For overlapping structured zonal boundaries, this keyword may be used to freeze the solution at fringe points during a cycle. Solution values at these points are then updated at the end of the cycle.

FRINGE FROZEN must be used for double-fringe boundaries. Running a double-fringe grid without this keyword may lead to erroneous results.

FRINGE FROZEN may also be specified for single-fringe boundaries, but is not required. The default for single-fringe boundaries is to update the solution at fringe points every iteration.

Using double fringes increases the solution accuracy at overlapping boundaries. Note, though, that by freezing the solution over a cycle, the solution at fringe points will lag in time behind the solution at interior points. This could potentially lead to instabilities during transients, or a reduction in the convergence rate if a large number of iterations per cycle are specified. However, testing on a limited number of cases has not revealed any problems when using up to five iterations per cycle.

Double-fringe boundaries may be created with GMAN, by first selecting double-fringe mode using (in GMAN's graphical mode) MODIFY BNDY > GENERATE FRINGE > SET FRINGE-MOD > DOUBLE FRINGE, then generating the fringe.