Transient analysis may be achieved with the unsteady SIMPLE algorithm, wherein the transient term is expressed as a source term in the momentum equation and the regular segregated flow solver is used to perform inner iterations.
- Set the physical time-step. The physical time-step will generally be governed by the transient phenomenon being simulated. The time-step size should at least satisfy the Nyquist sampling criterion; more than two time-steps per period are required.
- Set the maximum inner iterations stopping criterion. Generally, this will need be determined experimentally. You should select a certain number of inner iterations, say 10, and then see if the results are affected by refining or coarsening this number.
Smaller physical time-steps generally mean the solution is changing less from one time-step to the next, so that fewer inner iterations are required. There will clearly be an optimal balance of time-step size, under-relaxations factors and the inner iteration convergence level for a given problem and desired transient accuracy.