hello I am using fluent+gambit to model flow over an airfoil at low Reynolds number(.15 to .7 Milion) . I used allmost all combinations of turbulance modles(laminer and invisid also) - invisid gives lower drag than Exp. (as expacted) - laminer and turbulant models (found in fluent) gave about double the experimantal value I notced that at low AoA (say zero , but lift is not zero), I notced that viscous drag alone is close to the experimantal data and when pressure drag is added darg will be become twise the exp. value.

I need expalnation OR refrence OR example that I can compare with.

Thanks to all

Are you putting the correct value of velocity under Reference Values in Fluent? How fine is your mesh? is it a tri or quad mesh?

I would expect CFD to overestimate the lift (and subsequently drag - by a lesser extent) of the airfoil. unless you mesh is incredibly fine near the airfoil surface, CFD has trouble predicting the separation point (usually further aft than reality), thus leading to increased lift. If your getting significantly different results, there's probably something more than this contributing to the error.

Also a low Reynolds number would be 50, and not in the millions.