# RESTART

Restarts from a previously solved problem.

AcuSolve Command

## Syntax

RESTART {parameters}

## Qualifier

This command has no qualifier.

## Parameters

from_problem or problem (string) [no default]
Name of the problem from which to restart. If not specified, the name of the current problem is used.
from_directory or dir (string) [no default]
Name of the working directory from which to access the restart files. If not specified, the name of the current working directory is used. An absolute or relative path to the working directory of the previous run is allowed. The path should be surrounded by quotes in the input deck.
from_run or run (integer) >=0 [=0]
Run ID from which to restart the problem. If 0, the program restarts from the latest available run.
from_time_step or step (integer) >=0 [=0]
Time step from which to restart the problem. If 0, the program restarts from the latest available time step.
reset_time_step (boolean) [=off]
Flag specifying whether to reset the time step and time to zero.
reset_time_increment (boolean) [=off]
Flag specifying whether to reset the time increment.
type (enumerated) [=incremental]
Type of restart to perform.
incremental
Performs an incremental restart where all settings from the previous run are retained with the exception of those that are present after the RESTART{} command in the input file that is being processed.
full
Performs a full restart where no commands from the previous run are retained. The only data that is carried forward across the restart is solution data associated with nodal results, rigid bodies, flexible bodies, and so on. When using this type of restart, the input file must contain a fully populated set of commands followed by the RESTART command.

## Description

This functional command restarts the problem from a previously solved problem. For example,
RESTART { }

loads in the restart data of the last time step of the last run of the current problem in the current working directory.

The from_problem parameter specifies the name of the problem from which to restart. If it is unspecified, the name of the current problem is used. The current problem name is provided by the command line option, problem; see the AcuSolve Programs Reference Manual for a description of command line options.

The from_directory parameter specifies the working directory of the problem from which to restart. This directory may be either relative to the current directory or an absolute address. If the from_directory parameter is unspecified, the working directory of the current problem is used. This directory is given by the command line option working_directory; see the AcuSolve Programs Reference Manual for a description of command line options.

Every time the solver is run, a new run identification number is assigned to the problem. Within each run there might be one or more restart time steps. The from_run and from_time_step parameters are used to load the appropriate restart data. If both parameters are non-zero, then the proper file is loaded. If the restart data for such a run and time step does not exist, an error occurs. One or both of these parameters may be set to 0, indicating the use of the latest values. To be specific, if from_run is non-zero, but from_time_step is zero, then the latest restart time step of the given run is loaded. If from_time_step is non-zero, but from_run is zero, the latest run ID with the given time step is loaded. If both parameters are zero, then the latest run with the latest time step is loaded. In all cases, if no restart data are found, an error occurs.

The simplest input file with the restart option is
RESTART {
type = incremental
}
RUN { }

Here, the latest restart data of the current problem is loaded. Then the data are prepared for the solver by the RUN command.

Once the restart data are read, one or more problem parameters may be changed. In principal, all parameters may be changed at the restart, with the exception of the number of nodes given in the nodal coordinates.

For example, assume that the original problem included all boundary conditions, material models, and other needed data for solving the temperature field, but only the flow equations were solved. At restart, the solution of the flow equations may be used to solve for the temperature field, by modifying or adding TIME_SEQUENCE and STAGGER commands, as shown in the following input file:
RESTART {
type      = incremental
}
TIME_SEQUENCE {
staggers  = { "temp" }
}
STAGGER( "temp" ) {
equation  = temperature
...
}
RUN {
}

The RESTART command sets the unknown nodal fields to those written by the RESTART_OUTPUT command.

These solutions may be overwritten by including a NODAL_INITIAL_CONDITION command after the RESTART command. For example,
RESTART {
type         = incremental
}
NODAL_INITIAL_CONDITION( temperature ) {
}
RUN {
}

replaces the restart temperature values with those in the file temp.new.

Setting reset_time_step to on sets both the step number and time to zero. The default behavior is to use the step number and time from the previous solution.

A restart of type = full deviates from one of type = incremental in the sense that it does not retain any of the commands from the previous run. Instead, it requires you to fully specify the new run in the current input file, and simply extracts the solution information from the previous run. When using this command, it is typically placed at the end of the input file. For example, the following sequence of input commands can be used to run a transient simulation by restarting from a steady state case.
ANALYSIS {
title                    = "test problem"
type                     = transient
}
EQUATION {
flow                     = navier_stokes
turbulence               = spalart_allmaras
}
AUTO_SOLUTION_STRATEGY{
initial_time_increment   =  .001
min_stagger_iterations   = 2
max_stagger_iterations   = 3
}
COORDINATE{
}
SIMPLE_BOUNDARY_CONDITION("wall"){
...
}
...
RESTART{
type                     = full
reset_time_step          = on
reset_time_increment    = on
}
RUN {}
Care must be used when exercising functional commands in combination with the RESTART command. The specific instances of INCLUDE, ASSIGN and AUTO_SOLUTION_STRATEGY commands are not recorded in the file that contains the primary run information. Instead, they are evaluated immediately, and the values they produce are stored along with the data describing the run. This has important implications when restarting from a run that contains commands such as AUTO_SOLUTION_STRATEGY. When you rely on the AUTO_SOLUTION_STRATEGY command to set the solution strategy, it should be explicitly re-issued whenever a RESTART is used and the parameters impacting it are changed. For example, if a new equation is added to the restart case, it is necessary to have a new instance of the AUTO_SOLUTION_STRATEGY command in the restart file to properly populate all of the solution strategy commands. For example, a species equation can be added upon restart using the following set of commands:
RESTART {
type                = incremental
}
EQUATION{
}