26.10.1 Steps in Using FMG Initialization

You can access the FMG initialization procedure using the text user interface (TUI) once the standard flow initialization is performed (see this section in the separate Theory Guide) or if valid flow data is available (i.e., through reading a data file).

To customize the FMG initialization, type the following command :

solve $\rightarrow$ initialize $\rightarrow$ set-fmg-initialization

You will be asked to enter:

• The number of multigrid levels for the FMG iteration (the default is 5).

  For small cases (100, 000 cells or less), it is recommended that you lower the number of multigrid levels to 3 or 4.

• For each level of multigrid, you will be asked to enter the residual reduction (the default value is 0.001), and the number of cycles per level (the defaults at each level are 10, 10, 50, 100, 500, and 500). In general, you should perform more iterations on coarse levels than fine levels. Level 0 is the finest level, which represents the original mesh.

• FMG iteration Courant-number (the default is 0.75). This will be the CFL value that the FAS multigrid will use for the FMG initialization.

• Enabling verbose mode (the default is no). By enabling this option, you will be able to monitor the convergence at each level.

  If you do not customize the FMG settings, then the default values will be used.

To perform the FMG initialization, type the following command :

solve $\rightarrow$ initialize $\rightarrow$ fmg-initialization

When you are prompted to Enable FMG initialization? [no], type yes.

When verbose mode is selected and the FMG initialization is being executed, ANSYS FLUENT will first output the multigrid level information followed by convergence history for the FAS multigrid cycle on each level. The normalized residual value is printed after ten FAS cycles or when the number of FAS cycles is reached. The output will indicate when convergence is reached on each level and when the solution is being interpolated to the next level.