# How does "lengthInt(<Expression>)" fuction work???

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 October 15, 2012, 10:01 How does "lengthInt()" fuction work??? #1 Senior Member   mohammad Join Date: Dec 2010 Location: Seoul, South korea Posts: 215 Rep Power: 10 Dear all, I have the results of a wind turbine's blade. I have created an airfoil-shape polyline (bye cross-sectioning the blade with a plane) I can easily know the pressure distribution values on this polyline and also I can calulate the Cp. Theoritically, If I want to use this Cp to calulate the "Lift" and "Pressure Drag" I need to calculate the integral of Cp. However, the CFX-Post itself has a command " lengthInt". As read from the command description, it calclualtes the weighted integral of a quantity over the line( in my caus: the airfoil) Now my question is thatI want to use this CFX-Post functions to directly calculate the force ( or force/lenght). Do you have any idea that this will be Lift force? or do you have any solution how to get the CL from Cp distribuion over the airfoil? Regards, Thanks in advance

 October 15, 2012, 17:32 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,214 Rep Power: 110 You cannot integrate pressure over a line. As a line has zero area the force is zero. Extrude your polyline to make a surface, or take a cross section of your airfoil to do this.

October 15, 2012, 23:19
#3
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Join Date: Dec 2010
Location: Seoul, South korea
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Rep Power: 10
Quote:
 Originally Posted by ghorrocks You cannot integrate pressure over a line. As a line has zero area the force is zero. Extrude your polyline to make a surface, or take a cross section of your airfoil to do this.
Dear Glenn,
I is possible.
for example, if you create a polyline (e.g. ployline1) then you can use the following command line in an expression:

lengthInt(pressure)@ polyline1

it gives the length integral of the pressure. Thus its dimention is ([N m^-2][ m]=[J m^-2]

However I am not sure about correctness of this integral over a close loop(airfoil).

Regards,

Last edited by mohammad; October 16, 2012 at 06:32.

 October 16, 2012, 06:29 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,214 Rep Power: 110 I guess you are right, it returns units of N/m which is force per unit length. So getting CL from this is straight forward.

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