Does a sail act as a turbine blade or a propeller blade?

Discussion in 'Boat Design' started by ThinAirDesigns, Jan 13, 2009.

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Does a sail act as a turbine blade or a propeller blade?

  1. Sail always acts as a turbine blade.

    22.2%
  2. Sail always acts as a propeller blade.

    0 vote(s)
    0.0%
  3. A sail can and does act as either depending on circumstances.

    77.8%
  1. spork

    spork Previous Member

    Sorry, but parts of what you say can be true depending on how you define a turbine or a prop. Parts are absolutely false. To say "all are the same" is definitely wrong. All you have to do is look at the flow through an engine driven prop vs. the blades of a windmill. Look at the stream tube upstream and downstream of the disk. They differ in very material ways. I personally think the best and most practical definition states that a turbine has high pressure on the upstream side of the disk (upstream relating to relative flow through the disk), and a prop has the high pressure on the downstream side of the disk.

    This is absolutely not true. Induced drag can be said to be similar to lift, but parasite drag is qualitatively a completely different animal.

    And I suggest you take your own advice.
     
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  2. alex folen
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    alex folen Flynpig

    This is fun! For what it’s worth and if I’m understanding the “simple” and quick question, and with the help of "Akums Razors" theory, the simplest explanation is usually correct” It’s the turbine.
     
  3. spork

    spork Previous Member

    As Albert Einstein said "Everything should be made as simple as possible, but no simpler."

    I suppose it would be great if it were so simple that we could just say "it's a turbine". But it's not.
     
  4. alex folen
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    alex folen Flynpig

    I guess we’ll have to split up the prize PAR.
    A Propeller is designed to produce thrust using some sort of power. Turbines, the wind. Hello? A two question question. …Put you calc books up… like I did years ago.
     
  5. alex folen
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    alex folen Flynpig

    "So many people today — and even professional scientists — seem to me like someone who has seen thousands of trees but has never seen a forest."


    Auhhh, This is Einstine also.
     
  6. jehardiman
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    jehardiman Senior Member

    JB: Foolish was perhaps a poor word choice but it was the one that came to hand and I did reject "ignorant" for the reasons you stated. Realisticly, regardless of what some might argue, there is no difference in the mathematical formulation between work in (props) and work out (turbines) units, and as was pointed out, a sail could be either. The very attempt to compare a sail to one of the other is the fault in your premise, it is neither because they are just specific cases of the general situation, just like wings, kites, dynamic catenarys, jets, venturi, manometers, Flettlner Rotor, etc. A sail is a sail, and should be approached from the general equation, discarding all the constant or trivial terms rather than being compared to a degenerate form of the equation that needs to be added to. For some basics see the following websites that I was quickly able to google-up. For more detailed information, and the full expansion of the equations, get a good fluids text as I proposed.
    http://en.wikipedia.org/wiki/Euler's_equations_of_inviscid_motion
    http://www.roymech.co.uk/Related/Fluids/Fluids_Flow.html

    Mike: Pedantical? Yeah, well, but "flow" over foils is one of those things, like "hull speed" calculations, that most people misunderstand because they only learn the contrived equations to get a useful engineering answer, not the correct analysis on which other development can be based. That is one of my hot buttons. For insight into a mathematical contrivince that is now a believed "truth", read Lanchester's and Prandtl's original works for a glimpse into how Gamma was determined.

    Spork: I have neither the time, the inclination, nor is it my place to instruct you to be a better fluid dynamicist. All analysis of fluid devices start from the same equation, and wether "+ W" ends up as positive or negative depends on how you draw the flow, energy boundries, and assumptions on what to ignore (viscious heating perhaps?). Even your comments admit to that with "upstream" and "downstream" references that show you understand that basic. If you don't belive that a prop and a turbine are the same devices just carry out a prop calculation from J <<-J(opt) to J>>J(opt) or a centrifical pump calculation for negative specific speeds (i.e. negative omega). The fact that a turbine makes a poor prop and a prop makes a poor turbine and they both make poor glider wings does not make them different things than a sail.
     
  7. alex folen
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    alex folen Flynpig

    “Does a sail act as a Turbine blade or a propeller blade”
    ...If you were stranded on an island and your sails blew away, which would you use if available, a turbine prop blade or an airplane prop blade, assuming they were the same length and weight. I'd grab the turbine blade. …BUT! If winds should exceed 400 MPH and I had some coconuts to counterbalance the weight of the other end of the propeller blade I might choose the prop blade. Am I not understanding the poll question above? I might prefer foolish over ignorant in this response.
     
  8. chabrenas
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    chabrenas Mike K-H

    OK. I see where you're coming from - and given your profession I can perhaps guess at some of the encounters that have hardened your attitude :)=>

    This forum really does take me back to university lounge and coffee bar days - for which I'm grateful. And, as others have pointed out, its seems to be frequented by more mathematicians and scientists than certain 'physics' fora.
     
  9. chabrenas
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    chabrenas Mike K-H

    Which suggests that the design of each is optimised to work efficiently in that range of values of the variables in the equations you mentioned which defines its designed working environment.

