# Trouble with airfoil drag prediction.

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 March 25, 2009, 03:11 Trouble with airfoil drag prediction. #1 New Member   Yury Timofeyev Join Date: Mar 2009 Location: Latvia Posts: 16 Rep Power: 8 Hello. I know quite a few questions of this sort have been posted here, so I looked through some pages but found nothing useful for my case. I try to calculate Cl and Cd for an airfoil at Re=2e6. I get more or less accurate results for Cl but Cd is nit correct no matter what I'm doing to the mesh. I tried over 20 different mesh sizes, Inflation settings and so on but it doesn't seem to work... Could you please suggest some "example" settings or something like that for CFX-mesh and CFX-Pre so I have a starting point?

 March 25, 2009, 17:48 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,275 Rep Power: 88 Hi, Can you describe what you are trying to model? Is the wing stalled? Does it have small separations or fully attached flow? Is turbulence transition important? 2D or 3D? Compressible or incompressible? What Mach number? Glenn Horrocks

 March 26, 2009, 03:06 #3 New Member   Yury Timofeyev Join Date: Mar 2009 Location: Latvia Posts: 16 Rep Power: 8 Sorry It is a NACA 23020 airfoil with 1.2m long chord. It is at 0degree angle of attack. The simulation is 2D, the velocity of flow is 20m\s, normal pressure, density = 1.184. Using k-omega turbulence model, Eddy Length Scale in Global Initialisastion is set to 0.1m. Y-plus is somewhere around 1 in dense meshes but not more than 12. I need only accurate Cd and Cl that are close to the experimental data. Right now I'm getting Cl = 0.04-0.06 and Cd = 0.01, but it should be: Cl = 0.06 and Cd = 0.006. Yesterday I somehow achieved almost correct results but then I decreased the mesh scale (0.5 which led to 700 000+ element mesh) and the results were incorrect (similar to above).

March 30, 2009, 10:34
#4
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Join Date: Mar 2009
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Quote:
 Originally Posted by Zmur Sorry It is a NACA 23020 airfoil with 1.2m long chord. It is at 0degree angle of attack. The simulation is 2D, the velocity of flow is 20m\s, normal pressure, density = 1.184. Using k-omega turbulence model, Eddy Length Scale in Global Initialisastion is set to 0.1m. Y-plus is somewhere around 1 in dense meshes but not more than 12. I need only accurate Cd and Cl that are close to the experimental data. Right now I'm getting Cl = 0.04-0.06 and Cd = 0.01, but it should be: Cl = 0.06 and Cd = 0.006. Yesterday I somehow achieved almost correct results but then I decreased the mesh scale (0.5 which led to 700 000+ element mesh) and the results were incorrect (similar to above).
Try SST turbulece model

March 30, 2009, 13:47
#5
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Yury Timofeyev
Join Date: Mar 2009
Location: Latvia
Posts: 16
Rep Power: 8
Quote:
 Originally Posted by ahlo7 Try SST turbulece model
I did... Same results.

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