[Andre1742]

Good afternoon, everyone,

I am currently involved in a university project where I am studying a car carrier truck. I started with the motorbike tutorial, going through all the dicts.

My mesh consists of approximately 5,000,000 cells. Based on my computer's RAM (16 GB), I estimate that the maximum number of cells I can handle is around 10,000,000. My mesh includes four refinement boxes, allowing for gradual refinement. Although I consider my mesh to be quite coarse, it is acceptable for the type of study I am conducting.

My main issue is with my residuals, which are a bit too high (see the attached picture). Does anyone have suggestions for reducing these values? Additionally, any advice on improving my mesh would be greatly appreciated.

Here is my fvsolution, fvschemes, and snappyhexmeshdict:


/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: v2306 |
| \\ / A nd | Website: 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
{
Bisarca.stl
{
type triSurfaceMesh;
name Bisarca;
}
refinementBox1
{
type box;
min (-5.0 0.0 0.0);
max ( 45.0 5.0 9.0);
}
refinementBox2
{
type box;
min (-4.0 0.0 0.0);
max (40.0 4.0 8.0);
}
refinementBox3
{
type box;
min (-3.0 0.0 0.0);
max (35.0 3.0 7.0);
}
refinementBox4
{
type box;
min (-2.0 0.0 0.0);
max (30.0 2.0 6.0);
}
}


// 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 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 100000000;

// 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;

// 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 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 "Bisarca.eMesh";
level 5;
}
);



// 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
{
Bisarca
{
// Surface-wise min and max refinement level
level (5 6);

// Optional specification of patch type (default is wall). No
// constraint types (cyclic, symmetry) etc. are allowed.
patchInfo
{
type wall;
inGroups (BisarcaGroup);
}
}
}

// 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
{
refinementBox1
{
mode inside;
levels ((1E15 1));
}
refinementBox2
{
mode inside;
levels ((1E15 2));
}
refinementBox3
{
mode inside;
levels ((1E15 3));
}
refinementBox4
{
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.
locationInMesh (30.5 4.48 9.1);


// 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 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 5;

//- 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
{
"(lowerWall|Bisarca).*"
{
nSurfaceLayers 10;
}
}

// 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 0.01 ;

// 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 0.0001;


// 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 1.7.x! (didn't do anything in 1.7.x)
nGrow 0;

// Advanced settings

// When not to extrude surface. 0 is flat surface, 90 is when two faces
// are perpendicular
featureAngle 60;

// At non-patched sides allow mesh to slip if extrusion direction makes
// angle larger than slipFeatureAngle.
slipFeatureAngle 30;

// Maximum number of snapping relaxation iterations. Should stop
// before upon reaching a correct mesh.
nRelaxIter 4;

// Number of smoothing iterations of surface normals
nSmoothSurfaceNormals 2;

// Number of smoothing iterations of interior mesh movement direction
nSmoothNormals 4;

// Smooth layer thickness over surface patches
nSmoothThickness 8;

// 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
// Note: changed(corrected) w.r.t 1.7.x! 90 degrees corresponds to 130
// in 1.7.x.
minMedialAxisAngle 90;


// 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"

nSmoothScale 4;
errorReduction 0.75;
}



// 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;


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


/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 3.0.1 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
solvers
{
p
{
solver GAMG;
tolerance 1e-8;
relTol 0.001;
smoother GaussSeidel;
nPreSweeps 1;
nPostSweeps 2;
cacheAgglomeration on;
agglomerator faceAreaPair;
nCellsInCoarsestLevel 100;
mergeLevels 1;
}

Phi
{
$p;
}

U
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-8;
relTol 0.0005;
}

k
{
solver smoothSolver;
smoother symGaussSeidel;
tolerance 1e-9;
relTol 0.001;
nSweeps 1;
}

omega
{
solver smoothSolver;
smoother symGaussSeidel;
tolerance 1e-8;
relTol 0.001;
nSweeps 1;
}
}

SIMPLE
{
nUCorrectors 3;
nNonOrthogonalCorrectors 1;
pRefCell 0;
pRefValue 0;

residualControl
{
U 1e-06;
"p.*" 1e-05;
k 1e-06;
omega 1e-07;
}

consistent yes;
}

potentialFlow
{
nNonOrthogonalCorrectors 5;
}

relaxationFactors
{
equations
{
p 0.7;
U 0.7;
k 0.5;
omega 0.5;
}
}

cache
{
grad(U);
}

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


/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.4.0 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object fvSchemes;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

ddtSchemes
{
default steadyState;
}

gradSchemes
{
default cellMDLimited leastSquares 1;
grad(p) cellMDLimited Gauss linear 1;
grad(U) cellMDLimited leastSquares 1;
}

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

laplacianSchemes
{
//default Gauss linear corrected;
default Gauss linear limited 0.5;
}

interpolationSchemes
{
default linear;
}

snGradSchemes
{
//default corrected;
default limited 0.5;
}



wallDist
{
method meshWave;
}

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

[Tobermory]

Two quick suggestions - run longer (the residuals are still dropping), and try adjusting the relaxation factors.

[Andre1742]

Thank you for your consideration!

Any suggestion for the relaxation factor?

[Tobermory]

It's a case of trial and error I am afraid. Try restart from your old run and increase the p factor from 0.7 to 0.9 say, and see what effect that has. If this is beneficial and stable, then keep it, else return the value to 0.7. Next try raising the U factor. You could also try raise the turbulence factors, but they should be less important. Good luck.

[Andre1742]

thank you!

[Andre1742]

Since I still have problem with my residuals being too high, I am considering a solution restart from a convergence situation, so first order methods and then restarting using second order's.
this is my current fvschemes:

/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.4.0 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object fvSchemes;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

ddtSchemes
{
default steadyState;
}

gradSchemes
{
default cellMDLimited leastSquares 1;
grad(p) cellMDLimited Gauss linear 1;
grad(U) cellMDLimited leastSquares 1;
}

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

laplacianSchemes
{
//default Gauss linear corrected;
default Gauss linear limited 0.5;
}

interpolationSchemes
{
default linear;
}

snGradSchemes
{
//default corrected;
default limited 0.5;
}



wallDist
{
method meshWave;
}

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


and this is how i improved

/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.4.0 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object fvSchemes;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

ddtSchemes
{
default steadyState;
}

gradSchemes
{
default cellMDLimited leastSquares 1;
grad(p) cellMDLimited Gauss linear 1;
grad(U) cellMDLimited leastSquares 1;
}

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

laplacianSchemes
{
default Gauss linear corrected;
//default Gauss linear limited 0.5;
}

interpolationSchemes
{
default linear;
}

snGradSchemes
{
default corrected;
// default limited 0.5;
}



wallDist
{
method meshWave;
}

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




Any suggestion is appreciated. Thank you!!!