Swept Volume Theory

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Aug 2, 2022.

  1. latestarter
    Joined: Jul 2010
    Posts: 402
    Likes: 51, Points: 28, Legacy Rep: 233
    Location: N.W. England

    latestarter Senior Member

  2. Sailor Al
    Joined: Feb 2021
    Posts: 650
    Likes: 27, Points: 28
    Location: Sydney

    Sailor Al Senior Member

  3. latestarter
    Joined: Jul 2010
    Posts: 402
    Likes: 51, Points: 28, Legacy Rep: 233
    Location: N.W. England

    latestarter Senior Member

    Hi Al, I have only dipped into a couple of videos so far.
    He uses an App on some videos e.g.
    Sail Trim, Separation, and Stall


    I think I saw an actual wind tunnel video on another one but it was brief and probably less helpful than Babinsky's.
     
  4. Sailor Al
    Joined: Feb 2021
    Posts: 650
    Likes: 27, Points: 28
    Location: Sydney

    Sailor Al Senior Member

    Oh, that's disappointing. I thought you had proposed this series as a source for sailors to discover for the source of the lower pressure to leeward of the sail and higher pressure to windward.
    But when I find him talking about foils, flow fields and lift I realise that he's just another frustrated wing man!
    He uses some unidentified "wind tunnel app" to show a sail at zero AoA:
    upload_2022-11-18_8-45-26.png
    That is not a sail!
    Any sailor knows that a sail in that configuration would flap like a flag!
     
  5. gonzo
    Joined: Aug 2002
    Posts: 16,790
    Likes: 1,714, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Perhaps sailors should learn some math and physics to be able to understand the behavior of air flowing around a foil. When I started sailing in the late 60's sailors talked about the "wind tunnel" between the jib and main. It was supposed to accelerate the air flow at that area, in spite of all the physical evidence. I would look at the telltales on the leeward shrouds hanging limply between the jib and the main and wondered what they were talking about. Do you believe in the wind tunnel between the jib and main?
     
  6. Sailor Al
    Joined: Feb 2021
    Posts: 650
    Likes: 27, Points: 28
    Location: Sydney

    Sailor Al Senior Member

    You have clearly demonstrated many of the problems I also have with the current explanations!
    And like you, I am very confused about the interaction between the jib and the main. Certainly, looking at the telltales doesn't help explain what is going on!
    Which "wind tunnel between the jib and main" do you want me to believe in? (And please, don't offer a computer-generated simulation.)
     
  7. gonzo
    Joined: Aug 2002
    Posts: 16,790
    Likes: 1,714, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    As far as I know it doesn't exist. However, the winning sailors kept on talking about it.
     
  8. Sailor Al
    Joined: Feb 2021
    Posts: 650
    Likes: 27, Points: 28
    Location: Sydney

    Sailor Al Senior Member

    It goes to 63 pages! Where is the post that explains the source of the lower pressure to leeward of the sail and higher pressure to windward?
    Or the wind tunnel experiment explaining the interaction between main and jib?
     
  9. Sailor Al
    Joined: Feb 2021
    Posts: 650
    Likes: 27, Points: 28
    Location: Sydney

    Sailor Al Senior Member

    Well why are you asking me to believe in something that even you don't believe exists? I don't follow.
     
  10. Sailor Al
    Joined: Feb 2021
    Posts: 650
    Likes: 27, Points: 28
    Location: Sydney

    Sailor Al Senior Member

    Here's your post from the thread @Doug Halsey mentioned. I think you may have a good idea here. Did you develop it?
    The bit I like is that it separates pressure from wind speed.
    And I think you are agreeing that the wind feels slower in the slot. I have measured it with a hand-held anemometer, it is slower than the apparent wind.
    But we know from experience that the jib does make the main work better.
    I don't think we know why, but I think, if we stop linking wind speed and pressure, and just think about pressure, we'll get closer to the answer.
     
  11. Sailor Al
    Joined: Feb 2021
    Posts: 650
    Likes: 27, Points: 28
    Location: Sydney

    Sailor Al Senior Member

    Gentry claims to have invented the Circulation theory in 1975 which was after Talay/ NASA's Introduction to the Aerodynamics of Flight published it in the same year. It's nothing to do with Circulation.
    And anyway, I'm looking for a publication for sailors to understand sails, not wings!
     
  12. Sailor Al
    Joined: Feb 2021
    Posts: 650
    Likes: 27, Points: 28
    Location: Sydney

    Sailor Al Senior Member

    And no, it really isn't .
    His opening line is "The net forces on the boat are equal to the net change in momentum of the two fluids - that's conservation of momentum."
    That's "Momentum theory" which doesn't explain the source of the lower pressure to leeward of the sail of the higher pressure to windward.
     
  13. Sailor Al
    Joined: Feb 2021
    Posts: 650
    Likes: 27, Points: 28
    Location: Sydney

    Sailor Al Senior Member

    Here's a later post from the OP in that thread:
    I wonder what happened to this OP? I wish I had found this thread earlier.
    His view of the crazy theories propounded by the aerodynamics writers resonates soundly.
    Anyone who suggests that vortices are infinite is clearly smoking some strange weed, as is anyone who believes it's true. And that is the basis of the starting vortex theory that is still to be found in the recommended reading for university students.
    [EDIT]
    And if anyone doubts that it's still in the curriculum, I refer you to Anderson's Fundamentals of Aerodynamics Section 4.6, Page 344 of Sixth Edition:
    "However, the starting vortex has already been formed during the starting process, and it moves steadily downstream with the flow forever after." (my emphasis) [/EDIT]
     
    Last edited: Nov 17, 2022
  14. gonzo
    Joined: Aug 2002
    Posts: 16,790
    Likes: 1,714, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    The jib, particularly overlapping ones, by cheating the sail area formula. It also help balancing the CE by separating the foils. A second use, that may be advantageous, is that it deflects the air flow and the main can be sheeting tighter. However, a main alone can be more efficient but only at a narrow angle of sail.
     

  15. Sailor Al
    Joined: Feb 2021
    Posts: 650
    Likes: 27, Points: 28
    Location: Sydney

    Sailor Al Senior Member

    You are making four interesting points:
    1. The jib, particularly overlapping ones, by cheating the sail area formula.
    2. It also help balancing the CE by separating the foils.
    3. it deflects the air flow and the main can be sheeting tighter.
    4. a main alone can be more efficient but only at a narrow angle of sail
    4. is clearly wrong. Try it. A sloop is never better upwind without its jib.
    3. may be true, but needs a lot of work.
    2. What??
    1. which sail area formula?

    What about your really useful post from 2003?
     
    Last edited: Nov 19, 2022
Loading...
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.