Peter, I appreciate your efforts to this problem.
Blockmesh, here are some suggestions:
1. Please decide whether you have a laminar flow or turbulent flow (It looks like you have to go for laminar flow, highly viscous fluid (0.9 Ns/m2, I assume fluid is stationary, calculated Re is close to 1000. For internal flows, if Re<2000,flow would be laminar). Looks like it is a classic case of Couette flow (please see verification test case 004 under Fluent manual)
2. If you treat your flow to be turbulent (unlikely) then please investigate your mesh and suitable turbulence model. Y+=1 for all turbulence models and sub-types is not recommended. Near wall function defines your range of recommended Y+ value. More info is available in Fluent manual.
3. Boundary conditions: you can't treat inlet and outlet as walls, Instead I would go with periodic boundary condition?
4. Only solve half of the case with planer condition. I would not take symmetry edge in my geometry. If taken, please treat symmetry edge as a wall in boundary type and provide same plate velocity (just to avoid modifying geometry again)
5. Wall shear stress X area of plate = drag force, so you could focus on calculating wall shear stress (Tw) (N/m2) across plate.
6. Theoretical Tw (per Newtons law of Viscosity) = dynamic viscosity*velocity/distance between two plates = 0.9*5/0.04= 112.5, for your case Tw=112.5*2=225.