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-   -   Fluent Good Lift coefficient BAD drag coefficient (http://www.cfd-online.com/Forums/main/14833-fluent-good-lift-coefficient-bad-drag-coefficient.html)

Rif February 22, 2008 23:27

Fluent Good Lift coefficient BAD drag coefficient
 
Hi , I'm running an airfoil flow simulation in FLUENT. I managed to get a good agreement with the experimental results for the lift coefficients but the drag coefficient differs alot. Is there a way to correct this?

Cheerios Rif

ag February 23, 2008 11:43

Re: Fluent Good Lift coefficient BAD drag coeffici
 
Typically this is an indication of poor resolution of the viscous boundary layer or a problem with the computation of the viscous contribution to the drag. Look at those two areas.

Anton Lyaskin February 26, 2008 06:20

Re: Fluent Good Lift coefficient BAD drag coeffici
 
You can look for the materials of AIAA Drag Prediction workshops to find out what do you need to predict drag correctly - there are plenty of recommendations on grid resolution, turbulence models and etc.

Ortherion March 9, 2010 06:09

Actually, I'm looking for a solution to this problem too. I don't think AIAA could report would be any help, in my case I've been following numerous advices, tutorials, various models and settings], all that kind of stuff, still, my Drag coefficient is ALWAYS 10x bigger than it should be according to experimental data. [even in negative values a few times ]. I think I got best results when I copied A PROCEDURE FOR NUMERICALLY ANALYZING AIRFOILS AND WING SECTIONS simulation, Cl error was 5%, Cd 600%

agd March 9, 2010 10:52

As stated, this is usually an indication of a screwed-up representation of the viscous effects or the boundary layer. What is the maximum y+ of the first grid point off the airfoil surface? What turbulence model are you running? Do you even need a turbulence model? Are you resolving the boundary layer correctly?


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