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

al3x · August 30, 2015, 16:53

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)
https://drive.google.com/open?id=0B8...0V4TlViYkNGT3c

Lower Surface Cp & Airfoil
https://drive.google.com/open?id=0B8...XQzZV8zbmkzV28

Airfoil Cp
https://drive.google.com/open?id=0B8...FVnZTNBNGRqSG8

adrin · August 30, 2015, 19:58

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?

adrin

al3x · August 31, 2015, 16:31

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.

Script:
https://drive.google.com/file/d/0B8G...ew?usp=sharing

fluid23 · September 1, 2015, 13:22

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?

ManojM · October 5, 2017, 11:56

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?

August 30, 2015, 16:53	Problems finishing implementation of 2D Linear-Strength Vortex Method (Airfoil)	#1
al3x New Member Alexandros Kontogiannis Join Date: Dec 2014 Location: Athens Posts: 2 Rep Power: 0	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) https://drive.google.com/open?id=0B8...0V4TlViYkNGT3c Lower Surface Cp & Airfoil https://drive.google.com/open?id=0B8...XQzZV8zbmkzV28 Airfoil Cp https://drive.google.com/open?id=0B8...FVnZTNBNGRqSG8 Last edited by al3x; August 31, 2015 at 16:45.

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August 30, 2015, 19:58		#2
adrin Senior Member adrin Join Date: Mar 2009 Posts: 115 Rep Power: 17	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? adrin

August 31, 2015, 16:31		#3
al3x New Member Alexandros Kontogiannis Join Date: Dec 2014 Location: Athens Posts: 2 Rep Power: 0	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. Script: https://drive.google.com/file/d/0B8G...ew?usp=sharing

September 1, 2015, 13:22		#4
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	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?

October 5, 2017, 11:56		#5
ManojM New Member Manoj Join Date: Oct 2017 Posts: 1 Rep Power: 0	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?