Hello DanHeHe
There is no need to create a surface to separate a face in 2 different faces. If you select the underlying face, switch to sketch mode and create a closed sketch (like a rectangle), switching back to 3D mode will separate the sketch region as a seperate face from the underlying face. There won't be a new surface body created.
Regarding the imprinted faces not getting transferred to fluent, for that you need to create Named Selection for the imprinted faces. Select the face, then go to "Manage Groups" panel and select "Create NS". See below snapshot.
You are navigating away from the AIS Discovery experience