Hi all Am modelling a NACA 0009 foil at Re=9e+06. Lift co-efs,pressure dists. etc are fine, but drag values are not as expected. Specifically, they are negative, and even then the absolute value would be too big. The pressure force in the x direction is being given as -ve, leading to the -ve drag values, but the figures aren't anywhere near what they should be. I've checked the grid, volumes are all +ve, and the Y+ values are good too. I presume someone out there has done simple modelling of foil sections? Any help would be gratefully appreciated (and will prevent me getting more grey hairs!) Regards FP

(1). Simple question indeed. But unfortunately, the state of the art of CFD does not permit a simple answer yet. (2). To avoid getting more grey hairs, try to use only the validated portion of your analysis.

The drag of the airfoil is composed of two parts. -) friction drag -) pressure drag

friction drag comes from the shear forces acting on the surface. pressure is due to the displacement effect of the boundary layer.

My observation is, that it is much easier to get a good friction drag whereas it is much harder to get a good answer for the pressure drag.

fluent is giving you the friction and the pressure drag. for you airfoil I would guess that friction drag contributes about 80-90% to the overall drag. The rest is pressure drag. So I assume that you have to decrease pressure drag.

Try the following: -) you 2nd order schemes -) insert more cells in the boundary layer region. but do not use the fluent's adapt functionality. Do this in your grid generator but keep the y+ in the limits prescribed by your turbulence modell.

good luck Detlef