Hi Wolfgang, Thanks for the clarification. This answers my questions. Thanks, Isaac. On Wednesday, October 4, 2023 at 11:56:39 PM UTC-4 Wolfgang Bangerth wrote:
> Isaac: > > On 10/4/23 16:05, Isaac Bannerman wrote: > > I am using dealii for a CFD solver and the solution vector which is > passed to > > the linear algebra solver is a BlockVector where block zero is the > velocity > > and block 1 is the pressure. I would like to obtain the global DOF > numbers of > > all the entries in the block vector; I would therefore like to know if > the > > the indices of the solution vector (especially for block zero) > corresponds > > to the global DOF numbers. The following lines (ignoring other > details) > > illustrates the question I am asking: > > > > //solution vector > > BlockVector<double> newton_update; > > > > //the solution steps > > SolverFGMRES<BlockVector<double>> gmres(solver_control, vector_memory); > > gmres.solve(system_matrix, newton_update, system_rhs, *preconditioner); > > > > I want to know for instance if index "44" used in the snippet below will > > correspond to global dof number 44. > > > > newton_update.block(0)[44] > > > > In the case where the answer to the above question is no, could someone > please > > suggest a way to obtain the global DOF number of each entry in the > BlockVector? > > At the end of the day, the connection between DoF numbers and vector > elements > is what you make it. If you take a look at what step-3 does, for example, > you > see in the assembly this: > > // get DoF indices > cell->get_dof_indices(local_dof_indices); > > for (const unsigned int i : ...) > // Use DoF indices as index into a vector > system_rhs(local_dof_indices[i]) += cell_rhs(i); > > So yes, generally the DoF indices correspond 1:1 to vector entries. > > Of course, when you split a vector into blocks, the indices *within these > blocks* do not necessarily correspond to DoF indices. So in your example > above, > newton_update[44] > corresponds to DoF index 44. The use of > newton_update.block(0)[44] > also corresponds to DoF index 44, but only because block 0 starts at index > 0. > On the other hand, > newton_update.block(1)[44] > does *not* correspond to the DoF with index 44, but to the DoF with index > newton_update.block(0).size() + 44 > > > > My ultimate goal is the obtain the DOF and real coordinates (x,y,z) of > each > > entry in the block vector. Fortunately, I already obtained the DOFs and > their > > corresponding coordinates in the real space using > > DoFTools::map_dofs_to_support_points(...) ; I want to get the DOFs for > the > > BlockVector entries so I can match the the coordinates to the entries in > the > > BlockVector. Any suggestion on a better way to do it will also be much > > appreciated. > > As shown above, if you index into a block, you need to take into account > the > offset of that block within the vector as a whole. > > Best > W. > > > -- > ------------------------------------------------------------------------ > Wolfgang Bangerth email: bang...@colostate.edu > www: http://www.math.colostate.edu/~bangerth/ > > > -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en --- You received this message because you are subscribed to the Google Groups "deal.II User Group" group. To unsubscribe from this group and stop receiving emails from it, send an email to dealii+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/dealii/5cbced8b-a55c-4b40-8794-35e5f4924efen%40googlegroups.com.
