Reply To: UDF: Segmentation fault when using F_AREA

[Rob]

Interior faces should also have an ID, click on the surface in the GUI and in the Boundary Conditions Task Page you should see an ID. F_AREA is for facet data (surface) and C_stuff is for volumes. 

For flux into a porous zone I'd look at the bounding surface (interior) rather than the cells. You can loop over cells and then facets on a cell to find a boundary but that can be very inefficient as that loop may be called every iteration/timestep depending on the code.