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Problems finishing implementation of 2D Linear-Strength Vortex Method (Airfoil)

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Old   August 30, 2015, 15:53
Default Problems finishing implementation of 2D Linear-Strength Vortex Method (Airfoil)
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Alexandros Kontogiannis
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Hello everyone,

I would really appreciate any help to complete my script implementing a linear-strength vortex method as described by Katz and Plotkin in Low Speed Aerodynamics (Section 11.4.2 and Program no.7 in Appendix). This is my first script implementing a panel method and I have faced some (not only)programming difficulties.

So, I was trying to get a Cp plot of an airfoil but I always received a random fuzzy line (just garbage). It's been 3 days now that I'm trying to correct this and still no luck. BUT, in my surprise, I gave as input just the upper surface of the airfoil and I received the true Cp plot and the same for the lower. That being said, I can compute the two airfoil surfaces alone but if I input the full airfoil.dat file I get garbage.

Are the upper and lower vortex-sheets independent? The collocate points of the lower vortex-sheet should not be affected by the vortices of the upper vortex-sheet (and vice-versa)? It was my idea that the airfoil works as one vortex-sheet alone.. do I have to separate them?

Thanks in advance!

The links below correspond to the photos of the Cp plots with the two surfaces alone (correct) and the airfoil (fuzzy line - garbage):

Upper Surface Cp & Airfoil (S1210)

Lower Surface Cp & Airfoil

Airfoil Cp

Last edited by al3x; August 31, 2015 at 15:45.
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Old   August 30, 2015, 18:58
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You clearly have a bug. Yes, the entire vortex sheet system should represent the body in question (not just its constituent parts). Are you sure you have a consistent coordinate system and your local grid-normals all point outward (or inward) all around? Have you tried a simpler problem (say potential flow over a cylinder) and looked at the matrix values? For example, are the row sums (or column sums depending on implementation) of the matrix giving you correct values?

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Old   August 31, 2015, 15:31
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Alexandros Kontogiannis
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Yes adrin, I have evaluated the results with the vectors pointing outwards and inwards and all the other possible arrangements of normal and tangential vectors and I really can't make it work.. For evaluation I check the vortex-strength solution on a symmetrical airfoil for symmetry and I run in another tab of the terminal xfoil to crosscheck the Cp plots.

I almost even copy the Program no.7 (p. 562 ->Appendix of Katz and Plotkin "Low Speed Aerodynamics") and still doesn't work. Can anyone validate if this program works in Fortran (with a copy-paste from book) and let me know?

I will provide a link to the script if it's possible for anyone to check it and let me know of possible wrong implementations.

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Old   September 1, 2015, 12:22
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This looks like a simple numerical issue to me. I created a similar program for part of my thesis work a couple years ago. Check your math. I will look around later this evening and see if I can track down the code. Have you implemented any boundary layer methods yet or is this potential flow?
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Old   October 5, 2017, 10:56
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I am facing a similar problem.

I am trying to develop a linear vortex panel code, following Katz and Plotkin. I am getting fuzzy CP distribution.

al3X were you able to solve this issue?
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airfoil, panel, sheet, vortex

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