Node128 - CFD SUPPORT

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addLayersControls

This block defines parameters for adding boundary layer:

    relativeSizes true;

relativeSizes this parameters enables usage of relative values instead of absolute:
- true – sizes relative to the cell size next to the patch for which adding layers is defined
- false – sizes in absolute values

Figure: relativeSizes: true – false.

layers
    {
        C 
        {
            nSurfaceLayers  3;
            expansionRatio  1.5;
        }
        F 
        {
            nSurfaceLayers      2;
            // Wanted thickness of the layer next to the wall.
            // If relativeSizes this is relative to undistorted size of cell
            // outside layer.;
            firstLayerThickness 0.2;
        }
        D 
        {
            nSurfaceLayers 3;
            thickness      1.0;
        }
    }

layers block includes list of patches for which boundary layer is being added
- patch name or regular expression can be used
- the block contains
  - nSurfaceLayers – number of layers to be added
  - layer parameters (described below)
Boundary layer is correctly defined by following couple of parameters:
- firstLayerThickness, thickness
- firstLayerThickness, expansionRatio
- finalLayerThickness, expansionRatio
- finalLayerThickness, thickness
- thickness, expansionRatio
where
- firstLayerThickness – is the thickness of the first layer in the boundary layer (next to the wall)
- finalLayerThickness – is the thickness of the last layer in the boundary layer
- thickness – is the overall thickness of the whole boundary layer
- expansionRation – expansion ration of neighbouring layers in the boundary layer

Figure: Boundary layer parameters.

  expansionRatio 1.2;

    finalLayerThickness 0.3;

Boundary layer parameters for all defined patches (any parameter defined within the layers block prefers to parameters defined here).
```
    minThickness 0.05;
```
minThickness – minimum layer thickness below which the layer is not added
```
    nGrow 0;
```
nGrow – very experimental – best results with value
```
        featureAngle 150;
```
featureAngle – layers are collapsed at the feature edges which see angle above this value, an example of different setups is visualized in Figure .

Figure: Boundary layer parameters.

The remaining parameters are very advanced and set to default values. Please read a basic description in the file:

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


    // Patch displacement

        // Number of smoothing iterations of surface normals
        nSmoothSurfaceNormals 10;

        // Smooth layer thickness over surface patches
        nSmoothThickness 10;



    // Medial axis analysis

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

        // Reduce layer growth where ratio thickness to medial
        // distance is large
        maxThicknessToMedialRatio 0.3;

        // Number of smoothing iterations of interior mesh movement direction
        nSmoothNormals 15;

        // Optional: limit the number of steps walking away from the surface.
        // Default is unlimited.
        // nMedialAxisIter 10;

        // Optional: smooth displacement after medial axis determination.
        // default is 0.
        // nSmoothDisplacement 90;

        // (wip)Optional: do not extrude a point if none of the surrounding points is
        // not extruded. Default is false.
        // detectExtrusionIsland true;


    // Mesh shrinking

        // Optional: at non-patched sides allow mesh to slip if extrusion
        // direction makes angle larger than slipFeatureAngle. Default is
        // 0.5*featureAngle.
        slipFeatureAngle 30;

        // Maximum number of snapping relaxation iterations. Should stop
        // before upon reaching a correct mesh.
        nRelaxIter 25;

        // 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 10;

        // Max number of iterations after which relaxed meshQuality controls
        // get used. Up to nRelaxedIter it uses the settings in
        // meshQualityControls,
        // after nRelaxedIter it uses the values in
        // meshQualityControls::relaxed.
        nRelaxedIter 25;

        // 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;

For this particular case, one can modify following parameters to achieve better layer coverage (note that for another case it can significantly impair mesh quality):
- maxFaceThicknessRatio 5.0;
- maxThicknessToMedialRatio 3.0;
- slipFeatureAngle 130;

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