Grid Creation
To run MFIX-Exa you must specify n_cell
in the inputs file –
this is the number of cells spanning the domain in each coordinate direction at level 0.
Users often specify max_grid_size
as well. The default load balancing algorithm then divides the
domain in every direction so that each grid is no longer than max_grid_size
in that direction.
If not specified by the user, max_grid_size
defaults to 128 in 2D and 32 in 3D (in each coordinate direction).
Another popular input is blocking_factor
. The value of blocking_factor
constrains grid creation in that in that each grid must be divisible by blocking_factor
.
Note that both the domain (at each level) and max_grid_size
must be divisible by blocking_factor
If not specified by the user, blocking_factor
defaults to 8 in each coordinate direction.
The typical purpose of blocking_factor
is to ensure that the grids will be
sufficiently coarsenable for good multigrid performance.
There is one more default behavior to be aware of. There is a boolean refine_grid_layout
that defaults to true but can be over-ridden at run-time.
If refine_grid_layout
is true and the number of grids created is less than the number of processors
(Ngrids < Nprocs), then grids will be further subdivided until Ngrids >= Nprocs.
Caveat: if subdividing the grids to achieve Ngrids >= Nprocs would violate the
blocking_factor
criterion then additional grids are not created and the
number of grids will remain less than the number of processors
Note that n_cell
must be given as three separate integers, one for each coordinate direction.
However, max_grid_size
and blocking_factor
can be specified as a single value
applying to all coordinate directions, or as separate values for each direction.
if
max_grid_size
(orblocking_factor
) is specified as multiple integers then the first integer applies to level 0, the second to level 1, etc. If you don’t specify as many integers as there are levels, the final value will be used for the remaining levels.if different values of
max_grid_size
(orblocking_factor
) are wanted for each coordinate direction, thenmax_grid_size_x
,max_grid_size_y
andmax_grid_size_z
(orblocking_factor_x
,blocking_factor_y
andblocking_factor_z
) must be used. If you don’t specify as many integers as there are levels, the final value will be used for the remaining levels.note that
max_grid_size
is just an upper bound; withn_cell = 48
andmax_grid_size = 32
, we will typically have one grid of length 32 and one of length 16.
The grid creation process at level 0 proceeds as follows (if not using the KD-tree approach):
The domain is initially defined by a single grid of size
n_cell
.If
n_cell
is greater thanmax_grid_size
then the grids are subdivided until each grid is no longer thanmax_grid_size
cells on each side. Theblocking_factor
criterion (ie that the length of each side of each grid is divisible byblocking_factor
in that direction) is satisfied during this process.Next, if
refine_grid_layout = true
and there are more processors than grids at this level, then the grids at this level are further divided until Ngrids >= Nprocs (unless doing so would violate theblocking_factor
criterion).