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January 20, 2011, 13:32 
Negative drag coefficient...

#1 
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Bruno F.
Join Date: Feb 2010
Location: Boulder, CO
Posts: 17
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Greetings.
I'm trying to model a NACA 0006 airfoil to compare it experimental data (from Theory of Wing Sections, I. Abbott & V. Doenhoff). I'm trying to replicate in this case the flow for angle of attack of 4 degrees (so low alpha), and a Reynolds number of 3e6. The flow is incompressible and moves along the +x direction, and I'm running it using simpleFOAM and the SpalartAllmaras model for turbulence. The lift coefficient matches the experimantal data (~0.4), but the predicted drag coefficient is negative (i.e, producing thrust), and the solution seems to have converged after 500 iterations. The wierd thing is that I get a positive Cd value (as expected) when the angle of attack is zero, but as soon as I give it an angle of attack, the Cd becomes negative. I must be doing something wrong on the settings. I'm including the entire validation case as *.gz file. Any clues anyone? Regards, Bruno 

January 20, 2011, 23:45 

#2 
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Bruno F.
Join Date: Feb 2010
Location: Boulder, CO
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Solved. Dumb mistake. I had set the angle of attack by specifying <u,v> components of velocity (horizontal and vertical components) while the airfoil was perfectly horizontal. I still had my liftDir and dragDir pointing along the (0,1,0) and (1,0,0) directions which is wrong. This will give you the normal and axial forces on the airfoil, not the lift and drag. Two fixes would be:
1  Just rotate liftDir and dragDir axial components by whatever angle of attack you are using. 2  Or use the following equation to calculate it directly: Cl=Cn cos(alpha)  Ca sin(alpha) Cd=Ca cos(alpha) + Cn sin(alpha) Where Ca and Cn are the axial and normal forces that are computed when using dragDir along (1,0,0) and liftDir (0,1,0) with libforces. Bruno 

April 9, 2011, 06:41 

#3 
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Aamer Shahzad
Join Date: Mar 2010
Posts: 58
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Hello Bruno.....
lets say that i am translating an airfoil in x direction and the airfoil has an angle of attack of 40 degree. lets assume that instead of using u and v components of velocity, i have rotated my whole grid by 40 degree (and thus airfoil orients by 40 degree), in such case what should be used for Cl and Cd.... should it be (0,1,0) and (1,0,0) respectively now? 

April 9, 2011, 16:38 

#4 
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Bruno F.
Join Date: Feb 2010
Location: Boulder, CO
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Good afternoon Aamer. Yes, if you rotate the mesh by 40 deg, and maintain the flow only along the xdirection, then the lift will be along (0 1 0) and the drag will be along (1 0 0). Just remember, by definition, that the lift is always perpendicular to the flow and the a drag is always in the same direction of the flow. Let me know if this is not clear...
Cheers Bruno 

April 10, 2011, 04:51 

#5 
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Aamer Shahzad
Join Date: Mar 2010
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Dear Bruno.....
if the airfoil is moving in still air, then there is no flow around it.... so how can we find out the direction of flow then..... if airfoil is moving in x direction (in still air), do we have to assume that the flow direction will be in +x direction....? 

April 10, 2011, 11:03 

#6 
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Bruno F.
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Location: Boulder, CO
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Good morning Aamer.
It doesn't matter if the airfoil is moving or if it is static with wind blowning at the same velocity upon it. The same relative wind velocity is encountered for the airfoil in both cases. Perhaps this link will help you better understand: http://www.allstar.fiu.edu/aero/flight33.htm Cheers Bruno 

April 10, 2011, 12:10 

#7 
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Aamer Shahzad
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Thanks Bruno.... for the explanation and the link


October 28, 2012, 20:28 

#8 
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R. P.
Join Date: Jul 2010
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Hi everyone,
I am trying to calculate the drag, lift and pitchingmoment coefficients for a flow over a probe. The code structure below works fine when the flow is perpendicular to the probe. However, when I changed the angle of attack I got a rude difference for lift and pitchingmoment when compared with experimental data (the drag values is OK). I guess, this difference is related to the vectors (liftDir (0 1 0); dragDir (1 0 0); pitchAxis (0 0 1)). I this this vectors values is just valid for a parallel flow, isn't it ? Anybody knows how to change this vectors according to a certain angle of attack ? My mesh still the same and I just rotate my geometry in 10 degree. Thank you very much. Rophys forces { type forces; enabled true; functionObjectLibs ( "libforces.so" ); outputControl outputTime; patches (probe); directForceDensity true; fDName fDMean; CofR (0.02 0 0); log on; } forceCoeffs { type forceCoeffs; functionObjectLibs ("libforces.so"); patches (probe); outputControl outputTime; fDName fDMean; rhoInf 1.73e5; CofR (0.02 0 0); liftDir (0 1 0); dragDir (1 0 0); pitchAxis (0 0 1); magUInf 1503.1; lRef 0.05; Aref 9.817e4; } 

January 28, 2015, 13:14 

#9  
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Hasan K.J.
Join Date: Dec 2011
Location: Bristol, United Kingdom
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Quote:
Can someone tell me what are the normal and axial forces he is talking about here the Ca and Cn ??? is it the Cd and Cl computed from when using dragDir along (1,0,0) and liftDir (0,1,0) or something else because when I use the Cd and Cl calculated from computations in the place of Ca and Cn to get the actual Cd and Cl I am getting a list of negative and positive values for a range of angle of attacks from 5 to 20 Edit: Answer: yes the Ca and Cl are the computed lift and drag using dragDir along (1,0,0) and liftDir (0,1,0) and the reason I was getting a mixture of positive and negative values is because I was calculating in Excel and I did not use Radians. you have to use RADIANS when using sin and cos to get the right answer. Thanks, Hasan K.J
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"Real knowledge is to know the extent of one's ignorance."  Confucius Last edited by Alhasan; February 1, 2015 at 09:48. 

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