Stalling behaviors of foils based on aspect ratio or chord?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Jan 7, 2023.

  1. dustman
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    dustman Senior Member

    I keep reading that high aspect ratio foils can have issues with stalling.

    Is it the actual chord, or the aspect ratio, that determines the angle at which a foil will stall?
     
  2. BlueBell
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    BlueBell . . . . .

    All foils have "issues with stalling".
     
  3. jehardiman
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    jehardiman Senior Member

    The real answer is "it all depends".... First of all, it is never the cord alone, it is the thickness ratio and section shape. So for reasonably shaped foils of reasonable (approximately <4) aspect ratio, generally a thicker foil will stall at a higher angle of attack. However this says nothing about the Lift to Drag ratio; so it becomes important when high lift to drag is required, such as a lifting foil for hydrofoiling where high L/D is desired.

    What actually happens in real world high aspect ratio foils is that the long span causes tip motions with large(r) AoA changes at the tip which leads to tip stalling and stall rolling up the foil, greatly decreasing the L/D very quickly, and the CP rapidly moving spanwise, both leading to a loss of lift "crash and burn". Aircraft avoid this by adding "washout" to the wing (i.e. the static AoA is less at the tip than at the root causing stall to occur at the root first) allowing ailerons to maintain control. However washout only works with high Va monodirectional lifting foils and is problematic in a running seaway.
     
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  4. Skyak
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    Skyak Senior Member

    In the case of keels or rudders, people do high aspect ratio foils to reduce the wetted surface area needed for a given lift. The cavate is that the calculation is done for sailing at speed. At low speeds the planform area can be deficient.
    The right way to design the foils is to consider the Reynolds number in operation and have a plan for getting up to speed.
     
  5. dustman
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    dustman Senior Member

    I woke up from a nightmare this morning screaming "it all depends, it all depends!" :confused:

    Thanks for following it up! It seems the answer is yes, but with qualifiers. The picture I'm getting from you is that very high aspect ratio foils need more careful consideration in the design process. Aside from structural considerations.

    So can one make a generalization that, within reasonable limits, both greater chord and thickness ratio means that a foil will stall at a higher angle of attack. A higher aspect ratio will have a greater lift for a given area up to its stall angle. Both smaller thickness ratio and higher aspect ratio create less drag for a given lift.
    So this is due to the actual physical changes in its orientation, wouldn't happen in an infinitely rigid foil?
     
  6. dustman
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    dustman Senior Member

    In a paper I was reading it seemed to be saying that a larger chord operates at a higher reynolds number, regardless of aspect ratio, which means to me that there is a greater velocity differential between the two sides of the foil, so the water "gets up to speed" at a lower speed? So with a smaller chord the velocity is higher, this ultimately means that the foil will stall at a lower angle of attack due to the fluid not being able to "bend the corner"? I imagine that section shape plays a large role in when this occurs?
     
  7. dustman
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    dustman Senior Member

    o_OWhy?o_O
     
  8. DCockey
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    DCockey Senior Member

    Incorrect. The pressure difference across the foil is basically* independent of the Reynolds number.

    Reynolds number = velocity * length / kinemetic viscosity
    velocity is a representative velocity. For airfoils and hydrofoils the approach velocity is almost always used.
    length is a representative length. For airfoils and hydrofoils the chord is almost always used.
    kinematic viscosity is a fluid property. For a given fluid it varies with temperature and pressure (or density and pressure, or temperature and density, etc).​

    *Reasons for the qualifier "basically" in the first sentence.
    As the Reynold's number increases the boundary layer becomes thinner. The thickness of the boundary layer affects the flow over the foil. The effect on pressure difference and lift is negligible except at very low Reynolds numbers.
    Reynold's number can influence separation, if and where it occurs. Separated flow past a foil can be very different than attached flow. When a foil stalls the flow over the upper surface is separated. A higher Reynolds number delays separation.​

    Incorrect. A higher Reynolds number results in stall occurring at a higher angle of attack, not a lower angle of attack. Look at the plots of experimental data for lift vs angle of attack in Theory of Wing Sections by Abbott and Von Doenhoff.
     
  9. BlueBell
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    BlueBell . . . . .

    Aerodynamics.
    There are entire books written on it.
    Read them.
     
