[MohdShaeq]

Hello respected members of the OpenFOAM community,

I want to investigate the injection characteristics of hydrogen in a hydrogen engine and for that I am have generated a 3D-mesh for the hydrogen engine using the snappyHexMesh utility.

Although the generated mesh is okay for the combustion chamber, it is terrible for the parts which are the most important for my simulations, i.e. nozzle, injector fluid chamber and valve needle. I generated the 2D triangular mesh in Salome and used the resulting STL file to generate the 3D hexahedral mesh.

I have read several threads regarding skewed faces and also tweaked my snappyHexMeshDict, unfortunately to no avail. I have attached the photos of the generated 2D and 3D meshes for nozzle (Duese) and injector fluid chamber (Injektor Fluidkammer) in this post and for valve needle (Nadel) in the following one.

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.surface is of class
// searchablecylinder.
geometry
{
	Wasserstoffmotor // STL filename
	{
		type triSurfaceMesh;
		file "Wasserstoffmotor.stl";
		
		regions
		{
			Ansaugrohr {name Ansaugrohr; }
			Basalkruemmer {name Basalkruemmer; }
			Duese {name Duese; }
			InjektorFluidkammer {name InjektorFluidkammer; }
			InjektorFluidvolumen {name InjektorFluidvolumen; }
			Kammervolumen { name Kammervolumen; }
			Nadel {name Nadel; }
		}
	}
		refinementBox
    {
        type searchableBox;
        min (-1.0 -0.7 0.0);
        max ( 8.0  0.7 2.5);
    }


/*
	cone
	{
		type	searchableCylinder ;
		point1	(0.0252573 0 0.0119241) ; // location of the cente at the bottom
		point2 (0.0255827 0 0.0128696) ;  // location of the center at the top
		radius 0.015;			  // radius of the cylinder
	}
*/
}
// 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; //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 10000000; //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
	features
	(
		{
		file "Wasserstoffmotor.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
	{
		Wasserstoffmotor
		{
		// Surface - wise min and max refinement level
		level (2 3) ;
		regions
		   {
			Ansaugrohr	 { level (2 2); patchInfo { type wall; }}
			Basalkruemmer { level (2 2); patchInfo { type patch; }}
			Duese	{ level (5 5); patchInfo { type patch; }}
			InjektorFluidkammer	  { level (2 2); patchInfo { type patch; }}
			InjektorFluidvolumen { level (2 2); patchInfo { type patch; }}
			Kammervolumen { level (2 2); patchInfo { type wall; }}
			Nadel { level (5 5); patchInfo { type patch; }}
		   }
		}
	 }
	// 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
	{
       	 refinementBox
	 {
            mode inside;
            levels ((1.0 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.
        locationInMesh (0.01 0.01 -0.06); // Inside Point
/*
	locationsInMesh 
	(
		((0  0  0) 	          	  liner)
		((0  0 0.01)	   	   cylinderHead)
		((-0.025 -0.02 0.05)	 Valve_stem_exh)
		((-0.025 0.02 0.05)	 Valve_stem_exh) 
		((-0.005 0 0.005)	       H2_inlet)
	);
*/

	// Outside Point (external Mesh)(40  40 55) ; // Inside Point //(0 0 0); // Outside Point (external Mesh) 
	
	// 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
    {
        Wasserstoffmotor
        {
            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"

	maxNonOrtho 65;
    maxBoundarySkewness 20;
    maxInternalSkewness 4;
    maxConcave 80;
    minVol 1e-18;
    minTetQuality 1e-9;
    minArea -1;
    minTwist 0.05;
    minDeterminant 0.001;
    minFaceWeight 0.05;
    minVolRatio 0.01;
    minTriangleTwist -1;

    // Advanced
    nSmoothScale 4;
    errorReduction 0.75;

    relaxed
    {
        maxNonOrtho 75;
    }

    // Advanced
    nSmoothScale 4;
    errorReduction 0.75;

    //- 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;


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

As a beginner in OpenFOAM, I kindly request you to point out the mistakes I have made or changes that I should make. I would be highly grateful for your kind help.

