Nonlinear Solver
Default Settings for Heat Transfer interfaces
Nonlinear solver settings depend on the heat transfer model and on the study type.
Fully Coupled Solver Attribute
Heat transfer models use a fully coupled nonlinear solver attribute by default. The Jacobian update is set to minimal. A Newton nonlinear method is set by default with
Segregated Solver Attribute
The segregated solver attribute is set by default in the following cases:
Radiation in participating media using the Discrete ordinates method defines a large number of dependent variables (up to 80), which are placed in segregated groups. The number of dependent variables per segregated group and the nonlinear method settings depend on the Performance index parameter available in the heat transfer interface settings in the Participating Media Settings section.
The Thermal Damage subfeature (added under Biological Tissue feature) defines an additional variable alpha that is placed in a dedicated segregated group.
Default Settings for Moisture Transport interfaces
A Newton nonlinear method is set by default with a constant damping factor (0.5). The Jacobian update is set to On every iteration. The termination technique is Iterations, with Number of iterations set to 2.
Tuning the nonlinear solver
Default solver settings are defined to handle efficiently classical configurations. For particular applications, the default settings may need modifications to improve the robustness and performance of the solver.
Optimize Nonlinear Solver for Robustness
When the nonlinear solver fails or converges erratically, different options can be considered:
Using the Automatic highly nonlinear (Newton) option forces to start the computation with a very low damping factor and increases it carefully. Alternatively a low constant damping factor can be used. The damping factor ranges between 0 and 1. A constant damping factor equal to 0.1 is a very low value and should be robust but slow to converge. For low values of the damping factor, it is thus usually needed to increase the number of nonlinear iterations. If the nonlinear solver is unstable with such a damping factor then the automatic option should be used because it makes it possible to start with a lower damping factor and gradually increases it.
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Note that it is sometimes easier to update the boundary conditions than the initial condition to get consistent initial settings (see the Heat Conduction in a Finite Slab model).
Optimize Convergence Speed
Low convergence can be improved by following ways: