# Mesh and gridding

## Mesh, grids and tiles

### Mesh

There are some settings we can specify for the mesh and the automatic mesh refinement algorithm. These settings must be preceded by “amr.” in the input file.

| Description | Type | Default
--- | --- | --- | ---
max_level | Maximum level of refinement allowed (0 when single-level) | Int | 0
grid_eff | Threshold value to ensure grids do not contain too large a fraction of un-tagged cells. Applicable only when mesh refinement is enabled (“max_level” > 0). | 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. Applicable only when mesh refinement is enabled (“max_level” > 0). | Int | 1
n_cell | Number of cells at level 0 in each coordinate direction | Ints | 0 0 0

### Grids

The following inputs must be preceded by “amr.” and determine how we create the grids.

| Description | Type | Default
--- | --- | --- | ---
max_grid_size_x | Maximum number of cells at level 0 in each grid in x-direction | Int | 32
max_grid_size_y | Maximum number of cells at level 0 in each grid in y-direction | Int | 32
max_grid_size_z | Maximum number of cells at level 0 in each grid in z-direction | Int | 32
blocking_factor_x | Each grid must be divisible by blocking_factor_x in x-direction | Int | 8
blocking_factor_y | Each grid must be divisible by blocking_factor_y in y-direction | Int | 8
blocking_factor_z | Each grid must be divisible by blocking_factor_z in z-direction | Int | 8

The following inputs must be preceded by “particles.”

| Description | Type | Default
--- | --- | --- | ---
max_grid_size_x | Maximum number of cells at level 0 in each grid in x-direction for grids in the ParticleBoxArray if dual_grid is true | Int | 32
max_grid_size_y | Maximum number of cells at level 0 in each grid in y-direction for grids in the ParticleBoxArray if dual_grid is true | Int | 32
max_grid_size_z | Maximum number of cells at level 0 in each grid in z-direction for grids in the ParticleBoxArray if dual_grid is true. | Int | 32

### Tiles

The following inputs must be preceded by “fabarray_mfiter.” and determine how we create the logical tiles:

| Description | Type | Default
--- | --- | --- | ---
tile_size | Maximum number of cells in each direction for (logical) tiles (3D CPU-only) | IntVect | 1024000 1024000,8,8

The following inputs must be preceded by “particles.”

| 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

## Load balancing

Note that when running a granular simulation, i.e., no fluid phase, `mfix.dual_grid` must be 0. Hence, the `particles.max_grid_size` (in each direction) have no meaning. Therefore the fluid grid and tile sizes should be set for particle load balancing. It may also be necessary to set the blocking factors to 1.

The following inputs must be preceded by “mfix.” and determine how we load balance:

| Description | Type | Default
--- | --- | --- | ---
dual_grid | If true then use the “dual_grid” approach for load balancing | Bool | False
regrid_int | How often to regrid (in number of steps at level 0) if regrid_int = -1 then no regridding will occur | Int | -1
load_balance_fluid | Only relevant if (dual_grid); if so do we also regrid mesh data | Int | 0
load_balance_type | What strategy to use for load balancing Options are “KnapSack”, “SFC”, or “Greedy” | String | KnapSack
knapsack_weight_type | What weighting function to use if using Knapsack load balancing Options are “RunTimeCosts” or “NumParticles”” | String | RunTimeCosts
knapsack_nmax | Maximum number of grids per MPI process if using knapsack algorithm | Int | 128
greedy_dir | The direction in which the greedy algorithm cuts overloaded boxes | Int | 0
greedy_min_grid_size | The minimum particle grid size in the greedy load balance algorithm | Int | 2
greedy_3d | Partition particle grids in 3D with the greedy algorithhm | Bool | False
overload_tolerance* | The ratio between the maximum workload and the average workload in the greedy algorithm | Real | 1.2
underload_tolerance* | The ratio between the minimum workload and the average workload in the greedy algorithm | Real | 0.8
grid_pruning | Remove all covered grids from the base mesh; this may result in disjoined grids | Bool | False

* The greedy partitioning algorithm uses the tolerances to set the expected workload range for each rank but doesn’t strictly enforce them. The algorithm creates a partition as balance as possible even if the partition doesn’t meet the tolerances.

To allow a user to verify the breakdown of fluid grids created before running a full simulation, an input option, `mfix.only_print_grid_report` is supported. By default, it is `False`. When set to `True`, the run uses minimal memory to print the grid coverage report and exits immediately after that.