Dear Subramanya,
The first option does not tie you to solving fully coupled problems. In
fact, you can do any kind of staggered/coupled approach as you want,
because MatrixFree is conceptually just the data container collecting
various things, whereas you then define one or more operators on top of
it, like the LaplaceOperator in step-37. So when you go with the first
approach (what I recommend), you first set up a single MatrixFree object
with both DoFHandler objects (and all other relevant information, you
can also give different quadrature formulas, for example). Then you
create two classes, which are both using the same MatrixFree object (or
via the same shared_ptr), where the vmult() function in one class uses
"FEEvaluation<dim,....> eval_stress(matrix_free, 0)" as the evaluator
object and the vmult() function in the other class uses
"FEEvaluation<dim,....> eval_temp(matrix_free, 1)". And then you have a
coupling routine (organized in any way you like, but again with access
to the main MatrixFree object) that has both these FEEvaluation objects
in its evaluation function and does whatever you want to do, e.g.,
extract the values from one field (via read_dof_values()/evaluate()) and
consuming them as "coefficient" in the other field (via
integrate()/distribute_local_to_global()) on the other field, which
happens at the quadrature points.
As long as they are in the same MatrixFree object, everything is SIMD
vectorized and assembled in an optimized way.
Best regards,
Martin
Am 05.01.25 um 04:30 schrieb Subramanya G:
Thanks Martin for all the help!
If I understand correctly, the first approach would assume that both
the problems are solved in a fully coupled manner, with all fields
updated simultaneously at each solve.
For a staggered solution , where I solve the temperature first, and
then the stress - I'd either have to manually compute the cell
matching between the two objects, before assembling the vector, or
abandon the SIMD vectorization, and compute the vector the
"old-fashioned" way just for the coupling where information from the
two vectors needs to be accessed simultaneously.
Subramanya.
सुब्रह्मण्य .
On Sun, Jan 5, 2025 at 1:51 AM 'Martin Kronbichler' via deal.II User
Group <dealii@googlegroups.com> wrote:
Dear Subramanya,
The actual cells associated with the integer indices 'cell' (that
are batches in case of SIMD vectorization) are not necessarily the
same if you create two separate MatrixFree objects and initialize
each with the respective DoFHandler for the two fields. If you
intend to use coupled problems like the thermal-stress problem you
describe, you should use the MatrixFree::reinit function that
takes both DoFHandler objects as the arguments to an
std::vector<DoFHandler*> and respectively for the affine
constraints. Then, you can initialize FEEvaluation objects using
the 'dof_no' argument in the constructor:
https://www.dealii.org/developer/doxygen/deal.II/classFEEvaluation.html#ade40e452949cbefd4f7e6d5580af81c8
- it is the second argument next to MatrixFree.
You can also try to run with separate MatrixFree objects, but then
you need to manually link the two objects and one FEEvaluation
needs to be initialized with an array of cells (that you
constructed manually). I have done that in some applications, and
there is at least one code inside deal.II that does this:
https://github.com/dealii/dealii/blob/8f97fd0dca1eb878bc7ae9922ac89430cab800db/include/deal.II/multigrid/mg_transfer_global_coarsening.templates.h#L4041-L4068
- but it is much less obvious, so I really recommend the first
variant.
I hope this helps!
Best regards,
Martin
Am 03.01.25 um 17:14 schrieb subramanya gautam:
I am trying to implement a staggered thermal-elastic solver using
the matrix-free multigrid solver approach from step-37. I got an
elasticity solver and a thermal solver working and now I am
trying to associate the two. I have a shared triangulation, with
two separate dof_handlers for the thermal and stress problems.
The question I have is when I do the assembly loop, does the cell
refer to the same cell for both the thermal and elastic problems?
So from step-37, can I just call phi_temp.reinit(cell), and
phi_stress.reinit(cell), and have cell refer to the same physical
cell in the triangulation?
for (unsigned int cell = 0;
cell < system_matrix.get_matrix_free()->n_cell_batches();
++cell)
{
phi.reinit(cell);
for (const unsigned int q : phi.quadrature_point_indices())
phi.submit_value(make_vectorized_array
<https://www.dealii.org/current/doxygen/deal.II/vectorization_8h.html#ad7d7e08942faeecf438c75a254e06cbe><double>(1.0),
q);
phi.integrate(EvaluationFlags::values
<https://www.dealii.org/current/doxygen/deal.II/namespaceEvaluationFlags.html#a9b7c6d689cb76386839d0d13640f59aeaf9825c682f693a6a200094641a0d6a58>);
phi.distribute_local_to_global(system_rhs);
}
Thanks!
Subramanya.
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