snappyHexMesh in BARAM

baramMesh is a program that uses snappyHexMesh to generate meshes.

This section explains how the various settings and execution of baramMesh drive snappyHexMesh.

1. Geometry

When you import a geometry using “Import” or create one using “Add,” a volume and a surface are generated; you then specify the type for each of these.

Type of volume

• Select None or cellZone
• For cellZone, set as faceZone in surface of castellatedMeshControls.refinementSurfaces

Type of surface

• Select None, Boundary or Interface(with Non-Conformal and Inter-Region option)
• Set as faceType in surface of castellatedMeshControls.refinementSurfaces
  • None : Internal
  • Interface
    • Non-conformal : boundary
    • Conformal : baffle

//snappyHexMeshDict
...
  refinementSurfaces
  {
    cellZone_surface
    {
      faceZone cellZone_surface;// if None, this line is omiited
      faceType boundary;
      patchInfo
      {
        type patch;
      }
      level      (0 0);
    }
    ...

feature line

When reading an STL file or adding an object, use vtkFeatureEdges of vtk to generate feature lines → Save as an OBJ file in the constant/triSurface folder

---

2. Regions

To define a region, you can either enter coordinates directly within the region, move the pointer with the mouse, or select a volume. In all cases, the coordinates of a single point are obtained.

Since snappyHexMesh does not have a function to create regions, you should first create the region as a cellZone and then use OpenFOAM utilities and the cellZones to divide it into a multi-region mesh during export.

• Set point coordinate to castellatedMeshControls.locationInMesh
• For multi-region, it is used in the insidePoint field of the topoSetDict file to convert the section set to region into a cellZone.

//topoSetDict
...
    {
      name Region_1;
      type cellSet;
      action new;
      source regionToCell;
      insidePoints ((-2.28338 -0.46829 -1.36195));
    }
... 

---

3. Base Grid

Numbers in Grid Span

• The numbers displayed represents the size of the bounding box for the entire geometry; the actual base grid uses a value that is 0.1% larger than this, but if the “use Hex6” option is selected, the displayed value is used as-is.
• Set to vertices in blockMeshDict

Number of Cells per Direction

• Set to blocks blockMeshDict

//blockMeshDict
vertices
(
  (-0.200286 -0.299694 -0.300286)
  (0.800286 -0.299694 -0.300286)
  (0.800286 0.299694 -0.300286)
  (-0.200286 0.299694 -0.300286)
  (-0.200286 -0.299694 0.300286)
  (0.800286 -0.299694 0.300286)
  (0.800286 0.299694 0.300286)
  (-0.200286 0.299694 0.300286)  
);
blocks
(
  hex   (0 1 2 3 4 5 6 7) (35 20 20)  simpleGrading  (1 1 1)
);

Run

• Execute blockMesh utility

---

4. Castellation

These settings are used for the following properties of snappyHexMeshDict.

Configuration

• Number of Cells between Levels : castellatedMeshControls.nCellsBetweenLevels
• Feature Angle Threshold : castellatedMeshControls.resolveFeatureAngle
• Keep Non-manifold Edges & Keep Open Edges : vtkFeatureEdges

Advanced

• Max. Global Cell Count : castellatedMeshControls.maxGlobalCells
• Max. Local Cell Count : castellatedMeshControls.maxLocalCells
• Min. Refinement Cell Count : castellatedMeshControls.minRefinementCells
• Max. Load Unbalance : castellatedMeshControls.maxLoadUnbalance
• Allow Free Standing Zone Faces : castellatedMeshControls.allowFreeStandingZoneFaces

Surface/Feature Refinement

• Surface Refinement
  • Minimum Level : castellatedMeshControls.refinementSurfaces – <surface name> – first number of level
  • Maximum Level : castellatedMeshControls.refinementSurfaces – <surface name> – second number of level
• Feature Edge Refinement Level : castellatedMeshControls.features – <feature name> – level
• Curvature Refinement : castellatedMeshControls.refinementSurfaces – <surface name> – curvatureLevel
  • Number of Cells per Radius of Curvature : first number of curvatureLevel
  • Maximum Cell Levels : third number of curvatureLevel
  • Do not refine sharp surface : fourth number of curvatureLevel – if off, set to 0
    • Minimum Radius of Curvature : fourth number of curvatureLevel – if on, size level for the curvature

    ...
    propeller
    {
      patchInfo
      {
        type patch;
      }
      level (2 2);
      curvatureLevel (10 0 7 1);
    }
    ...

