simplify 3D to 2D
 

In stress analysis, we have two theory like plane-stress and plane-strain.
In CFD analysis, Flow-3d manual says that we only need to set the number of mesh cell in Z-direction by one. It works for conditions such as shallow water in which the Z-scale is small enough. I guess that it is similar to plane-stress simplication.
So if the Z-scale is infinitely large just like plane-strain, does it work well that we only set the number of mesh cell in Z-direction by one?