) 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.) 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.
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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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If Tolerance is selected, enter a Maximum number of iterations 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 Tolerance or Iterations or tolerance is selected, enter a Tolerance factor to modify the tolerance used for termination of the segregated iterations. The actual tolerance used is this factor times the value specified in the Relative tolerance field in the General sections of the Stationary Solver and Time-Dependent Solver nodes.
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If Tolerance or Iterations or tolerance is selected, choose a Termination criterion to control how the Newton iterations are terminated. Select:
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Solution to terminate the Newton iterations on a solution-based estimated relative error.
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Residual to terminate the Newton iterations on a residual-based estimated relative error.
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Solution or residual to terminate the Newton iterations on a solution-based estimated relative error and a residual-based estimated relative errors. Enter a scalar Residual factor multiplying the residual error estimate. The default is 1000.
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If Iterations or Iterations or tolerance is selected, enter a Number of iterations to specify a fixed number of iterations to perform. The default is 1.
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Pseudo time-stepping 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:
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The Limit to target CFL number checkbox is selected by default. This setting controls how the CFL number is changed once the Target CFL number is reached. If you clear this checkbox, the CFL number can continue to increase until the error tolerance is fulfilled.
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Select the Anderson acceleration checkbox to activate Anderson acceleration for the pseudo time-stepping method.
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Select the Override Jacobian update for steps checkbox to override updates of the Jacobian for the segregated steps. The CFL threshold value (default: 100), which is the value of the CFL number where overriding of the Jacobian update becomes active. That is, the overriding becomes active for larger CFL numbers than the threshold. From the Jacobian update list, choose On first iteration (the default) or Minimal, 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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Anderson acceleration, 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 Dimension of iteration space field (default: 10) and the mixing parameter as a value between 0 and 1 using the Mixing parameter field (default: 1.0). The Iteration delay field (default 0) contains the number of iterations between pseudo time stepping becomes inactive and Anderson acceleration becomes active. Enter a threshold value in the Threshold for Anderson step field (default: 10). This threshold value controls if the Anderson step or the Newton step is used in the nonlinear step. If the norm of the new step is less than the threshold times the norm of the previous step, the Anderson step is used. Otherwise, the Newton step is used. Lowering the value for the Threshold for Anderson step can improve robustness at the price of performance.
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