[malaboss]

Hi FOAMers !
I am now studying an airfoil and I would like to create the mesh around it with snappyHexMesh. When I ran some simulations, it gave me a high drag.
Looking at the profile more precisely I saw that the stream line left the profile quite early (at least earlier than they should, which could be the cause of this higher drag. Looking more precisely, I saw that the profile did not respect well the NACA geometry which is normal as snappyHexMesh does not generate curves cells.

I spent several hours looking for the parameters I should change to improve the precision of the snap to the surface and I could not find the solution.


Do you know how can I improve the snapping ?


You can find attached the bad surface I get and my snappyHexMeshDict File.

Castellated

castellatedMeshControls
{
	allowFreeStandingZoneFaces true;
    // 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 100000000;

    // 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 200000000;

    // 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 20; //10

    // Number of buffer layers between different levels.
    // 1 means normal 2:1 refinement restriction, larger means slower
    // refinement.
    nCellsBetweenLevels 20;   // 2;



    // 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 "surfacemesh.eMesh";
            level 8;
        }
    );



    // 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
    {
        stlSurface
        {
            // Surface-wise min and max refinement level
            level (3 3);  // 5 6
        }
    }

    // Resolve sharp angles
    resolveFeatureAngle 30;   // 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
    {
        refinementBox
        {
            mode inside;   // inside;
            levels ((1.0e+15 0));  //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 (-1.39263 1.8424 0.0477);
    //locationInMesh (-3023.55 4000 -103.606); 
	locationInMesh (-3 11 0); 

}

Snap

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 5;

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

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

addLayers

addLayersControls
{
    relativeSizes true;  //true;   // v 1.6
    // Per final patch (so not geometry!) the layer information
    layers
    {
        //minZ
        //{
        //    nSurfaceLayers 3;
        //}
        stlSurface_patch0   // a_surface_name
        {
            nSurfaceLayers 15;//2;
        } 
    }

    // Expansion factor for layer mesh
    expansionRatio 1.03;   // 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.
    //finalLayerRatio FINALLAYERRATIO;   // 0.3; 
    finalLayerThickness 0.8;   // 0.3;    // v 1.6

    //- 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 0.001;   // 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;   // 1;


    // Advanced settings

    //- When not to extrude surface. 0 is flat surface, 90 is when two faces
    //  make straight angle.
    featureAngle 180;   // 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 500;//5

    // Number of smoothing iterations of interior mesh movement direction  
    nSmoothNormals 50;//3

    // Smooth layer thickness over surface patches
    nSmoothThickness 100;//10;

    // Stop layer growth on highly warped cells 
    maxFaceThicknessRatio 0.8;//0.5;

    // Reduce layer growth where ratio thickness to medial 
    // distance is large 
    maxThicknessToMedialRatio 0.5;//0.3;

    // Angle used to pick up medial axis points
    minMedianAxisAngle 180;//130;

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

    // Overall max number of layer addition iterations   // v 1.6
    nLayerIter 10;//50;   // v 1.6
}

Every help is welcome !
Thanks in advance !