Coarsening grid from Abaqus imported mesh

[Thang W Pham]

Hello everyone,

I am new to deal.II and I am playing around with the grid of L-shape which is imported from Abaqus input file using GridIn::read_abaqus(). The grid has been successfully read as well as the boundary indicators ( from the surface with the name of "SS1" and "SS2"). However, when I try to do coarsening the grid by setting cell->set_coarsen_flag(), nothing happened. I also tested with adaptive refinement as in step-6, only a figure of the new cell has been generated, none has been coarsened even I set the coarsening ratio to high value. 

I wonder what is wrong here or is there something that I misunderstand with the code. Thanks in advance. 

[blais...@gmail.com]

Dear Truong,

My understanding is that the triangulation in deal.II works like a quad/oct-forest.That is, the mesh that is generated or read serves as the forest and each cell is a tree. Consequently, it is not possible to coarsen a mesh more than it's initial configuration.

For a lot of meshes generated by deal.II, this is not a problem since they contain a very low number of cells (for example, a cubic grid has 1 cell in it's coarsest level, hence it is a forest with once tree). However, if you read your mesh from an external source, you cannot make it coarser than it is when you have read it.

My suggestions would be to make a coarse abacus mesh, then you refine it within deal.II. This way you would have a coarsest level with as few cell a possible, then you could refine it adaptively. As far as I know, it is not possible to coarsen a mesh below it's level 0, because this would imply "merging cell".

I hope that was sufficiently clear.

Best of luck!
Bruno

[Thang W Pham]

Thanks for your instant and helpful answer. So, the grid from other mesh generator cannot be made coarser at all. As I referred from step-5 tutorial, I guess a procedure like associating the surface boundaries (from meshing software) to the manifolds would also only works for meshing refinement, not for mesh coarsening. 

Vào lúc 00:09:00 UTC+9 ngày Thứ Tư, 1 tháng 9, 2021, blais...@gmail.com đã viết:

[Jean-Paul Pelteret]

Dear Truong,

To supplement Bruno’s remarks, and answer this question in particular:

 I guess a procedure like associating the surface boundaries (from meshing software) to the manifolds would also only works for meshing refinement, not for mesh coarsening. 

It would work for mesh coarsening as well, but only up to the coarsest mesh that is read into deal.II. So if you do some initial refinement within deal.II itself, then both refinement and subsequent coarsening respect the attached manifold geometries.

The point to be understood is that deal.II does not, and cannot, have any more fundamental knowledge of the discretisation other than what you pass it in the original mesh. It does not know the steps that you took to get that mesh — the underlying CAD geometry, whether the mesh is cartesian or structured, has an even or odd number of divisions on each edge, etc. All of this information, which would be critical to determine how the mesh might be coarsened using the child-parent cell approach to adaptivity that deal.II adopts. So one always needs to consider a read in mesh to be “flattened”, i.e. at its coarsest state, and the benefits of mesh adaptivity may then be maximised when the read in grid is as coarse as possible and one is able to produce a satisfactorily refined mesh using the built in refinement tools (and possibly connecting the mesh to a CAD model or manifold geometries to provide information of boundary and interface curvature).

Best,

Jean-Paul

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[Thang W Pham]

Now I understand more about the mechanism of refinement tool with import mesh/grid. Thank you for such clear and detailed explanation and the suggestion as well.

Regards,

Thang. 

Vào lúc 14:00:30 UTC+9 ngày Thứ Tư, 1 tháng 9, 2021, Jean-Paul Pelteret đã viết: