Flowfield calculations in javafoil

Discussion in 'Hydrodynamics and Aerodynamics' started by johan gronvall, Dec 30, 2014.

  1. daiquiri
    Joined: May 2004
    Posts: 5,372
    Likes: 239, Points: 73, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Humberto, I truly think that you have done a terrific job with this interfacing of your software with the Javafoil.

    However, you have to pardon me if I insist that there is an incongruence between the outputs shown in the post #21. I am doing it in order to understand what's going on. If my comments are right, you will have an improved analysis tool after we have ended this discussion. If I am wrong, I will have learned something new. It is a win-win situation for both of us. :)

    So, let's take the two bottom-left curves from your post #21 and make an overlay. Here it is:
    Lift distribution comparison.jpg
    The Autocad's "Area" tool says that the net difference between smooth and sawtooth areas is approximately 5%. So, IMO, the difference in lift coefficients of the finite wing (or keel) has to be 5% too. And you say that currently it is just 1%. Something doesn't add-up here. Sure, it remains a rather small difference, but if you want to use that tool for the optimization of keels you need to be sure that the integration module of the software works properly.

    In fact, the overlayed finite-wing characteristic curves are almost identical:
    Aero curves comparison.jpg

    So this has to be sorted out, IMO.
    Perhaps I am wrong, and perhaps I am missing some other info here. But I really think that you should look deeper into this before fully relying on the results of these calculations. Once you have found and fixed the glitch, or (if there is no glitch) once you've found a valid explanation for this behavior, you will have a great tool for the analysis there. :)
    Cheers
     
  2. Humberto
    Joined: Mar 2008
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    Location: Spain

    Humberto Junior Member

    Let me re-check and I will try to summarise and post a document with the equations
     
  3. tspeer
    Joined: Feb 2002
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    Likes: 185, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    I think you may want to work on the distribution of your points a bit more. You appear to be distributing the points according to the curvature of the contour. That's not a bad approach, but I think you could use some more points toward the trailing edge. And there are some abrupt changes in point density. Maybe an approach that sets a lower bound on point density would help.

    The attached file shows how XFOIL distributes the points on a similar section.
     

    Attached Files:


  4. Humberto
    Joined: Mar 2008
    Posts: 47
    Likes: 2, Points: 0, Legacy Rep: 55
    Location: Spain

    Humberto Junior Member

    Sure. I am ware of this, but I do the sampling directly from a NURBS surface, and the procedure I follow is curvature dependent. I am going to check the knots distribution in the trailing edge. I will try to follow your suggestion
    Humberto
     
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