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May 13, 2013, 16:58 
NACA 0012 validation Drag problems

#1 
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Good Night
I'm simulating the NACA 0012 at 10º and Re6000000. The setup is: Solver  Pressure Based Kepsilon  Realizable  Enhaced Wall Function Velocity Inlet (x68.93m/s y12.1 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 

May 13, 2013, 20:00 

#2 
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Generally use kw models for outer flow. Did you forget to set the direction vector for cl & cd according to the AoA?


May 13, 2013, 23:11 

#3 
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Yes the components are correct. Cd 1x // 0y // 0z Cl 0x// 1y // 0z.
thanks 

May 13, 2013, 23:27 

#4 
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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 xaxis in your simulation. 

May 14, 2013, 17:12 

#5 
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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 ! 

May 14, 2013, 21:18 

#6 
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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 xdirection is because in aviation the flow is horizontal (xdirection) . Now that the free stream flow in your simulation has an angle with the xdirection you have to calculate your lift & drag according to the freestream direction rather than the x/y direction.


May 15, 2013, 12:17 
Kw

#7  
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Quote:
I gave the Kw a chance. I used the Kw, 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 Kw setup for subsonic simulations ? thanks !! 

August 8, 2014, 14:18 
Result intrepretetion

#8 
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Arun
Join Date: Jun 2014
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wat fluent gives is body force tat is CAcd and CNcl 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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