[MohdShaeq]

Hello,

I have been trying very hard to generate a hexahedral mesh for a relatively simple geometry with snappyHexMesh. Even though I have now gained a fair bit of experience in working with the utility, however, I have not been able to generate the desired mesh as not all the features/surfaces are correctly recognised. I have tried several values of level under the features and refinementSurfaces sub-dictionaries, unfortunately to no avail.

Attached herewith are the images of the STL-surfaces and the generated mesh. The snappyHexMeshDict is as follows:-

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v1912                                 |
|   \\  /    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
{
	Geometrie // STL filename
	{
		type triSurfaceMesh;
		file "Geometrie.stl";

		regions
		{
			Brennkammer
			{
				name Brennkammer;
			}

			Einlass
			{
				name Einlass;
			}

			Kanal_aussen
			{
				name Kanal_aussen;
			}

			Kanal_innen
			{
				name Kanal_innen;
			}
		}
	}

	Verfeinerungszylinder1
	{
		type      searchableCylinder;
		point1    (0 0 -0.03);
		point2    (0 0 0.07);
		radius    0.001652;
	}

	Verfeinerungszylinder2
	{
		type      searchableCylinder;
		point1    (0 0 0);
		point2    (0 0 -0.02);
		radius    0.043;
	}
};

// Settings for the castellatedMesh generation.	
castellatedMeshControls		//sub-dictionary of controls for castellated mesh.
{ 
	// 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 300000; //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 30000000; //1000000;
	
	// 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; //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/triSurface for now.
	features
	(
		{
		file "Geometrie.eMesh";
		level 5; //4; //5; //6
		}
	);

	// 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
	{
		Geometrie
		{
			level (1 1); // default (min max) refinement for whole surface

			regions
			{
				Brennkammer
				{
					level (0 0);
					patchInfo
					{
						type wall; // Optional specification of patch type (default is wall). No constraint types (cyclic, symmetry) etc. are allowed.
					}
				}

				Einlass
				{
					level (6 6);
					patchInfo
					{
						type patch; // Optional specification of patch type (default is wall). No constraint types (cyclic, symmetry) etc. are allowed.
					}
				}

				Kanal_aussen
				{
					level (3 3);
					patchInfo
					{
						type wall; // Optional specification of patch type (default is wall). No constraint types (cyclic, symmetry) etc. are allowed.
					}
				}

				Kanal_innen
				{
					level (3 3);
					patchInfo
					{
						type wall; // Optional specification of patch type (default is wall). No constraint types (cyclic, symmetry) etc. are allowed.
					}
				}
			}
		}
	}

	// 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
	{
		Geometrie
		{
			mode distance;
			levels ((0.005 3));
		}

		Verfeinerungszylinder1
		{
			mode inside;
			levels ((1E15 3));
		}

		Verfeinerungszylinder2
		{
			mode distance;
			levels ((0.02 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 -0.02);

	// 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 false;
}
 
 // 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 2.0;

	//- 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 10;

		// - 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 true ;
 }
 
 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
    {
	   Geometrie
       {
		   nSurfaceLayers 2;
	   }
    }

    // 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 0.3;

    // 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.
    // 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 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
    // 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"

	maxNonOrtho 65;
    maxBoundarySkewness 20;
    maxInternalSkewness 4;
    maxConcave 80;
    minVol 1e-13;
    minTetQuality 1e-13;
    minArea -1;
    minTwist 0.05;
    minDeterminant 0.001;
    minFaceWeight 0.05;
    minVolRatio 0.01;
    minTriangleTwist -1;

	/*
	// Optional : some meshing phases allow usage of relaxed rules.
    // See e.g. addLayersControls::nRelaxedIter.
    relaxed
    {
        //- Maximum non-orthogonality allowed. Set to 180 to disable.
        maxNonOrtho 75;
    }

    // Advanced

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

    // Advanced
    nSmoothScale 4;
    errorReduction 0.75;

    relaxed
    {
        maxNonOrtho 55;
    }

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


// Write flags
writeFlags
(
    scalarLevels	// write volScalarField with cellLevel for postprocessing
    layerSets		// write cellSets, faceSets of faces in layer
    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;


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

I kindly entreat those reading this thread to provide me with suggestions and/or improvements. I would be highly thankful to you all.

Kind regards,
Mohd Shaeq

[MohdShaeq]

I have been able to solve this problem. The trick lay in, as far as I can tell, employing inGroups under refinementSurfaces under the castellatedMeshControls sub-dictionary. The snappyHexMeshDict now reads as follows:-

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2112                                 |
|   \\  /    A nd           | Website:                                        |
|    \\/     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
{
    Geometrie
    {
        type triSurfaceMesh;
        file "Geometrie.stl";
        regions
        {
            Brennkammer    { name Brennkammer;}
            Einlass        { name Einlass;}
            Kanal_aussen   { name Kanal_aussen;}
            Kanal_innen    { name Kanal_innen;}
        }
    }

    Verfeinerungszylinder1
	{
		type      searchableCylinder;
		point1    (0 0 -0.03);
		point2    (0 0 0.07);
		radius    0.0025;
	}

	Verfeinerungszylinder2
	{
		type      searchableCylinder;
		point1    (0 0 0);
		point2    (0 0 -0.02);
		radius    0.043;
	}
};

// Settings for the castellatedMesh generation.
castellatedMeshControls		//sub-dictionary of controls for castellated mesh.
{
	// 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 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/triSurface for now.
    features
    (
        {
            file "Geometrie.eMesh";
            level 4;
        }
    );



    // 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
    {
		Geometrie
        {
            level (3 3); // Surface - wise min and max refinement level
            patchInfo
            {
                type wall;
                inGroups (GeometrieGroup);
            }

            regions
		    {
                Einlass         { level (1 1); patchInfo { type patch;} }
                Kanal_aussen	{ level (6 6); patchInfo { type wall;} }
                Kanal_innen		{ level (6 6); patchInfo { type wall;} }
		    }
		 }
     }

    // 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
    {
        Verfeinerungszylinder1
		{
			mode inside;
			levels ((1E15 1));
		}

		Verfeinerungszylinder2
		{
			mode distance;
			levels ((0.02 3));
		}
    }

    // 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 (0 0 -0.02);

    /*locationsInMesh
(
    (( 0 0 -0.02) Brennkammer)
    (( 0  0.00199  0.0068) Kanal_aussen)
);*/

    // 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
 {
	nSmoothPatch 3;
	tolerance 2.0;
	nSolveIter 30;
	nRelaxIter 5;
	// Feature snapping
		nFeatureSnapIter 10;
		// - 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 true ;
 }

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
    {
        Geometrie
        {
            nSurfaceLayers 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 0.3;

    // 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.
    // 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 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
    // 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"


    // Advanced

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

}

// 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;

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

I hope that this thread maybe of help to someone in the future.

Kind regards,
Mohd Shaeq