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I wonder, how do I mesh a car?

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Old   May 2, 2011, 15:39
Default I wonder, how do I mesh a car?
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Mads Reck
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Hey fellow foamers,

for a bit of fun I have now, for some time, been trying to do an OpenFOAM simulation on a car. I am trying to use snappyHexMesh for this whilst I know there are, commercial, alternatives.

Anyways, I struggle a bit - like many others it seems in these forums - with the layers. For aerodynamic simulations, a good resolution of the boundary layer is of course essential, so the ultimate goal is to resolve down to a max(y+) of around 2. To start with we could try to just make some layers without thinking too much about the resolution. After that maybe we can get the layer sizes down. This is what I tried below where I've taken a car geometry from a website (mentioned below), I hope there are no copyright issues there. I tried to take the motobike tutorial from OpenFOAM and modify it to accommodate the car. Other than that, I didn't do much besides making a boundingBox-mesh where the car floats freely in. Obviously it should touch the lower wall, but let's keep it very simple for now. I tried to show all the steps below.

Anyways, this sounds all nice and dandy - but the layer part is not really robust. I rarely get my wanted 8-10-15 layers consistently around the geometry, which I expect would be needed to get the low max(y+) I want. And it is really hard when I try to get really thin layers.

All help and tricks are greatly appreciated

best regards
Mads
ps. I edited this post a bit, because initial problems were solved (updated in the code below)

-------------------

cd <your working directory>
cp -r $FOAM_TUTORIALS/mesh/snappyHexMesh/motorBike/ firstCarTest
cd firstCarTest/

####download car
wget http://www.dmi3d.com/models/porche911.zip
unzip porche911.zip

####blender part
blender
<import Porche 911.lwo, export as an STL named porsche911.stl>
<move porsche911.stl to <your working directory>/firstCarTest/constant/trisurface

####make sure the STL is closed and nice
cd <your working directory>/firstCarTest/constant/triSurface
admesh porsche911.stl -a porsche911_clean.stl


####edit the newly, clean STL file so the first line and last line reads:
solid myCar
[...]
endsolid myCar

####let's edit the blockMeshDict to our new boundingBox
Code:
/*---------------------------------------------------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  1.7.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/

FoamFile
{
    version         2.0;
    format          ascii;

    root            "";
    case            "";
    instance        "";
    local           "";

    class           dictionary;
    object          blockMeshDict;
}

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


convertToMeters 1;

vertices
(
    (-10 -20 -10) //EDIT ADD
    ( 10 -20 -10) //EDIT ADD
    ( 10  100 -10) //EDIT ADD
    (-10  100 -10) //EDIT ADD
    (-10 -20  50) //EDIT ADD
    ( 10 -20  50) //EDIT ADD
    ( 10  100  50) //EDIT ADD
    (-10  100  50) //EDIT ADD
 /*EDIT DELETE  
    (-5 -4 0)
    (15 -4 0)
    (15  4 0)
    (-5  4 0)
    (-5 -4 8)
    (15 -4 8)
    (15  4 8)
    (-5  4 8)
    EDIT DELETE*/
);

blocks
(
    //EDIT DELETE hex (0 1 2 3 4 5 6 7) (20 8 8) simpleGrading (1 1 1)
    hex (0 1 2 3 4 5 6 7) (20 120 60) simpleGrading (1 1 1)  // EDIT ADD
);

edges
(
);

patches
(
    patch frontAndBack
    (
        (3 7 6 2)
        (1 5 4 0)
    )
    patch inlet
    (
        (0 4 7 3)
    )
    patch outlet
    (
        (2 6 5 1)
    )
    wall lowerWall
    (
        (0 3 2 1)
    )
    patch upperWall
    (
        (4 5 6 7)
    )
);

// ************************************************************************* //
####let's edit the snappyHexMeshDict to the new geometry
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  1.7.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
|    \\/     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
{
//EDIT DELETE   motorBike.stl
porsche911_clean.stl  //EDIT ADD
    {
        type triSurfaceMesh;
        // EDIT DELETE name motorBike;
        name myCar; // EDIT ADD
    }

//EDIT DELETE   refinementBox
//EDIT DELETE   {
//EDIT DELETE       type searchableBox;
//EDIT DELETE       min (-1.0 -0.7 0.0);
//EDIT DELETE       max ( 8.0  0.7 2.5);
//EDIT DELETE    }
};



