over the last few weeks I've been trying to understand how to use ISSM, in particular how to use the Full-Stokes model. I understand that FS is tricky, so I've given it some time to test around with different discretizations and settings. Unfortunately, I don't quite trust the results I've been getting with FS, details below. We had a project meeting today, and we agreed we'd really want to use FS but that it doesn't seem to work thus far. My background is in model order reduction, so it's an obvious choice because of its cost, especially long term.
I guess what I'm looking for at this point is more details about how the FS model is solved internally, and if there are any settings that might help with stability. For instance, what's the difference between MINI and MINI-condensed? I've tried Taylor Hood, but the solver wouldn't converge regardless of mesh size. I've been going through the documentation on the website, but I couldn't find much in regard to computational details outside the use of ISSM.
A big question that came up today was mass conservation in the stressbalance equations. If I understand correctly, for SSA and HO the pressure is computed from the velocity such that mass is conserved. For the Stokes problem this is only weakly enforced unless the finite element scheme is chosen appropriately e.g. such that the pressure space contains P0 (e.g. https://link.springer.com/article/10.1007/s10915-011-9549-4 ). For the FE schemes implemented in ISSM, is mass conservation guaranteed at the element level? I don't think Taylor-Hood (P2-P1) does it, but maybe the MINI schemes? Have you tried using P0 or P1+P0 as pressure space?
I'm happy to discuss per email if there's a specific contact person on the ISSM side.