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NACA 0012 validation Drag problems

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Old   May 13, 2013, 16:58
Default NACA 0012 validation Drag problems
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Good Night

I'm simulating the NACA 0012 at 10 and Re-6000000. The setup is:

-Solver -- Pressure Based
-K-epsilon - Realizable - Enhaced Wall Function
-Velocity Inlet (x-68.93m/s y-12.1 m/s) Pressure Outlet.
-Simple - Green Gauss Node Based - Second order Upwind for the rest.
-Standart initialization from inlet.

The Cl is very close from the validation but the Cd is negative ?!?!

I attached some images.

What's the setup you guys use for this case ? Is my Mesh the problem ? The airfoil is 20c from the walls.

thanks
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File Type: jpg pergunta.jpg (95.4 KB, 19 views)
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Old   May 13, 2013, 20:00
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Generally use k-w models for outer flow. Did you forget to set the direction vector for cl & cd according to the AoA?
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Old   May 13, 2013, 23:11
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Yes the components are correct. Cd- 1x // 0y // 0z Cl- 0x// 1y // 0z.

thanks
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Old   May 13, 2013, 23:27
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Thank you for posting the values of your cl&cd direction vectors. The components are incorrect. The correct value should be
cd:< cos(alpha), sin(alpha), 0 >
cl: <-sin(alpha), cos(alpha), 0>

I assume that the chord of your airfoil is parallel with the x-axis in your simulation.
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Old   May 14, 2013, 17:12
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THANK YOU !!!

It worked !! The Cd is now very close to experimental ones.

I used your "formula" and it worked very well but why the "sin" in lift is negative ?

thank you !
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Old   May 14, 2013, 21:18
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The lift & drag is in regard to the free stream flow. It is just a simple coordinate transformation. The drag in most of the sketches is in parallel with x-direction is because in aviation the flow is horizontal (x-direction) . Now that the free stream flow in your simulation has an angle with the x-direction you have to calculate your lift & drag according to the free-stream direction rather than the x-/y- direction.
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Old   May 15, 2013, 12:17
Default K-w
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Quote:
Originally Posted by blackmask View Post
The lift & drag is in regard to the free stream flow. It is just a simple coordinate transformation. The drag in most of the sketches is in parallel with x-direction is because in aviation the flow is horizontal (x-direction) . Now that the free stream flow in your simulation has an angle with the x-direction you have to calculate your lift & drag according to the free-stream direction rather than the x-/y- direction.
Thank you blackmask !

I gave the K-w a chance. I used the K-w, but the solution went crazy with turbulance viscosity and reversed flow on pressure outlet messages, besides the variation of 10^7 in cd and Cl coefficients.

I did the mesh again taking care or the y+ but it didn't work.

Any recomendation of K-w setup for subsonic simulations ?

thanks !!
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Old   August 8, 2014, 14:18
Default Result intrepretetion
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wat fluent gives is body force tat is CA-cd and CN-cl for zero AOA only(axial and normal force coeff).SO
Cl=CN*cos(aoa)-CA*sin(aoa)
CD=CA*cos(aoa)+CN*sin(aoa)
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