shell mesh missing！
 

I am working with an airplane model with high-lift devices. Due to the geometry complexity, I first choose ICEM Octree option to generate tetra meshes. During this process, I encounterd some confusing problems.

The shell mesh on the slat are missing. Actually the tetra meshes get into the slat（Fig. 1）.

I knew I could get the slat meshed if a larger "max elment size" was set on appropriate entities. In my case, to get finner mesh around the boundaries like the trailing edges, I grouped bounding curves in a single part seperated with all other surfaces, and set small sizes on these curves. From the geometry view, I got a watertight surface (all red lines) after buiding geometry topologies. The element size I set on the curves and surfaces also seemed to be larger than the topology tolerances (the trailing edge thickness is around 1, the building topo is 0.1, and the max element size on the trailing edge curves is 8). I don't know why the slat can't be meshed with small sizes set on trailing edge curves.

I also thought warning box "your geometry has a hole" would pop up as if there were any geometry leakage I didn't found, but nothing happend. After I created a ORFN material in the slat as once suggested in this forum(Fig. 2), there were still not any leakage message; however, in the log window below I saw "material point ORFN 1 can reach material point fluid 1". Finally, I found that if I toggled off "fix holes" in volume mesh set up, that leakage waring box will appear(Fig. 3). I just could not figure out the reason again.

Are there any underlying geometry flaws with this model, which can't be discovered easily? For a given geometry, does that mean the meshes will get into the inner space through the geometry seams if element size are decreased to a certain value？

Options...
 

Yes, if your geometry has flaws (gaps or holes), and your mesh goes finer than those gaps or holes, the flood fill process will fill it with mesh.

If you don't have a material point inside the "void", it will see the same volume on both sides and remove the shells (exactly what you were seeing).

These images are not enough to know if there is a problem with your geometry. You could even have a problem with your material point locations (they should be in the volumes and not too close to the surfaces to ensure a good flood fill). The fact that it worked for the coarser mesh does suggest that you have a tolerance issue. Below a certain mesh size, you have leakage. You could run Build topo with the tolerance set to smaller than your smallest mesh size and see what happens, but I would check the leakage path first.

Creating a material point (ORFN or SOLID) should warn you about leakage... Use that opportunity to see where the leakage path is. The jagged line connects the material points thru the centroid of each elements along the flood fill leakage path. It will pass directly thru the gap or hole. Once you find it, you could fix it.

Another good option would be to use the "Only Shells" option. This will delete all the volume mesh, but leave you the shells. Then you could run simple checks, such as for single edges (suggests holes). Once you have the surface mesh fixed up nice and watertight, you could run a bottom up tetra method such as Advancing front or Delaunay with the TGlib-AF option...



Simon

Appreciate your reply very much, Simon! I still have some unresolved questions.

First, I did run "build topologies " with a small tolerance compared with mesh size. In this model, the maximum element size on surface was set to 256, and those on curves were set to 16 except curves surrounding the thin trailing edge (0.99 thick), the maximum element sizes of which were set to 4. My building topo is only 0.1, and I got all red lines around.

Which parameters determin the smallest mesh size you mentioned? Min size limit? I also set min size limit to 4 in global mesh setup. Does that mean my smallest mesh size will not exceed 4? If in that way, why do tetra meshes go inside this watertight model (tolerance 0.1)?

Second, I have checked the material point. I lies in the fluid domain and far away form the slat surface. Even after I put an ORFN inside the model, I didn't get leakage warning untill I check off "fix holes" in volume mesh set up. It is really beyond my previous experience. Can building topo with small tolerance resolve all geomerty flaw problems?