Volume Refinement

• Volume Refinement Level : castellatedMeshControls.refinementRegions – <volume name> – levels, second number
• Gap Refinement : castellatedMeshControls.refinementRegions – – gapLevel
  • Min. Cell Layers in a gap : first number of gapLevel
  • Gap Detaction Start Level : second number of gapLevel
  • Max. Refinement Level : third number of gapLevel
  • Direction : gapMode
  • Include surface’s own gap : gapSelf
• Level Increment : castellatedMeshControls.refinementRegions – – levelIncrement
  • Split Count per Direction – x, y, z : third tuple of levelIncrement
  • Min. Level : first number of levelIncrement
  • Max. Level : second number of levelIncrement

    ...
    volumeName
    {
      mode inside;
      levels ((1000000000000000.0 1));
      gapLevel (4 1 3);
      gapMode mixed;
      gapSelf true;
      levelIncrement (0 10 (1 0 0));
    }
    ...

Run for Single region

• Set only castellatedMesh to true and run snappyHexMesh

Run for Multi-region

• Set only castellatedMesh to true and run snappyHexMesh
• Inter-Region Interface의 faceType은 baffle(conformal일 때) 혹은 boundary(non-conformal일 때)로, Inter-Region이 아닌 Interface의 faceType을 internal로 주고 snappyHexMesh 실행
  • Region이 아닌 cellZone 내부 영역에 격자가 만들어지지 않는 것을 방지하기 위해 internal로 설정

---

5. Snap

snapControls에서 설정

• Interation Count
  • Smoothing for Surface : nSmoothPatch
  • Smoothing for Internal : nSmoothInternal
  • Mesh Displacement Relaxation : nSolveIter
  • Snapping Relaxation : nRelaxIter
• Feature Snapping
  • Snapping Relaxation : nFeatureSnapIter
  • Feature Snap Type : implicitFeatureSnap
  • Multi-surface Feature Snap : multiRegionFeatureSnap
• Tolerance : tolerance
• Concave Angle : concaveAngle
• Min. Area Ratio : minAreaRatio
• Buffer Layer : solver – displacementPointSmoothing
  • Point Smoothing Method : displacementPointSmoothingCoeffs.pointSmoother
  • Number of Point Smoother Iteration per Time step : displacementPointSmoothingCoeffs.nPointSmootherIter
  • GETMe Transformation Parameter : displacementPointSmoothingCoeffs.transformationParameter

snapControls
{
  nSmoothPatch 1;
  nSmoothInternal 0;
  tolerance 3;
  nSolveIter 30;
  nRelaxIter 5;
  implicitFeatureSnap false;
  explicitFeatureSnap true;
  nFeatureSnapIter 15;
  multiRegionFeatureSnap no;
  concaveAngle 45;
  minAreaRatio 0.3;
  solver displacementPointSmoothing;
  displacementPointSmoothingCoeffs
  {
    pointSmoother geometricElementTransform;
    nPointSmootherIter 10;
    transformationParameter 0.667 ;
  }
}

Single region일 때 실행

• snap 만 true로 주고 snappyHexMesh 실행

Multi-region일 때 실행

• Inter-Region이 아닌 interface의 faceType을 internal로 주고 snappyHexMesh를 실행
• topoSet
  • 각 region의 locationInMesh Point를 topoSet의 insidePoints로 사용해 각 리전을 셀존으로 나눈다
• Inter-Region이 아닌 interface가 있을 때, 이 Interface들의 faceType을 baffle(conformal일 때) 혹은 boundary(non-conformal일 때)로 변경하고 snappyHexMesh를 “*-*overwrite” 옵션과 함께 실행

//topoSetDict
actions
(
  {
    name Region_1;
    type cellSet;
    action clear;
  } 
  {
    name Region_1;
    type cellSet;
    action new;
    source regionToCell;
    insidePoints  ((-2.28338 -0.46829 -1.36195));
  }
  {
    name Region_1;
    type cellZoneSet;
    action new;
    source setToCellZone;
    sourceInfo
    {
      set Region_1;
    }
  }
  {
    name Region_2;
    type cellSet;
    action clear;
  } 
  {
    name Region_2;
    type cellSet;
    action new;
    source regionToCell;
    insidePoints  ((0 0 0));
  }
  {
    name Region_2;
    type cellZoneSet;
    action new;
    source setToCellZone;
    sourceInfo
    {
      set Region_2;
    }
  }
);

