Over predicted drag for airfoils, any help?

Yasser

Hello everybody: I am trying to obtain the drag force for a NACA0012 airfoil (2D model) and I get the drag over predicted compare to the experimental data (for about 30% or more) even in angle of attack of zero! The problem is with the viscous drag which is really over predicted. Although it looks a very simple problem, but I am not able to obtain an acceptable drag for turbulent flows. I have ensured the following facts:

1. The Reynolds number matches the experiments and is not in transition Reynolds numbers (Re=2-5 million) 2. I have used all the turbulence models and wall functions available in 2ddp solver 3. I am using a fine grid close to the airfoil and I keep the y+ value in the proper range for different wall functions 4. I have verified the reference values 5. I am using a very high quality structured grid with a maximum skewness of 0.12 6. The domain is large enough, so it has no effect on the flow field around the foil 7. The flow is incompressible (M=0.085). Both pressure farfield and velocity inlet have been tried.

I really appreciate it if anyone can help.

Thanks

Yasser