Under relaxation

The PCG solver can optionally use an under-relaxation factor between 0.0 and 1.0 to improve convergence. In general a low value will lead to conver­gence, but at a slower convergence rate (i.e. with many SZ iterations). Higher values will increase the convergence rate, but at the risk of non-convergence.

Automatic (dynamic) estimation of under-relaxation factors

If the automatic estimation of the under-relaxation factor is allowed, the under-relaxation factor is calculated automatically as part of the outer itera­tion loop in the PCG solver. The algorithm determines the factors based on the minimum residual-2-norm value found for 4 different factors. To avoid numerical oscillations the factor is set to 90% of the factor used in the previ­ous iteration and 10% of the current optimal factor.

The time used for automatic estimation of relaxation factors may be signifi­cant when compared to subsequently solving the equations and the option is only recommended in steady-state cases.

Under-relaxation by user-defined constant factor

This option allows the user to define a constant relaxation factor between 0.0 and 1.0. In general a value of 0.2 has been found suitable for most simula­tions.