how to find mid-edge/mid-surface between two successive turbine blade
 

Hi guys,

First please find attached a picture of my problem.

I just imported geometry from Solidedge into Ansys designmodeler 15. Now I want to specify and sketch a line/edge or surface exactly between two successive blades in order to define flow region and apply periodic boundary.
As it is depicted in attachment.

the thing is I tried mid surface but it didnt work out.

Have you got any solution for that ?!

Cheers