Defining Coupled Equations Controls

You can define controls to fine-tune the coupled equations solver.

Important: These controls are intended for expert users.

This task shows you how to:

Before you begin:

Enable the coupled solver when you define the fluid physics of your scenario. For more information, see Defining the Fluid Physics of a Flow Simulation and About Coupled Solving Versus Segregated Solving.
Define a steady-state flow step or a transient flow step. For more information, see Defining a Steady-State Flow Step or Defining a Transient Flow Step, respectively.
Expand the Coupled equations controls section of your step.

Define Controls Using the AMG-BiCGStab Linear Solver

Enter a value for the Maximum CFL, which is the pseudo time step used to define the subiterations.
From the Linear solver options, select AMG-BiCGStab.
The app calculates the pre-conditioner matrix using the algebraic multi-grid bi-conjugate gradient stabilized (AMG-BiCGStab) method. The AMG-BiCGStab linear solver is recommended for most simulations.
Enter a value for the Iteration limit, which is the maximum number of iterations for the linear solver.
Enter a value for the Convergence check frequency, which is the interval (in linear iterations) to check linear convergence.
Enter a value for the Linear convergence limit, which is the point at which the iterative solver stops.
The iterative solver stops when the relative residual norm of the system of equations and the relative correction of the solution norm fall below this convergence limit.
Select one of the following patterns for filtering errors from the AMG cycle type options:
Option Description
V Filters errors using a V-cycle pattern.
W Filters errors using a W-cycle pattern.

The cycle type pattern filters the error from the linear solver to accelerate the convergence.

Select one of the following techniques for removing high-frequency errors from the Residual smoother options:

Option	Description
ILU	Removes high-frequency errors using the incomplete lower-upper factorization smoother.
SSOR	Removes high-frequency errors using the symmetric successive over-relaxation smoother.

Enter a value for the Maximum size of coarsest level.
Enter a value for the Maximum number of levels, which is the maximum number of coarse grid problems.
Enter a value for the Number of pre-smoothing steps, which is the number of pre-relaxations for the fixed multi-grid cycles.
Enter a value for the Number of post-smoothing steps, which is the number of post-relaxations for the fixed multi-grid cycles.

Enter a value for the Maximum CFL, which is the pseudo time step used to define the subiterations.
From the Linear solver options, select ILU-FGMRES.
The app calculates the pre-conditioner matrix using the incomplete lower-upper factorization flexible generalized minimum residual (ILU-FGMRES) method.
Enter a value for the Iteration limit, which is the maximum number of iterations for the linear solver.
Enter a value for the Convergence check frequency, which is the interval (in linear iterations) to check linear convergence.
Enter a value for the Linear convergence limit, which is the point at which the iterative solver stops.
The iterative solver stops when the relative residual norm of the system of equations and the relative correction of the solution norm fall below this convergence limit.
Enter a value for the Number of restart vectors.
The number of restart vectors is the number of times the solution is projected into a subspace before the subspace is rebuilt.