Vortices as source of aerodynamic force?

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Mar 26, 2021.

  1. CT249
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    CT249 Senior Member

    Sailor Al, why do you think your diagram is better for learning and sharing ideas about how to make a boat go faster than Tom's diagram?

    Personally, I find it very much more effective to consider lift and drag as separate factors. Sometimes we want to maximise one at the expense of the other, and using the concept separately makes it easy to consider the tradeoffs. This can be particularly useful when changing from one craft to another; it's easy to remind yourself that with low drag hulls you should also shift to a lower drag/lower lift rig trim, and vice versa.

    I still can't see why you don't like to use the term aero drag, since it appears to me that it's something that I can physically feel when holding a windsurfer rig that is not sheeted in at an angle of attack. Something that can be extremely significant in many efficient craft, in which aero drag is probably the major drag force, is surely something we should consider.
     
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  2. Sailor Al
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    Sailor Al Senior Member

    Precisely because it avoids getting into silly discussions like this.
    What you are feeling is the net aero force of the wind on the sail.
    There is only one force resulting from the wind on the sail(s).
    You can't feel "aero drag", and you can't detect "aero lift".
    You can detect leeway/heel by observing the heel of the boat and the amount you have to hike out to keep it flat, and, if you are observant, by watching the the amount of leeway by watching the angle of your wake.
    You can detect thrust by watching the boat speed: more thrust, more speed.
     
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  3. Sailor Al
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    Sailor Al Senior Member

    And anyhow, it's hard to get two two people to agree on the definition of "aero lift".
    Is it the component of net aero force resolved perpendicular to a) the chord of the sail or b) the incoming airstream, and if b), is that b.1) the apparent wind angle (AWA), or b.2) the AWA minus the sheeting angle, and anyhow, is the AWA relative to the C/L or the direct of travel (including the leeway angle)? And what about the upwash angle?
     
  4. DCockey
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    DCockey Senior Member

    In aerodynamics literature "lift" has a clear and unambiguous standard definition. Lift is the component of aerodynamic force normal to the direction of the oncoming flow. The oncoming flow direction is as seen by an observer traveling with the airfoil, wind or other object, not as seen by an observer fixed relative to the earth. This definition of lift is independent of the direction of the chord (and the definition of "chord") or how the angle of the oncoming flow is measured. This is the definition of lift used by Marchaj in Sailing Theory and Practice and his subsequent books including Aero-hydrodynamics of Sailing. Marchaj also refers to lift as the "cross-wind force". Similarly the standard definition of "drag" in aerodynamics literature is the component of aerodynamic force parallel with the direction of oncoming flow.

    My opinion: "Lift" and "drag" are useful when discusing the aero-hydrodynamics of sailboats because the allow the use of knowledge of aerodynamic forces from the general aerodynamics literature.

    On a forum such as this one the defintion of a term such as lift can be anything an individual poster wishes it to be. But use of non-standard defintions frequently leads to confusion.

    Sailor Al, why don't you just ignore any discussions involving the use of "lift"?
     
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  5. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Defining "lift" as the force perpendicular to the apparent wind-direction and "drag" as the force in the direction of the apparent wind is very practical and straight-forward when you determine these forces in e.g. a wind-tunnel measurement or when you calculate these forces using some computer-program, e.g. X-foil. In the case of a wind-tunnel measurement you usually have the parts of the sailing yacht above the water isolated, so you have no info about the leeway angle and adjust the apparent wind-direction by rotating the model, so it is therefore practical to orient the reference frame to the apparent wind, that is defined by the wind-tunnel walls.
    When you later apply the results on the sailing yacht, moving on the water, you can easily transform the aerodynamic force to forward thrust and heeling force, that may be most practical in that context.
    One sailing context where it is reasonable to consider just lift and drag is ice-sailing, where you usually go some 3 or 4 times the true wind, so the apparent wind-angle is some 15°, i.e. much smaller than for most sailboats. Here the speed is almost proportional to the L/D ratio. Also the current America's Cup class sails at some 3 times the true wind, so lift and drag is also relevant in that context.
     
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  6. Sailor Al
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    Sailor Al Senior Member

    Can you give such an example in the context of sail trimming, rig tuning or match racing please?
    I have no direct experience of wind tunnel testing, I'm just an amateur sailor trying to find a lexicon that my peers intuitively understand and with which to discuss sail trimming, rig tuning and match racing tactics.
    Lift and drag are terms from aeronautics where you have thick wings, low angles of attack, fixed cambers and high speed airflows - very different from sailing!
    And from hydrodynamics where again you have thick foils, low angles of attack, fixed cambers but heavy, viscous flow and low speed fluid flows. In this context the discussion of the forces on the keel may be relevant. (Reynolds number is relevant).
     
  7. CT249
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    CT249 Senior Member

    Done much windsurfing? In boards I'm fairly sure you CAN feel the effect of aero drag and lift, because the freestanding rig means that the fore and aft forces can be felt. Similarly, in craft with high lateral stability but less fore-and-aft stability, like some multis, some say that they can feel the bow lift when the rig is developing excess drag.

