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25.10.3 Turning On FAS Multigrid

As discussed in Section  25.6, FAS multigrid is an optional component of the density-based explicit formulation, while AMG multigrid is always on, by default for the density-based implicit formulation. Since nearly all coupled explicit calculations will benefit from the use of the FAS multigrid convergence accelerator, you should generally set a non-zero number of coarse grid levels before beginning the calculation. For most problems, this will be the only FAS multigrid parameter you will need to set. Should you encounter convergence difficulties, consider applying one of the methods discussed in Section  25.11.2.

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Note that you cannot use FAS multigrid with explicit time stepping (described in Section  25.3.2) because the coarse grid corrections will destroy the time accuracy of the fine grid solution.



Setting Coarse Grid Levels


As discussed in Section  25.6.4, FAS multigrid solves on successively coarser grids and then transfers corrections to the solution back up to the original fine grid, thus increasing the propagation speed of the solution and speeding convergence. The most basic way you can control the multigrid solver is by specifying the number of coarse grid levels to be used.

As explained in Section  25.6.4, the coarse grid levels are formed by agglomerating a group of adjacent "fine'' cells into a single "coarse'' cell. The optimal number of grid levels is therefore problem-dependent. For most problems, you can start out with 4 or 5 levels. For large 3D problems, you may want to add more levels (although memory restrictions may prevent you from using more levels, since each coarse grid level requires additional memory). If you believe that multigrid is causing convergence trouble, you can decrease the number of levels.

If FLUENT reaches a coarse grid with one cell before creating as many levels as you requested, it will simply stop there. That is, if you request 5 levels, and level 4 has only 1 cell, FLUENT will create only 4 levels, since levels 4 and 5 would be the same.

To specify the number of grid levels you want, set the number of Multigrid Levels in the Solution Controls panel (Figure  25.10.1) under Solver Parameters.

Solve $\rightarrow$ Controls $\rightarrow$ Solution...

You can also set the Max Coarse Levels under FAS Multigrid Controls in the Multigrid Controls panel.

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

Changing the number of coarse grid levels in one panel will automatically update the number shown in the other.

Coarse grid levels are created when you first begin iterating. If you want to check how many cells are in each level, request one iteration and then use the Grid/Info/Size menu item (described in Section  6.6.1) to list the size of each grid level. If you are satisfied, you can continue the calculation; if not, you can change the number of coarse grid levels and check again.

For most problems, you will not need to modify any additional multigrid parameters once you have settled on an appropriate number of coarse grid levels. You can simply continue your calculation until convergence.



Using Residual Smoothing to Increase the Courant Number


In the density-based explicit formulation, implicit residual smoothing (or averaging) is a technique that can be used to reduce the time step restriction of the solver, thereby allowing the Courant number to be increased. The implicit smoothing is implemented with an iterative Jacobi method, as described in Section  25.5.4. You can control residual smoothing in the Solution Controls panel.

Solve $\rightarrow$ Controls $\rightarrow$ Solution...

By default, the number of Iterations for Residual Smoothing is set to zero, indicating that residual smoothing is disabled. If you increase the Iterations counter to 1 or more, you can enter the Smoothing Factor. A smoothing factor of 0.5 with 2 passes of the Jacobi smoother is usually adequate to allow the Courant number to be doubled.


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