Kind regards,
Shaeq

[MohdShaeq]

The screenshots of the generated 2D and 3D meshes for the valve needle (Ventilnadel) are attached herewith.

[MohdShaeq]

/*---------------------------------------------------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v1912                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
Build  : _f3950763fe-20191219 OPENFOAM=1912
Arch   : "LSB;label=32;scalar=64"
Exec   : checkMesh
Date   : Sep 03 2023
Time   : 15:15:47
Host   : localhost.localdomain
PID    : 24512
I/O    : uncollated
Case   : /home/MohdShaeq/Desktop/Wasserstoffmotor-Studie/grobesNetz
nProcs : 1
trapFpe: Floating point exception trapping enabled (FOAM_SIGFPE).
fileModificationChecking : Monitoring run-time modified files using timeStampMaster (fileModificationSkew 10)
allowSystemOperations : Allowing user-supplied system call operations

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Create time

Create mesh for time = 0

Time = 0

Mesh stats
    points:           1331
    faces:            3300
    internal faces:   2700
    cells:            1000
    faces per cell:   6
    boundary patches: 1
    point zones:      0
    face zones:       0
    cell zones:       0

Overall number of cells of each type:
    hexahedra:     1000
    prisms:        0
    wedges:        0
    pyramids:      0
    tet wedges:    0
    tetrahedra:    0
    polyhedra:     0

Checking topology...
    Boundary definition OK.
    Cell to face addressing OK.
    Point usage OK.
    Upper triangular ordering OK.
    Face vertices OK.
    Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
    Patch               Faces    Points   Surface topology                  
    walls               600      602      ok (closed singly connected)      

Checking faceZone topology for multiply connected surfaces...
    No faceZones found.

Checking basic cellZone addressing...
    No cellZones found.

Checking geometry...
    Overall domain bounding box (-0.1 -0.1 -0.2) (0.1 0.1 0.1)
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
    Mesh has 3 solution (non-empty) directions (1 1 1)
    Boundary openness (9.48677e-18 -3.72694e-18 1.01644e-18) OK.
    Max cell openness = 1.21069e-16 OK.
    Max aspect ratio = 1.5 OK.
    Minimum face area = 0.0004. Maximum face area = 0.0006.  Face area magnitudes OK.
    Min volume = 1.2e-05. Max volume = 1.2e-05.  Total volume = 0.012.  Cell volumes OK.
    Mesh non-orthogonality Max: 0 average: 0
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 5.55352e-15 OK.
    Coupled point location match (average 0) OK.

Mesh OK.

--> FOAM Warning : 
    From function virtual Foam::polyMesh::readUpdateState Foam::polyMesh::readUpdate()
    in file meshes/polyMesh/polyMeshIO.C at line 215
    Number of patches has changed.  This may have unexpected consequences.  Proceed with care.
Time = 2.5e-06

Mesh stats
    points:           598371
    faces:            1384437
    internal faces:   1210087
    cells:            399274
    faces per cell:   6.4981
    boundary patches: 8
    point zones:      0
    face zones:       0
    cell zones:       0

Overall number of cells of each type:
    hexahedra:     323774
    prisms:        0
    wedges:        0
    pyramids:      0
    tet wedges:    0
    tetrahedra:    0
    polyhedra:     75500
    Breakdown of polyhedra by number of faces:
        faces   number of cells
            6   27643
            8   2252
            9   29480
           10   754
           11   1597
           12   9609
           13   303
           14   542
           15   2957
           16   42
           17   74
           18   242
           20   3
           21   1
           22   1

