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32.4.2 Solve/Controls/Multigrid...

The Solve/Controls/Multigrid... menu item opens the Multigrid Controls panel.



Multigrid Controls Panel


The Multigrid Controls panel allows you to set parameters related to the multigrid solver. See Sections  25.22.3 and 25.11.2 for details about the items below.

figure

Controls

Cycle Type   contains a drop-down list for each equation that is being solved. From this list you can select the multigrid cycle type ( Flexible, V-Cycle, W-Cycle, or F-Cycle). See Section  25.11 for details.

Note that, for the density-based solvers, the Pressure, Momentum, and Energy equations will not appear individually. They will instead be grouped together and called Flow.

Furthermore, for the density-based explicit solver, the Cycle Type choices for the Flow equations will be limited to V-Cycle and W-Cycle, while the choices for Flow for the density-based implicit solver will be limited to V-Cycle and F-Cycle.

Termination   specifies the termination criterion for each equation that is being solved using algebraic multigrid. See Section  25.11 for details.

Restriction   specifies the residual reduction criterion for each equation that is being solved using the Flexible algebraic multigrid cycle. See Section  25.11 for details. (This item will not appear for an equation that is using a V-Cycle, W-Cycle, or F-Cycle.)

AMG Method   contains the drop-down list to choose between two AMG solvers: aggregative or selective. See Section  25.11 for details.

Aggregative   enables aggregative AMG solver.

Selective   enables selective AMG solver. The selective AMG solver is available only for scalar equations, and is not available in parallel FLUENT.

Stabilization Method   contains the drop-down list to choose the stabilization method.

BCGSTAB   enables bi-conjugate gradient stabilized method.

Algebraic Multigrid Controls   contains parameters related to the algebraic multigrid solver. See Sections  25.6.3 and 25.22.3 for details.

Scalar and Coupled Parameters   contain parameters that you can set, If you are using the density-based explicit solver or the pressure-based solver with any of the segregated schemes, described in Section  25.4.3 and Section  25.9.1, you will only set Scalar Parameters. If you are using the density-based implicit or the pressure-based coupled scheme, described in Section  25.4.3, then you can set the Coupled Parameters.

Fixed Cycle Parameters   contains parameters that control the V, W, and F cycles. You can set the number of Pre-Sweeps and Post-Sweeps, and the Max Cycles. Normally one post-sweep is performed and no pre-sweeps are done. See Section  25.11.1 for details about using the items below.

Pre-Sweeps   sets the number of sweeps to perform before moving to a coarser level.

Post-Sweeps   sets the number of sweeps to perform after coarser level corrections have been applied.

Max Cycles   sets the maximum number of V, W, or F cycles to be performed. The multigrid solver will continue to solve the set of equations until either the maximum number of cycles has been performed, or the Termination criteria are satisfied.

Coarsening Parameters   contains parameters that control the grouping of equations in the algebraic multigrid algorithm. See Section  25.11.1 for details.

Max Coarse Levels   is the maximum number of coarse levels 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 nodes 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 multigrid solver.

Coarsen by   controls the number of equations on each successively coarser grid level. By default, this parameter is set to 2, indicating that the number of equations on each level will be 1/2 of the number on the previous level. In general, the number of equations on each coarser grid level will be equal to 1/ $n$ of the number on the previous level, where $n$ is the value set for the Coarsen by parameter.

Smoother Type   consist of two types:

Gauss-Seidel   is the simplest smoother type and is recommended when using the pressure-based segregated solver.

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 solver.

Flexible Cycle Parameters   contains parameters that control the flexible multigrid cycle.

Sweeps   specifies the number of times to apply the smoothing method each time a relaxation is performed.

Max Fine Relaxations   sets the maximum number of relaxations to be performed on the Level 1 grid (fine grid level). This parameter eliminates the possibility that the Gauss-Seidel solver will get "stuck'' on the fine grid level, unable to reduce the residuals by the fraction ( $\alpha$) required by the Termination criteria.

