Guidelines for Solving Multiphysics problems
Multiphysics Models
Unless the model contains a multiphysics node that defines a coupling between a Heat Transfer interface and another interface (see Multiphysics Couplings below), each physics interface defines default solver settings that are merged.
The Heat Transfer interfaces always define a dedicated segregated group that uses a linear solver optimized for the heat transfer equations. For strongly coupled models, it may be efficient to merge two (or more) segregated steps. In this case, a unique linear solver must be chosen for the fully coupled solver or the new segregated group.
Time-dependent settings from different physics interfaces may compete. When the different settings are merged the strictest one is kept.
Multiphysics Couplings
When a Heat Transfer interface is coupled with another physics interface through a multiphysics coupling feature, additional predefined default settings are loaded. The next two paragraphs describes the subtleties of the Nonisothermal Flow, Electromagnetic Heating, and Heat and Moisture Transport interfaces.
Nonisothermal Flow
The Nonisothermal Flow multiphysics coupling controls the solver settings for the flow and the temperature dependent variables.
When it assumes a weak coupling between the flow and the heat interfaces (typically no Volume Force feature in the flow interface), the default solver contains dedicated segregated groups for heat and flow dependent variables. Each uses the default linear solver of the corresponding interface, except that the presmoother and postsmoother in the GMRES iterative solver switches from SOR to SCGS.
When a strong coupling is assumed (at least one Volume Force feature in the flow interface), the default solver merges the temperature, pressure, and velocity. In this case, the linear solver corresponds to the default linear solver of the flow interface. The Jacobian is updated once per time step.
Electromagnetic Heating
The Electromagnetic Heating multiphysics interfaces (Joule heating, Laser Heating, Induction Heating, and Microwave Heating) define default settings that solve the temperature and the electromagnetic fields using a coupled step. It can be the fully coupled nonlinear solver if there is no additional variable to solve for, otherwise it is a segregated step containing the temperature and the electromagnetic variables. However when radiation in participating media or damage variable are solved they are placed in a separate group as described above.
Heat Transfer with Surface-to-Surface Radiation
The Heat Transfer with Surface-to-Surface Radiation multiphysics coupling forces a strong coupling between temperature and radiosity variables solved by a Heat Transfer interface and a Surface-to-Surface Radiation interface, respectively. The default solver suggestion ensures that the temperature and the radiosity dependent variables are solved together, either by a fully coupled nonlinear solver, or by placing them in a same segregated group when other segregated groups are generated by some other physics interfaces.
Local Thermal Non-Equilibrium
The Local Thermal Non-Equilibrium multiphysics coupling forces a strong coupling between both the solid and the fluid temperature variables involved in the porous medium model. The default solver suggestion ensures these two temperature dependent variables are solved together, either by a fully coupled nonlinear solver, or by placing them in a same segregated group when other segregated groups are generated by some other physics interfaces.
Heat and Moisture Transport
The Heat and Moisture Transport interface defines dedicated segregated groups for heat and moisture dependent variables. Each uses the default linear solver of the corresponding interface. A Lower Limit node is added for the relative humidity to enforce a positive value.