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[snappyHexMesh] Problem in results with snappyHexMesh between refinement levels

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Old   November 8, 2018, 04:46
Unhappy Problem in results with snappyHexMesh between refinement levels
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Phil
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Hi,

i ran into a problem when solving a case for external flows around a car with pisoFoam and snappyHexMesh.

You can clearly see a big influence from the mesh between different refinement levels (see attached pictures)

Above the Car one can clearly see the edges of the different refinement levels in the velocity profile.

I have the same problem in the transition between the layers and the surrounding mesh at the front of the car (see attached picture).

Has anyone an idea what seems to be the problem?
I also did the motorbike tutorial to see if this problem happens there too. And in some Areas you can clearly see those boundary lines in the velocity profile.

I dont think that this is a Problem with snappyHexMesh itself, because it is commonly used. But what am I doing wrong?

I write the content from fvSchemes fvSolution and SnappyHexMesh in the next post.


Hope one of you can help me


U_profile_above_car.jpg

U_profile_above_car_with_mesh.jpg

car_front_U_profile.png
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Old   November 8, 2018, 04:48
Default fvSchemes
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fvSchemes



ddtSchemes
{
default Euler;
}

gradSchemes
{
default leastSquares;
}

divSchemes
{
default none;
div(phi,U) Gauss linearUpwindV grad(U);
div(phi,k) Gauss upwind;
div(phi,kl) Gauss upwind;
div(phi,kt) Gauss upwind;
div(phi,omega) Gauss upwind;
div((nuEff*dev2(T(grad(U))))) Gauss linear;
}

laplacianSchemes
{
default Gauss linear corrected;
}

interpolationSchemes
{
default linear;
}

snGradSchemes
{
default corrected;
}

wallDist
{
method meshWave;
}




fvSolution


solvers
{
p
{
solver GAMG;
tolerance 1e-6;
//relTol 0.01;
smoother GaussSeidel;
nPreSweeps 0;
nPostSweeps 1;
nFinestSweeps 2;
cacheAgglomeration true;
nCellsInCoarsestLevel 20;
agglomerator faceAreaPair;
mergeLevels 1;
}

pFinal
{
$p;
tolerance 1e-06;
relTol 0;
}

"(U|k|kl|kt|omega)"
{
solver GAMG;
smoother GaussSeidel;
tolerance 1e-06;
//relTol 0.01
nPreSweeps 0;
nPostSweeps 1;
nFinestSweeps 2;
cacheAgglomeration true;
nCellsInCoarsestLevel 20;
agglomerator faceAreaPair;
mergeLevels 1;
}

"(U|k|kl|kt|omega)Final"
{
$U;
tolerance 1e-06;
relTol 0;
}
}

PISO
{
nOuterCorrectors 1;
nCorrectors 2;
nNonOrthogonalCorrectors 1;
pRefCell 0;
pRefValue 0;
}

relaxationFactors
{
equations
{
p 0.3; // 0.9 is more stable but 0.95 more convergent
"(U|k|kl|kt|omega)" 0.7; // 0.9 is more stable but 0.95 more convergent
}
}




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
{
Estate_quer_3d.stl
{
type triSurfaceMesh;
name estate;
}

//- Refine a bit extra around the small centre hole
refinebox
{
type searchableBox;
min (-500.0 0 -80);
max (3000 1000 500);
}

refinebox1
{
type searchableBox;
min (-300 0 -80);
max (1000 1000 450);
}
refinebox2
{
type searchableBox;
min (-200 0 -80);
max (800 1000 400);
}
};



// 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 20000000;

// 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;

// Number of buffer layers between different levels.
// 1 means normal 2:1 refinement restriction, larger means slower
// refinement.
nCellsBetweenLevels 10;



// 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 "Estate_quer_3d.extendedFeatureEdgeMesh";
level 7;
}
);



// 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
{
estate
{
// Surface-wise min and max refinement level
level (7 7);
}
}

resolveFeatureAngle 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
{
refinebox
{
mode inside;
levels ((1E15 2));
}

refinebox1
{
mode inside;
levels ((1E15 3));
}
refinebox2
{
mode inside;
levels ((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.
// This is an outside point locationInMesh (-0.033 -0.033 0.0033);
locationInMesh (2100 0.1 700); // 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 true;
}



// 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
{
estate
{
nSurfaceLayers 3;
}
}

// Expansion factor for layer mesh
expansionRatio 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 0.5;

// Minimum thickness of cell layer. If for any reason layer
// cannot be above minThickness do not add layer.
// See relativeSizes parameter.
minThickness 0.25;

// 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 70;

// 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 120;

// 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 5;

// 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;
}

Last edited by Phil_WO; November 8, 2018 at 04:56. Reason: Attached everything in this post
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Old   November 8, 2018, 04:49
Default fvSolution
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solvers
{
p
{
solver GAMG;
tolerance 1e-6;
//relTol 0.01;
smoother GaussSeidel;
nPreSweeps 0;
nPostSweeps 1;
nFinestSweeps 2;
cacheAgglomeration true;
nCellsInCoarsestLevel 20;
agglomerator faceAreaPair;
mergeLevels 1;
}

pFinal
{
$p;
tolerance 1e-06;
relTol 0;
}

"(U|k|kl|kt|omega)"
{
solver GAMG;
smoother GaussSeidel;
tolerance 1e-06;
//relTol 0.01
nPreSweeps 0;
nPostSweeps 1;
nFinestSweeps 2;
cacheAgglomeration true;
nCellsInCoarsestLevel 20;
agglomerator faceAreaPair;
mergeLevels 1;
}

"(U|k|kl|kt|omega)Final"
{
$U;
tolerance 1e-06;
relTol 0;
}
}

PISO
{
nOuterCorrectors 1;
nCorrectors 2;
nNonOrthogonalCorrectors 1;
pRefCell 0;
pRefValue 0;
}

relaxationFactors
{
equations
{
p 0.3; // 0.9 is more stable but 0.95 more convergent
"(U|k|kl|kt|omega)" 0.7; // 0.9 is more stable but 0.95 more convergent
}
}
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