Hello all, (apologies in advance for the double post, looks like I wasn't subscribed properly earlier),
Ive been working on some libMesh/GRINS/PETSc Multigrid code and have been getting some unexpected convergence results that don't quite line up with theory. Currently I am uniformly refining a simple square grid with Dirichlet BCs and manually element-wise constructing L2 projected interpolation matrices between mesh levels while constraining each projection using the four argument version of constrain_element_matrix. As a naive first attempt I turned asymmetric_constrain_rows to false, as it helped me get past the `!constraint_row.empty()' assertion which I didn't quite understand. Having now seen some of the convergence results I think maybe I was too hasty in turning this asymmetric switch off as my iterative solvers are having some difficulties on some mesh levels while being OK on others, my hunch is this might be related to projections of hanging boundary nodes I introduce while refining - but this leaves me stuck on the above assert. Could someone please provide some kind of introductory reference on how this constraint process works as a whole, or what might be causing me to trip this assert? I notice that this assert is disabled in the 3 argument version and I guess am just wondering what is the best way to understand why the DofConstraintRow might be empty in this situation. Thanks for your time, Boris Boutkov
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