.. sec:InputsTimeStepping:
Time stepping
=============
There are several ways that the inputs are used to determine the time step of particle-only,
fluid-only, and coupled fluid-particle systems.
.. rubric:: Fluid-only simulations
In fluid-only simulations, setting ``mfix.fixed_dt`` causes the fluid to advance with a
fixed (constant) time step. If ``mfix.fixed_dt`` is not defined, then an appropriate time
step is computed based on the advective CFL condition.
.. caution::
**The adaptive time step based on the advective CFL condition is always computed
and is strongly recommended for most cases.**
* For a fixed time step simiulation, if the computed step size is smaller than
``mfix.fixed_dt`` for a fixed, the simulation aborts with the message:
.. highlight:: c++
::
amrex::Abort::0::"Fixed dt is too large for fluid solve !!!
* For simulations using the adaptive time step:
* If the computed time step is larger than ``mfix.dt_max``, then the
time step is set to ``dt_max``
* If the computed time step is smaller than ``mfix.dt_min``, then the
simulation aborts with the message:
.. highlight:: c++
::
amrex::Abort::0::"Current dt is smaller than dt_min !!!
.. rubric:: Particle-only DEM simulations
In particle-only DEM simulations, the time step used to advance particles, ``dtsolid``,
is determined by computing the collision time ``tcoll`` from particle properties, then dividing
the result by the number of steps per collision, ``tcoll_ratio``:
.. highlight:: c++
::
dtsolid = tcoll / tcoll_ratio;
.. note::
``mfix.fixed_dt`` must be defined for particle-only DEM simulations, however it is only
used to determine the frequency of outputs and has no effect on the particle advance. If
a positive value is not specified for ``mfix.fixed_dt``, then the code aborts with the
following message:
.. highlight:: c++
::
amrex::abort::0::if running particle-only must specify fixed_dt in the inputs file !!!
.. rubric:: Coupled fluid-DEM simulations
In a coupled fluid-DEM simulation, the fluid time step, ``dt``, and the particle time step,
``dtsolid``, are computed the same as in fluid-only and DEM-only simulations, respectively.
If ``dt < dtsolid``, then the particle sub-time step, ``subdt``, is set to the fluid time
step and one particle advance is taken. However, if ``dt > dtsolid`` (which is usually the
case), then the number of sub-steps needed for DEM particles to advance the same total time
as the fluid, ``nsubsteps``, is computed by dividing ``dt`` by ``dtsolid`` and the particle
sub-time step is computed by dividing ``dt`` by ``nsubsteps``.
.. highlight:: c++
::
int nsubsteps = 1;
amrex::Real subdt = dt;
if (dt > dtsolid) {
nsubsteps = amrex::Math::ceil(dt / dtsolid);
subdt = dt / nsubsteps;
}
The following inputs are defined using the ``mfix`` prefix.
+----------------------+-----------------------------------------------------------------------+-------------+--------------+
| 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 | Flag to use a fixed time step | Int | 0 |
+----------------------+-----------------------------------------------------------------------+-------------+--------------+
| cfl | CFL constraint (dt < cfl * dx / u) | Real | See note |
| | | | |
| | * Defaults to 0.5 if ``mfix.advection_type = MOL`` | | |
| | * Defaults to 0.9 if ``mfix.advection_type = Godunov`` | | |
| | * ignored if ``mfix.fixed_dt = 1`` | | |
+----------------------+-----------------------------------------------------------------------+-------------+--------------+
| dt_min | Abort if ``dt`` gets smaller than this value | Real | 1.e-6 |
+----------------------+-----------------------------------------------------------------------+-------------+--------------+
| dt_max | Maximum value of ``dt`` if ``fixed_dt`` is not 1 | Real | 1.e14 |
+----------------------+-----------------------------------------------------------------------+-------------+--------------+
| tcoll_ratio | DEM time step 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 | String | "" |
| | 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 | | |
+----------------------+-----------------------------------------------------------------------+-------------+--------------+
| clean_exit | This input represents the name of a file that, if found in the run | String | "" |
| | folder, makes the code stop and perform a clean exit | | |
+----------------------+-----------------------------------------------------------------------+-------------+--------------+
In the case of unsteady flow, the simulation will stop when either the number of steps
reaches ``max_step`` or time reaches ``stop_time``.
The following inputs are defined using the ``mfix`` prefix and are only relevant if running a steady state simulation.
+-----------------------+-----------------------------------------------------------------------+-------------+------------+
| 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 | Real | None |
| | | | |
| | (Must be set if steady_state = 1) | | |
+-----------------------+-----------------------------------------------------------------------+-------------+------------+
| steady_state_maxiter | Maximum number of allowed iterations to converge to steady state | Int | 100000000 |
+-----------------------+-----------------------------------------------------------------------+-------------+------------+