[Edolig]

Hi everyone, I m simulating an automotive rear wing. It's one of the first times I'm using Openfoam and Snappy and I'm trying to understand how some parameters of the mesh can affect the results of the simulation. In particular as it is said in the title I've tried to play with the number of cells between the levels in order to produce a smoother transition, however I've noticed that the results are affected by the parameter nCellsBetweenLayers.

Simulationg with nCellsBetweenLayers=2 the results are for the downforce -3.2386 and drag 0.7336
Simulationg with nCellsBetweenLayers=3 the results are for the downforce -3.1708 and drag 0.7239

So a difference of 2.1% on downfoarce and 1.32% on drag, so no big differences however I'm struggling to choose which can be better. Probably 3 as value is more accurate than 2 but at the same time the computational cost and time is higher since the number of cells increases.

here there is the SnappyHexMeshDict:
[

PHP Code:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  8                                     |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
#include "../initialConditions"
// 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
{
    MAIN.stl
    {
        type triSurfaceMesh;
        name wing_MAIN;
    }
    FLAP.stl
    {
        type triSurfaceMesh;
        name wing_FLAP;
    }
    ENDPLATE.stl
    {
        type triSurfaceMesh;
        name wing_ENDPLATE;
    }
    
    refinementBox
    {
        type searchableBox;
        min $min1;
        max $max1;
    }

    refinementBox2
    {
        type searchableBox;
        min $min2;
        max $max2;
    }

    refinementBox3
    {
        type searchableBox;
        min $min3;
        max $max3;
    }
    
    refinementBox4
    {
        type searchableBox;
        min $min4;
        max $max4;
    }

}



// 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 250000000;

    // 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 250000000;

    // 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;

    // 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 0.05;


    // 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 "MAIN.eMesh";
            level $MAIN_feature;
        }
        {
            file "FLAP.eMesh";
            level $FLAP_feature;
        }
        {
            file "ENDPLATE.eMesh";
            level $ENDPLATE_feature;
        }
          
    );



    // 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_MAIN{
            level $MAIN_RefLvl;
            patchInfo{
                type wall;
                inGroups (wing_Group);
            }
        }
        wing_FLAP{
            level $FLAP_RefLvl;
            patchInfo{
                type wall;
                inGroups (wing_Group);
            }
        }

        wing_ENDPLATE{
            level $ENDPLATE_RefLvl;
            patchInfo{
                type wall;
                inGroups (wing_Group);
            }
        }
       
    }
    // Resolve sharp angles
    resolveFeatureAngle 10; //90


    // 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 (($lvl1 $lvl1));
        }

        refinementBox2
        {
            mode inside;
            levels (($lvl2 $lvl2));
        }
        
        refinementBox3
        {
            mode inside;
            levels (($lvl3 $lvl3));
        }
        
        refinementBox4
        {
            mode inside;
            levels (($lvl4 $lvl4));
        }

    };


    // 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 0.1 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 false;
}


// Settings for the snapping.
snapControls
{
    //- Number of patch smoothing iterations before finding correspondence
    //  to surface
    nSmoothPatch 5;

    //- Relative distance for points to be attracted by surface feature point
    //  or edge. True distance is this factor times local
    //  maximum edge length.
    tolerance 2.5;

    //- 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 30;

    //- 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 false;

    // Per final patch (so not geometry!) the layer information
    layers
    {
        wing_MAIN          {   nSurfaceLayers $MAIN_nLayers;     }
        wing_FLAP          {   nSurfaceLayers $FLAP_nLayers;       }
        wing_ENDPLATE      {   nSurfaceLayers $ENDPLATE_nLayers;       }
         
    }

    // Expansion factor for layer mesh
    expansionRatio 1.2; //1.2

    // 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 0.001;
    
    firstLayerThickness 0.0002;    //0.00005

    // 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 0.000001;    //0.000001

    // 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 359;

    // At non-patched sides allow mesh to slip if extrusion direction makes
    // angle larger than slipFeatureAngle.
    slipFeatureAngle 10;

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

    // Number of smoothing iterations of surface normals
    nSmoothSurfaceNormals 10;

    // Number of smoothing iterations of interior mesh movement direction
    nSmoothNormals 10;

    // Smooth layer thickness over surface patches
    nSmoothThickness 20;

    // Stop layer growth on highly warped cells
    maxFaceThicknessRatio 2;

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

    // Angle used to pick up medial axis points
    // Note: changed(corrected) w.r.t 17x! 90 degrees corresponds to 130 in 17x.
    minMedianAxisAngle 45;


    // 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{
    #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;


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


So I'm asking for your kind opinion about this problem

Best regards

(ps: I would like to attach the two images of the meshes so that you can have a look but I receive an error about "forbidden permission". It's the first time I post on the forum so if someone have some advice about this error let me know and i provide to attach the images)