Nonlinear Solver
Default Nonlinear 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 On every iteration for Stationary studies, and to On first iteration for Time-Dependent studies. 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 512 for a single wavelength), 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. When multiple wavelength are considered, the variables relative to distinct wavelengths are not mixed together in the segregated groups.
The Thermal Damage subfeature (added under Biological Tissue feature) defines an additional variable alpha that is placed in a dedicated segregated group.
Default Nonlinear Settings for Moisture Transport Interfaces
Nonlinear solver settings depend on the study type.
Fully Coupled Solver Attribute
A fully coupled solver attribute is set by default. A Newton nonlinear method with a constant damping factor is set by default, with the following settings:
The Damping factor is set to 0.7.
The Jacobian update is set to Once oer time step for Time-Dependent studies, and to On every iteration for Stationary studies or when the liquid concentration on surfaces, cl_evap, is solved.
The Maximum number of iterations is set to 5 for Time-Dependent studies, and to 25 for Stationary studies.
The Tolerance factor is set to 0.01 for Time-Dependent studies, and to 0.1 for Stationary studies.
Segregated Solver Attribute
The segregated solver attribute is set by default when another physics interface is solved together with moisture transport. The dependent variables of the Moisture Transport interface are placed in a separate segregated group.
A Newton nonlinear method with a constant damping factor is set by default, with the following settings:
The Damping factor is set to 0.7.
The Jacobian update is set to On first iteration for Time-Dependent studies, and to On every iteration for Stationary studies or when the liquid concentration on surfaces, cl_evap, is solved.
The termination technique is Iterations, with Number of iterations set to 2.
Default Nonlinear Settings for The Lumped Thermal System Interface
When a Lumped Thermal System interface is coupled to one or many heat transfer interface using Lumped Thermal Connector nodes, the lumped thermal system dependent variables and the temperature variables are gathered in the same segregated nodes if other degrees of freedom are solved for or in a Fully Coupled node.
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: