[Snacker]

Hello everyone,

I am encountering an issue while trying to simulate a winglet using OpenFOAM. Specifically, my problem lies with SnappyHexMesh, as it is failing to generate surface layers around the entire airfoil surface of the winglet. This is hindering the accuracy and reliability of my simulation results.

To provide some context, I am attempting to model a winglet using OpenFOAM for computational fluid dynamics (CFD) analysis. However, despite my efforts, SnappyHexMesh seems to be neglecting certain portions of the airfoil surface when generating surface layers.

I have attached an image highlighting the problem for better understanding.

I have thoroughly reviewed my mesh settings and boundary conditions, but I am unable to pinpoint the exact cause of this issue. I have ensured that the geometry of the winglet is correctly defined and free from errors.

I would greatly appreciate any insights or suggestions from the community regarding how to troubleshoot and resolve this problem with SnappyHexMesh. If anyone has encountered a similar issue or has expertise in meshing with OpenFOAM, your guidance would be invaluable.

Thank you in advance for your assistance. I look forward to your responses and insights.

Cheers

/*--------------------------------*- C++ -*----------------------------------*\
========= |
\\ / F ield | OpenFOAM: The Open Source CFD Toolbox
\\ / O peration | Website: https://openfoam.org
\\ / A nd | Version: 8
\\/ 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
{
motorBike
{
type triSurfaceMesh;
file "motorBike.stl";
}

refinementBox
{
type searchableBox;
min (-1 0 -2);
max ( 5 4 2);
}

//refinementBox2
//{
// type searchableBox;
// min (-1 0 -2);
// max ( 5 2 2);
//}

};

// min (-1.0 -0.7 0.0);
// max ( 8.0 0.7 2.5);

// 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 20000000; // 10 000 000

// 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 2000000000; // 200 000 000

// 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 8; // 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 "motorBike.eMesh";
level 7; // 6
}
);



// 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
{
motorBike
{
// Surface-wise min and max refinement level
level (6 6); // 6 6

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

// Resolve sharp angles
resolveFeatureAngle 15;


// 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)); // 4
}
//refinementBox2
//{
// mode inside;
// levels ((1E15 4)); // 4
//}

motorBike
{
mode distance;
levels ((0.04 11));
}






}


// 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 (-2.0 0.0 0.0);


// 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 50;

//- Detect (geometric only) features by sampling the surface
// (default=false).
implicitFeatureSnap false;

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

//- 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 //14
layers
{
"(motorBike).*"
{
nSurfaceLayers 10;
}
}

// Expansion factor for layer mesh
expansionRatio 1.00; //1.09

// 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 0.000375; //0.3 mot // 0.46 naca

// 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 0.000375; //0.15

// 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 178;

// 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 70;
}



// 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]

SnappyHexMesh is notorioulsy unstable when it comes to boundary layers. You will find a high fraction of threads in this forum is about problems with introducing high quality layers. You can look through them to find some general recommendations. AFAIK people who really depend on hq layers choose other mesh generators like Fluent Meshing, ICEM, ... .

[Snacker]

Originally Posted by AtoHM

SnappyHexMesh is notorioulsy unstable when it comes to boundary layers. You will find a high fraction of threads in this forum is about problems with introducing high quality layers. You can look through them to find some general recommendations. AFAIK people who really depend on hq layers choose other mesh generators like Fluent Meshing, ICEM, ... .

sadly i do not have the opportunity to use paid software.

[Yann]

Hello,

As AtoHM already pointed out, layers addition is not snappy's strong suit.
You may want to try the OpenCFD branch (openfoam.com) which allows layer-by-layer addition and alternative mesh motion solver.

https://www.openfoam.com/news/main-n...#snappy_layers

In my experience it's not perfect but it can significantly improve layer coverage.

[Alczem]

Hey,


I second Yann's comment about the layer-by-layer addition, and would like to mention that someone posted their conclusions about the best settings for snappyHexMesh. You can find the thread here. I have had good results with the layer coverage with these settings.