SnappyHexMesh中的使用问题

[秋山结月]

目前的问题主要是两方面：

1. 最终得到的贴体网格不够光滑，存在翘曲

2.边界层随缘加密，有时候能加上，有时候不能加上。

我个人的使用流程是这样的：
①用Solidwork画好实体，生成STL
②画背景网格并局部加密
③用surfaceFeatureExtract提取实体的边。
④SnappyHexMesh，将实体投射进网格。

结果实际制作出来的网格是坑坑洼洼的，完全不光滑，例如...

这样的效果还是在本人将Smooth和Snap的iteration调的非常大之后才能实现的，用200核的机器跑都需要15小时以上，然后跑出来结果就是这样的网格。

另外一个问题就是，我是的确选择了施加边界层的，但是到最后并没有出现边界层，依然只是局部加密。以下是我的SnappyHexMeshDict，不知有人愿意指点一下吗？谢谢

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  plus                                  |
|   \\  /    A nd           | Web:      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
{
    body.stl
    {
        name floatingObject;
        type triSurfaceMesh;

    }

};

// 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 3000000;

    // 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 50;

    // 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 2;


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

    // Specifies a level for any cell intersected by explicitly provided
    // edges.
    // This is a featureEdgeMesh, read from constant/triSurface for now.
    // Specify 'levels' in the same way as the 'distance' mode in the
    // refinementRegions (see below). The old specification
    //      level   2;
    // is equivalent to
    //      levels  ((0 2));

    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
    {
        floatingObject
        {
            level (1 2);

        }

    }

    // Feature angle:
    // - used if min and max refinement level of a surface differ
    // - used if feature snapping (see snapControls below) is used
    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
    //   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
    {
        floatingObject
        {
            mode distance;
            levels ((2 2) (4 1));
        }
    }  

    // 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 0.0 0.1);

    // 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 120;

    // Maximum relative distance for points to be attracted by surface.
    // True distance is this factor times local maximum edge length.
    // Note: changed(corrected) w.r.t 17x! (17x used 2* tolerance)
    tolerance 2.0;

    // Number of mesh displacement relaxation iterations.
    nSolveIter 300;

    // Maximum number of snapping relaxation iterations. Should stop
    // before upon reaching a correct mesh.
    nRelaxIter 80;

    // Feature snapping

        // Number of feature edge snapping iterations.
        // Leave out altogether to disable.
        nFeatureSnapIter 80;

        // Detect (geometric only) features by sampling the surface
        // (default=false).
        implicitFeatureSnap true;

        // Use castellatedMeshControls::features (default = true)
        explicitFeatureSnap true;

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

    // Layer thickness specification. This can be specified in one of four ways
    // - expansionRatio and finalLayerThickness (cell nearest internal mesh)
    // - expansionRatio and firstLayerThickness (cell on surface)
    // - overall thickness and firstLayerThickness
    // - overall thickness and finalLayerThickness

        // Expansion factor for layer mesh
        expansionRatio 1.2;

        // 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 0.03;

        // Wanted thickness of the layer next to the wall.
        // If relativeSizes this is relative to undistorted size of cell
        // outside layer.
        //firstLayerThickness 0.3;

        // Wanted overall thickness of layers.
        // If relativeSizes this is relative to undistorted size of cell
        // outside layer.
        //thickness 0.2;


    // Minimum overall thickness of total layers. If for any reason layer
    // cannot be above minThickness do not add layer.
    // If relativeSizes this is relative to undistorted size of cell
    // outside layer..
    minThickness 0.001;


    // Per final patch (so not geometry!) the layer information
    // Note: This behaviour changed after 21x. Any non-mentioned patches
    //       now slide unless:
    //          - nSurfaceLayers is explicitly mentioned to be 0.
    //          - angle to nearest surface < slipFeatureAngle (see below)
    layers
    {
        floatingBody
        {
            nSurfaceLayers      8;
            firstLayerThickness 0.02;
        }
    }

    // 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 360;

    // 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 80;

    // Number of smoothing iterations of surface normals
    nSmoothSurfaceNormals 100;

    // Number of smoothing iterations of interior mesh movement direction
    nSmoothNormals 50;

    // Smooth layer thickness over surface patches
    nSmoothThickness 100;

    // Stop layer growth on highly warped cells
    maxFaceThicknessRatio 0.5;

    // Reduce layer growth where ratio thickness to medial
    // distance is large
    maxThicknessToMedialRatio 0.9;

