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Aerodynamics of HAWTs

  1. Jun 7, 2007 #1

    DaveC426913

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    The wind turbine I pass everday seems to have its blades almost flat to the wind. So, I want to make sure I understand exactly what the wind turbine is doing, and if I have the airfoil components right - see my diagram.

    It seems to me that, as the angle of attack is increased, the resultant lifting force on the foil is turned farther and farther backwards, meaning less force is put into rotating the rotor and more is put into trying to bend the turbine's spine. (Well, I guess that rearward component is actually drag isn't it?)

    Contrarily, the lower the angle of attack is, the more the lifting force of the foil is inline with the preferred direction that will rotate the rotor. but the tradeoff is that a lower AoA means less lifting force.

    I think, despite all this, the most efficient point is reached at the highest angle of attack before separation of boundary layer. Is this correct?
     

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  3. Jun 7, 2007 #2
    Wind turbines are designed to rotate at an optimal speed. If the speeds become to great the blades will feather, thus rotating slower not to put excessive loads on the generator, bearings etc. On slow wind days, the props will increase the angle of attack. On days the wind is too fast, the blades will angle themselves such that the turbine actually stops. Too much wind is a bad thing just as too little wind is a bad thing.


    Edit: (I had something wrong in here, sorry)

    The blades stall with the angle of attack. You're right that the blades create more lift as the angle of attack increases, but also you get closer to stalling.

    However, as you incline the blade more, you get a larger and larger component of drag, which will push the blades back and cause a large moment at the foundation.

    Ideally, you want to spin at the optimal design speed (RPM) while creating the least drag because it will allow your components to live longer. Its goal is not to create thrust, so it does not have to spin fast with high angles of attack. It just has to spin at the right speed with the minimal amount of effort required.
     
    Last edited: Jun 7, 2007
  4. Jun 7, 2007 #3
    Maybe this helps.

    [​IMG]

    [​IMG]
     
  5. Jun 8, 2007 #4

    DaveC426913

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    So, when I'm seeing the blades almost perpendicular to the wind, that's a very high angle of attack. i.e. lots of lift but also lots of drag.



    Ah. Another piece of the puzzle: stronger wind/lift doesn't result faster rotation (as it would if left alone). The system steps up the gear ratio so that rotation remains in the optimal range - the consequence of a higher gear ratio being a larger counterforce against the rotation (i.e. harder to "push"). And that's what the stronger wind is doing - pushing a "heavier" blade.
     
  6. Jun 8, 2007 #5
    That makes great physical sense Dave. The change in gear ratios would greatly increase the inerta of the blades.
     
  7. Jun 8, 2007 #6

    russ_watters

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    Actually, the thing you are missing is that your angle of attack is wrong in your diagram. If the blade is moving right-to-left in your diagram, then the wind is not vertical, it's coming in from-upper left to lower-right. This is called relative wind. So the pitch needs to be adjusted as the turbine speeds up, to maintain the proper angle of attack: the faster it spins, the flatter the blades.

    Now as the blades speed up, the optimal angle of attack gets lower (just as a plane in flight lowers its angle of attack as it flies faster and is more efficient at high speed than low speed). So you end up with a very large lift component and a very small drag component to your vector, making the resultant torque higher despite the fact that the angle is more "into" the turbine instead of perpendicular to it.

    Now a related concept - you may notice that fans, propellers, turbines, etc. are twisted - the angle of attack is much higher near the center and the blades are much flatter at the tips. The reason for this may be obvious by now: The linear speed of the blades is higher at the tips than near the center, so the relative wind is more parallel to the direction of motion. In other words, if your entire fan were nearly flat, you'd generate virtually no lift at the center and if it were steeply pitched, you'll stall the tips.

    Relative wind is most often seen in sailing (see: how can boats travel faster than the wind?): http://www.physclips.unsw.edu.au/jw/sailing.html
     
  8. Jun 8, 2007 #7

    russ_watters

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    So hopefully you can apply my last post's info to this and see that when the blades are perpendicular to the true wind, it must be because the turbine is spinning slowly in light wind. The angle of attack will be relatively high in this case, and your l/d ratio isn't as good, but it is ok because the lift force is almost all perpendicular to the axis of rotation. Also, at low speed, the pressure component of drag isn't as bad for a high angle of attack as it would be for high speed.
     
  9. Jun 10, 2007 #8
    I think I agree with russ_waters abowe.

    The wind speed and the angle of attach that the wind mill blade will see while turning around, is not that one that you will see while sitting resting at the ground.

    If you takes your seat at the tip of the wind mill blade and make some rotations with it, then you will see the angle of attach and the relative airspeed the the blade will see.

    As mentioned above, wind mill blades are normally tvisted as the wind speed component due to rotation will increase as the radius increase.
     
  10. Jun 10, 2007 #9

    DaveC426913

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    Slaps forehead. I'd always seen that but it never occurred to me that the reason was so simple.
     
  11. Jun 10, 2007 #10

    DaveC426913

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    OK, so let's approach this from the reverse. I'm looking at a HAWT where the blades are very nearly flat and its rotating at optimum speed. I presume the axis of the prop is pointed directly into the wind (which I can tell by the flag nearby). What does the diagram look like? Or I guess the operative question is: how fast is the wind going for the blades to be be almost flat?
     
  12. Jun 10, 2007 #11

    DaveC426913

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    Yes, I am familiar with relative wind.

    When out on my intermediate certification, I observed this:
    In a close haul on a port tack, you can't get head up much better than 30 degrees to the wind. If you then tack to a starboard close haul, you will again be at about 30 degrees to the wind.

    So you've changed your heading by 60 dgerees, right?

    Wrong. That tack will have you moving through a heading change of 90 degrees.:rolleyes:
     
  13. Jun 11, 2007 #12

    russ_watters

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    That's a question I can't answer - the optimal AOA/efficiency depends on the particulars of the power transmission system and the airfoil. My guess would be that wind turbines are designed for a specific range of speeds. Just like on a sailboat, then, if they start to exceed the optimal wind speed, they feather (luff) the prop to bleed-off energy so they don't snap the blades off (roll the boat).

    I don't know if GE publishes such information or not - it may be proprietary or dependent on the particular installation.
     
  14. Jun 11, 2007 #13
    "My guess would be that wind turbines are designed for a specific range of speeds. Just like on a sailboat, then, if they start to exceed the optimal wind speed, they feather (luff) the prop to bleed-off energy so they don't snap the blades off (roll the boat)."

    I think that's right. At least this was how I got it explained of someone in the windmill business.
     
  15. Jun 11, 2007 #14
    By the way, the emergency control system of some jet turbine propeller engines does the same. If some critical condition is reached then the propeller blades just "feather".
     
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