Checking topology...
    Boundary definition OK.
    Cell to face addressing OK.
    Point usage OK.
    Upper triangular ordering OK.
    Face vertices OK.
    Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
                   Patch    Faces   Points                  Surface topology
                   walls    15735    15915      ok (closed singly connected)
              Ansaugrohr    11739    13844  ok (non-closed singly connected)
           Basalkruemmer    11628    12818  ok (non-closed singly connected)
                   Duese      883      980  ok (non-closed singly connected)
     InjektorFluidkammer     1984     2764  ok (non-closed singly connected)
    InjektorFluidvolumen      192      256  ok (non-closed singly connected)
           Kammervolumen   131405   145376  multiply connected (shared edge)
                   Nadel      784      810  ok (non-closed singly connected)

Checking faceZone topology for multiply connected surfaces...
    No faceZones found.
  <<Writing 44 conflicting points to set nonManifoldPoints

Checking basic cellZone addressing...
    No cellZones found.

Checking geometry...
    Overall domain bounding box (-0.1 -0.1 -0.2) (0.1 0.1 0.1)
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
    Mesh has 3 solution (non-empty) directions (1 1 1)
    Boundary openness (4.41082e-16 1.55119e-16 -5.88725e-15) OK.
    Max cell openness = 2.03288e-16 OK.
    Max aspect ratio = 1.5008 OK.
    Minimum face area = 3.90625e-07. Maximum face area = 0.0006.  Face area magnitudes OK.
    Min volume = 3.66016e-10. Max volume = 1.2e-05.  Total volume = 0.0119673.  Cell volumes OK.
    Mesh non-orthogonality Max: 31.0121 average: 12.0054
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 1.00107 OK.
    Coupled point location match (average 0) OK.

Mesh OK.

Time = 5e-06

Mesh stats
    points:           566222
    faces:            1342867
    internal faces:   1210087
    cells:            399274
    faces per cell:   6.39399
    boundary patches: 8
    point zones:      0
    face zones:       0
    cell zones:       0

Overall number of cells of each type:
    hexahedra:     295821
    prisms:        12555
    wedges:        0
    pyramids:      0
    tet wedges:    1334
    tetrahedra:    0
    polyhedra:     89564
    Breakdown of polyhedra by number of faces:
        faces   number of cells
            4   5242
            5   10124
            6   26772
            7   1850
            8   1378
            9   28941
           10   354
           11   1179
           12   9722
           13   182
           14   508
           15   2959
           16   41
           17   66
           18   241
           20   4
           22   1

Checking topology...
    Boundary definition OK.
    Cell to face addressing OK.
    Point usage OK.
    Upper triangular ordering OK.
    Face vertices OK.
    Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
                   Patch    Faces   Points                  Surface topology
                   walls    15735    15915      ok (closed singly connected)
              Ansaugrohr    10565    13819  ok (non-closed singly connected)
           Basalkruemmer     9093    10756  ok (non-closed singly connected)
                   Duese      667      820  ok (non-closed singly connected)
     InjektorFluidkammer     1554     2607  ok (non-closed singly connected)
    InjektorFluidvolumen      185      243  ok (non-closed singly connected)
           Kammervolumen    94347   117298  multiply connected (shared edge)
                   Nadel      634      685  ok (non-closed singly connected)

Checking faceZone topology for multiply connected surfaces...
    No faceZones found.
  <<Writing 44 conflicting points to set nonManifoldPoints

Checking basic cellZone addressing...
    No cellZones found.

Checking geometry...
    Overall domain bounding box (-0.100007 -0.100072 -0.200014) (0.100009 0.100025 0.1)
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
    Mesh has 3 solution (non-empty) directions (1 1 1)
    Boundary openness (-1.23824e-16 3.34021e-17 -5.3738e-15) OK.
    Max cell openness = 3.61057e-16 OK.
    Max aspect ratio = 21.1498 OK.
    Minimum face area = 1.33193e-09. Maximum face area = 0.000644089.  Face area magnitudes OK.
    Min volume = 9.72437e-12. Max volume = 1.31486e-05.  Total volume = 0.011967.  Cell volumes OK.
    Mesh non-orthogonality Max: 64.9997 average: 14.9734
    Non-orthogonality check OK.
    Face pyramids OK.
 ***Max skewness = 6.08013, 14 highly skew faces detected which may impair the quality of the results
  <<Writing 14 skew faces to set skewFaces
    Coupled point location match (average 0) OK.