// 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 1000000;

    // 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 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.
//EDIT DELETE    nCellsBetweenLevels 3;
    nCellsBetweenLevels 1; //EDIT ADD



    // 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 "someLine.eMesh";
        //    level 2;
        //}
    );



    // 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
    {
   //EDIT DELETE     motorBike
        myCar //EDIT NEW
        {
            // Surface-wise min and max refinement level
  //EDIT DELETE          level (5 6);
            level (4 4);  //EDIT NEW - very coarse
        }
    }

    // Resolve sharp angles
    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
    {
//EDIT DELETE        refinementBox
//EDIT DELETE        {
//EDIT DELETE            mode inside;
//EDIT DELETE            levels ((1E15 4));
//EDIT DELETE        }
    }


    // 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 (3 3 0.43);
}



// 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 4.0;

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

    //- Maximum number of snapping relaxation iterations. Should stop
    //  before upon reaching a correct mesh.
    nRelaxIter 5;
}



// 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
    {     
      myCar_myCar                 //EDIT ADD
      {                     //EDIT ADD
        nSurfaceLayers 8;   //EDIT ADD
      }                     //EDIT ADD


/*
EDIT DELETE        
       // I DELETED ALL THE motorBike surface layer things
EDIT DELETE 
*/
    }

    // Expansion factor for layer mesh
    expansionRatio 1.0;

    //- Wanted thickness of final added cell layer. If multiple layers
    //  is the thickness of the layer furthest away from the wall.
    //  See relativeSizes parameter.
    finalLayerThickness 0.3;

    //- 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 0.1;

    //- 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;


    // Advanced settings

    //- When not to extrude surface. 0 is flat surface, 90 is when two faces
    //  make straight angle.
    featureAngle 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
    minMedianAxisAngle 130;

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

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



// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
meshQualityControls
{
    //- Maximum non-orthogonality allowed. Set to 180 to disable.
    maxNonOrtho 65;

    //- Max skewness allowed. Set to <0 to disable.
    maxBoundarySkewness 20;
    maxInternalSkewness 4;

    //- Max concaveness allowed. Is angle (in degrees) below which concavity
    //  is allowed. 0 is straight face, <0 would be convex face.
    //  Set to 180 to disable.
    maxConcave 80;

    //- Minimum projected area v.s. actual area. Set to -1 to disable.
    minFlatness 0.5;

    //- Minimum pyramid volume. Is absolute volume of cell pyramid.
    //  Set to a sensible fraction of the smallest cell volume expected.
    //  Set to very negative number (e.g. -1E30) to disable.
    minVol 1e-13;

    //- Minimum face area. Set to <0 to disable.
    minArea -1;

    //- Minimum face twist. Set to <-1 to disable. dot product of face normal
    //- and face centre triangles normal
    minTwist 0.02;

    //- minimum normalised cell determinant
    //- 1 = hex, <= 0 = folded or flattened illegal cell
    minDeterminant 0.001;

    //- minFaceWeight (0 -> 0.5)
    minFaceWeight 0.02;

    //- minVolRatio (0 -> 1)
    minVolRatio 0.01;

    //must be >0 for Fluent compatibility
    minTriangleTwist -1;


    // Advanced

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


// Advanced

// Flags for optional output
// 0 : only write final meshes
// 1 : write intermediate meshes
// 2 : write volScalarField with cellLevel for postprocessing
// 4 : write current intersections as .obj files
debug 0;


// Merge tolerance. Is fraction of overall bounding box of initial mesh.
// Note: the write tolerance needs to be higher than this.
mergeTolerance 1E-6;


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

### there is an error in the tutorial, at least in my version. Let's fix it
mv system/fvSolution.org system/fvSolution

####lets' blockMesh
blockMesh

####lets' snappyHexMesh
snappyHexMesh

#### wait a bit and then let's look at the result
foamToVTK -latestTime

### load into paraview
paraview
<in paraview open the folder VTK and then open the file firstCarTest_0.vtk>
<create an x-slice and make sure that surface with edges is turned on>
<turn the view to see, you will see a picture as the one I attached, maybe you can see: the layers are quite uneven>

That was a lot of uploading, I hope I got it right
Attached Images
File Type: jpg meshView.jpg (69.4 KB, 130 views)
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car aerodynamics, snappyhexmesh, three-dimensional

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