[petr.f.]

Nice Tuesday evening to all!

I'm trying to simulate external aerodynamics of a car. It's a pretty simple task, however, I'm a bit confused about snappyHexMesh. I've started with the motorBike tutorial. The problem are prism layers on the lowerWall.

If I want to model the lowerWall as a no-slip wall, I have to use the prism layers. But when the relativeSizes are set to true I'll get the same mesh that you can see on the picture. Then, when using wall functions, the y+ is OK (~ 30 - 50) on the motorBike, but totally off-limits (~ 3500) near the inlet and outlet from the computational domain.

Now If I try to set the relativeSizes to false and mesh my car case with layers satisfying y+ ~ 30 (then in my case delta y = 0.8 mm near the wall), snappyHexMesh fails with floating point exception. If I try to mesh the case with relativeSizes true, but set a different layers parameters for the car and different for lowerWall then the layers on the lowerWall are either not generated at all or they look like in the motorBike tutorial and I get bad y+ ...

I could consider the lowerWall patch as a slip wall, but that's not how it is in reality. What is your experience with this problem?

About the meshing proces I'm using - it's quite standard.
1) blockMesh -> surfaceFeatureExtract -> decompoePar -> renumberMesh (optional) -> snappyHexMesh for castellated and snap mesh.
2) checkMesh, if the snap mesh is OK (I only get skewness ~ 6 on boundaries errors, nothing serious) I proceed with the addLayer phase. I've taken the snappy configuration from the motorbike tutorial, only changed the mesh resolution around the model (my base size is 0.672 m, so the levels on the car body are 7 - 8)

I'm using these snap, addLayers and meshQuality controls (and latest OpenFOAM 2.3.x):

snapControls
{
        globalFeatureEdges      true;
        nSolveIter              30;
        nSmoothPatch            3; // default 3
        tolerance               2.0; // def. 2, 
        nRelaxIter              5; // def. 5
        nFeatureSnapIter        10; // def. 10
        implicitFeatureSnap     false;
        explicitFeatureSnap     true;
        multiRegionFeatureSnap    false;
}

addLayersControls
{
    layers
    {
        body                {nSurfaceLayers 5;}
        chassis             {nSurfaceLayers 5;}
        frontFender         {nSurfaceLayers 3;}
        frontSuspension     {nSurfaceLayers 3;}
        frontWheelContact   {nSurfaceLayers 3;}
        frontWheel          {nSurfaceLayers 5;}
        rearFender          {nSurfaceLayers 3;}
        rearSuspension      {nSurfaceLayers 3;}
        rearWheelContact    {nSurfaceLayers 3;}
        rearWheel           {nSurfaceLayers 5;}

        lowerWall
        {
            nSurfaceLayers 3;

            expansionRatio 1.5;
            firstLayerThickness 0.001;
            minThickness 0.01;
            
//             relativeSizes false;
//             expansionRatio 1.5;
//             firstLayerThickness 0.002;
//             minThickness 5e-4;
//             featureAngle 50;
        }
    }

    relativeSizes true;
    expansionRatio 1.2;
    firstLayerThickness 0.157; 
    minThickness 0.01; 

//     relativeSizes false;
//     expansionRatio 1.2;
//     firstLayerThickness 0.0008;
//     minThickness 1e-5;

    nGrow 0;
    featureAngle 150; 
    slipFeatureAngle 30;
    nRelaxIter 3; 
    nSmoothSurfaceNormals 1;
    nSmoothNormals 3;
    nSmoothThickness 10;
    maxFaceThicknessRatio 0.5;
    maxThicknessToMedialRatio 0.3;
    minMedianAxisAngle 90;
    nMedialAxisIter 10;
    nBufferCellsNoExtrude 0;
    nLayerIter 50;
}

meshQualityControls
{
    maxNonOrtho 75; //def. 65
    maxBoundarySkewness 20;
    maxInternalSkewness 4;
    maxConcave 80;
    minFlatness 0.5;
    minVol 1.0E-13;
    minTetQuality 1.0e-30;
    minArea -1.0;
    minTwist 0.02;
    minDeterminant 0.001; // def. 0.01
    minFaceWeight 0.02; // def. 0.05
    minVolRatio 1e-4; // def. 0.01
    minTriangleTwist -1.0;
    nSmoothScale 4;
    errorReduction 0.75;
}

snappyHexMesh errors - floating point exception

Layer addition iteration 0
--------------------------

Determining displacement for added points according to pointNormal ...
Detected 0 points with point normal pointing through faces.
Reset displacement at 0 points to average of surrounding points.

