I am running a model in both 2D and 3D with the same X & Y dimensions. After iterations based on a constant pressure or velocity inlet boundary conditions, I observed that the resulting velocity vector plots are similar but the volumetric flow rates at the inlets and outlets are dramatically different. This totally makes sense. But I'm curious as to how exactly FLUENT treats the 2D model. I can't seem to find that information anywhere. Also, can I obtain any scaling factor to convert the 2D flow data into 3D flow data (for faster simulations)? Thanks!

2D geometry is slice for the 3D geometry....

means physically it is assumed that the width is infinite long.....

like rectangle channel in 3D can be represented by 2D plane rectangle geometry provided that the width of 3D channel is large enough to consider as not affecting the flow....

2D is not infinitly wide. If it was you would have an infinite inlet area and infinite flow rate.

By default, it is unit depth (1 metre) but you can change it in the reference values as far as I can remember. So, to calculate you inlet area it is simply the height*1m depth.

If your problem is cyclindrical shape you should use the 2D axisymmetric model.

sorry..... you are right it is a unit length....

In cylindrical shape isn't the unit depth defined as one radian?