Time Stepping

Note that if both are specified, both criteria are used and the simulation still stop when the first criterion is hit. In the case of unsteady flow, the simulation will stop when either the number of steps reaches max_step or time reaches stop_time. In the case of unsteady flow, the simulation will stop when either the tolerance (difference between subsequent steps) is reached or the number of iterations reaches the maximum number specified.

The following inputs must be preceded by “mfix.”:

Key	Description	Type	Default
max_step	Maximum number of time steps to take	Int	-1
stop_time	Maximum time to reach (s)	Real	-1.0
fixed_dt	Should we use a fixed timestep?	Int	0
cfl	CFL constraint (dt < cfl * dx / u) if fixed_dt not 1	Real	0.5
dt_min	Abort if dt gets smaller than this value	Real	1.e-6
dt_max	Maximum value of dt if calculating with cfl	Real	1.e14
tcoll_ratio	DEM timestep equals the min collision time divided by this value	Real	50.0
walltime_limit	Runtime limit specified with format HH:MM:SS. When the runtime has almost reached the limit (approaching is computed by considering the average runtime needed for each step) then the simulation ends and a clean exit is performed	String	“”
clean_exit	This input represents the name of a file that, if found in the run folder, makes the code stop and perform a clean exit	String	“”

The following inputs must be preceded by “mfix.” and are only relevant if running a problem to steady state. Currently, the criterion for setting “steady_state” to true is if “dt” is undefined in mfix.dat

Key	Description	Type	Default
steady_state	Are we running a steady-state calculation?	Int	0
steady_state_tol	Tolerance for checking if we have reached steady state (Must be set if steady_state = 1)	Real	None
steady_state_maxiter	Maximum number of allowed iterations to converge to steady state	Int	100000000

Setting the Time Step

There are several ways that the inputs are used to determine what time step is used in the evolution of the fluid-particle system in MFIX-Exa.

1) In a pure particle case, the mfix.fixed_dt, if specified, is only used to determine the frequency of outputs, it has no effect on the “dtsolid” used in the particle evaluation. If you do not specify a positive value of mfix.fixed_dt then the code will abort.

amrex::Abort::0::If running particle-only must specify fixed_dt in the inputs file !!!

The particle time step “dtsolid” is determined by computing the collision time “tcoll” from particle properties, then setting “dtsolid” to be “tcoll / 50”.

2. In a pure fluid case, there are two options:

• If you want to fix the dt, simply set mfix.fixed_dt = XXX and the fluid time step will always be that number.

• If you want to let the code determine the appropriate time step using the advective CFL condition, then set mfix.cfl = 0.7 for example, and the fluid time step will be computed to be dt = 0.5 * dx / max(vel).

  • If dt as computed in the compute_dt routine is smaller than the user-specified `mfix.dt_min` then the code will abort:

    amrex::Abort::0::"Current dt is smaller than dt_min !!!
    

  • If dt as computed in the compute_dt routine is larger than the user-specified `mfix.dt_max` then dt will be set to the minimum of its computed value and dt_max

  • Note that the cfl defaults to 0.5 so it does not have to be set in the inputs file. If neither mfix.cfl nor fixed_dt is set, then default value of cfl will be used. If mfix.fixed_dt is set, then it will override the cfl option whether mfix.cfl is set or not.

These options apply to steady state calculations as well as unsteady runs.

3. In a coupled particle-fluid case, dt is determined as in the pure-fluid case. In this case the particle time step “subdt” is first computed as in the particle-only case (“dtsolid”), then is adjusted so that an integral number of particle steps fit into a single fluid time step.