Max Coarse Relaxations   sets the maximum number of relaxations to be performed on each grid level above Level 1 (i.e., the coarse grid levels). This parameter eliminates the possibility that the Gauss-Seidel solver will get "stuck'' on a coarse grid level, unable to reduce the residuals on that level by the fraction ( $\alpha$) required by the Termination criteria. If the iterative solution on a given grid level is unable to meet the accuracy constraint of the Termination criteria, the correction equation will be deemed "converged'' when this maximum number of relaxations on that grid level has been performed.

Options   contains additional multigrid parameters.

Verbosity   controls the amount of information that is printed out by the multigrid solver for monitoring purposes. See Section  25.11.1 for details.

FAS Multigrid Controls   contains parameters related to the FAS multigrid solver. See Sections  25.6.4 and 25.11.2 for details. As noted in the title of this panel section, the FAS multigrid solver is used only for the Flow equations (pressure, momentum, and energy).

This section of the panel appears only when the density-based explicit solver is used.

Fixed Cycle Parameters   contains parameters that control the V, W, and F cycles of the FAS multigrid solver.

Pre-Sweeps   sets the number of iterations of the multi-stage solver to be performed on a given grid level before proceeding to a coarser grid level (the value of $\beta_1$ described in Section  25.6.2). Typically, this is set to 1.

Post-Sweeps   sets the number of multigrid cycles to be performed on a given grid level before proceeding back up to the finer grid level (the value of $\beta_2$ described in Section  25.6.2). A value of 1 results in V-cycle multigrid, and a value of 2 results in W-cycle multigrid.

Coarsening Parameters   contains parameters that control the grouping of cells in the FAS multigrid algorithm.

Max Coarse Levels   sets the maximum number of grid levels to be used in the multigrid process. A value of 0 disables multigrid (no coarse grid levels). If the coarse grids do not already exist, they are created automatically when you start iterating; you cannot create them by clicking the OK button. See Section  25.10.3 for details.

Coarsen by   controls the number of cells in each successively coarser grid level. By default, this parameter is set to 2, indicating that the number of cells on each level will be 1/2 of the number on the previous level. In general, the number of cells on each coarser grid level will be equal to 1/ $n$ of the number on the previous level, where $n$ is the value set for the Coarsen by parameter.

Relaxation Factors   are provided to moderate and stabilize the multigrid corrections.

Courant Number Reduction   sets the factor by which to reduce the Courant number for coarse grid levels (i.e., every level except the finest). Some reduction of time step (such as the default 0.9) is typically required because the stability limit cannot be determined as precisely on the irregularly shaped coarser grid cells.

Correction Reduction   sets the factor by which to reduce the magnitude of the multigrid corrections transferred from one level to the next finer level. A typical value with $\beta_1 = 1$ is 0.6. If two Pre-Sweeps and two Post-Sweeps are performed, this value can often be increased to 1.0 (i.e., full correction transfer).

Species Correction Reduction   sets the factor by which to reduce the magnitude of the species corrections to stabilize the multigrid calculation. This item appears only when species transport is being modeled.

Correction Smoothing   sets the correction smoothing factor used to interpolate corrections from a coarser grid to a finer grid. For multigrid on structured meshes, corrections can be interpolated up to a finer mesh "smoothly'' by using, for example, tri-linear interpolation. For unstructured meshes there is no analogous simple, algebraic procedure that can be used to interpolate without introducing substantial high frequency "noise''. Instead, the corrections are first interpolated, and then subjected to a smoothing pass. The default Correction Smoothing value of 0.5 should be acceptable for all cases; you should not need to change it.

Options   contains additional multigrid parameters.

Verbosity   controls the amount of information that is printed out by the multigrid solver for monitoring purposes.

Default   sets the fields to their default values, as assigned by FLUENT. After execution, the Default button becomes the Reset button.

Reset   resets the fields to their most recently saved values (i.e., the values before Default was selected). After execution, the Reset button becomes the Default button.


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