[Dorian1504]

Hello,

I want to create a mesh around a NACA wing in order to calculate aerodynamic coefficients.
i want to perform a supersonic simulation (Mach 3), so I wanted to use k-omega SST turbulence model. I've heard that I need to have a y+=1 for this model. I've used a calculator to determine the thickness of the first layer and find 4,78.10^-7 m.

I estimated the viscous layer using the formula delta/x=0,16/(Re^1/7) and found delta=0.0122. I tried to implement 35 layers with an expansion ration of 1.3 to have a total thickness equal to 0.0155. However no layer is created.

4.78.10^-7m seems to be really small, can the problem come from this value ?

What have I done wrong ?

Thank you for your help

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.1                                   |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}

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

convertToMeters 1;

xmin -15;
xmax 15;
ymin -10;
ymax 10;
zmin -5;
zmax 5;

deltax 0.2;
deltay 0.2;
deltaz 0.2;

lx #calc "$xmax - $xmin";
ly #calc "$ymax - $ymin";
lz #calc "$zmax - $zmin";

xcells #calc "round($lx/$deltax)";
ycells #calc "round($ly/$deltay)";
zcells #calc "round($lz/$deltaz)";

vertices
(

    ($xmin $ymin $zmin)
    ($xmax $ymin $zmin)
    ($xmax  $ymax $zmin)
    ($xmin  $ymax $zmin)
    ($xmin $ymin $zmax)
    ($xmax $ymin $zmax)
    ($xmax  $ymax $zmax)
    ($xmin  $ymax $zmax)
);

blocks
(
    hex (0 1 2 3 4 5 6 7) ($xcells $ycells $zcells) simpleGrading (1 1 1)
);

edges
(
);

boundary
(
    box
    {
        type patch;
        faces
        (
            (3 7 6 2)
            (1 5 4 0)
        (0 3 2 1)
        (4 5 6 7)
        );
    }
    inlet
    {
        type patch;
        faces
        (
            (0 4 7 3)
        );
    }
    outlet
    {
        type patch;
        faces
        (
            (2 6 5 1)
        );
    }
);

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

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  4.1                                   |
|   \\  /    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. 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
{
    airfoil.stl
    {
        type triSurfaceMesh;
        name airfoil;
    }

};



// 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 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 0;

    // Number of buffer layers between different levels.
    // 1 means normal 2:1 refinement restriction, larger means slower
    // refinement.
    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 "airfoil.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
    {
        airfoil
        {
            // Surface-wise min and max refinement level
            level (4 4);
        }
    }

    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
    {
    }


    // 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.
    // This is an outside point locationInMesh (-0.033 -0.033 0.0033);
    locationInMesh (-0.000001348 -0.001247 0); // Inside point

    // 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 1.0;

    //- 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) features by sampling the surface
        implicitFeatureSnap false;

        //- Use castellatedMeshControls::features
        explicitFeatureSnap true;

        //- Detect features between multiple surfaces
        //  (only for explicitFeatureSnap, default = false)
        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 false;

    // Per final patch (so not geometry!) the layer information
    layers
    {
        airfoil
        {
            nSurfaceLayers 35;
        }
    }

    // Expansion factor for layer mesh
    expansionRatio 1.3;

    firstLayerThickness 0.0000004785;

    // Minimum thickness of cell layer. If for any reason layer
    // cannot be above minThickness do not add layer.
    // See relativeSizes parameter.
    minThickness 0.0000004;

    // 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 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 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
    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
{
    #include "meshQualityDict"

    // 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

    //- Number of error distribution iterations
    nSmoothScale 4;
    //- Amount to scale back displacement at error points
    errorReduction 0.75;
}


// Advanced

// Write flags
writeFlags
(
    scalarLevels    // write volScalarField with cellLevel for postprocessing
    layerSets       // write cellSets, faceSets of faces in layer
    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;

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

[Dorian1504]

I've finally decided to use the wall Functions, so my y+ should be equal to 30-50. That means to have a first layer thickness equal 1.5E-5 m.
I tried to generate the layers with only 5 layer without success. The calculations are done but nothing appears on paraView (maybe I can't see it ?)

I precise when i choose a much bigger thickness the layers appear.

Any idea why snappyHexMesh refuses to create these layers when I decrease the first layer thickness ?

Thank you

[Flowkersma]

Hi, I would suggest not to use snappyHexMesh to create a mesh around an airfoil. I would use either blockMesh or some other meshing tool such as construct2d or Overture. See this thread.

Regards,
Mikko

[Dorian1504]

Thank you for your answer,

But I need to simulate 3D wing, not an airfoil. Does blockMesh also suitable for this case?
Moreover, I just use the NACA airfoil to have an easier geometry and validate the simulation. The other wing geometries are created by CST method and convert into stl file. I thought snappyHexMesh was a good option since it's an unstructured mesher easy to automate.

Regards,
Dorian

[me3840]

I really doubt snappyHexMesh is good enough to produce that number of layers. Many commercial codes would struggle with that. I would suggest you start with a lower number of layers (5 or so) and work your way up.

If snappy does not produce layers it's because doing so would cause too many quality problems. You have to find out why that is. The easy way is to just reduce the number and gradually build finer meshes, learning from each iteration.