snappyHexMesh - mergeAcrossPatches fails on symmetryPlane

Summary

The new feature of snappyHexMesh "mergeAcrossPatches" works well on patch boundaries and greatly improves layer coverage. Unfortunately it damages a symmetry plane.

Steps to reproduce

Generate mesh for a case where the geometry intersects the symmetry plane.

Example case

Is attached.

What is the current bug behaviour?

When mergeAcrossPatches is true, not only the faces at patch boundaries on the geometry are merged (which is very desirable for layer coverage), but also the faces at the boundary between geometry and symmetry plane. Consequently, the symmetry plane is substantially not planar. See attached images.

What is the expected correct behavior?

The faces should only be merged at patch boundaries on the geometry. Intersections between the geometry and the domain boundaries should be treated as features where the faces won't be merged.

For a symmetry plane the current behavior leads to an error (as it is not planar), but the same applies for boundaries which are patches, e.g. the lowerWall boundary in the motorbike tutorial.

Relevant logs and/or images

Attachements:

• mergeAcrossPatches-false.png - No merged faces; symmetry plane planar
• mergeAcrossPatches-true.png - merged faces; "crimpled" edge between symmetry plane and geometry.
• mergeAcrossPatches.tgz - example case. Use Allrun to run (runs a few seconds). (/uploads/9917f61abe18891cca38d55325d8db74/mergeAcrossPatches.tgz)

Environment information

• OpenFOAM version : v1906
• Operating system : ubuntu
• Hardware info :
• Compiler : gcc

Possible fixes

Ideas:

• Explicitly exclude certain patches from the merge mechanism
• Identify feature at intersection of geometry with domain boundary.[mergeAcrossPatches.tgz]

Update: The damage happens in the layer addition phase (though no layers are created in the example). It can be avoided if the parameter addLayersControls -> featureAngle is reduced:

    // When not to extrude surface. 0 is flat surface, 90 is when two faces
    // are perpendicular
    featureAngle 5;

Unfortunately, this leads to hardly any layers being added in real cases.

By Ole Hympendahl on 2019-07-02T13:26:44 (imported from GitLab project)

Many thanks @ohy , we will have a look at it when we can spare time.

@ohy Could you please upload the test case again? It seems the case didn't make through.

mergeAcrossPatches.tgz

@Prashant Hope it works now. Thanks for having a look, this is a great feature!

By Ole Hympendahl on 2019-07-03T06:36:09 (imported from GitLab project)

Dear @ohy

The lowerWall was set as 'wall' type in your case. So the code doesn't treat it as a symmetryPlane.

You need to change the type in blockMeshDict to be symmetry.

Also change in $FOAM_SRC/dynamicMesh/polyTopoChange/polyTopoChange/combineFaces.C following lines to be changed (thanks to @Mattijs )

        (
            p0 != -1
         && p1 != -1
//         && !patches[p0].coupled()
//         && !patches[p1].coupled()
         && !(
                polyPatch::constraintType(patches[p0].type())
             || polyPatch::constraintType(patches[p1].type())
            )          
        )

This should work as attached screenshot. However we need to review it further to generalize it for two symmetryPlanes, or any other combination where we want to merge or not to merge...

mentioned in commit e3de84e5fe0805c2da038ccbcde02554e4d4405a

Dear @Prashant, dear @Mattijs, Many thanks for pointing out this solution! Finally I tested this and it works very well. Best regards Ole

By Ole Hympendahl on 2019-09-23T12:33:57 (imported from GitLab project)

closed

By Ole Hympendahl on 2019-09-23T12:33:58 (imported from GitLab project)