GGU-2D-SSFLOW: Notes on bandwidth, units and equation solvers

After pointing to the "System/Analyse" or, alternatively, pressing [F5], the equation system bandwidth is examined and a message box displayed where necessary. If you do not agree with the bandwidth optimisation the analysis is cancelled.

NOTE ON BANDWIDTH:
Finite-element methods give rise to equation systems which, depending on the system, only have values different to "0" in the region of the principle diagonals. The iterative equation solver contained in the FEM applications uses this in order to save memory and thus shorten computation time. The maximum distance between the principle diagonal and the system's non-zero element most distant from the principle diagonal is known as the bandwidth. In principle, iterative equation solvers do not require bandwidth optimisation. However, the results are much more stable and the computation time shorter if the bandwidth is small.

NOTE ON UNITS:
The program works true to your units. If you want to work with common length units (metres), it follows that all length input must then be made in metres. Beside the permeability and discharge, time is employed as an additional unit. Here too, only one unit of time T, e.g. seconds [s], may be used. The results are provided in accordance with the input units used.

In transient analyses it is often useful to define permeability not in m/s, but in m/hour or m/day, for example. If you have entered appropriately modified permeability values the program converts these permeability values back to m/s for display in the legend only. This saves unnecessary explanations when submitting analyses.

Following the bandwidth optimisation a dialog box opens where "Cholesky" or "Iterative" can be selected as equation solver. Information on the differences between the two equation solvers can be read by pressing the "Equation solver info" button.