Skin Friction Lines - Some Comparisons

Discussion in 'Hydrodynamics and Aerodynamics' started by Leo Lazauskas, Feb 15, 2013.

  1. jehardiman
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    jehardiman Senior Member

    Sorry I'm getting into this late,but before we get too carried away, lets put some perspective up.

    A swimmer is Rn 1x10^6
    A 3m row boat is Rn 7x10^6
    A 10m sail boat is Rn 3x10^7
    A 10m power boat is Rn 9x10^7
    A 100m ship is Rn 6x10^8
    A CVN is Rn 3x10^9

    As far a Naval Architecture is concerned anything below 1x10^7 is not of interest and realisticly for commericial applications anything below 1x10^8 is a "just throw power at it" problem. Throw in wave orbitals which cause an order of magnitude change over the length of a ship and the fact that there is no laminar flow on rigid bodies in open water above 1x10^6 and you can see that this will devolve quickly into bottle washing and button sorting.
     
  2. Remmlinger
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    Remmlinger engineer

    Just take a yacht with a length of 10m, which has a fin keel with a chord length of 0.8m. The yacht is sailing at 4 Knots. Now you want to test this case in a towing tank with a model of 2m length. You measure the total force and you want to subtract the viscous resistance of the keel to get the resistance of the canoe body. How would you calculate the viscous resistance of the keel? The chord based Reynolds number for the model is in this case 1.4 * 10^5
     
  3. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks, I guess, for bringing reality into it. :)

    But for some us non-NA's there are many applications where it
    does matter. I have interests in under-water turbines, struts, fins
    and keels, and UUV, and almost no interest in laminar flow.

    On a slightly related note, have you come across any measurements of
    the kinematic eddy viscosity of water in the open sea? From what I
    can gather it can vary by up to 6 orders of magnitude.

    Yeah well you know? That's just like your opinion man.
    I thought a NA's primary function was to provide adequate free-board.
     
  4. Remmlinger
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    Remmlinger engineer

  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks, Uli.
    I knew of it but I didn't have it. It seems to be an earlier version of some
    papers that I (and you) already have by those three authors. I think some
    of the work on the wake parameter (Uppercase Pi) have been reconsidered
    in the later works.

    If you are collecting papers, I would also look at some more recent work by
    Ramis Orlu. His thesis discusses much of the same stuff and includes some more experiments.
    http://www.mech.kth.se/mech/info_staff.jsp?ID=198
     
  6. jehardiman
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    jehardiman Senior Member

    I know that there was some ADCP and LDVL data taken in the littorials in the last few years. I bet if you were to go ask for some raw REMUS ADCP data you would see it. Processed data won't work but the raw feed (like for Teledyne RDI waves array) will.

    Nope, it is to give the captian a good place to stand and be seen when sailing into port.
     

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  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Perfect, jeh. "REMUS" was the key to narrowing down the search.
     
  8. Mikko Brummer
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    Mikko Brummer Senior Member

    I have been thinking so but can you give some evidence for the claim?
     
  9. haribo
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    haribo Junior Member

    IMO this "fakt" is only correct for stady flow over straight surfaces

    not necessarily for flow with acceleration (speedup) thus unstady flow and
    not necessarily for also rigid but curved bodies

    centrifugal forces may hold the waterparts longer in a laminar pathway....
     
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  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    And slight oscillations in surge, yaw and pitch, as well as ambient waves (small
    chop or large swell) and turbulence could prevent that flow from staying laminar.
    It is a very complicated matter! :)
     
  11. haribo
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    haribo Junior Member

    and the fast not rigid-body swimmer in the water are quite clever in uses their benefits in this case
     
  12. Mikko Brummer
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    Mikko Brummer Senior Member

    Does somebody know what would be a typical turbulence intensity in the sea surface layer?
     
  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Follow jehardiman's suggestion in an earlier post and search for REMUS etc.
    There are some papers and graphs that show how it varies with depth, salinity,
    upwelling, proximity to shore etc etc.
     
    Last edited: Feb 21, 2013
  14. Mikko Brummer
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    Mikko Brummer Senior Member

    You would look up the corresponding cdp of your keel profile, at the given Re and given CL. I seem to use in my VPP

    Cdp =15/(KeelRe)^0,2*(0,008+0,017*(RootChord+TipChord)/2+0,0042*CL^2)

    (don't ask where it comes from)
     

  15. Mikko Brummer
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    Mikko Brummer Senior Member

    Thanks.
     
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