }
};

refinementRegions
    {
        a_box_for_refinement
        {
            mode inside;
            levels ((1.0 4)); // refine to level 4 inside the box
        }
    }

More details in the documentation: https://doc.openfoam.com/2306/tools/...inementRegions

Regards,
Yann

[Nafiz375]

Hi Yann,
Thanks again for clearing up many of my questions. I actually owe it to you, as you have answered many of my doubts.
I actually tried something like this.

1.PNG

That's why I added another block to the block mesh. Is it possible to do that with just the refinement box, as you mentioned earlier?

Reagrds,
Nafiz

[Yann]

Hello Nafiz,

Originally Posted by Nafiz375

That's why I added another block to the block mesh. Is it possible to do that with just the refinement box, as you mentioned earlier?

Sure, just as I described on my previous post.
Now if you aim to reproduce the results of a reference paper, the best you can do is to setup you case as close as possible from your reference.

Regards,
Yann

[Nafiz375]

Hello Yann,
I solved that issue and made a refinement zone by adding a refinement box. But, while zooming in on the sectional view of the cylinder, I found some additional meshing happening, as shown in the picture.
1.jpg
I tried to resolve that by adjusting different levels of meshing in both refinementSurfaces and refinementRegions. But I couldn't improve.

Any suggestions for that?

Best wishes,
Nafiz

[Yann]

Hello Nafiz,

It might be a matter of alignment between blockMesh, the cylinder geometry and your refinement regions.
The cells at the limit of your refinement zone can get selected for refinement or not depending on their position relatively to the refinement zone. Sometimes you can get this kind of thing when cells are just at the verge of the zone.

Have you used refinement regions with distance mode on your cylinder or did you define box around it?

You can try to slightly change the box position or refinement distance to see if it allow to avoid picking these cells for refinement.

Regards,
Yann

[Nafiz375]

Hello Yann,
I solved this issue by following your suggestions.
Now, I'm trying to extract data like stream-wise (U_x) and span-wise (U_y) velocities for the box as shown in the image. A plot_over_line just gives me a particular line.

vel_mag.jpg

How can I extract all the associated values for that whole region? Any suggestions?

Best wishes,
Nafiz

[Yann]

Hello Nafiz,

You can use the clip filter with the clip type: box to get the data of the region of interest.

Now what do you mean by "extract data"? If you want to save these data in a file, you can click on your clip filter in the pipeline, then file>save data

Regards,
Yann

[Nafiz375]

Hello Yann,
Thanks for your awesome advice. I tried the clip feature, as you mentioned. But while I tried to save data, I found something unusual: I specified one box with specific coordinates (x axis: 378 to 1008, y axis: 283.5 to 787.5) in the attached picture. But in the Find Data feature, while I checked the values for velocities, I found both x and y axis coordinates exceeded my given range (2nd picture).

1.jpg
2.PNG

Any suggestions how can I find the data only for my specified region of interest?

Regards,
Nafiz

[Yann]

Hello Nafiz,

Are you sure your clip kept the inside of the box and not the outside?

Regards,
Yann