    You also haven't shown that this is a silly discussion; for example you haven't addressed the issue that when one is moving across different types of craft, breaking down the lift and drag can be a good way of understanding why optimum sail depth changes so much. For example, in an F18 cat you have lots of righting moment available and therefore could easily increase sail camber to say, as much as a Laser - but if you do that, you'll be low and slow because that setup creates too much drag. Similarly, a Windsurfer LT rig has to be set up to completely different twist and camber figures to a slalom rig, because the LT platform requires more lift and will not reach high enough apparent winds for aero drag to be as significant.

    In small craft, it's not always easy to "watch boatspeed" because you don't have polars etc, and if you spend too much time comparing yourself to others you may be reacting too slowly. It can be very useful for some of us to be able to understand that in flat water, for example, we have less hull drag and therefore require less lift from the sails and can move to a flatter, higher-pointing shape.

    I recently ordered a new jib for the 36'er. I knew that we weren't going to be looking for something to operate at the same sheeting angles as a TP52 jib because I understood the need to match the lift/drag characteristics of the rig with that of the hull. How would one break that down as easily and usefully by just referring to thrust?
     
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  8. CT249
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    CT249 Senior Member

    Here's an example of questions that many of us could understand if they were answered in terms of lift and drag;

    "Al, why are you changing the prebend today? The boat isn't any more stable than it was yesterday and the wind is just as strong."

    "Al, why do you want the traveller further down and more mainsheet tension than you did at the start of the beat?"

    "Al, why are you setting this J/24 up so much differently than the Farr 40?"
     
  9. DogCavalry
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    DogCavalry I aim to misbehave.

    Blessed be the name Marchaj, and his revelations delivered unto us, the faithful, in his tomes of Wisdom.
     
  10. Sailor Al
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    Sailor Al Senior Member

    All reasonable questions, but lacking context, and lacking your answer with reference to lift/drag.
    Q1) How was I changing prebend? Increasing prebend flattens the sail and so reduce its power.Presumably expecting heavier conditions where there is enough thrust from a flattened sail with reduced heel. On my F40, we adjust mast bend during the race with the backstay as the wind strength changes. From 12 kts upward we are over powered, so manage heel with the backstay.
    Q2) To manage the twist: earlier we had too much power generating too much heel.
    Q3) Don't have an answer to that: they are totally different boats.
     
  11. Sailor Al
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    Sailor Al Senior Member

    And even he explains that Lift and Drag are values measures in the wind tunnel, but driving force FS and heeling force FH are those which control the boat's behaviour.

    The mystery is why, for the remainder of the book, which he introduces in the preface with: "..my book is addressed to the general readers rather than the scientist...", (my emphasis) , he continues to reference the scientific, wind tunnel characteristics of lift and drag.
    I can only conclude that his role as an academic in aeronautics an astronautics, being a chartered engineer and a glider pilot overruled the knowledge of sailing that he gleaned from being a national Finn champion.
     

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    Last edited: Apr 8, 2021
  12. DogCavalry
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    DogCavalry I aim to misbehave.

    Well, he understood how engineering works. But why are you unhappy with resolving one net force into it's extremely useful components (l/d) but happy with resolving that same net force into two other useful components (Fs/Fh)? Engineers understand perfectly well the sum of all local vectors, and find the math required to express that sum differently for different aspects of the whole engineering challenge to be trivially simple. It is far too easy to visualize something incorrectly, and get lost in the math.
     
  13. Sailor Al
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    Sailor Al Senior Member

    OK, let me lay it out.
    On Page 7, in his opening paragraph of the first section of the book, he states: "This course [the windward leg] more than any intensifies the conflict between aerodynamic efficiency ...and the hydrodynamic efficiency..."

    He defines aerodynamic efficiency on P9 as:
    "The angle εA, between the lift L and the total aerodynamic force FT may serve as the index of aerodynamic efficiency of the sail".
    upload_2021-4-10_7-36-53.png
    So he's not just choosing to resolve Ft around the direction of the airstream as an academic exercise, he is using that resolution as a fundamental indicator of the efficiency of the sail.
    You and I, as sailors, know that the efficiency of the sail is its ability to provide the most thrust with the least lateral force and moment (leeway and heel).
    (sorry, having issues pasting images)
     

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  14. Doug Halsey
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    Doug Halsey Senior Member

    One thing you're missing is that it's very important that the sail produce a side force for the keel or centerboard to lift against.

    If you carry your definition of efficiency to an extreme, and the side force becomes zero, then the hydrodynamic drag angle becomes 90°. In that case, you couldn't sail to windward at all, and you wouldn't necessarily be very fast reaching either.

    Remember that the apparent wind angle is equal to the sum of the two drag angles (Lanchester's Theorem), so if you make the side force too small you can never bring the apparent wind angle forward of the beam.
     
    Last edited: Apr 10, 2021
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  15. Sailor Al
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    Sailor Al Senior Member

    I went off sailing on a real sailing boat yesterday just after I saw this post and expected that by this morning, 22 hours later, you, Doug, would have seen the flaw and deleted it or, given this chat room's name is "Hydrodynamics and Aerodynamics" that at least one other forum member would have responded.
    Then I thought:"It must be a twisted ruse to see who's watching".
    But then I saw that you had compounded it by editing it a few hours later, so I guess it wasn't a ruse.
    I think I'll wait patiently to see how long it takes for the penny to drop.
     
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