airfoils in tandem configuration
 

I have two airfoils in tandem configuration in front of an air stream in mach0.3
I used spalart-almaras as a viscous model to achieve the results.I tried all boundary conditions u think.BUT nothing I could find,I mean I can not achieve the correct periodic pattern of drag coefficient.what are the correct boundary conditions? where is my problem?:confused:
I'll be so glad in advance for your help.
tnx:)