# Restart File

This page only applies to SU2 version 6.2.0 and lower.
The new documentation section can be found here.

The SU2 solution format has the extension .dat, and the files are (by default) in binary format.

The restart files are used to restart the code from a previous solution and also to run the adjoint simulations, which require a flow solution as input. In order to run an adjoint simulation, the user must first change the name of the restart_flow.dat file to solution_flow.dat in the execution directory (these default file names can be adjusted in the config file). It is important to note that the adjoint solver will create a different adjoint restart file for each objective function, e.g. restart_adj_cd.dat.

To restart a simulation the RESTART_SOL flag should be set to YES in the configuration file. If performing an unsteady restart the UNST_RESTART_ITER needs to be set to the iteration number which you want to restart from. For instance if we want to restart at iteration 100 and run the unsteady solver with 2nd-order dual time stepping method, we will need to specify UNST_RESTART_ITER = 100 and have the restart files solution_flow_00098.dat and solution_flow_00099.dat.