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Incoherent values of Cl in 2D calculation

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Old   May 1, 2011, 20:23
Default Incoherent values of Cl in 2D calculation
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Charles
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Hi there,

I'm blowing my mind on a Fluent problem. I've done some calculations on an airfoil with and without an airbrake. I gave up the 3D because I had some strange results (like 25% more lift with the airbrake up !).

So I made my 2D model using Catia, meshed with gambit, and processed with Fluent. The residuals are converging in like 1200 iterations. But for 1mē of my naked, 1m chord, FX66 airfoil, I get some impossible results :
- Drag : 8.69 N
- Lift : 608.41 N

That gives me 70 lift to drag ratio instead of a normal 25 for this airfoil. I wouldn't mind to have like too much or too few lift AND drag because I know it's a reference value thing, but in that case this is the drag to lift ratio which is not possible.
I tried changing things in the reference values but the Force reports is still exactly similar. It's even more frustrating that my flow looks nice.

http://img830.imageshack.us/i/airfoil.jpg/

Did any of you encounter such a problem ? Do you have a clue for the solution ?

Thanks, greetings from France
Charles
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Old   May 1, 2011, 21:15
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Charles
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I might have a clue.
I used a Size function to refine the mesh around the airfoil in Gambit. Is that regular in 2D ? I tried to compute this airfoil using basic "Tri" element without any refining and I get a 11 lift to drag ratio which is plausible at a 0° angle of attack.

If the size function is not appropriate in 2D and is the source of my problem ? What would you recommand to mesh my boundary layer ?

Thanks
Charles
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