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snappyHexMesh castellation step creates gaps between curved triangulated surfaces

Summary

In multiRegion cases, the castellation step of snappyHexMesh creates small gaps between curved triangulated surfaces (specified by stl files exported from CAD software AutoDesk Invecntor) when they are not exactly equal, even when stl files are exported at very high resolution (highest resolution available in the CAD software).

Steps to reproduce

The issue can be reproduced by applying snappyHexMesh on a multiregion case with a curved interface between two regions, and with the different regions specified by different stl files exported from CAD software.

Example case

Attached is a minimal working example consisting of two concentric cylinders corresponding to a fluid and a solid region. Only the castellation step of snappyHexMesh is run. MWE.tar.gz

What is the current bug behaviour?

The castellation step of snappyHexMesh creates small gaps between two triangulated surfaces, i.e., on the interface between two regions. As a result, some faces on the interface are identified as walls instead of mappedWalls between the two surfaces. In the snapping step, the gaps are closed but the patch type of some faces remains wall instead of mappedWall.

What is the expected correct behavior?

snappyHexMesh should not remove cells located between two triangulated surface when the two triangulated surfaces coincide within a certain tolerance. The tolerance should be such that coinciding surfaces exported by CAD software can be meshed without creating small gaps. Possibly, this tolerance could be a user input.

Relevant logs and/or images

Attached is a log file (log.txt) of the provided minimal working example (logs of surfaceFeatureExtract - blockMesh - snappyHexMesh - splitMeshRegions) as well as some figures showing the case setup and the gaps on the interface.

Overview of the multiRegion geometry. MWE_fig1

Detailed view of the interface. MWE_fig2

View of the interface, showing some faces that are identified as type wall (white) instead of mappedWall (red) MWE_fig3

Environment information

  • OpenFOAM version : v1906
  • Operating system : CentOS Linux release 7.6.1810
  • Hardware info :
  • Compiler : intel

Possible fixes