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Fluent vs. xFoil result for E423 airfoil: Cd is way off

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Old   November 17, 2017, 12:55
Default Fluent vs. xFoil result for E423 airfoil: Cd is way off
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klu
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My friends and I are currently studying the lift and drag of Eppler E423 high lift airfoil as a practice to use Fluent (we are all new to CFD). We were trying to match the Cl and Cd vs. alpha curves generated from xFoil shown in this link.
http://airfoiltools.com/airfoil/deta...e423-il#polars

I started from the high Re cases (1M and 500K). So far, my Cl values seem to match pretty well, but my Cd values are much higher (xFoil ~ 0.01, my Fluent results are 0.03~0.07). Has anyone tried to do such comparison before? Were there some fundamental difference in the methods behind xFoil and Fluent, so the drag prediction could be so different? (If this question is too stupid, a good reading reference would be greatly appreciated.)

For my Fluent model, I used the following settings.
Transient analysis
pressure-based model
k-epsilon, realizable, standard wall treatment
SIMPLE solver
The y+ around the airfoil is above 30 (some in hundreds)
Results: Cl match pretty well, but Cd is much higher (See attached figure where blue is my Fluent results, and the light brown is xFoil.)

I also tried refining my mesh near the airfoil and used a different turbulent model.
Transient analysis
pressure-based model
k-omega, SST
PISO solver
The y+ is around 1 or slightly below 1
Results: Cl is less than what I got with k-epsilon model (not matching), Cd even higher (not matching xFoil).

If I am only interested in Cl and Cd for low Mach scenario (with various Re), what would be the more appropriate settings? Thank you for your help!
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Old   September 22, 2023, 23:56
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Francisco Herbert
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This is an old but relevant question. It deserves an answer. Long story short: your modeling in Fluent is assuming that the free-stream flow is turbulent, and XFOIL is assuming near laminar free-stream flow. For more information have a look at the following review: https://www.hindawi.com/journals/mpe/2015/854308/
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