Solver restart facility
NSU3D provides the capability for restarting both steady-state and time-dependent runs using the restart.out or restart.# directories generated by previous NSU3D runs. Note that the solution.# directories cannot be used to restart NSU3Druns since they do not contain the required information for restarting runs. (They can however be used to visualize the solution).
To restart a steady state run, specify the restart directory name in the input parameter file under the RESTART FILE heading (link).
The restart facility is then activated using the parameters RESTARTF and RESTARTT (link).
RESTARTF = 0.0 bypasses the restart function and flow variables are initialize to freestream.
RESTARTF = 1.0 results in the flow field being initialized by the values in the specified restart file/directory.
RESTARTT = 1.0 is used to specify the initial turbulence values from the restart file/directory (only when RESTARTF=1.0)
RESTARTT = 0.0 is used to set initial turbulence values as freestream values.
RNTCYC is no longer functional and should be set = 0.0
To produce an exact restart, use RESTARTF = 1.0 and RESTARTT = 1.0
To restart only flow field values and use freestream turbulence values use RESTARTF = 1.0 and RESTARTT = 0.0
UNSTEADY or TIME-DEPENDENT RUNS:
The restart procedure for time dependent runs is similar to that described above for steady-state runs. However, for time-dependent runs using ITACC=2 or 3 (link) (BDF2 or BDF3) additional time level information is required to produce an exact restart with no loss in time accuracy. If the specified restart file was produced from a run using ITACC=2 or 3, then the necessary information for a fully time accurate restart run is available in the restart file and the restart will be exact.
However, if the restart file was generated from a previous steady state run, or from a run with an ITACC value smaller than the current ITACC value, then the restart file does not contain sufficient information to enable an exact restart. In this case, a restart using a lower time accuracy on the first time step can be achieved by specifying: RESTARTF = -1.0 For example, if restarting from a steady-state solution but specifying ITACC=2.0 RESTARTF= -1.0 must be used. In this case, the first time step will consist of a BDF1 time step (i.e. first order accuracy in time, equivalent to ITACC=1.0) and all subsequent time steps will revert to BDF2 (ITACC=2) time accuracy.
In the case where the restart file was generated using the same ITACC value as currently set in the new run, but a different time step value was used, RESTARTF = -1.0 must also be specified, resulting in the first time step being computed using BDF1.
The type of time discretization at each time step (BDF1, BDF2, BDF3) is output to standard out, providing a means for checking the operation of the restart facility on the first several time steps.