GGU-3D-SSFLOW: "Model" menu item

After pointing to the "System/Model" 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.

The following dialog box is displayed subsequent to bandwidth optimisation:

Open

The following check boxes and information can be provided in the dialog boxes:

• Iteration deviation
  If unsaturated zones are present within the system and calculation of the phreatic line is required an iteration process will be necessary. The required iteration deviation for computation of the phreatic line is defined here. The program aborts the iteration if the deviation of the potentials at all nodes is below this limit value for two consecutive iterations. It avoids a negative iteration values in horizontal-plane systems.

• Damping
  Oscillation around the actual solution can occur during computation of the phreatic line in complicated groundwater systems. This oscillation is damped by the damping factor. Values between 0.0 and 0.95 make the most sense. A value of 1.0 (complete damping) makes no sense and will therefore not be accepted by the program. Oscillations occur primarily when the curve of the kr(u) function is very steep (e.g. as for sand).

• System with phreatic line
  Specify whether a phreatic line should be computed for the system.

• "Dynamic damping"
  In extreme groundwater systems it may not be possible to reach a solution using normal damping. An analysis using dynamic damping may then be carried out. After every five
  iteration steps the damping value is increased by 0.05 up to a maximum of 0.99.

You will be informed on progress during the computation. After computation is complete you will see a message box showing the computation time. You can then move on to the "Evaluation" menu.