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Old   April 7, 2012, 19:24
Default Lift Coefficient CFX
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David Delgado
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Hi everybody

I want to calculate the lift and drag coefficients of a NACA Profile. For that I made a 2D mesh in ICEM and run the calculation in CFX (saved as Fluent Mesh and automatically converted to 2.5 D mesh for CFX). In the CFD Post I have calculated the forces in X and Y direction, and with the angle of attack my drag and lift forces. My question is which area should I take to calculate the lift coefficient?. CFX caclulates the area of the Profile (2.5D Profile), but I've read that the area that I need is Chordlength* Span. This two values are different.

If somenoe can help me, I will be very thankfull

Best regads
David
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Old   April 15, 2012, 19:09
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Glenn Horrocks
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Drag and lift coefficients are defined on a reference area. For some applications (eg bluff bodies, like a car) the area is the frontal projected area. For others, generally streamlines bodies (eg airfoils) the area is the planform area.

So you have to deteremine the appropriate area for your application. Obviously if you are comparing to published results you have to match their definition.
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Old   April 15, 2012, 22:17
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David Delgado
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thank you very much for your answer. one last question. I am doing the simulations in a steady state. Till an angle of attack of 20 I get very good results, very similar to realized experiments. After this angle, the converguence oscilates in 0,001 and the lift and drag coefficients vary from the experiments. Do you recomend me to do in this case a transcient simulation?

Thank you for your time and answers

Best regards
David
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Old   April 17, 2012, 02:08
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Glenn Horrocks
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Simulations in the separated region are tricky to get accurate so do not expect easy answers.

You will certainly require transient flow, and may require an LES/DES/SAS approach if the flow is turbulent.
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