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-   -   How does "lengthInt(<Expression>)" fuction work??? (http://www.cfd-online.com/Forums/cfx/108121-how-does-lengthint-expression-fuction-work.html)

mohammad October 15, 2012 10:01

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

ghorrocks October 15, 2012 17:32

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.

mohammad October 15, 2012 23:19

Quote:

Originally Posted by ghorrocks (Post 386772)
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,

ghorrocks October 16, 2012 06:29

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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