Flow Simulation solves the time-dependent form of the Navier-Stokes equations. For steady flow problems Flow Simulation starts the calculation from initial conditions defined by the user. The solver iterates (“time-marches”) on the variables until there is no appreciable change, i.e. the solution converges. You can facilitate shorter computation times by specifying initial conditions that are close to the final results.
Although this practice is recommended, it is not usually required. For External problems the initial conditions will be the Ambient Conditions of the undisturbed fluid stream around the body.
For unsteady (“transient”, or Time-dependent) problems Flow Simulation “time marches” the solution from initial conditions for the problem’s physical time that you specify. Unlike steady flow problems, the initial conditions must be precise, with the exception of unsteady problems, which have a steady periodic solution (e.g. in the case of periodic boundary conditions) that can be obtained from arbitrary initial conditions, but additional time will be required to eliminate the influence of specified initial conditions.
Steady-state problems are solved by marching the solution in time using time steps determined locally, i.e. at each computational mesh cell independently, which are based on the fluid flow properties of each cell. By default, the time step for solving time-dependent fluid flow problems is specified by Flow Simulation automatically, based on the fluid flow properties only. If you want either to better resolve a problem’s time-dependent solution (by specifying a smaller time step than the automatically selected one, e.g. for resolving periodic solutions of too small period) or to calculate a heat transfer in solids faster (by specifying a larger time step than the automatically selected one, e.g. if the fluid flow does not changed), it is expedient to specify the time step manually. If you solve a time-dependent problem with heat transfer in solids only, i.e., without calculating a fluid flow (the Fluid Flow option is disabled and the Conduction option is enabled) a manual specification of the time step is preferable.
Alternatively, after passing the Wizard, you can enable the Time-dependent option in General Settings dialog and specify the Maximum physical time for finishing the calculation (see “Finishing the Calculation”), as well as strategy and moments of saving results during calculation (see “Saving Results and Backup Files”) in the Calculation Control Options dialog box.
To specify time-dependent boundary conditions, use the Dependency dialog (see “Dependency”).