Pre-conditioned Conjugate Gradient

Advanced Settings

Gradual activation of SZ drainage - To prevent numerical oscillations the drainage constant may be adjusted between 0 and the actual drainage time constant defined in the input for SZ drainage. The option has been found to have a dampening effect when the groundwater table fluctu­ates around the drainage level between iterations (and does not entail reductions in the drain flow in the final solution). For the steady-state solver and the transient solver the option is by default turned ON.

Horizontal conductance averaging between iterations - To prevent poten­tial oscillations of the numerical scheme when rapid changes between dry and wet conditions occur a mean conductance is applied by taking the conductance of the previous (outer) iteration into account. By default this option is enabled for both steady-state and transient simu­lations.

Under-relaxation - Under-relaxation factors can be calculated automatically as part of the outer iteration loop. 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 determined as 90% of the factor used in the previous iteration and 10% of the current optimal factor.

The second option is to define a constant relaxation factor between 0 and 1. In general a low value will provide convergence, but at a low convergence rate - i.e. with many SZ iterations. Higher values increases the convergence rates, but also the risk of non-convergence. As a general rule a value of 0.2 has been found suitable for most set-ups.

The time used for automatic estimation of relaxation factors may be significant compared to subsequently solving the equations and the option is only recommended in steady-state cases. In transient simula­tions, ’No under-relaxation’ is recommended.