[Kryoleiche]

Hello everyone,

as a new user to OpenFOAM, I am preparing the pre-processing for my case study of internal streaming through an S-shaped pipeline. I have created a set of STL files using Blender. The geometry consists of an inlet and outlet plane, a circular channel and two 90° junctions, connecting the channel with the in- and outlet. The structure appears waterproof in Paraview.

However, when applying snappyHexMesh to this geometry, the cells on the inlet face are strangely distributed in a fragmented manner, partly to the inlet itself, and partly to the adjoined inlet junction (see screenshots 1 and 2 for clarification). This has consequences for the simulation, as the boundary condition for the inlet velocity can not correctly be applied. Peculiarly, the problem does not affect the outlet (see screenshot 3), even though inlet and outlet are defined in an identical manner in the snappyHexMeshDict (see below). As far as I can see, the STL file seems to be intact, as all vertices of the inlet have the same Z component.

Any ideas on what causes the trouble?

snappyHexMeshDict:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  9
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    object      snappyHexMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

// Which of the steps to run
castellatedMesh true;
snap            true;
addLayers       false;


// 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
{
        CircleRInlet.stl 
        {
        	type triSurfaceMesh; 
        	name inlet;
        }
        CircleRInletJunction.stl 
        {
        	type triSurfaceMesh; 
        	name inletJunction;
        }
	CircleRChannel.stl 
	{
		type triSurfaceMesh; 
		name channel;
	}
        CircleROutletJunction.stl 
        {
        	type triSurfaceMesh;
        	name outletJunction;
        }
        CircleROutlet.stl 
        {
       	type triSurfaceMesh; 
        	name outlet;
        }
         CircleR.stl 
        {
        	type triSurfaceMesh;
        	name all; 	
        }
        
    	/*refinementBox
   	{
        	type searchableBox;
        	min (-26.0 -3 -8);
        	max ( 26.0  3 8);
    	}*/
};



// 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 100000;

    // 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.
    nCellsBetweenLevels 1;



    // Explicit feature edge refinement
    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    // Specifies a level for any cell intersected by its edges.
    // This is a featureEdgeMesh, read from constant/geometry for now.
    features // taken from STL from each .eMesh file created by "SurfaceFeatureExtract" command
    (
        {file "CircleRInlet.eMesh"; level 3;} 
        {file "CircleRInletJunction.eMesh"; level 3;} 
        {file "CircleRChannel.eMesh"; level 3;} 
        {file "CircleROutletJunction.eMesh"; level 3;} 
        {file "CircleROutlet.eMesh"; level 3;} 
        //{file "CircleR.eMesh"; level 3;}
    );



    // 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
    {
	inlet 
	{
		level (1 1);
		patchInfo
		{
			type patch;
		}
	}
	inletJunction 
	{
		level (5 5);
	} // was 4 4
	channel 
	{
		level (3 3);
	} 
	outletJunction 
	{
		level (5 5);
	} 
        outlet 
        {
        	level (1 1);
        	patchInfo
		{
			type patch;
		}
        }// was 4 4
	/*all 
	{
		level (5 5);
	}*/
    }

    // Resolve sharp angles
    resolveFeatureAngle 80;



    // 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
    {
        all				// !!! wichtig, hier zusammengesetztes Volumen verwenden und 						Punkt im Inneren setzen
        {
            mode	distance;
            levels 	((0.1 3));//((0.001 4) (0.003 3) (0.01 2));
        }
    }


    // 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 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 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 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;
    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 false;

    // Per final patch (so not geometry!) the layer information
    layers
    {
    	inletJunction
		{nSurfaceLayers 3;} // was 3
	channel
         	{nSurfaceLayers 3;} // was 3
	outletJunction
		{nSurfaceLayers 3;} // was 3
    }

    // 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.
    // Relative to undistorted size of cell outside layer.
    // See relativeSizes parameter.
    finalLayerThickness 0.1;

    // 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 0.05;

    // 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 80;

    // 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 2;

    // 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 90;

    // 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;
    minTetQuality 1e-30;

    //- 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

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


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

surfaceFeaturesDict:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  9
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    object      surfaceFeaturesDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

surfaces
(
	"CircleRInlet.stl"
	"CircleRInletJunction.stl"
	"CircleRChannel.stl"
	"CircleROutletJunction.stl"
	"CircleROutlet.stl"
	//"CircleR.stl"
);

// Identify a feature when angle between faces < includedAngle
includedAngle       150;

subsetFeatures
{
    // Keep nonManifold edges (edges with >2 connected faces)
    nonManifoldEdges       no;

    // Keep open edges (edges with 1 connected face)
    openEdges       yes;
}

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

blockMeshDict:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  9
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    format      ascii;
    class       dictionary;
    object      blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

convertToMeters 1.0;

vertices
(
    (-30 -5 -10)	// 0
    (30 -5 -10)	// 1
    (30 5 -10)	// 2
    (-30 5 -10)	// 3
    (-30 -5 10)	// 4
    (30 -5 10)	// 5
    (30 5 10)	// 6
    (-30 5 10)	// 7

    
);

blocks
(
	hex (0 1 2 3 4 5 6 7) (60 10 20) simpleGrading (1 1 1)		// Inlet (520 60 160)

);

edges
(
);


patches
(   
	patch bottom
    	(
		(0 1 2 3)
	)
	patch top
    	(
		(7 6 5 4)
	)
	patch end
    	(
		(0 4 5 1)
	)
	patch right
    	(
		(1 5 6 2)
	)
	patch front
    	(
		(3 2 6 7)
	)
	patch left
    	(
		(3 7 4 0)
	)	
);

mergePatchPairs
(
);

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

[Kryoleiche]

Problem solved after re-doing the geometry in Blender. It probably was malfunctioning due to copy-and-paste related issues.