### 17.3.2. Linear Solver Parameters

`mg solver option`
Type | Integer
Default Value | 5 (automatic)

Description

For hydrodynamic equations (u, v, w, p), determines the coarsening algorithm for algebraic multigrid of the fluid coupled equations.

A value of 5 (automatic) has the following behavior:

- If the simulation involves multiphase flow with the free surface model activated, or with coupled volume fractions, option 2 is used.

- Otherwise, option 4 is used.

A value of 4 means that the anisotropic coarsening is designed to give robust convergence for the widest possible range of flow conditions.

Alternative coarsening algorithms are also available for comparison purposes.

A value of 3 means that coarsening is based on the velocity coefficient.

A value of 2 means that the anisotropic coarsening is based on the pressure coefficient. This was the previous default value.

A value of 1 sets an isotropic coarsening based on topology only (similar to conventional geometric multigrid).

`multigrid solver`
Type | Logical
Default Value | t
Description | Control whether multigrid is used in CFX-Solver.
`overlap relaxation fluids`
Type | Real
Default Value | 1.0
Description | Linear solver under-relaxation of overlap equations in a parallel run for hydrodynamics equations.
`solver relaxation fluids`
Type | Real
Default Value | 0.9
Description | The under-relaxation value for the coupled U, V, W and P linear smoother within the multigrid solver. On a very poor mesh, coupled U, V, W and P linear solver failure can be helped by increasing this under-relaxation to, for example, 0.75.
`solver relaxation scalar`
Type | Real
Default Value | 0.9
Description | The under-relaxation value for the scalar linear smoother within the multigrid solver. On a very poor mesh, scalar equation linear solver failure can be helped by increasing this under-relaxation to, for example, 0.75.
`solver target reduction fluids`
Type | Real
Default Value | 0.1
Description | The linear solver for the coupled mass-momentum system iterates until the final RMS residual for the continuity equation is below this number times the initial RMS continuity residual. It is occasionally useful, for testing purposes for instance, to drive the linear solver to a tighter tolerance (for example, to machine round-off).
`solver target reduction scalar`
Type | Real
Default Value | 0.1
Description | The linear solver for the coupled mass-momentum system iterates until the final RMS residual for the continuity equation is below this number multiplied by the initial RMS continuity residual. It is occasionally useful, for testing purposes for instance, to drive the linear solver to a tighter tolerance (for example, to machine round-off).

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