  10. Skyak
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    Skyak Senior Member

    Look at that paper again. Cord is a FACTOR in the Reynolds number. If your boat is going between 0 and 6knots and you are considering rudders of equal area but different aspect ratio (say 1 and 3) you will find that the high aspect foil runs out of its Reynolds number range at a higher velocity than the low aspect foil. High aspect foils are more efficient, but they have a higher stall speed.

    All foils stall. Foil profiles generally have 10 to 15 degrees of linear lift -and that's the infinite length case. Thicker foils of the same series just have smoother paths for the low pressure flow to higher angles.

    Like I said in my previous post, design your foils for the right Reynolds number so it stays in the "drag bucket" where L/D is good. All keels and rudders stall at low speed. Is it tolerable? Can it be avoided with skills? Is performance more important or is forgiveness more important for beginners?
     
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  11. dustman
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    dustman Senior Member

    I haven't had that ahah moment with reynolds number yet. Just to be sure I understand... Reynolds number is the speed of the fluid relative to the length of the surface it is flowing over, and the density and viscosity of the fluid determines how energy is exchanged between the fluid molecules and surface, which in turn determines the behavior of the fluid. The molecules of the fluid (and the surface it is flowing over) are bound to each other by whatever forces, and when these forces are overcome the bonds are basically ripped apart causing disturbances in the flow, and it is somewhat of a chain reaction because once turbulence is created you have fluid flowing every which way which increases the differentials in velocity between fluid particles, thus causing more tearing, and it takes more energy to break more bonds.

    It seems like greater velocity over a surface or between water particles would cause more turbulence, and that the higher the speed of the particles the more momentum they would have, and thus would have more propensity to keep going in the same direction as opposed to following the surface of a foil. Why is it that a higher reynolds number would delay separation? My intuition tells me the opposite would be true.

    In reference to my original question, a longer chord length would raise the reynolds number, and since a higher reynolds number delays separation, a lower aspect ratio and thus greater chord length would mean you could maintain lift at a lower boat speed or greater angle of attack, relative to a higher aspect ratio foil of the same area but smaller chord? But has lower overall lift due its lower efficiency for a given area.

    So with a lower aspect ratio you have a greater range of control, but you sacrifice efficiency, and you would need more area to achieve a given amount of lift. Vice versa with higher aspect ratio.

    This all means that you have to be more attentive with a higher aspect keel and rudder.

    As long as I have plenty of righting moment wouldn't I be better off just adding a little more sail and going with a lower aspect keel and rudder? I will be sailing in shallow water a lot.
     
  12. dustman
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    dustman Senior Member

    In summary, a lower aspect ratio and thicker foil will give me a greater range of control, but at a loss of efficiency?

    My plan is:

    ~1000lb displacement

    120ft2 sail area

    4.5ft2 keel and rudder area, which is 3.75% of SA. That would be 3ft2 divided between two keels, and 1.5ft2 between two rudders.

    I'm struggling deciding on aspect ratio for the keels and rudders. I really want as small a draft as possible considering the kind of sailing I want to do. I'm considering between 2 to 4 with aspect ratio. Is the difference between 2 and 4 big enough to make it worth it? Would I even notice much of a difference in performance, and would the difference in lift/drag of the keel and rudder relative to the rest of the drag on the vessel affect my windward performance to any significant degree?
     
  13. dustman
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    dustman Senior Member

    I HAVE read probably over 30 books, papers, and articles on the subject in the last few years, and watched just many videos. I am here to make sure I understand them properly, which apparently I don't fully. I don't have the mathematical ability to understand everything presented, or to prove or disprove the hypothesis I formed based on what they are saying. Basically I want to concrete my understanding so I can move on to application and it is apparent that many people on this forum are capable of helping me with that process. Sometimes I'm a little slow on the uptake, but once I understand something I understand it well. You can contribute to my education, or not, your choice.

    And if you want to give me links to a list of good literature on the subject I will gladly read it all, or at least what is relevant to my application. My work will suffer because I will be up into the wee hours of the morning reading and pondering what I have read.
     
  14. DCockey
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    DCockey Senior Member

    The mistake is the assumption of some sort of significant "bonds" between molecules or fluid "particles" which take energy to break.
     

  15. dustman
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    dustman Senior Member

    I'm trying to visualize what's happening on the microscopic level, if I can do this it really helps me understand what's going on.

    Am I on the right track otherwise?
     
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