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Negative Drag coefficient naca 0012 airfoil using fluent 15

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Old   March 2, 2015, 22:36
Question Negative Drag coefficient naca 0012 airfoil using fluent 15
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Lucas Vinicius Fernandes Coelho
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Hi,

I'm new in CFD and I have a problem, in my simulation the drag coefficient of a naca 0012 is negative, as I increase the speed angle of attack, the drag coefficient decreases and becomes negative, I don't now why, I'm pretty sure that my velocity components are OK. I'm use the k-omega model for a Re=3e06. I need help. And I want to know if fluent have a way to rotate only the airfoil or the mesh, without I need to use the velocity components to change the angle of attack.

Thanks

p.s.: Sorry for my english
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Old   March 7, 2015, 12:01
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Jesus Ramirez
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I a similar problem. Initially I performed a naca 0012 simulation on OpenFOAM and I got good results of Cd. However when I just modified the profile the Cd became negative. Is there any physical reason for this?
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Old   March 9, 2015, 12:51
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Anthony McCarthy
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Is it definitely negative coefficient you are recording or are you referring to a negative force? If it is the latter this is correct
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Old   March 9, 2015, 13:40
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Within the Drag Monitor you have to modify Force Vector (on the bottom of the Drag Monitor window) which is by default X=1 and Y=0. If your velocity is parallel to the X-axis then this is OK (X=1 and Y=0) but if you change AoA (velocity vector) you also need to modify this Force Vector within Drag Monitor so X=cos(AoA) and Y=sin(AoA) (for positive AoA). Drag force should be calculated as parallel to the free stream velocity. Also don't forget to modify Lift Monitor accordingly (lift force should be calculated perpendicular to free stream velocity).

Try to change (if you already didn't) and see if it helps.

Best regards,
CFDBuddha
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Old   March 9, 2015, 13:54
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Anthony McCarthy
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Also as an extension to what cfdbuddha pointed out, manually calculating your co-efficients for the forces leads to less confusion when changing your x-y for different angles of attack
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Old   March 13, 2015, 15:18
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Lucas Vinicius Fernandes Coelho
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Thanks CFDBuddha, but now I have another problem, the Cd must be something like 0.015 to 10 degrees, but my Cd is to high, something like 0.027, how I can correct this?
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Old   March 13, 2015, 15:26
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Do you use the exact same conditions as for the Cd value you are trying to acquire? What simulation or test gave you that value of Cd?

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Old   March 13, 2015, 15:35
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Lucas Vinicius Fernandes Coelho
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Yes, for 1 , 2 , 3 degrees its ok, but for high degrees, the Cd increases. All the Cl are OK
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Old   March 13, 2015, 15:43
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It makes sense for the low AoA, but for higher AoA, many unsteady phenomena could cause differences in your calculations. Is your time integration time-accurate or steady state? You should use the exact same simulation specifications as the for the Cd you are trying to acquire. Also, keep in mind that there may be fluctuations to your Cd value, even for the steady-state case. You should first average over your simulation cycles and then compare.

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