Mesh, grids, and tiles

Mesh

Level-0 mesh

The following inputs are defined using the amr prefix.

	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
n_cell	Number of cells at level 0 in each coordinate direction	ints	0 0 0

The level 0 mesh spacing is computed for each direction by dividing the domain length by the number of cells. The mesh spacing is required to be the same in all directions:

\[\frac{\text{prob_hi[0] - prob_lo[0]}}{\text{n_cell[0]}} = \frac{\text{prob_hi[1] - prob_lo[1]}}{\text{n_cell[1]}} = \frac{\text{prob_hi[2] - prob_lo[2]}}{\text{n_cell[2]}}\]

The inputs for defining the mesh for a single-level simulation are demonstrated in the following example and illustrated in Fig. 4. In this example, the domain is a \(4 \times 1 \times 1\) cuboid, and there are \(32 \times 8 \times 8\) cells in the X, Y, and Z directions, respectively. The result is a uniform mesh spacing of \(0.125\) m in all three directions.

Listing 4 Snippet of intpus for mesh example. This is not a complete input file.

# Define periodicity and domain extents
# -------------------------------------------------------------
geometry.coord_sys   =  0           # Cartesian coordinates
geometry.is_periodic =  0   0   0   # periodicity for each direction
geometry.prob_lo     =  0.  0.  0   # lo corner of physical domain.
geometry.prob_hi     =  4.  1.  1.  # hi corner of physical domain

# Define the maximum level of refinement and number of cells
# -------------------------------------------------------------
arm.max_level = 0
arm.n_cell = 32  8  8

domain used with box embedded boundary — Fig. 4 Example of a single-level mesh.

Warning

MFIX-Exa simulations with a non-uniform mesh will not run.

Mesh refinement

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

	Description	Type	Default
grid_eff	Threshold value to ensure grids do not contain too large a fraction of un-tagged 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.	int	1

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.

Grids

The following inputs are defined using the amr prefix.

	Description	Type	Default
max_grid_size_x	Maximum number of cells at level 0 in each grid in X	int	32
max_grid_size_y	Maximum number of cells at level 0 in each grid in Y	int	32
max_grid_size_z	Maximum number of cells at level 0 in each grid in Z	int	32
blocking_factor_x	Each grid in X must be divisible by `blocking_factor_x`	int	8
blocking_factor_y	Each grid in Y must be divisible by `blocking_factor_y`	int	8
blocking_factor_z	Each grid in Z must be divisible by `blocking_factor_z`	int	8

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.

Tiles

The following inputs are defined using the fabarray prefix.

	Description	Type	Default
mfiter_tile_size	Maximum number of cells in each direction for (logical) tiles	IntVect	1024000,8,8

The following inputs are defined using the particles prefix.

	Description	Type	Default
tile_size	Maximum number of cells in each direction for (logical) tiles in the ParticleBoxArray if dual_grid is true.	IntVect	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.