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-   -   NACA 0012 validation Drag problems (http://www.cfd-online.com/Forums/fluent/117691-naca-0012-validation-drag-problems.html)

leo_NM May 13, 2013 16:58

NACA 0012 validation Drag problems
 
2 Attachment(s)
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

blackmask May 13, 2013 20:00

Generally use k-w models for outer flow. Did you forget to set the direction vector for cl & cd according to the AoA?

leo_NM May 13, 2013 23:11

Yes the components are correct. Cd- 1x // 0y // 0z Cl- 0x// 1y // 0z.

thanks

blackmask May 13, 2013 23:27

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.

leo_NM May 14, 2013 17:12

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 !

blackmask May 14, 2013 21:18

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.

leo_NM May 15, 2013 12:17

K-w
 
1 Attachment(s)
Quote:

Originally Posted by blackmask (Post 427573)
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 !!

ARUN.AVIATOR90 August 8, 2014 14:18

Result intrepretetion
 
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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