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25.11.1 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 Controls panel (Figure  25.11.1).

Solve $\rightarrow$ Controls $\rightarrow$ Multigrid...

figure   

When using the density-based explicit formulation or the pressure-based solver with any of the segregated algorithms, described in Section  25.4.3 and Section  25.9.1, only Scalar Parameters are set in the Multigrid Controls panel. If you use the density-based implicit or the pressure-based coupled algorithm, described in Section  25.4.3, 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 Section  25.6.2). 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.

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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.

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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, refer to Section  25.6.3.



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 Controls panel (Figure  25.11.1) under Flexible Cycle Parameters.

Solve $\rightarrow$ Controls $\rightarrow$ Multigrid...

Figure 25.11.1: The Multigrid Controls Panel
figure



Setting the Verbosity


The steps for monitoring the solver are as follows:

1.   Set multigrid Verbosity to 1 or 2 in the Multigrid Controls panel.

Solve $\rightarrow$ Controls $\rightarrow$ Multigrid...

2.   Request a single iteration using the Iterate panel.

Solve $\rightarrow$ Iterate...

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

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 FLUENT's default settings, you can click the Default button in the Multigrid Controls panel. 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.


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