Grids and tiles

Grids

The following inputs are defined using the prefix amr:

	Description	Type	Default
max_grid_size_x	Maximum number of cells at level 0 in each grid in X	Ints	32
max_grid_size_y	Maximum number of cells at level 0 in each grid in Y	Ints	32
max_grid_size_z	Maximum number of cells at level 0 in each grid in Z	Ints	32
blocking_factor_x	Each grid in X must be divisible by blocking_factor_x	Ints	8
blocking_factor_y	Each grid in Y must be divisible by blocking_factor_y	Ints	8
blocking_factor_z	Each grid in Z must be divisible by blocking_factor_z	Ints	8
refine_grid_layout	If true, AMReX will attempt to chop new grids into smaller chunks ensuring at least one grid per MPI process, provided this does not violate the blocking factor constraint.	Bool	true

Note, the default for max_grid_size is 64 for GPU runs.

The domain is decomposed into grids by dividing the number of cells by the max grid size for each direction (e.g., n_cells[0]/max_grid_size_x). The blocking factor ensures that the grids will be sufficiently coarsenable for good multigrid performance; therefore, the max_grid_size must be divisible by the corresponding blocking_factor.

Note

The AMReX documentation contains a significant amount of information on grid creation and load balancing. Users are strongly encouraged to read the relevant sections.

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Tiles

The following inputs are defined using the prefix fabarray:

	Description	Type	Default
mfiter_tile_size	Maximum number of cells in each direction for (logical) tiles.	Ints<3>	1024000 8 8

The following inputs are defined using the prefix particles:

	Description	Type	Default
tile_size	Maximum number of cells in each direction for (logical) tiles in the ParticleBoxArray if load_balance is DualGrid	Ints<3>	1024000 8 8

When running on shared memory machines using an OpenMP enabled executable, grids are subdivided into tiles and iterated over to improve data locality by cache blocking.

Note

MFIX-Exa disables tiling when using GPU accelerators.

Mesh refinement

The following inputs are defined using the prefix amr. These inputs control the automatic mesh refinement algorithm and are only applicable when amr.max_level > 0.

	Description	Type	Default
max_level	Maximum level of refinement. The default value, 0, means that there is one cell size covering the entire domain (i.e., single-level).	Int	0
grid_eff	Threshold value to ensure grids do not contain too large a fraction of untagged cells.	Real	0.7
n_error_buf	Controls the number of tagged cells before grids are defined. Used to ensure coarse/fine boundaries are not too close to tagged cells.	Ints	0

Caution

Mesh refinement limitations:

• MFIX-Exa simulations with particles currently do not support mesh refinement.

See the AMReX documentation for details on the adaptive mesh refinement algorithms.

The following inputs are defined using the prefix mfix. These inputs control the automatic mesh refinement algorithm and are only applicable when amr.max_level > 0.

	Description	Type	Default
regrid_int	How often to regrid (in number of steps at level 0). If regrid_int <= 0 then no regridding will occur.	Int	0

Tagging for refinement

The following inputs are defined using the prefix mfix.tag. They control how cells are tagged for the mesh refinement algorithm and are only applicable when amr.max_level > 0.

Cells intersected by the embedded boundary are always tagged for refinement, regardless of these settings.

	Description	Type	Default
rho	Tag cells for refinement when the fluid density is greater than the specified value. To tag cells when density is less than the specified value, set mfix.tag.rho.less_than = true. Values may be specified per level. If fewer than amr.max_level values are provided, the last value is used for all higher levels.	Reals	None
rho.less_than	Tag cells for refinement when the fluid density is less than the specified value.	Bool	false
grad_rho	Tag cells for refinement when the difference in fluid density between two adjacent cells is greater than the specified value. To tag cells when the difference is less than the specified value, set mfix.tag.grad_rho.less_than = true. Values may be specified per level. If fewer than amr.max_level values are provided, the last value is used for all higher levels.	Reals	None
grad_rho.less_than	Tag cells for refinement when the difference in fluid density between two adjacent cells is less than the specified value.	Bool	false
vel	Tag cells for refinement when the magnitude of fluid velocity is greater than the specified value. To tag cells when the difference is less than the specified value, set mfix.tag.vel.less_than = true. Values may be specified per level. If fewer than amr.max_level values are provided, the last value is used for all higher levels.	Reals	None
vel.less_than	Tag cells for refinement when fluid velocity magnitude is less than the specified value.	Bool	false
grad_vel	Tag cells for refinement when the difference in any fluid velocity component between two adjacent cells is greater than the specified value. To tag cells when the difference is less than the specified value, set mfix.tag.grad_vel.less_than = true. Values may be specified per level. If fewer than amr.max_level values are provided, the last value is used for all higher levels.	Reals	None
grad_vel.less_than	Tag cells for refinement when the difference in any fluid velocity component between two adjacent cells is less than the specified value.	Bool	false
regions	A list of predefined regions used to identify sections of the domain for mesh refinement.	Strings	None