.. _inputs_mlmg:

Section: MLMG options
~~~~~~~~~~~~~~~~~~~~~

This section specifies the Multi-Level Multi-Grid (MLMG) options for each type
of linear solve. There are three types of linear solves performed in amr-wind
"diffusion" which is a cell based Helmholtz like solve to advance the momentum
equations, "nodal_proj" is a node based pressure projection, and "mac_proj"
projects velocities to faces. The options are the same for each and the prefix
determines which MLMG option is being specified. Below the diffusion options are
described but the same options apply to "nodal_proj" and "mac_proj". It is also
possible to specify diffusion solver options for specific equations such as
temperature, to do that use "temperature_diffusion" as your prefix.

**Linear operator options**
   
.. input_param:: diffusion.max_coarsening_level

   **type:** Integer, optional, default = 100
   
   This parameter sets the max number of multigrid coarsening allowed at the lowest amr 
   level to the solver. 
   Typically setting to a large number means coarsen as much as possible until the grid 
   can not be coarsened anymore.
      
.. input_param:: diffusion.max_order

   **type:** Integer, optional, default = 2
   
   Order of the one-sided stencil applied near physical boundaries and fine/coarse boundaries.

**MLMG options**

.. input_param:: diffusion.verbose

   **type:** Integer, optional, default = 0

   Sets the verbosity of the MLMG solver.

.. input_param:: diffusion.maxiter

   **type:** Integer, optional, default = 200

   Sets the max number of multigrid iterations. If :input_param:`diffusion.do_fixed_iters`
   is set to True, then AMReX will not abort if specified tolerance is not met
   after max iterations, otherwise it will abort.

.. input_param:: diffusion.do_fixed_iters

   **type:** Boolean, optional, default = true

   If ``true``, then AMReX will not abort if the specified tolerance is not met
   even after :input_param:`diffusion.maxiter` iterations have completed.

.. input_param:: diffusion.mg_rtol

   **type:** Real, optional, default = 1.0e-11
   
   Set the relative tolerance for the linear solver
   
.. input_param:: diffusion.mg_atol

   **type:** Real, optional, default = 1.0e-14
   
   Set the absolute tolerance for the linear solver

.. input_param:: diffusion.fmg_maxiter

   **type:** Integer, optional, default = 0

   Sets the number of F-cycle MG iterations to perform before switching to V-cycle MG.

.. input_param:: diffusion.num_pre_smooth

   **type:** Integer, optional, default = 2

   Number of pre smoothing steps

.. input_param:: diffusion.num_post_smooth

   **type:** Integer, optional, default = 2

   Number of post smoothing steps

.. input_param:: diffusion.num_final_smooth

   **type:** Integer, optional, default = 8

   Number of final smoother steps applied

.. input_param:: diffusion.num_bottom_smooth

   **type:** Integer, optional, default = 0

   Number of smoother steps applied during bottom solve.

**Bottom solver options**
   
.. input_param:: diffusion.bottom_solver

   **type:** String, optional, default = "bicgstab"
   
   Set the bottom solver type. Current bottom solver options 
   include: smoother, bicgstab, cg, bicgcg, cgbicg, hypre, and petsc. 
   The hyper and petsc options will require compiling with those libraries.

.. input_param:: diffusion.bottom_verbose

   **type:** Integer, optional, default = 0

   Sets the verbosity of the bottom solver within MLMG.

.. input_param:: diffusion.bottom_rtol

   **type:** Real, optional, default = 1.0e-4

   Set the relative tolerance for the bottom solver for convergence.

.. input_param:: diffusion.bottom_atol

   **type:** Real, optional, default = -1.0

   Set the absolute tolerance for the bottom solve. Setting a negative number
   disables absolute tolerance check.

.. input_param:: diffusion.bottom_maxiter

   **type:** Integer, optional, default = 200

   Maximum number of iterations for the bottom solver

.. input_param:: diffusion.hypre_interface

   **type:** String, optional, default = ``ij``

   The hypre interface to use when :input_param:`diffusion.bottom_solver` is set
   to ``hypre``. Valid choices are: ``ij``, ``semi_structured``, and
   ``structured``.

.. input_param:: diffusion.hypre_namespace

   The ParmParse ``prefix`` where the hypre options must be read from for this
   solver. For example, to set hypre options for NodalProjector

   ..
      nodal_proj.hypre_namespace = "nodal_proj.hypre"
      nodal_proj.hypre.hypre_solver = GMRES
      nodal_proj.hypre.hypre_preconditioner = BoomerAMG