[DriesAllaerts]

Hi all,

I'm trying to use snappyHexMesh to create meshes for multiregion cases. I have a separate stl file for every region, and I start from a background mesh generated by blockMesh. I'm only using the castellatedMesh step, snap and addLayers is set to false. snappyHexMesh works fine when the interface between different regions is flat. When the interface is curved, however, some holes appear in the mesh on the interface (see MWE_fig2 for example). I think this problem is related to the fact that curved surfaces are triangulated in the stl files, and very small differences in triangulation are conceived by snappy as a hole in the geometry which does not need to be meshed.

I created a minimal working example to illustrate the issue. The example consists of two concentric cylinders corresponding to a fluid and a solid region (see MWE_fig1). Figure MWE_fig2 shows a close up of the mesh at the interface, showing that there are several holes in the mesh. Figure MWE_fig3 shows the interface patch solid_to_fluid in red, and the white line across the interface correspond to faces that are erroneously indentified as a boundary face with patchtype wall instead of a mappedWall between solid and fluid.

The main consequence of these holes is that part of the interface is converted into insulatedWalls, so the effective area through which heat is exchanged between fluid and solid is reduced. A secondary consequence is that some of the faces are assigned to the wrong region: the solid has a patch fluid_patch0 that originates from the fluid stl file, and the fluid has a patch solid_patch0 which originates from the solid stl file. I attached a log file with the output of blockMesh, surfaceFeatureExtract and splitMeshRegions.

I have tried several things to solve this issue but without succes. Increasing the original block mesh resolution does not solve the issue. Adding refinement levels does not solve the issue. The stl files are extracted from AutoDesk Inventor; increasing the resolution of the surface triangulation in the stl file reduces the amount of holes, though even at the highest resolution there are still holes in the mesh. Turning on the snapping phase closes the holes, but the faces corresponding to the deleted cells remain boundary faces with patch type wall, which means that the effective interface area is still reduced. I also tried nCellZoneErodeIter, handleSnapProblems, and increasing the mergeTolerance, but none of them has any effect. Setting gapLevelIncrement makes the problem worse.

Has anyone encountered similar issues and did you find a solution? Should this be reported as a bug in snappyHexMesh?

Thanks for looking into this case.
Dries Allaerts