[tkeskita]

Hi,

I think I've managed to find a nice layer addition procedure for SnappyHexMesh. Otherwise the result looks fine, but the layer addition seems to remove some edges at sharp features, see the top part of this figure:

with_layers.jpg

The top edges were fine after snapping phase, before layer addition:

without_layers.jpg

Does anyone know why this happens? It's possible to fix this issue by separating the surfaces into different patches, but it's a cumbersome workaround..

BR,
Tuomo

[Yann]

Hello,

If you are using the OpenCFD branch (openfoam.com), try using the mergePatchFacesAngle parameter in addLayersControls:

// When to merge patch faces. Default is featureAngle. Useful when featureAngle is large.
mergePatchFacesAngle 45;

source: https://develop.openfoam.com/Develop...ref_type=heads

[tkeskita]

Hi,

Originally Posted by Yann

If you are using the OpenCFD branch (openfoam.com), try using the mergePatchFacesAngle parameter in addLayersControls:

thanks, that helped (when using openfoam.com v2306)!

I've been testing the layer addition, and so far the best results I've got when I've used the settings below, and then grown layers incrementally by rerunning snappyHexMesh with only the layer addition option enabled, adding only 2 layers at a time. Between the runs I only decrease finalLayerThickness and minThickness. This approach seems to produce layers almost everywhere nicely, but I have no idea why this stepwise growing approach seems to work, but growing all layers in one go does not.. any insights?

addLayersControls
{
    // Layer sizing
    relativeSizes   true;
    expansionRatio  1.3;
    finalLayerThickness 0.3; // 0.15;
    minThickness    0.1; // 0.05;
    nGrow           0;

    // Mesh dependencies
    featureAngle    130;
    mergePatchFacesAngle 45; // OpenFOAM.com option
    layerTerminationAngle -180; // OpenFOAM.com option
    maxFaceThicknessRatio 0.5;

    // Patch displacement
    nSmoothSurfaceNormals 1;
    nSmoothThickness 10;

    // Medial axis analysis
    minMedialAxisAngle 90;
    maxThicknessToMedialRatio 0.3;
    nSmoothNormals  3;
    slipFeatureAngle 30;

    // Mesh shrinking
    nRelaxIter      3;
    nBufferCellsNoExtrude 0;
    nLayerIter      50;
    nRelaxedIter    0;
    nOuterIter      3; // OpenFOAM.com option

    layers
    {
        inlet
        {
             nSurfaceLayers 0;
        }
        outlet
        {
             nSurfaceLayers 0;
        }
        walls
        {
             nSurfaceLayers 2;
        }
    }
 }

meshQualityControls
{
    maxNonOrtho     35;
    relaxed { maxNonOrtho 75; } // for layer addition
    maxBoundarySkewness 20;
    maxInternalSkewness 4;
    maxConcave      80;
    minVol          1e-15;
    minTetQuality   1e-15;
    minArea         -1;
    minTwist        0.02;
    minDeterminant  0.001;
    minFaceWeight   0.05;
    minVolRatio     0.01;
    minTriangleTwist -1;
    nSmoothScale    4;
    errorReduction  0.75;
}

[Yann]

Have you tried the multi pass layer addition feature implemented since v2206 release? It looks like you're doing the same thing manually.
https://www.openfoam.com/news/main-n...hexmesh-layers

Snappy computes the total layer thickness and then moves the mesh away from the surface in order to insert the layers. When doing so, it has to morph the existing mesh and it sometimes results in bad cell quality which prevent it to add layers (to maintain mesh quality).
The stepwise approach might help thanks to the smaller mesh motion it requires. Just my feeling about it though.

[tkeskita]

Hi,

thanks again @Yann! Turned out that the problem was in openfoam.org version of snappyHexMesh. Openfoam.com version seems to work nicely! Yes, the multi pass layer addition of openfoam.com is very useful, I could get all the layers in one go! I ended up using nOuterIter = maximum number of layers divided by 2. Here is an example:

sharp_edge.png

I'm wondering about the layers near 90 degree corners like this one below, would it be better not to have layers here? I mean, might there be some numerical trouble when the flow is in right to left direction..? Openfoam.org version of Snappy does not make layers in this area, maybe there is still some parameter to be tuned for openfoam.com?

corners.png

BR,
Tuomo

[Yann]

Originally Posted by tkeskita

I'm wondering about the layers near 90 degree corners like this one below, would it be better not to have layers here? I mean, might there be some numerical trouble when the flow is in right to left direction..? Openfoam.org version of Snappy does not make layers in this area, maybe there is still some parameter to be tuned for openfoam.com?

Attachment 97651

I don't know about numerical trouble, it really depends on your flow. Probably you should try and see how it behaves.

To avoid getting layers here, you could set the featureAngle below 90°, but it will not add layers in any sharp angle on your geometry. The only way to avoid adding layers in this specific place would be to have a separate patch for it so you can control the layers parameters for this patch.

Regards,
Yann

[tkeskita]

FYI I've now added what I've learned about snappyHexMesh boundary layer addtion to my Blender SnappyHexMesh GUI add-on (see the discussion thread) for ease of use!

[tkeskita]

Hi,

optimized layer parameters further, here is the result when using Feature Angle 130. Turned out that nSmoothDisplacement iterations are really important!

Screenshot_2023-12-16_19-45-13.png

and this one using Feature Angle 45. For some reason collapsing happens at second edge and not the first from the corner.

Screenshot_2023-12-16_19-51-29.png

This one is with Feature Angle 95, seems like something in between..

Screenshot_2023-12-16_20-19-07.png

[Yann]

Hello Tuomo,

I like the first one
Out of curiosity, have you tried to use your settings on another geometry?

[tkeskita]

Hi Yann,

Originally Posted by Yann

I like the first one

Ok, but the real question is, does your solver like it too?

Originally Posted by Yann

Out of curiosity, have you tried to use your settings on another geometry?

Well I tried a few other ones, including the OF flange geometry. Maybe try the motorBike geo at some point, that might pose a challenge..

BR,
Tuomo