Drag on part of a wallzone which contains hanging nodes
 

I have 3d model and its solution, in which there is flow between two different shaped walls. I want to calculate the drag force on of the walls. "Report>force>drag", gives the drag force on the whole wallzone (both walls together). I can't separate the wallzone using "Grid>separate" because there are hanging nodes in result of adapting.
Are there any other methods to to calculate the drag force or separate the wallzone?

It'll take me near a month to rerun all of my models, so your answer would be really appreciated.