From: <an...@tn...> - 2004-03-15 19:24:14
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Benjamin KIRK, BENJAMIN (JSC-EG) (NASA) writes: > I have attached a description of the XDA format, hopefully this will clear > up how the boundary conditions are read from file. Thanks, this explains quite a bit. Apparently it is not possible to give nodal boundary conditions with specific values. To reproduce results from other FE Codes I would like to impose varying nodal quantities, such as Quantity Node-nr value 0 42 0.0 0 46 0.3 0 52 0.7 1 42 0.0 1 101 100. .... Of course it is easily possible to read these data in from additional files, but it would be nice to be able to provide numerical values of imposed boundary conditions. This seems to be missing in the scheme you described in the definition of the XDR files. > As for the boundary mesh: > > The main reason it exists is for extracting the boundary of the mesh for > post-processing purposes. The idea is that you can extract the entire > boundary of a simulation and then look at surface quantities of interest. > With that said, the only thing *I* have ever used it for is extracting faces > to make sure that boundary conditions are properly specified. meshtool.cc > probably shows this most clearly. The boundary mesh should be one dimension > lower than the primal mesh... I'll fix this so that it is properly > enforced. OK, this makes sense. I will also dig into meshtool.cc > As for specifying boundary conditions, the BoundaryInfo class simply > provides a way for you to group nodes or sides together. It is up to you to > use this grouping (presumably in your matrix assembly routine) to impose > meaningful boundary conditions. There are no 'magic' boundary conditions > (like 1 = no-slip, 2 = constant temperature, etc...). This would require > libMesh to know too much about the systems you are solving and would limit > your flexibility. So, are the boundary descriptors inherited on a refined mesh (ok, I should try that out)? Then it seems reasonable not to prescribe nodal values, or one would have to interpolate on a refined mesh. Thanks for your time Martin |