## 25.15.1 Overview of FMG Initialization

FMG initialization utilizes the FLUENT FAS Multigrid technology (see Section  25.6.4) to obtain the initial solution. Starting from a uniform solution (after performing standard initialization), the FMG initialization procedure constructs the desirable number of geometric grid levels using the procedure outlined in Section  25.6.4. To begin the process, the initial solution is restricted all the way down to the coarsest level. The FAS multigrid cycle is then applied until a given order of residual reduction is obtained or the maximum number of cycles is reached. The solution is then interpolated one grid level up and the FAS multigrid cycle is applied again between the current level all the way down to the coarsest level. This process will repeat until the finest level is reached. The FMG initialization iteration is illustrated in Figure  25.15.1.

Since FMG initialization does most of the work on coarse levels, this initialization procedure is computationally inexpensive and, for large problems, a good initial solution can be obtained in a fraction of the time spent to converge on a final solution. Note that FMG initialization can be used with the pressure-based and density-based solvers.

When FMG initialization is started, the algorithm will perform the following steps:

1.   Records the current solver selection and all current solver parameters.

2.   Switches from the selected solver to the density-based explicit formulation.

3.   Performs one FMG iteration using the FMG parameters given in the text command interface (see below).

4.   Switches back to the initially selected solver and resets all solver parameters back to the original solver settings.

In the FMG iteration, the inviscid Euler equations are solved using first order-discretization to obtain the approximate solution. If species are present, then the FMG initialization will solve the species equations. However, turbulence equations or any other transport scalars are not solved in the FMG initialization.

Limitations of FMG Initialization

• FMG initialization is not available for unsteady flows.

 If an initial solution is needed for an unsteady calculation, then you must first switch to the steady state solver, perform FMG initialization and other necessary iterations to get an initial solution, and then perform unsteady calculations.

• FMG will not initialize turbulence or other transport equations field variables.

 If your are solving for turbulent flow, then you can still use FMG initialization, however the FMG will not initialize the turbulence field variables. For this reason, a good initial guess of the turbulence field quantities are important for achieving fast convergence after the FMG initialization has been performed.

• FMG should not be used with multiphase flow.

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