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khor_albert April 5, 2010 07:58

Cl/Cd problem which has been troubling me for months
Hi there

Part of my project requires modelling with airfoil. I used C-grid, NACA 4424 and k-e turbulence model and strange enough, the Cl/Cd keeps increasing from 15 to 30 degrees. The 30 deg Cl/Cd value is quite close to experimental value but the trend is totally different. I tried rotating the model instead of the flow but it didn't work although it helped a bit. I made small modifications on the airfoil, but Cl/Cd should get smaller as angle of attack gets too large, shouldn't it?

Please advise. This problem has been troubling me for months. What could be the cause of this inaccuracy? All my other simulations produce value within 20% difference from experimental value - only this particular case with the largest modifications. Just give me anything - it just might help!

Thanks mates! :)

cfdman April 6, 2010 02:39

Dear Shin

I think you should check near wall treatment. is Y+ values appropriate for k-e turbulence model? Near wall treatment is very important for accurate solutions on aerodynamic calculations.
Std. k-e model is known to be insufficient in regions where flow seperation occurs.
You should check both y+ values and seperation while using k-e model as these two are the main sources of erroneous results.
Maybe you can use Spalart Almaras which gives accurate solutions on external aerodynamic. If you have fine boundary layer mesh you can use realizeable k-e.

Good luck

DoHander April 6, 2010 11:34

Check these:

1. That your projection vectors (for calculating Cl and Cd) are properly calculated, you must manually change these vectors when you change the incidence.

2. Check your y+ value. This should be about 100 or smaller then 1 (for enhanced wall treatment). Intermediary values for y+ will give you a bad estimation for Cd. However your Cl should decrease after a certain incidence (typically between 12-20 degrees).

3. Try with a different turbulence model (k-omega maybe) or Spalart-Almaras


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