# Discretization

The following inputs must be preceded by “mfix.”

Key | Description | Type | Default
--- | --- | --- | ---
advection_type | Predictor-Corrector Method of Lines (“mol”) or Godunov (“godunov”) | String | Godunov
redistribution_type | Use flux (“FluxRedist”), state (“StateRedist”) or no (“NoRedist”) redistribution | String | StateRedist
redistribute_before_nodal_proj | Redistribute the velocity field before the nodal projection | Bool | True
redistribute_nodal_proj | Redistribute the velocity field after the nodal projection | Bool | False
use_drag_coeff_in_proj_gp | Algebraically consistent p coeff in proj or (default) simplified form | Bool | False
use_drag_in_godunov | Include a drag term in the Godunov flux or (default) not | Bool | False
correction_small_volfrac | Threshold volume fraction for correcting small cell velocity at the end of the predictor and corrector | Real | 1.e-4
deposition_redist_type | Redistribute excess solids using max packing (“MaxPack”) or state (“StateRedist”) algorithms. | String | MaxPack
deposition_redist_vfrac | The threshold cell volume fraction when using “StateRedist” for deposition redistribution. | Real | 0.1

Notes: The code was originally developed with MOL and FluxRedist. Preliminary tests show that the new single-step Godunov method is roughly twice as fast as the predictor-corrector MOL at the same time step (e.g., CFL limited to 0.5). Further, the Godunov method allows for roughly twice the time step, CFL should be limited to 0.9 for stability. Finally, it is recommended that the Godunov method be used in conjunction with StateRedist. While not fully vetted, early tests also show increased stability in complex geometries for a StateRedist- Godunov scheme compared to the previous FluxRedist-MOL scheme.