Segregated

The node (

) is an attribute that handles parameters for a segregated solution approach. This attribute makes it possible to split the solution process into substeps. Each substep uses a damped version of Newton’s method.

The attribute can be used together with the Stationary Solver and Time-Dependent Solver nodes. An alternative to the segregated approach is given by the coupled solver, which is handled with the Fully Coupled attribute node. Although several and attribute nodes can be attached to an operation node, only one can be active at any given time.

	The convergence properties of a model might depend on the order of the segregated steps. You can move the Segregated Step nodes to change the order in which the solver runs each step.

	Using the segregated solver, you can impose lower limits and upper limits for the field variables by adding Lower Limit and Upper Limit subnodes. See Lower Limit and Upper Limit.

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Termination Criterion for the Fully Coupled and Segregated Attribute Nodes

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to terminate the segregated iterations after a fixed number of iterations.

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If is selected, enter a to limit the number of segregated iterations (default: 10). When the maximum number of iterations has been performed, the segregated method is terminated even if the tolerance is not fulfilled.

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If or is selected, enter a to modify the tolerance used for termination of the segregated iterations. The actual tolerance used is this factor times the value specified in the field in the sections of the and nodes.

to terminate the Newton iterations on the minimum of the solution-based and residual-based estimated relative errors. Then enter a scalar (default: 1000) that multiplies the residual error estimate.

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If or is selected, enter a to specify a fixed number of iterations to perform. The default is 1.

With a Time-Dependent Solver, also select the check box to force the nonlinear solver to terminate as soon as the convergence is estimated to be too slow. Enter a limit on the convergence rate in the accompanying field.

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to use a pseudo time-stepping method to stabilize convergence toward steady state for a stationary solver. Pseudo time stepping is not available for time-dependent solvers. See Pseudo Time Stepping for more information. For the pseudo time-stepping method, specify the following controller parameters:

Select the check box to override updates of the Jacobian for the segregated steps. The value (default: 100), which is the value of the CFL number where overriding of the Jacobian update becomes active. From the list, choose (the default) or , which updates the Jacobian at least once and then only when the nonlinear solver fails during parameter stepping. It reuses the Jacobian for several nonlinear systems whenever deemed possible.

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, which is a nonlinear convergence acceleration method that uses information from previous Newton iterations to accelerate convergence. The Anderson acceleration method is primarily intended for acceleration of nonlinear iterations in transport problems involving, for example, crosswind diffusion stabilization. It is useful is for solving linear or almost linear problems using the segregated solver, where convergence can be improved and the performance increased. You can control the number of iteration increments to store using the field (default: 10) and the mixing parameter as a value between 0 and 1 using the field (default: 1.0). The field (default 0) contains the number of iterations between pseudo time stepping becomes inactive and Anderson acceleration becomes active. When used for pseudo time stepping, you can also enter a value (default: 100), which is the value of the CFL number where Anderson acceleration becomes active and pseudo time stepping becomes inactive.