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: bange...@colostate.edu
www: http://www.math.colostate.edu/~bangerth/
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