The default initial condition for time-dependent problems has all variables set to zero. In many situations, you will want to calculate a steady-state solution first, and then use it as an initial condition for the time-dependent problem. This steady-state solution will be saved in a results file (called res, by default). See Starting an Ansys Polyflow Calculation from an Existing Results File for information about starting from an existing results file.

If you start from a time-dependent results file, Ansys Polyflow can also read the time derivative of the solution from the corresponding restart file (which is automatically saved during a time-dependent calculation and called rst by default). See Starting an Evolution or Time-Dependent Calculation from Existing Results and Restart Files for details.

If you start without the rst file, Ansys Polyflow will not be able to perform a first-order or second-order continuation, and will not find the old value of .

If you want to continue the calculation starting from the res file, you have to first modify the initial values of and in the Numerical Parameters menu. Specifying the values that correspond to the res file is almost equivalent to using an res file and an rst file. The only difference is that the -derivative of the solution is not available and the convergence of the continuation may be different.

If an rst file is available, the extrapolation scheme can be restarted exactly at the level where the program was interrupted before. Typically, when the second-order Crank-Nicolson/Adams-Bashforth scheme is used, the time derivatives at two previous time steps are used to extrapolate the solution. When the first-order explicit Euler scheme is used, the derivative at the previous time step is used for the extrapolation.

You should never modify program data such as model parameters or boundary conditions when starting from an rst file. Any deviation will result in an ill-posed problem.