    // Angle used to pick up medial axis points
    // Note: changed(corrected) w.r.t 17x! 90 degrees corresponds to 130 in 17x.
    minMedianAxisAngle 150;

    // 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 80;

    // 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 80;

    // Additional reporting: if there are just a few faces where there
    // are mesh errors (after adding the layers) print their face centres.
    // This helps in tracking down problematic mesh areas.
    //additionalReporting true;
}

// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
meshQualityControls
{
    // Maximum non-orthogonality allowed. Set to 180 to disable.
    maxNonOrtho 70;

    // Max skewness allowed. Set to <0 to disable.
    maxBoundarySkewness 4;
    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 80;

    // 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. This has to be a positive number for tracking
    // to work. Set to very negative number (e.g. -1E30) to
    // disable.
    //    <0 = inside out tet,
    //     0 = flat tet
    //     1 = regular tet
    minTetQuality 1e-15;

    // 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 -2;

    // minimum normalised cell determinant
    // 1 = hex, <= 0 = folded or flattened illegal cell
    minDeterminant 0.001;

    // minFaceWeight (0 -> 0.5)
    minFaceWeight 0.00;

    // minVolRatio (0 -> 1)
    minVolRatio 0.01;

    // must be >0 for Fluent compatibility
    minTriangleTwist -1;

    //- If >0 : preserve single cells with all points on the surface if the
    //  resulting volume after snapping (by approximation) is larger than
    //  minVolCollapseRatio times old volume (i.e. not collapsed to flat cell).
    //  If <0 : delete always.
    //minVolCollapseRatio 0.5;

    // Advanced

    // Number of error distribution iterations
    nSmoothScale 200;
    // amount to scale back displacement at error points
    errorReduction 0.8;

    // 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 70;
    }
}

// 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;

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

[秋山结月]

但是更加奇怪的是我这种质量的网格依旧能够通过checkMesh...
这是为什么啊...

[李东岳]

snappyHexMesh生成出来这种网格很正常。需要更好的网格需要更细心的调试，目前没有好的建议告诉你。不过这种网格没问题

[秋山结月]

@东岳 谢谢东岳老师，不过想请问一下没问题是什么意思呢？
是指对敛散性不会有大的影响还是指对精度不会有太大影响呢？

[李东岳]

都不会有大影响，如果你的求解器比较简单的话，比如单相流之类

[秋山结月]

@东岳 好的 明白了 不过咱的求解器是基于InterFoam自己改的...我试试算的效果如何吧
谢谢老师

[chaoscfd]

个人的一个经验是善用refinementRegions关键字，在需要加密的地方建立refinementRegions，将level提高些，应该可以优化一下网格。还有一个地方是可以试试调整surfaceFeatureDict中的includedAngle，可以优化角度变化比较剧烈处的网格，比如你第一张图那里。另外可以适当将maxLocalCells和maxGlobalCells提高些。这个还是熟能生巧，多试试改变那些参数。
SHM是分步来处理网格的，先是snap，最后才是加边界层。所以边界层网格在最后一个时间步内，不知道你有没有查看最后一个时间步的网格。

[秋山结月]

@chaoscfd
这里想请问您几个问题：

1. 建立refineRegions，我这里是用distance的办法定义的refineRegion，离模型xx米内用xx等级这样，我还需要另外用box对其局部定义吗？ 还是说我只需要提高distance的等级就可以了？
   2.第二就是includeAngle，我这里取得是90°，我之前尝试过取180°，的确新选取的边很多都是我想要的，但是也出现了好多我不想要的edge（比如明明就是个连续的面，结果突然出现一个edge斜着贯穿了这个面，类似一个正方型强行被分成2个三角形）。有没有什么办法，只捕捉到我想优化的edge，而抛弃一些莫名其妙捕捉到的edge呢

[秋山结月]

@chaoscfd

想请问一下我是这是什么回事呢。。。
好像snap阶段结束后..就直接真的没了...
@东岳 顺便请教一下东岳老师

[chaoscfd]

@秋山结月
1.对的，最好另外再建立新的几何体，用这个新的几何体包裹要refine的局部区域。
2.这个问题倒是没有遇到过，是不是stl文件的问题？
3.snap结束后不应该直接没掉啊，即便是没有需要施加边界层的区域，也会有“No layers to generate ...”，这样的信息。看看你后面有没有这样的提示信息？