Failed 1 mesh checks.

Time = 7.5e-06

Mesh stats
    points:           558196
    faces:            1336636
    internal faces:   1210087
    cells:            399274
    faces per cell:   6.37838
    boundary patches: 8
    point zones:      0
    face zones:       0
    cell zones:       0

Overall number of cells of each type:
    hexahedra:     294164
    prisms:        15576
    wedges:        0
    pyramids:      0
    tet wedges:    1962
    tetrahedra:    16
    polyhedra:     87556
    Breakdown of polyhedra by number of faces:
        faces   number of cells
            4   5156
            5   8370
            6   26971
            7   2237
            8   1510
            9   28276
           10   972
           11   632
           12   9636
           13   143
           14   393
           15   2941
           16   27
           17   52
           18   235
           19   3
           20   1
           21   1

Checking topology...
    Boundary definition OK.
    Cell to face addressing OK.
    Point usage OK.
    Upper triangular ordering OK.
    Face vertices OK.
    Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
                   Patch    Faces   Points                  Surface topology
                   walls    15735    15915      ok (closed singly connected)
              Ansaugrohr    10248    13256  ok (non-closed singly connected)
           Basalkruemmer     8860    10402  ok (non-closed singly connected)
                   Duese      625      776  ok (non-closed singly connected)
     InjektorFluidkammer     1468     2516  ok (non-closed singly connected)
    InjektorFluidvolumen      179      235  ok (non-closed singly connected)
           Kammervolumen    88809   110228  multiply connected (shared edge)
                   Nadel      625      674  ok (non-closed singly connected)

Checking faceZone topology for multiply connected surfaces...
    No faceZones found.
  <<Writing 38 conflicting points to set nonManifoldPoints

Checking basic cellZone addressing...
    No cellZones found.

Checking geometry...
    Overall domain bounding box (-0.100007 -0.100072 -0.200014) (0.100009 0.100025 0.1)
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
    Mesh has 3 solution (non-empty) directions (1 1 1)
    Boundary openness (-1.1779e-16 -2.92112e-17 -5.35009e-15) OK.
    Max cell openness = 3.61057e-16 OK.
    Max aspect ratio = 19.7306 OK.
    Minimum face area = 3.79651e-09. Maximum face area = 0.000644089.  Face area magnitudes OK.
    Min volume = 9.72437e-12. Max volume = 1.31486e-05.  Total volume = 0.0119668.  Cell volumes OK.
    Mesh non-orthogonality Max: 64.9997 average: 14.9447
    Non-orthogonality check OK.
    Face pyramids OK.
 ***Max skewness = 6.08013, 13 highly skew faces detected which may impair the quality of the results
  <<Writing 13 skew faces to set skewFaces
    Coupled point location match (average 0) OK.

Failed 1 mesh checks.

End

[MohdShaeq]

Dear members, kindly help me in alleviating this issue as I am unable to proceed forward. Your valuable suggestions will make it very easy for me to simulate my case successfully.

Best regards,
Shaeq

[AtoHM]

Originally Posted by MohdShaeq

The screenshots of the generated 2D and 3D meshes for the valve needle (Ventilnadel) are attached herewith.

You try to cover details that are one to two magnitudes smaller than the mesh cell size you preset. Increase the level for this region to get a better represenation. That is just to get the geometry right, you might need even more to get the physics correct there because of high gradients. Also we can see, that your STL is already a "rough" representation of the needle's cylindrical shape in some areas.