Xfoil vs javafoil?

Discussion in 'Hydrodynamics and Aerodynamics' started by sigurd, Jul 12, 2011.

  1. sigurd
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    sigurd Pompuous Pangolin

    Hi, I ran Tspeer´s P30108 (check out his .com domain) through javafoil and no matter what I do to transition model, Number of CRITters (surface finish), I can´t get the drag bucket within twice the drag of the xfoil model. How come?
    Xfoil (modeled by Tspeer): [​IMG]
    Javafoil:
     

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  2. daiquiri
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    daiquiri Engineering and Design

  3. sigurd
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    sigurd Pompuous Pangolin

    Great reading, thanks..
     
  4. sigurd
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    sigurd Pompuous Pangolin

    Is there any quick way to convert stuff like
    0.0 0.0
    ...........
    1.0 0.0

    0.0 0.0
    ............
    1.0 0.0

    to the format that xfoil accepts which is, apparently
    1.0 0.0
    ......
    0.0 0.0

    0.0 0.0
    .............
    1.0 0.0

    I think it would involve just inverting the order of one half of the lines. Many of the UIUC foils are useless to xfoil and javafoil.
     
  5. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    I use Excel for that purpose. The steps are:
    - import the data from a .txt file containing foil coordinates (if you have .dat file, simply change the filename suffix into .txt) into Excel
    - invert the rows (there's a command in Excel for that task: http://office.tizag.com/excelTutorial/excelsorting.php)
    - save it as a new .txt file
    - change again the suffix from .txt into .dat​
    and you get the XFoil-ready foil coordinates file. :)
    Cheers
     
  6. sigurd
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    sigurd Pompuous Pangolin

    great, thanks.. used "data: sort: by column" in open office (free excel clone). works a charm.
     
  7. sigurd
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    sigurd Pompuous Pangolin

    Hi daiquiri, I have a text editor called notepad ++. It is free and have a zillion functions and is quick so I use it all the time. I just found the (textFX) "sort" function. It is even quicker for flipping foil coordinates, no name change or import export.

    What are the xfoil Ncrit counterparts to Javafoil "smooth", "painted fabric", "NACA standard", "bugs and dirt"?

    Javafoil is a lot less picky, and brings a solution, whereas xflr sometimes won't converge on most AoA or simply not start the analysis. Or am I using it wrongly?

    What do you use to smooth out kinks? A lot of the rotorcraft foils at UIUC have a little bump on the upper surface. Are these typos or intentional BL trips? For example RAE2822 has a bump on the LE and a dimple on the middle of the upper surface.
     
  8. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Thanks for the tip about Notepad++. I might check it out, when I find some time for that. :)

    As about bumps on the airfoil surface, there should be none, so it could be a typo. This is what RAE2822 airfoil looks like, the bumps in the pic are just pressure probes for the boundary-layer examination: http://www.grc.nasa.gov/WWW/wind/valid/raetaf/raetaf.html

    There is a very active group at Yahoo dedicated to Xfoil-related exchange of info: http://groups.yahoo.com/group/xfoil/
    I think you chould join the group and benefit from the other people's knowledge and experiences. I know that Tom Speer is one of them, and he is an Xfoil power user, a living enciclopedia about the software usage (possibly second only to Xfoil's creator Mark Drela, of course). So perhaps you should address these questions to him. :)

    Always check out if the "solution" given by Javafoil corresponds to known experimental data. I've found on several occasions that the drag curve from Javafoil, for example, had a very different shape from what one can find in several reference texbooks with experimental data. So I'm generally not overly confident of Javafoil's accuracy.

    Cheers
     
  9. sigurd
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    sigurd Pompuous Pangolin

    Been a member for several years, since I planned to learn xfoil, but I always gave up on it (lazy), until I found xflr5. I'm sure most of my noob questions have been answered there, 10 years past.

    I'm going to experiment a bit to get rid of the kinks - the ghetto way is to reduce number of panels, then increasing them again.

    Sure, I've no idea how much I can trust either one of the simulators.
    I just found NLR-7223-62, xflr showed CDmin less than 0.001 at re 1.5million - is that even possible? Would be nice if I could get an idea of what is, and is not reasonable results. I guess I'll be looking around for empirical data.
     
  10. sigurd
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    sigurd Pompuous Pangolin

    Hi again,
    Why don't I get the shoulder in the upper surface cp at 0.5 chord that they got in the link of yours?
     

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  11. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    I understand that the picture above is taken from an example of the numerical simulation of a transonic flow (M = 0.73) it means that there will probably be a shockwave on the upper side of the airfoil, which was captured by the finite-element CFD sofware used by authors. XFoil, on the other hand, cannot handle shockwaves. It will automatically apply the Karman-Tsien compressibility correction to local Cp, but the results are valid only if the local Mach number is less than 0.70-0.75 .
    In the case of cited validation work, local Mach number spikes are surely supersonic (freestream M = 0.73) , so XFoil is useless here.
    Cheers
     

  12. sigurd
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    sigurd Pompuous Pangolin

    Ah, ofcourse - thanks again.
     
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