On Jun 21, 2012, at 7:42 AM, Matthew Knepley wrote: > On Wed, Jun 20, 2012 at 8:40 PM, Blaise Bourdin <bourdin at lsu.edu> wrote: > Hi, > > I want to change the way I handle boundary conditions in my finite elements > code: > Right now, I skip the rows and columns during the matrices assembly, using a > flag (a SectionInt). Using an IS denoting all dof with a BC and > MatZeroRowsColumns, MatZeroRowsColumnsIS or MatZeroRowsColumnsLocal would > make much more sense. > > The difficulty comes up when applying a Dirichlet boundary condition on only > one of the dof associated with a vertex (roller boundary conditions in > elasticity, for instance). What I can easily get from Sieve is the point > number associated with a vertex, for instance, from which I can obtain the > _local_ index of the dof associated with a given component of a field at this > point. For vectors, I can then set values on the local Vec or the Section. > For matrices, though I can't just use MatGetLocalSubMatrix since I need also > to modify the out of diagonal block terms. > > I guess I need the LocalToGlobalMapping or to renumber the IS describing my > BC and convert it to global indices. I am sure that this information is > somewhere in Sieve (in the overlap, I guess), but I was thinking that since > getting a Local to Global scatter is trivial, there may be a way to obtain > the mapping or renumbering the IS from the scatter. > > Now that I think about it more, the right way is most likely to get the > information I need from Sieve. > > Yes, in the new version there is jsut a LocalToGlobalMapping. In C++, you can > use the GlobalNumbering, > > numbering->getIndex(point) + dof offset
And for the record, in the old version, where a DMMesh can be associated with several data layout, the following seems to work (adapted from DMCreateLocalToGlobalMapping_Mesh): Obj<PETSC_MESH_TYPE::real_section_type> s; ALE::Obj<PETSC_MESH_TYPE> m; ierr = DMMeshGetMesh(dm, m);CHKERRQ(ierr); ierr = SectionRealGetSection(vsec,s);CHKERRQ(ierr); const ALE::Obj<PETSC_MESH_TYPE::order_type>& globalOrder = m->getFactory()->getGlobalOrder(m, s->getName(), s); offset = globalOrder->getIndex(point) + dof ; Blaise -- Department of Mathematics and Center for Computation & Technology Louisiana State University, Baton Rouge, LA 70803, USA Tel. +1 (225) 578 1612, Fax +1 (225) 578 4276 http://www.math.lsu.edu/~bourdin -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20120621/f2361635/attachment-0001.html>
