Skin Friction Lines - Some Comparisons

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

  1. jehardiman
    Joined: Aug 2004
    Posts: 3,035
    Likes: 549, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Go read Hoerner's discussion on boundary layer tripping as a function of velocity fluctuations in Fluid Dynamic Drag Chapter 2, Section 6.

    Nope, see the discussion above. Hoerner used data from plates, airships and spheres. Come on guys, if your laminar data comes from a wind tunnel with a 0.005% rms velocity fluctuations, real life performance will be different as you drive through seaweed, whale snot, and jetski wakes as Leo correctly points out below.

     
    Last edited: Feb 22, 2013
  2. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 150, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    Keep in mind what you are trying to do here and what it
    will be used for.
    If you are interested in the fundamental physics of the
    problem then differences of 1% are very important.
    If you are only interested in the engineering aspects then
    do what (I imagine) Petros did: take the stogie out of your
    mouth, point at the 7%-10% scatter in predictions and get
    back to work on the important aspects of the design. :)
     
  3. Mikko Brummer
    Joined: May 2006
    Posts: 547
    Likes: 67, Points: 28, Legacy Rep: 258
    Location: Finland

    Mikko Brummer Senior Member

    To answer my own question, from the looks of the equation I would bet it comes from Hoerner. This is only the profile drag, you also have to allow for induced drag, a function of effective draft. Effective draft is difficult to estimate, probably the most difficult/important factor in a VPP after the effective sailing length.

    As David Cockey elsewhere notes, you should remember that profile drag depends on CL - I was looking at the ORCi VPP, which appears to ignore that: see attached ORCi VPP appendage viscous drag. The canoe body viscous drag is still handled in the ORCi VPP by the ITTC 57-line... (see 2nd attach.)

    ORCi could have included the CL-dependent part of appendage drag into the induced drag part, but I could not find evidence for that. The ORCi effective draft is quite a construction by itself.
     

    Attached Files:

  4. DCockey
    Joined: Oct 2009
    Posts: 4,997
    Likes: 488, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    That formula includes a CL^2 term so it tries to account for the increase in viscous profile drag due to lift. However the coefficient is not universal. It probably originated as a "best fit" to some experimental data.
     

  5. sigurd
    Joined: Jun 2004
    Posts: 827
    Likes: 8, Points: 18, Legacy Rep: 65
    Location: norway

    sigurd Pompuous Pangolin

    All I know is that it becomes worse in a headwind
     

    Attached Files:

Loading...
Similar Threads
  1. rwatson
    Replies:
    38
    Views:
    6,163
  2. Slip
    Replies:
    3
    Views:
    1,328
  3. designing
    Replies:
    5
    Views:
    1,756
  4. Shayne Young
    Replies:
    12
    Views:
    2,871
  5. rwatson
    Replies:
    8
    Views:
    2,850
  6. dustman
    Replies:
    53
    Views:
    6,989
  7. jesdreamer
    Replies:
    88
    Views:
    15,961
  8. nine6
    Replies:
    5
    Views:
    1,579
  9. hprasmus
    Replies:
    11
    Views:
    6,118
  10. Leo Lazauskas
    Replies:
    50
    Views:
    8,297
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.