You can access and customize the FMG initialization procedure using the Solution Initialization task page (see Solution Initialization Task Page) after the standard or hybrid flow initialization has been performed or if valid flow data is available (such as from reading a data file).

Alternatively, FMG initialization can be accessed from the text user interface (TUI). To customize the FMG initialization, use the following command after performing standard or hybrid initialization or reading data:

solve → initialize → set-fmg-initialization

You will be asked to enter:

To perform the FMG initialization, type the following command after performing standard or hybrid initialization or reading data:

solve → initialize → fmg-initialization

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

When verbose mode is selected and the FMG initialization is being used, 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.

When setting the FMG initialization parameters, you should consider performing more iterations on the coarse levels than on the fine levels. However, remember that the purpose of FMG initialization is to obtain a good initial solution at a low cost. You should try to avoid unreasonable convergence tolerance that will make the FMG initialization expensive.

Turn on the verbose mode to help you determine if the flow is converging as expected during the FMG iterations. If the solution is not converging to the desired tolerance, consider increasing the number of FAS multigrid cycles at each level. If the solution is diverging during the FAS cycles, then consider lowering the FMG iteration Courant number since the default value is probably too aggressive and is likely causing the solution to diverge.

For turbulent flows, it is very important to first perform standard initialization with proper and realistic values of the turbulence variables (for example and ). This can be done by computing the average values based on the conditions defined at the inflow boundary or at all boundary zones. Then, you can proceed with FMG initialization. Unrealistic initialization of turbulence variables may cause convergence difficulties during the first few iterations on the fine mesh, thereby nullifying the benefit of FMG initialization.

Viscous FMG Initialization

By default, the FMG initialization procedure computes an initial solution based on the Euler equations of inviscid flows as described in Overview of FMG Initialization in the Fluent Theory Guide. The density-based solver offers the possibility of adding the viscous terms of the Navier-Stokes equation to the FMG initialization procedure. This allows boundary layers to develop as part of the initialization phase, thus mitigating the solution transients encountered upon startup. The FMG viscous terms are engaged with the following text command:

> solve/initialize/set-fmg-options/viscous-terms?
Enable viscous terms during FMG initialization [no] yes

Note that although the turbulence model equations are not solved during the initialization, the FMG viscous terms consider the effective viscosity, which includes the contribution of the turbulent viscosity . Since the turbulent viscosity obtained from the freestream initialization might not be sufficient to develop adequate boundary-layers, a default turbulent viscosity ratio of 100 is used during FMG initialization. This value can be modified with the following text command:

> solve/initialize/set-fmg-options/set-turbulent-viscosity-ratio
Set turbulent viscosity ratio used during FMG initialization [100] 100 

For conjugate heat-transfer cases, the FMG viscous terms also include heat conduction effects, which allow an initial temperature distribution to develop in solid zones.

Species-reaction Initialization

By default, the FMG initialization solves the transport species equations, but neglects volumetric reaction terms. The convergence of certain reactive flows can be improved by developing an initial solution that accounts for the production and destruction of species, and that is in thermo-chemical equilibrium. This is achieved by retaining the species reactions during the FMG initialization with the following text command, which is available after performing standard or hybrid initialization or reading data:

> solve/initialize/set-fmg-options/species-reaction?
Enable species volumetric reactions during FMG initialization [no] yes

FMG species reactions are currently restricted to the density-based solver. They are also not available with turbulence-chemistry interaction (TCI), Eddy-dissipation-concept (EDC) models, or with the CHEMKIN-CFD chemistry solver.