blockMeshと snappyHexMesh で二次元の円柱後方の流れ場モデルを作成する方法~その4~ snappyHexMesh

snappyHexMesh ヒョウ

① モデルの概要説明
② blockMeshの実行
③ freeCADによるジオメトリ(STLファイル)作成

今回は snappyHexMesh を説明します。snappyHexMeshの内容を構成するsnappyHexMeshDictにはいろいろな設定が含まれます。ここでは本ディクショナリファイルの全容を示して,castellatedMesh, snap, addLayersの要点を解説します。


・castellatedMeshの実行 ← blockMeshへのジオメトリ形状のはめ込み
・snapの実行      ← はめ込んだジオメトリ形状の輪郭および隣接メッシュの調整
・addLayersの実行    ←ジオメトリに隣接するメッシュへの境界層メッシュ作成


// Which of the steps to run  下3行のtrue or falseでそれぞれの実行要否を決めます
castellatedMesh true;
snap true;
addLayers true;    //←境界層メッシュを作成しない場合はtrueをfalseに修正

// 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
wall   //←作成したジオメトリ(STLファイル)の輪郭に対応する境界面の名前
type triSurfaceMesh;
file “wall.stl”;  //←”triSurface”ディレクトリに保存したSLTファイル名を入力

// Settings for the castellatedMesh generation.
castellatedMeshControls    //←calsellatedmeshの設定領域です
// 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 2;

// Number of buffer layers between different levels.
// 1 means normal 2:1 refinement restriction, larger means slower
// refinement.
nCellsBetweenLevels 4;

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

// Specifies a level for any cell intersected by its edges.
// This is a featureEdgeMesh, read from constant/triSurface for now.

// 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.
wall    //上述のgeometory{}内部で記入した境界条件の名前を入力
// Surface-wise min and max refinement level
level (0 0);
}    //ここではジオメトリ表面のメッシュを細分化しません。
}    //(0 2)と入力した場合は輪郭形状に応じて0~2回までの細分化を行います。

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.

{    //ジオメトリ周辺のメッシュを細分化する場合はここに入力します。

// 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 (-0.1 0 0.001); // Inside point

// 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    //←snapの設定領域です
//- 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 300;

//- 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) features by sampling the surface
implicitFeatureSnap false;
//- Use castellatedMeshControls::features
explicitFeatureSnap true;

//- Detect features between multiple surfaces
// (only for explicitFeatureSnap, default = false)
multiRegionFeatureSnap true;

// Settings for the layer addition.
addLayersControls    //←境界層メッシュ作成の設定領域です
// Are the thickness parameters below relative to the undistorted
// size of the refined cell outside layer (true) or absolute sizes (false).
relativeSizes true;   //境界層厚さの指定に関するサイズ指定を相対値とします

// Per final patch (so not geometry!) the layer information
“wall*”    //STLファイル名,境界面の名前と同じ名前を””内に入力し,最後に*を付けてください
nSurfaceLayers 3;    //境界層メッシュ数,ここでは3メッシュです

// Expansion factor for layer mesh
expansionRatio 1.0;    //境界層メッシュの成長率,均一の厚みとします

// 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;    //成長しない設定なのでこの入力値が厚み(比率)になります

// Minimum thickness of cell layer. If for any reason layer
// cannot be above minThickness do not add layer.
// See relativeSizes parameter.
minThickness 0.25;

// 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 30;

// Maximum number of snapping relaxation iterations. Should stop
// before upon reaching a correct mesh.
nRelaxIter 5;

// 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
minMedianAxisAngle 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;

// Max number of iterations after which relaxed meshQuality controls
// get used. Up to nRelaxIter it uses the settings in meshQualityControls,
// after nRelaxIter it uses the values in meshQualityControls::relaxed.
nRelaxedIter 20;

// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
#include “meshQualityDict”

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

// Advanced

// Write flags
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;


castellatedMeshの実行では,もともとのblockMeshで作成したメッシュ形状から,ジオメトリ(STLファイル)の輪郭に合わせてメッシュをくり抜く操作が行われます。ジオメトリの輪郭に,blockMeshのメッシュサイズに対して細かな凹凸がある場合はminRefinementLevelrefinementSurfacesLevel(0 0)を調整すれば輪郭を反映できるかもしれません。




addLayersではsnapで作成したジオメトリの境界面に境界層メッシュを作成します。ここでは”nSurfaceLayers 3“で境界層メッシュ数を3メッシュとしました。さらにサイズの指定を”relativeSizes true“として相対値で入力するように指定し,成長率を1.0(成長なし)として”finalLayerThickness 0.3“としました。もともとのblockMeshのメッシュサイズ30mmなので,境界層メッシュの厚さは9 mm (= 30 mm * 0.3)となります。






エラーなく実行できれば,以下の3つのディレクトリが生成します。生成するファイルのファイル名は/system/controlDictにあるdeltaTの数値の倍数になります。例えばdeltaTを0.05としている場合。生成するディレクトリ名は”0.05″, “0.1”, “0.15”の3つです。それぞれ以下の内容です。











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