26.5.1 Additional Algebraic Multigrid Parameters

There are several additional parameters that control the algebraic multigrid solver, but there will usually be no need to modify them. These additional scalar and coupled parameters are all contained in the Multigrid tab in the Advanced Solution Controls dialog box (Figure  26.5.1).

Solution Controls Advanced...

When using the density-based explicit formulation or the pressure-based solver with any of the segregated algorithms, described in this section in the separate Theory Guide and Section  26.3.1, only Scalar Parameters are set in the Multigrid tab. If you use the density-based implicit or the pressure-based coupled algorithm, described in this section in the separate Theory Guide , then you can set the Coupled Parameters.

Fixed Cycle Parameters

For the fixed (V, W, and F) multigrid cycles, you can control the number of pre- and post-relaxations ( $\beta_1$ and $\beta_3$ in this section in the separate Theory Guide). Pre-Sweeps sets the number of relaxations to perform before moving to a coarser level. Post-Sweeps sets the number to be performed after coarser level corrections have been applied. Normally, under Scalar Parameters, one post-relaxation is performed and no pre-relaxations are done (i.e., $\beta_3=1$ and $\beta_1=0$), but in rare cases, you may need to increase the value of $\beta_1$ to 1 or 2. Under Coupled Parameters, three post-relaxations are performed by default with no pre-relaxations.

If you are using AMG with V-cycle to solve an energy equation with a solid conduction model presented with anisotropic or very high conductivity coefficient, there is a possibility of divergence with a default post-relaxation sweep of $1$. In such cases you should increase the post-relaxation sweep (to say $2$) in the AMG section for better convergence when using the pressure-based segregated algorithms.

Coarsening Parameters

For all multigrid cycle types, you can control the maximum number of coarse levels ( Max Coarse Levels under Scalar or Coupled Parameters) that will be built by the multigrid solver.

Sets of coarser simultaneous equations are built until the maximum number of levels has been created, or the coarsest level has only 3 equations. Each level has about half as many unknowns as the previous level, so coarsening until there are only a few cells left will require about as much total coarse-level coefficient storage as was required on the fine mesh. Reducing the maximum coarse levels will reduce the memory requirements, but may require more iterations to achieve a converged solution. Setting Max Coarse Levels to 0 turns off the algebraic multigrid solver.

Another coarsening parameter you can control is the increase in coarseness on successive levels. The Coarsen by parameter specifies the number of fine grid cells that will be grouped together to create a coarse grid cell. The algorithm groups each cell with its closest neighbor, then groups the cell and its closest neighbor with the neighbor's closest neighbor, continuing until the desired coarsening is achieved. Typical values for the scalar parameters are in the range from 2 to 10, with the default value of 2 for the Gauss-Seidel smoother giving the best performance, but also the greatest memory use. For coupled parameters, a default value of 4 (for 2D) and 8 (for 3D) for the ILU smoother give the best performance. You should not adjust this parameter unless you need to reduce the memory required to run a problem.

Depending on the smoother type, Gauss-Seidel or ILU, the Coarsen by and Post-Sweeps settings should be changed as follows when selecting the non-default smoother type: ILU   : Post-Sweeps = 3 and Coarsen by = 8 Gauss-Seidel   : Post-Sweeps = 1 and Coarsen by = 2

Smoother Types

Two smoother types are available for scalar and coupled parameters. Gauss-Seidel is the simplest smoother type and is recommended when using the pressure-based segregated algorithm. ILU is more CPU intensive, but has better smoothing properties for block-coupled systems such as the pressure-based coupled solver and the density-based implicit formulation. In other words, the default scalar Smoother Type is Gauss-Seidel, while the coupled Smoother Type is ILU. For more information about the two smoother types, see this section in the separate Theory Guide.

Flexible Cycle Parameters

To change the maximum number of relaxations, increase or decrease the value of Max Fine Relaxations or Max Coarse Relaxations in the Multigrid tab in the Advanced Solution Controls dialog box (Figure  26.5.1) under Flexible Cycle Parameters.

Solution Controls Advanced...

Figure 26.5.1: The Multigrid Tab for the Flexible Cycle

Setting the Verbosity

The steps for monitoring the solver are as follows:

1.   Set multigrid Verbosity to 1 or 2 in the Multigrid tab in the Advanced Solution Controls dialog box.

Solution Controls Advanced...

2.   Request a single iteration using the Run Calculation task page.

Run Calculation

If you set the verbosity to 2, the information printed in the ANSYS FLUENT console for each equation will include the following:

• equation name

• equation tolerance (computed by the solver using a normalization of the source vector)

• residual value after each fixed multigrid cycle or fine relaxation for the flexible cycle

• number of equations in each multigrid level, with the zeroth level being the original (finest-level) system of equations

Note that the residual printed at cycle or relaxation 0 is the initial residual before any multigrid cycles are performed.

When verbosity is set to 1, only the equation name, tolerance, and residuals are printed.

A portion of a sample printout is shown below:

pressure correction equation:
 tol.  1.2668e-05
    0  2.5336e+00
    1  4.9778e-01
    2  2.5863e-01
    3  1.9387e-01

  multigrid levels:
     0    918
     1    426
     2    205
     3     97
     4     45
     5     21
     6     10
     7      4

Returning to the Default Multigrid Parameters

If you change the multigrid parameters, but you then want to return to ANSYS FLUENT's default settings, you can click the Default button in the Multigrid tab. ANSYS FLUENT will change all settings to the defaults, and the Default button will become the Reset button. To get your settings back again, you can click the Reset button.