Boundary-layer trip

Discussion in 'Hydrodynamics and Aerodynamics' started by Remmlinger, Oct 20, 2013.

  1. Remmlinger
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    Remmlinger engineer

    I use Rhino to calculate all the necessary hydrostatics from the IGES-files.
    The many leak-points are truly annoying.
    The input to my boundary-layer calculation is an extension of UliLines.

    I have just tried the following that might work:
    Read the IGES-file in Rhino
    export a VRML-1.0 file
    read this VRML file into Freeship
    under file-coordinates you can export a text file with x/y/z coordinates.
    I have not checked the quality yet.
    Delftship also has the "coordinate" option

    Uli
     
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  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    After much frigging around I've answered my own question to
    some extent. I needed to add a starboard side and then remove it.
    I'm not really sure why it worked but it would be a big effort
    to convert all DSYHS to Michlet or other useful formats.
    I'll try to produce some resistance curves to see how they match
    with Uli's Figure 12.

    Uli: Just some sunspots I noticed when reading the paper...
    The legend of Fig. 12 has a typo. It should be "residuary", not "resuduary".

    In Figure 7, I think you would be justified in removing one "sour" point at
    2200, 6000, as long as you noted that you did it in the text.
     
  3. Remmlinger
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    Remmlinger engineer

    Thanks !!
    I am flattered that you read my paper so carefully
     
  4. tspeer
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    tspeer Senior Member

    I haven't been working with DSYHS files, but I've been using Rhino to generate offsets for Michlet. My method is pretty labor intensive and agricultural, but it works.

    I put a Bounding Box around the surface, Explode it, and Ungroup it, then delete all the lines except for one rectangle around the surface. I Divide the edges of the box into 20 segments to provide guides for taking the offsets. The next step is to Section the surface vertically and horizontally, using point-snap to select the points on the bounding box generated by Divide as the start and end points for the Section command. Finally, I take the Intersection of all the sections to generate the offset points. Those I can Export to a text file. The last step is to use Excel to sort the points, extract just the Y values, and fill in the missing offsets with zeros.

    It ain't pretty, but it gets the job done. The general process would probably work with just about any CAD program.
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks, Tom, but us long-term "dole bludgers" can't afford Rhino.

    I think I can get some decent hulls to work with for my purposes. I only need
    the hull offsets a little way above the design WL so Flotilla can sink and trim
    the hull.
     
  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The attached graph show Michlet predictions for the total resistance
    of the DSYHS Model 72. The hull is a bit beamy for thin-ship theory,
    and there are difficulties with viscous effects and BL trips as Uli
    has noted.

    Rtexp is the measured resistance in Newtons.

    Rtmichlet is the predicted total resistance using Grigson's line for
    the skin-friction. It is clear that the resistance is over-predicted.

    Rtfunny is the predicted resistance using Grigson's line, No
    transverse wave resistance, and 0.81*Rwdiv, where Rwdiv is the
    diverging wave resistance. It would be quite amusing if these sort
    of manipulations with form factors on the wave resistance were
    elated in some way to hull geometric parameters.

    In many of his research papers, L.J. Doctors uses multiple form
    factors on the skin-friction and wave resistance. His wave
    resistance form factors are often less than 1.0, typically around 0.9.
    Here I have extended that idea, possibly to the point of silliness!
    On the other hand, it is well known that viscosity can have marked
    effects on the bow-wave, stern wave interference, especially for
    small hulls: for example, the usual large humps and hollows in
    the wave resistance coefficient curve are absent.

    When I get time I'll run the hull through Flotilla to see how
    predicted sinkage and trim compare to measured values.

    Thanks for giving us something to think about Uli. (I can find faults
    easily in other people's reports - it's mine that I am blind to!)
     

    Attached Files:

  7. daiquiri
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    daiquiri Engineering and Design

    The incredibly close match of the tweaked Michlet prediction curve to the experimental resistance curve raises more than one question regarding the physical meaning of the applied tweaks, imho.
    Leo, it would imo be interesting to see the same graph completed with the frictional resistance, and perhaps a view of the linesplan of the Delftship model too. ;)
     
  8. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I'll see what I can do. It's 2am here, but I'll push on :)

    The physical meaning could be similar to the infinite hollow trailing behind
    transom sterns in some ship wave theories, except here the "hollow" is
    filled with a viscous fluid that damps out waves. As I said in the original
    post, it would be amusing if some geometric basis could be found for these
    "wave form factors".
    I came up with the tweak because I am working on similar problems with
    transom sterns. Some people apply a large form factor increasing skin
    friction. I am of the opinion they should only be applied when the
    transom is wet, and that viscous form factors on a dry transom are
    meaningless because there is, in a sense, no BL separation.
    What can I say: every guy needs a hobby and I don't have a shed :)
     
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Here you go, Slavi.
    By the way, in my comment about finding a basis for the wave form factors I meant
    the whole series of hulls, not just this one particular instance.
     

    Attached Files:

  10. tspeer
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    tspeer Senior Member

    I used Rhino as an example, because that's what I'm using. But the principle of intersecting section lines should be something that could be adapted to free CAD software.

    I'm thinking maybe I should modify my technique to add planar surfaces with zero offset to fill out the rectangle before I compute the section lines. That would result in a full set of points and save me time in Excel.
     
  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks again, Tom.
    There is an annoying "feature" in Defltship known as "leak points". If these
    occur anywhere below the DWL the program won't allow hydrostatics to be
    calculated. Sometimes they can occur when lines are out by 1mm in 2000
    metres! Adjusting hundreds of them in 70 odd files is something I am
    not going to do.

    It might be easiest for me to write a simple program to take the x,y,z values
    supplied for the 70 odd DSYHS hulls and generate michlet offsets from them
    directly.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    That's the main problem I have with all translation from one program into another. When i dump lines into my Wolfson stability program, the number of lines becomes an issue and how they are produced...ggrr! Once in, it's fine. Similarly when dumping lines in my FE program..millions of points literally micro millilitres apart, yet each one is registered, which means removing them...wasting time!

    Some outputs can be tweaked to proved less accuracy, in the sense, less number of points. Which for dumping into my FE program, makes it easier. But I suspect that may remove "your" accuracy that you wish to keep?

    In the end i select a number of points on each curve, and remove the rest. It makes for a non-production faired hull, but, it wont really make any real difference to my outputs whether stability/stress related. I suspect the same would be true of Michlet? It is still time consuming though, even though I am very ruthless with deleting/remove vast number of points.

    Sounds like a plan :D
     
  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    It's not a huge problem for Michlet because it is based on integrals
    which are smoothing operations. Programs that depend on derivatives
    (or pressure integrations) would be badly affected.

    I think I misled myself with how easy it would be to reproduce some
    of Uli's results for the DSYHS because I assumed Delftship, which
    comes from the same Uni, would be able to read the files without
    needing any further tweaking. Silly me!
     
  14. DCockey
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    DCockey Senior Member

    Remember that points input into DelftShip are the locations of control points, and resulting hull surface in general will not pass through the points. Whether this is a concern depends on the shape of the hull, number of control points and desired level of accuracy.
     

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Sure.
    But Delftship also allows export of the points of the actual surface in a variety
    of formats. I was hoping that somebody had an easy way of going from reading
    an iges file to exporting a table of offsets or a Michlet file.

    Sorry for the late edit, but I thought I should add that Michlet produces files
    that Delftship can read. They are actual points on the surface, not control
    points.
     
    Last edited: Oct 24, 2013
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