GGU-3D-TRANSIENT: General notes on "fixed" boundary conditions

The GGU-3D-TRANSIENT program also imports the steady-state boundary conditions from the steady-state modelling record (henceforth also known as fixed boundary conditions). When analysing transient systems it is almost always the case that at least parts of these boundary conditions remain constant for the complete investigation duration. In principle, you could define the appropriate boundary condition polygons and assign nodes to these. However, this is onerous and superfluous. For this reason the program initially assumes all steady-state boundary conditions as being temporally constant. Naturally, if nodes are then assigned a boundary condition polygon, the temporal constancy at this node is deleted. If a node boundary condition is not to be applied during transient modelling, you must delete the fixed boundary condition (see below).

Do not misuse the fixed boundary conditions to assign a substantially different boundary condition value than that which applied in the steady-state condition. Otherwise a pressure shock may be the result, leading to the necessity for extremely small time increments to arrive at a solution. It is better to define a boundary condition polygon for the appropriate nodes with smooth transitions, which correspond better to the naturally occurring conditions.