---

6. Boundary Layer

addLayersControls에서 설정

Configurations

layers.<boundary name>의 설정

• Number of Layers : nSurfaceLayers
• Thickness Model Specification : thicknessModel
• Size Specification : relativeSizes
• First Layer Thickness : firstLayerThickness
• Expansion Ratio : expansionRatio
• Min. Total Thickness : minThickness
• Final Layer Thickness : finalThickness
• Total Thickness : totalThickness

  layers
  {
    propeller
    {
      nSurfaceLayers 3;
      thicknessModel firstAndExpansion;
      relativeSizes on;
      expansionRatio 1.2;
      firstLayerThickness 0.3;
      minThickness 0.3;
    }
  }

Advanced Configurations

• Number of Grow : nGrow
• Static Analysis of Starting Mesh
  • Feature Angle Threshold : featureAngle
  • Max. Thickness Ratio : maxFaceThicknessRatio
• Patch Displacement Smoothing
  • Number of Iterations : nSmoothSurfaceNormals
  • Smooth Layer Thickness : nSmoothThickness
• Medial Axis
  • Min. Axis Angle : minMedialAxisAngle
  • Max. Thickness Ratio : minThickness
  • Number of Smoothing Iter. : nSmoothNormals
  • Max. Snapping Relaxation Iter. : nRelaxIter
• Mesh Shrinking
  • Num. of Buffer Cells : nBufferCellsNoExtrude
  • Max. Layer Addition Iter. : nLayerIter
  • Max. Iter. Before Relax : nRelaxedIter

addLayersControls
{
  layers
  {
  }
  nGrow 0;
  featureAngle 60;
  maxFaceThicknessRatio 0.5;
  nSmoothSurfaceNormals 1;
  nSmoothThickness 10;
  minMedialAxisAngle 90;
  maxThicknessToMedialRatio 0.3;
  nSmoothNormals 3;
  nRelaxIter 10;
  nBufferCellsNoExtrude 1;
  nLayerIter 50;
  nRelaxedIter 20;
  thicknessModel finalAndExpansion;
  relativeSizes on;
  finalLayerThickness 0.5;
  expansionRatio 1.2;
  minThickness 0.3;
}

실행

• addLayers 만 true로 주고 snappyHexMesh 실행

---

7. Export

Export as BaramFlow project

• Single region mesh 일 때
  • topoSet 실행 – cellZone을 만드는 과정
  • createPatch 실행
• Multi-region mesh 일 때
  • splitMeshRegions -cellZoneOnly 실행
  • topoSet 실행 – cellZone을 만드는 과정
    • region 생성을 위해 만들었던 셀존은 삭제하고 사용자가 설정한 셀존을 생성
  • createPatch 실행

//topoSetDict file
actions
(
	{
	  name cube;
    type cellSet;
    action clear;
  }
  {
    name cube;
    type cellSet;
    action new;
    source surfaceToCell; //boxToCell; cylinderToCell; sphereToCell;
    file "constant/triSurface/cube.stl";
    useSurfaceOrientation true;
    includeInside true;
    nearDistance -1;
    curvature -100;
    outsidePoints ();
    includeCut false;
    includeOutside false;
  }
  {
    name cube;
    type cellZoneSet;
    action new;
    source setToCellZone;
    sourceInfo  {set cube;}
  }
);

2D Plane

• Boundary : extrudeMeshDict 파일의 sourcePatches와 exposedPatchName에 사용
• Thickness : extrudeMeshDict 파일의 thickness에 사용
• 실행
  • extrudeMesh -region <regionName> -dict system/extrudeMeshDict
  • collapseEdges -overwrite 실행

//extrudeMeshDict
...
constructFrom   patch;
sourceCase      ".";
sourcePatches   ( far_surface );
exposedPatchName far_surface;
extrudeModel    plane;
flipNormals     false;
mergeFaces      false;
thickness       1;
...

Axi-symmetry

• Source Boundary, P1 : extrudeMeshDict 파일의 sourcePatches에 사용
• Exposed Boundary, P2 : extrudeMeshDict 파일의 exposedPatchName에 사용
• Angle : extrudeMeshDict 파일의 wedgeCoeffs.angle에 사용
• Axis origin : extrudeMeshDict 파일의 wedgeCoeffs.point에 사용
• Axis direction : extrudeMeshDict 파일의 wedgeCoeffs.axis에 사용
• 실행
  • extrudeMesh -region <regionName> -dcit system/extrudeMeshDict

//extrudeMeshDict
...
constructFrom   patch;
sourceCase      ".";
sourcePatches   ( patchName );
exposedPatchName patchName-1;
extrudeModel    wedge;
wedgeCoeffs
{
    point   (0 0 0);
    axis    (1 0 0);
    angle   10;
}
flipNormals     false;
mergeFaces      false;
...