[radix]

Hi everybody,
I'm simulating a 2D VAWT Darrieus type with its blade profile NACA 0012. The dimensions of my design are as follow,
-chord lenght: 0,5 m
-diameter: 1 m
The freestream velocity in my simulation is 5 m/s. And the result is quite surprising because the maximum cp (coefficient of performance) exceed the Betz efficiency. The value of my maximum cp is nearly 21. After this peak point it went down to 7. It's still high right!?
I'm using S-A for the turbulence model and unsteady solver with moving mesh.
Could someone figure out the problem from my simulation?
Does it have something to do with the grid refinement?
Thank you,