A powerful implicit advection technique has been incorporated into FLOW-3D®, Version 9.2. This paper illustrates uses of this technique to show its advantages, but also indicates certain limitations related to the accuracy of implicit methods.
Using the implicit advection scheme requires the selection of an input parameter impadv that activates the scheme according to:

impadv = 0, no implicit advection (i.e., explicit, the default);
1, implicit advection, with limited advection at free surfaces
controlling the time-step size for accuracy;
2, implicit advection, with no advection limit on time-step size.

When using FLOW-3D® to simulate transient problems, especially those involving sharp free surfaces and/or fluid-fluid interfaces the best implicit advection option is impadv=1. In this case, the program will limit time-step size by those fluid velocities at a free surface where the velocity is normal to the surface and the fluid fraction at that location has changed by more than 5% in the preceding cycle. Otherwise, the surface velocities will not impose a limit on time-step size.