Dividing Patches
 

Dear Foamers,

How can I divide a patch into two separate patches relative to a STL file? For example, in the attached picture, I want to divide the cylindrical patch into two patches: one for the faces inside the red STL surface, and another for the other faces.

I tried using a series of setSet commands, and I can get the cells associated to each patch, but how can I extract the outer faces of the cell sets? Any alternative method?