displacementMedialAxis : Smoothing using Medial Axis ...
displacementMedialAxis : Reducing layer thickness at 34025 nodes where thickness to medial axis distance is large
displacementMedialAxis : Removing isolated regions ...
displacementMedialAxis : Number of isolated points extrusion stopped : 214133
displacementMedialAxis : Smoothing field ...
    Iteration 0   residual 1.49626e-05
displacementMedialAxis : Moving mesh ...
displacementMedialAxis : Iteration 0
Moving mesh using displacement scaling : min:1  max:1
Checking faces in error :
    non-orthogonality > 65  degrees                        : 273
    faces with face pyramid volume < 1e-13                 : 34
    faces with face-decomposition tet quality < 1e-30      : 2439
    faces with concavity > 80  degrees                     : 41
    faces with skewness > 4   (internal) or 20  (boundary) : 0
    faces with interpolation weights (0..1)  < 0.05        : 0
    faces with volume ratio of neighbour cells < 0.01      : 0
    faces with face twist < 0.02                           : 88
    faces on cells with determinant < 0.01                 : 335
displacementMedialAxis : Iteration 1
Moving mesh using displacement scaling : min:0.75  max:1
[9] #0  Foam::error::printStack(Foam::Ostream&) in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libOpenFOAM.so"
[9] #1  Foam::sigFpe::sigHandler(int) in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libOpenFOAM.so"
[9] #2
[9]  at sigaction.c:0
[9] #3  Foam::face::centre(Foam::Field<Foam::Vector<double> > const&) const in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libOpenFOAM.so"
[9] #4  Foam::face::sweptVol(Foam::Field<Foam::Vector<double> > const&, Foam::Field<Foam::Vector<double> > const&) const in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libOpenFOAM.so"
[9] #5  Foam::primitiveMesh::movePoints(Foam::Field<Foam::Vector<double> > const&, Foam::Field<Foam::Vector<double> > const&) in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libOpenFOAM.so"
[9] #6  Foam::polyMesh::movePoints(Foam::Field<Foam::Vector<double> > const&) in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libOpenFOAM.so"
[9] #7  Foam::fvMesh::movePoints(Foam::Field<Foam::Vector<double> > const&) in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libfiniteVolume.so"
[9] #8  Foam::motionSmootherAlgo::scaleMesh(Foam::List<int>&, Foam::List<Foam::Pair<int> > const&, Foam::dictionary const&, Foam::dictionary const&, bool, int) in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libdynamicMesh.so"
[9] #9  Foam::medialAxisMeshMover::shrinkMesh(Foam::dictionary const&, int, Foam::List<int>&) in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libautoMesh.so"
[9] #10  Foam::medialAxisMeshMover::move(Foam::dictionary const&, int, Foam::List<int>&) in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libautoMesh.so"
[9] #11  Foam::autoLayerDriver::addLayers(Foam::layerParameters const&, Foam::dictionary const&, Foam::List<int> const&, int, Foam::decompositionMethod&, Foam::fvMeshDistribute&) in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libautoMesh.so"
[9] #12  Foam::autoLayerDriver::doLayers(Foam::dictionary const&, Foam::dictionary const&, Foam::layerParameters const&, bool, Foam::decompositionMethod&, Foam::fvMeshDistribute&) in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libautoMesh.so"
[9] #13
[9]  in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/bin/snappyHexMesh"
[9] #14  __libc_start_main in "/lib64/libc.so.6"
[9] #15
[9]  in "/opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/bin/snappyHexMesh"
