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-   -   Numerical calculation of Cl, Cd, Cm from Cf and Cp (http://www.cfd-online.com/Forums/main/124768-numerical-calculation-cl-cd-cm-cf-cp.html)

mmisu October 12, 2013 11:39

Numerical calculation of Cl, Cd, Cm from Cf and Cp
 
Hello,

I have a boundary layer code coupled with an inviscid solver that gives me the pressure coefficient and the skin friction coefficient in "s" (a coordinate along the airfoil that starts from the stagnation point).

My question is how can I correctly integrate Cp and Cf in oder to get Cd, Cl and Cm. I've found some formulas in "Fundamentals of Aerodynamics" by Anderson. However, these formulas are in terms of Cartesian coordinates and suggest the integration from the leading edge to the trailing edge.

1. Is it correct to assume that these formulas must be applied on the two parts of the airfoil obtained after we find the stagnation point and split the airfoil in two parts ?

2. Or should I transfer Cp and Cf back to Cartesian coordinates and integrate on the geometrical lower and upper side ?

Thanks for your understanding and answers for what could be a basic question.

mmisu October 14, 2013 00:34

Solved
 
You could use the formulas from Anderson to integrate Cp and Cf on the airfoil in a simpler way:

1. Walk the airfoil from the TE, lower side, LE, upper side, TE. This is the way classical panel method store the airfoil geometry.

2. Keep only the terms with "u" indices from the Anderson formulas. This simplification will avoid the need to split the data in upper/lower side.

aalisha March 31, 2014 23:56

Hey can you please help me how to calculate Cp if I have cl, cd and cm. Please I am kind of new in the field. :)

lovecraft22 April 1, 2014 16:45

Quote:

Originally Posted by aalisha (Post 483122)
Hey can you please help me how to calculate Cp if I have cl, cd and cm. Please I am kind of new in the field. :)

If you have the Cp then you have the pressure. If you have the pressure you can integrate on the surface and multiply it by the surface local normal. This will provide you a vector for the two/three components of your simulation.

aalisha April 4, 2014 03:22

Quote:

Originally Posted by lovecraft22 (Post 483303)
If you have the Cp then you have the pressure. If you have the pressure you can integrate on the surface and multiply it by the surface local normal. This will provide you a vector for the two/three components of your simulation.

Thanks for your kind reply. However, I think there is a small confusion, I was to calculate Cp (power co-efficient) as I am trying to study the performance of a turbine.

How to use Cl, Cm, Cd to finally get the power co-efficient

cfdivan April 6, 2014 16:26

Hi aalisha,

There are some empirical formulas for the power coefficient of lift based machines:

http://upload.wikimedia.org/math/3/4...4223e5428b.png


where

http://upload.wikimedia.org/math/4/c...5131eaee1a.png


and lambda as TSR.


You may be able to find values lower than 0.6 (Betz limit, I guess)


Regards,

aalisha April 7, 2014 02:18

Thank you so much for your help. :) ! Just for knowledge does coefficient of moment also have some relation with power coefficient. :p


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