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[snappyHexMesh] SnappyHexMesh boundary refinement |
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#1 |
Member
Join Date: Jun 2017
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Hi everyone,
I am new to SnappyHexMesh and try to generate a mesh. It works quite well, but I want to add a refinement layer to the boundary. I have already refined the surface with a level of 3 and nCellsBetweenLevel 7. But I want to add a new layer to the boundary with nCellBetweenLevel 3 on top. Is this somehow possible? Thanks in advance! |
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#2 |
Member
Thomas Sprich
Join Date: Mar 2015
Posts: 76
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Hi,
I think you can use the following to achieve what you want: Code:
// 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 // increasing 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 { //box1x1x1 //{ // mode inside; // levels ((1.0 4)); //} //sphere.stl //{ // mode distance; // levels ((1.0 5) (2.0 3)); //} } Thomas |
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#3 |
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Hi and thanks for your reply.
This would be a refinement in a box or a sphere. But I want to refine my boundary layer also for other geometries who can be much more complicated. |
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#4 |
Member
Thomas Sprich
Join Date: Mar 2015
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Hi,
Have you tried it? The example showed Code:
sphere.stl The import part of the example I showed is: Code:
{ mode distance; levels ((1.0 5) (2.0 } Thomas |
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#5 |
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Join Date: Jun 2017
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I tried it but nothing at all hapoened? Here is my snappyHexMeshDict:
Code:
/*--------------------------------*- C++ -*----------------------------------*\ | ========= | | | \\ / F ield | OpenFOAM: The Open Source CFD Toolbox | | \\ / O peration | Version: 4.1 | | \\ / A nd | Web: www.OpenFOAM.org | | \\/ 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 { body.stl { type triSurfaceMesh; name body_local; } box { type searchableBox; min (0.3 -0.1 -0.1); max (0.7 0.3 0.1); } box2 { type searchableBox; min (0.385 -0.1 -0.1); max (0.75 0.3 0.1); } }; // 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 0; // Number of buffer layers between different levels. // 1 means normal 2:1 refinement restriction, larger means slower // refinement. nCellsBetweenLevels 7; // 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 "body.eMesh"; level 2; } ); // 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 { body_local { // Surface-wise min and max refinement level level (2 2); } } // Resolve sharp angles on fridges 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 { box { mode inside; levels ((1 1)); } box2 { mode inside; levels ((2 2)); } body.stl { mode distance; levels ((1 5) (2 7)); } } // 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 (0.41 0.051 0.005); // 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 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 10; // Feature snapping //Number of feature edge snapping iterations. nFeatureSnapIter 100; //Detect (geometric only) features by //sampling the surface (default=false). implicitFeatureSnap false; // Use castellatedMeshControls::features // (default = true) explicitFeatureSnap true; multiRegionFeatureSnap false; } // 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 true; // Per final patch (so not geometry!) the layer information layers { body { nSurfaceLayers 3; } } // Expansion factor for layer mesh expansionRatio 1.1; // 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 0.5; // 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. // See relativeSizes parameter. 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 17x! (didn't do anything in 17x) 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 20; // Number of smoothing iterations of surface normals nSmoothSurfaceNormals 10; // Number of smoothing iterations of interior mesh movement direction nSmoothNormals 10; // 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 16x! 90 degrees corresponds to 130 in 16x. 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; } // 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 70; //- 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; //- Minimum quality of the tet formed by the face-centre // and variable base point minimum decomposition triangles and // the cell centre. Set to very negative number (e.g. -1E30) to // disable. // <0 = inside out tet, // 0 = flat tet // 1 = regular tet minTetQuality 1; //- 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 0.05; //- Minimum normalised cell determinant // 1 = hex, <= 0 = folded or flattened illegal cell minDeterminant 0.001; //- minFaceWeight (0 -> 0.5) minFaceWeight 0.05; //- minVolRatio (0 -> 1) minVolRatio 0.5; //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; // ************************************************************************* // |
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#6 |
Senior Member
Yann
Join Date: Apr 2012
Location: France
Posts: 1,169
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Hello Friendly,
I'm not exactly sure to understand what you want to to in the first post. Nevertheless, about your last post, it doesn't work because you renamed your body.stl to body_local. Hence you have to use that name in your refinementRegions section : Code:
body_local { mode distance; levels ((1 5) (2 7)); } |
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#7 |
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Join Date: Jun 2017
Posts: 73
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Yeah you are right, I already fixed it.
Its just not clear for me, what is the difference between the refinement on a surface and the refinement of the region with the distance mode. Is there any difference? |
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#8 |
Senior Member
Yann
Join Date: Apr 2012
Location: France
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Yes there is a difference:
It might not make a big difference on a simple geometry, but it makes a big one on complex geometries involving a big range of refinement levels. Imagine a complex geometry such as a car: you might need to refine some surfaces up to...lets say level 10, to properly represent small details on the geometry. If you use a big value for nCellsBetweenLevels to have smooth transitions, it means this parameter will be applied to every level transition from 10 to 0 and these small refined details will have a huge impact on the size of your mesh. In this case, you should better use a smaller value for nCellsBetweenLevels and use refinementRegions to specify your own distance and level of refinement around your geometry. (And possibly use different parameters with different STL files to have a finer control on the zones you want to refine or not) Yann Last edited by Yann; June 14, 2019 at 06:40. Reason: typo |
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#9 |
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Thank you very much for the detailed explanation!
Its helped a beginner like me a lot! |
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