    So we can reasonably interpret the poll as asking whether the normal operating environment of a sail is defined by a set of values which is contained within the set defining the designed working environment of a turbine, or within the set defining the designed working environment of a propeller, or within one that intersects both sets.
     
  10. spork

    spork Previous Member

    Fortunately for you I do have the time and inclination to correct your flawed ideas about fluid dynamics. Unfortunately, I don't have that time at this very moment. I will say that you may be misunderstanding the question. The question is not whether an object such as a propeller CAN act as either a propeller or a turbine - it's a question of whether and when it acts as one or the other.

    If you consider a windmill planted in the ground, and operating in the normal way, are you actually suggesting that we can say it's vanes are acting as a propeller if we just look at it the right way!?
     
  11. jehardiman
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    jehardiman Senior Member

    Yes, useful devices are optimized for thier intent and operating conditions, but are described by constant equations.

    Think of a 1 kilo mass having a velocity (including zero) in a gravitational field. It always has mass, but it only has "weight" (an unbalanced force acting towards the center of the larger body) in certian conditions. In a balanced energy stable orbit "weight" = 0 only becuase the net force vector is perpendicular to the direction that "weight" is measured. In a negative energy unstable orbit it has positive "weight" force and '"falls" towards the larger mass, and we have the ability to extract some useful work from it or it is balanced by an outside force (the mass is supported), and no apparent work is done though the mass may have significant energy. In a positive energy unstable orbit it has "negative weight" or "centrifugal force" and moves away from the larger mass and requires work input or it is balanced by a "centripetal force", again doing no work though it may have significant energy. So from a semantic point of view the mass only has "weight" when we try to prevent if from going to a lower energy state, and only has "centripetal force" when we attempt to hold it at lower energy state.

    Most people would use Newtons equations to slove the above example. Now assume the mass is a fluid. All same conditions apply except now "weight" becomes "hydrostatic pressure" and "centripetal force" becomes "lift". It is all point of view

    Make the time, I will, as this ought to be good.

    IMHO it is rather obvious you have no real experience in applied fluid dynamics because of the example you chose. In real life applications the windmills blades will act as a propeller fairly often with any -del V. This is because there cannot be an instantainous change in momentum of the blade and work will be extracted from the blade into the air. A ships engineer would laugh in your face and point to the speed to turns warning placards if you told him that a propeller will never backdrive the main reduction gear. Or all the shaft generators sold.

    There is no massless, steady, inviscid, irrotational flow in the real world. The fact that those simplifing assumptions can lead to a plethora of degenerate situationaly specific equations does not make them or that simplifing process correct, regardless that the answers you get are within real world ability to resolve.

    Anyway, to get back on topic. It is rather obvious to the most casual observer that a sail can be either, and it can be demonstrated in a relatively simple way. If you cannot use the sail as an energy in propulsion device, why is there Rule 42? FWIW, I used to propell my 26' sail boat at about 2 knots in 0 wind by sallying her with the main sheeted well home.
     
    Last edited: Jan 22, 2009
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  12. ThinAirDesigns
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    ThinAirDesigns Senior Member

    Spork:
    jehardiman:
    Wow, just wow.

    You do see the "operating in the normal way" in sporks assertion don't you? -- we don't "normally" place wind generators up on windy hills just so they can be occasionally backdriven. I'm pretty sure you'll find that by "normal way", spork means in a steady state wind.

    At any rate, even by your own irrelevent example of a turbine being backdriven you are acknowledging that there is a fundamental difference between an airfoil being used as a prop and being used as a turbine -- a difference that isn't just related to frame of reference. It is this difference to which the OP/poll refers.

    Since that is one of the poll options, but yet not everyone picked it ... what makes the poll "foolish" in your mind?

    JB
     
  13. jehardiman
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    jehardiman Senior Member

    Actually I read "in the normal way" to imply passive, i.e. without power input. The problem is that the windmill stores energy like a flywheel, and bleeds that energy back into the airflow "in the normal way" in certian conditions. As you say it is an airfoil, neither a turbine or a propeller. Those are just transient states, not what the blade is.

    As far as the OP/poll is concerned the operative word is "always". That is what makes it "foolish". I believe the old saw is that "absolutes are for tyrants and dullards".
     
  14. ThinAirDesigns
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    ThinAirDesigns Senior Member

    There's three options there ... two "always" and one "depends". No one is being a tyrant forcing anyone to choose "always".

    I myself fell squarely in the "depends" camp well before posting the poll. I suspected that there would be some on the "always turbine" camp and hoped to engage in some interesting conversation in that regard. I hardly find that desire "foolish".

    JB
     

  15. ThinAirDesigns
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    ThinAirDesigns Senior Member

    You're apparently misreading the poll and OP question ... it doesn't ask if a sail *is* a turbine or prop. It asks what it *acts as*. In other words, which of the transient states you describe above and under what circumstances.

    Since we seem to agree that there truly is a way to determine when the airfoil (sail) is acting in one state or the other, perhaps you could use your understanding of this to educate me as to when a sail acts as a prop -- that is leaving a stream of air behind that is accelerated relative to the free stream.

    Please?

    JB
     
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