The Solvers
When solving a Multiphysics problem it is necessary to ensure that the solvers behave correctly. It is especially important to check if a model is solved Fully Coupled or in a Segregated way. COMSOL has built in logic to handle many multiphysics problems when the predefined multiphysics couplings are used ensuring that the model solves correctly. If many physics or couplings are added at the same time this logic can fail and manual settings should be performed. For large models COMSOL generates Iterative Solver Suggestions (see below).
See Introduction to Solvers and Studies and Solution Operation Nodes and Solvers in the COMSOL Multiphysics Reference Manual.
It is also possible to enable and disable both physics and couplings from within a Study Step. This can be done in the Physics and Variables Selection section in Settings. This functionality can be used to solve a model both coupled and uncoupled by adding two Studies : one where the Multiphysics coupling is active and one where it has been deactivated.
Iterative Solver Suggestions
Multiphysics models can in 3D easily grow rapidly in size and become difficult to solve with a direct solver, simply because you run out of RAM. In this case it can be necessary to switch to an iterative solver approach. For certain multiphysics couplings iterative solver suggestions are automatically generated. Set up a new solver configuration (first time the model is solved) or right click the study node and select Show Default Solver, then expand the Solver Configuration tree under Stationary Solver or Time-Dependent Solver. The predefined iterative solver suggestions that are automatically generated can be seen here. In the predefined cases a direct solver is used as default and iterative solvers are suggested and disabled (grayed out). To turn on one of these approached right-click the solver and select Enable (or press F4). Predefined suggestions exist for:
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Acoustic-Structure Boundary, gives one iterative solver suggestion.
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Thermoviscous Acoustic-Structure Boundary, generates two suggestions. The first is fast and can save 20% of memory while the second that uses domain decomposition (DD) is very robust and memory lean, but it can be slow.
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Combining Acoustic-Structure Boundary and Piezoelectric Effect yields a suggestion that can be used for modeling large piezoelectric transducer assemblies.
All solver suggestions have a tag that makes it possible to see which physics feature that has generated them, for example, (asb1) means that the suggestion comes from the Acoustic-Structure Boundary 1 feature.