Expert: Paul Soderman - 1/14/2009

Question
QUESTION: Is there any direct formula for drag coefficient(Cd) calculation than given below? If yes then please tell me.

Fd = Cd x 0.5 x rho x A x V^2

Thanks.

ANSWER: No

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QUESTION: Dear Paul!

Its Agnimitra once again. Another two more questions to go-

1. What does it mean if the lift-to-drag coefficient ratio of an airfoil comes out negative?

2. If both coefficients are negative; and also one negative (say lift) but the other positive (say drag)?

Thanks in advance.
Regards!

ANSWER: 1.  It means you made a mistake or the lift is negative.  That is, the wing loading is downward probably because of negative angle of attack.

2.  What is the question.
Paul

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QUESTION: Actually, I was trying to predict lift and drag coefficients of an airfoil having twist at the trailing end for different angle of attacks and Reynolds numbers.
I was using fluent 6.0 software for that. I used Realizable k-epsilon turbulence model with enhanced wall treatment.

I got,for some angle of attacks, negative lift coefficients; and also for some, negative drag coefficients.
The combination of both the coefficients came out to be like- positive-positive, negative-negative and also positive-negative. I was wondering if it could be possible.

Should I take the absolute values of the coefficients always or not consider all such negative coefficients. Another point I would like to highlight is that the negative coefficients did not always come for negative angle of attacks; I have checked it.

Please help!

Answer
I suppose you could have sufficient airfoil twist that at some angles of attack the lift is negative.  The integrated load would be down or negative.  I can't imagine how you could get negative drag - that is not physically possible when considering the entire airfoil.

It is more likely that you are using the code incorrectly.  You should not take the absolute values.  Talk to Fluent.
Paul