[ceprlx19:32502] *** Process received signal ***
[ceprlx19:32502] Signal: Floating point exception (8)
[ceprlx19:32502] Signal code:  (-6)
[ceprlx19:32502] Failing at address: 0x858800007ef6
[ceprlx19:32502] [ 0] /lib64/libc.so.6() [0x3548a32920]
[ceprlx19:32502] [ 1] /lib64/libc.so.6(gsignal+0x35) [0x3548a328a5]
[ceprlx19:32502] [ 2] /lib64/libc.so.6() [0x3548a32920]
[ceprlx19:32502] [ 3] /opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libOpenFOAM.so(_ZNK4Foam4face6centreERKNS_5FieldINS_6VectorIdEEEE+0x14f) [0x7f391720f40f]
[ceprlx19:32502] [ 4] /opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libOpenFOAM.so(_ZNK4Foam4face8sweptVolERKNS_5FieldINS_6VectorIdEEEES6_+0x3a) [0x7f391720fb0a]
[ceprlx19:32502] [ 5] /opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libOpenFOAM.so(_ZN4Foam13primitiveMesh10movePointsERKNS_5FieldINS_6VectorIdEEEES6_+0xdd) [0x7f39172c195d]
[ceprlx19:32502] [ 6] /opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libOpenFOAM.so(_ZN4Foam8polyMesh10movePointsERKNS_5FieldINS_6VectorIdEEEE+0x1f3) [0x7f39172a0e63]
[ceprlx19:32502] [ 7] /opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libfiniteVolume.so(_ZN4Foam6fvMesh10movePointsERKNS_5FieldINS_6VectorIdEEEE+0xbc) [0x7f391963d5ec]
[ceprlx19:32502] [ 8] /opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libdynamicMesh.so(_ZN4Foam18motionSmootherAlgo9scaleMeshERNS_4ListIiEERKNS1_INS_4PairIiEEEERKNS_10dictionaryESB_bi+0x201) [0x7f3917e114a1]
[ceprlx19:32502] [ 9] /opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libautoMesh.so(_ZN4Foam19medialAxisMeshMover10shrinkMeshERKNS_10dictionaryEiRNS_4ListIiEE+0x10c) [0x7f39178d379c]
[ceprlx19:32502] [10] /opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libautoMesh.so(_ZN4Foam19medialAxisMeshMover4moveERKNS_10dictionaryEiRNS_4ListIiEE+0x465) [0x7f39178e0e95]
[ceprlx19:32502] [11] /opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libautoMesh.so(_ZN4Foam15autoLayerDriver9addLayersERKNS_15layerParametersERKNS_10dictionaryERKNS_4ListIiEEiRNS_19decompositionMethodERNS_16fvMeshDistributeE+0xdc9) [0x7f39177b6de9]
[ceprlx19:32502] [12] /opt/OpenFOAM/OpenFOAM-2.3.x/platforms/linux64Gcc48DPOpt/lib/libautoMesh.so(_ZN4Foam15autoLayerDriver8doLayersERKNS_10dictionaryES3_RKNS_15layerParametersEbRNS_19decompositionMethodERNS_16fvMeshDistributeE+0x500) [0x7f39177ba0e0]
[ceprlx19:32502] [13] snappyHexMesh() [0x43951d]
[ceprlx19:32502] [14] /lib64/libc.so.6(__libc_start_main+0xfd) [0x3548a1ecdd]
[ceprlx19:32502] [15] snappyHexMesh() [0x4138a1]
[ceprlx19:32502] *** End of error message ***
--------------------------------------------------------------------------

Thanks a lot, Petr.

[KateEisenhower]

Hi Petr,

I am sorry I can't help you with your problem. Nethertheless I have a question: How do you get to know your y+ values? Do you just make a guess, do you measure it with the ruler tool in ParaView?

Thanks in advance,

Kate

[petr.f.]

Hi Kate,

no problem, I've more or less solved it by using three different snappyHexMeshDict files (one for castellated&snap phase, one for addLayers on the body and one for addLayers on the lower wall) with different settings for snapping and both addLayers.

About the y+ - there is a utility in OpenFOAM for that purpose - yPlusRAS (if you are using RANS modelling approach, yPlusLES if you are using LES...), which can do the job for you. However, the y+ computation through yPlusRAS gives you apparently y* value (although I haven't check the latest OpenFOAM release), for details see e.g.

http://www.cfd-online.com/Forums/ope...-testcase.html

Petr.