Drag Coefficient undepredicted by CFD?
 

I am simulating a 3D airfoil .

The values of Lift coefficients are within the bounds of 10% error compared to
the experimental values .

However , the values of drag coefficient I am getting have an error of above 20% compared to experimental values .
I am using SA turbulence model .

If the above discrepancy is due to the wake region/flow separation and wing tip vertices . How can I accurately capture these phenomena ?

For airfoil major contributor of drag is skin friction drag if you are dealing with sub-sonic flows. To get accurate result make sure you are using NS-equation. Quality of the grid should be good to get an accurate result and you should resolve the boundary layer especially laminar sub-layer to get accurate drag value. For grid size next to wall estimation, you can consider the following link.
http://www.pointwise.com/yplus/

Then gradually stretch to the boundary of the domain. I personally prefer SST-k-W turbulence model for drag calculations. I believe solution procedures and turbulence models have a little effect if your grid is fine and has good orthogonality, less skewness and uniformly stretched.

My conclusion is: do LES if you have computational power, else do RANS with SST-k-W turbulence model and make sure you are resolving laminar sublayer.

I hope you can get an accurate result if you are able to make good mesh no matter whether you are doing RANS or LES.

Thank you so much.