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[snappyHexMesh] How too improve y+ number at trailing edge and Wing tip

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Old   October 16, 2021, 04:27
Red face Improving y+ number at trailing edge and Wing tip
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Ricardo H
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Hello, I'm doing an analysis with sHM and SimpleFoam.
But when I mesh the wing the trailing edge and the tip doesnt seem very well meshed
Furthermore the y+ number in the trailing edge and the tip it's upper than expected
I alredy have tried to use different refinement levels from 7 to 9, as well as diferent resolve and feature angles.
I enclosed some pictures and my sHM code
What could I do?
Thanks!!!

SnappyHexMesh
PHP Code:
/--------------------------------- C++ -----------------------------------\
  =========                 |
  \\      /  
F ield         OpenFOAMThe 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 "Wing3.stl";
    }

    
refinementBox
    
{
        
type searchableBox;
        
min (-0.3 0 -0.075);
        
max 4 0.8 0.2);
    }
};



// 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 20000000;

    
// 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
    
(
        {
            
file "Wing3.eMesh";
            
level 5;
        }
    );



    
// 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
    
{
        
wing
        
{

            
// 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 (wingGroup);
            }
        }
    }

    
// Resolve sharp angles
    
resolveFeatureAngle 60;


    
// 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));
        }
    }


    
// 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 (0.5 0.1 0.043);


    
// 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 300;

    
//- 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 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
    
layers
    
{
        
"(lowerWall|wing).*"
        
{
            
nSurfaceLayers 5;
        }
    }

    
// Expansion factor for layer mesh
    
expansionRatio 1.5//D:

    // 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 0.253125//D:

    // 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//D:

    // 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 30;

    
// 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
{
    
maxNonOrtho 65;

   
maxBoundarySkewness 20;
   
maxInternalSkewness 4;

   
maxConcave 80;

   
minVol 1e-13;

   
minArea 1e-13;

   
minTetQuality 1e-30;

   
minTwist 0.05;

   
minDeterminant 0.001;

   
minFaceWeight 0.02;

   
minVolRatio  0.01;

   
minTriangleTwist -1;

   
minvolCollapseRatio 0.5;//

  //Advanced 

   
nSmoothScale 4;

   
errorReduction 0.75;

   
relaxed
   
{
    
    
maxNonOrtho 75;
   }
}


// 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;


// ************************* // 
Attached Images
File Type: jpg Imagen2.jpg (27.7 KB, 13 views)
File Type: jpg Imagen1.jpg (21.0 KB, 14 views)

Last edited by AlbatrossR; October 17, 2021 at 22:04. Reason: misspeled words
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Old   October 17, 2021, 04:54
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This is probably caused by boundary layers collapsing around these edges. If you look around the forum, this is the most common question asked about snappy. Creating a good coverage for the whole body is quite hard or not possible at all for some geometries. Closely look at your mesh and see if this is the case and check out other threads here in the forum for ideas of how to improve.
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Old   October 17, 2021, 21:49
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Indeed,I tried with the feature angle of 180 from the addLayersControls and it helped now my y+ it's on 230
but at the tip lower surface there are some inconsistencies
Do you have any idea of how to improve it?
I have searched at the forum but with no seccess yet
Attached Images
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File Type: jpg qqqqq111.jpg (17.5 KB, 12 views)
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Old   October 18, 2021, 04:33
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It could help to increase cellsBetweenLevels. At inclined faces, the spatial offset where the level changes from surface-spacing to the background mesh spacing sits closer to the wall sometimes. This can prevent snappy from extruding the layers there.

Careful: this increases the mesh size maybe significantly. Another option would be to make a refinement box with the same level as the surface spacing at the tip and trailing edge (might be also good in terms of covering flow gradients in the wake region?). Then there is no level change close to the wall and it might be easier for snappy to create layers. Hope this helps.


I would recommend to investigate real closely the mesh in Paraview, instead of running a simulation for each try. If the layers collapse, your y+ will be high.
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