DDWFTTW - Directly Downwind Faster Than The Wind

Discussion in 'Propulsion' started by Guest625101138, Jan 4, 2009.

  1. kerosene
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    kerosene Senior Member

    this all have been gone through dozen times in this very thread.

    too tired to type same stuff over and over.
     
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  2. DogCavalry
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    DogCavalry I aim to misbehave.

    Yes, maybe the moderator can lock it.
     
  3. tlouth7
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    tlouth7 Senior Member

    I think this is a really nice illustration of the principle. In my experience the bit that is hard to accept is that the thrust generated by the propeller can be greater than the drag on the wheels.

    The reason that is possible is because the wheels are running on a fast moving belt, whereas the propeller is pushing against stationary air. A good transmission will transfer almost all power from the wheels to the propeller, and power is force x velocity. So a small force x high velocity at the wheels = large force x low velocity of the airstream being created by the propeller. Even once the cart starts moving forwards on the treadmill (equals faster than the wind downwind) the velocity of the ground moving past the wheels will always be faster than the velocity of the air moving past the propeller.

    This also shows why the principle would not work in still air - there would be no difference in velocity and so the drag would be greater than the thrust.
     
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  4. gonzo
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    gonzo Senior Member

    The ground does not input any energy into the system (cart). For example, a windmill takes energy from the wind, which is the difference in velocity between the air and the ground. However, the cart and the wind both have the ground as the fixed frame of reference. What you guys describe is a system that creates energy, which I am very skeptical of. I see where your confusion is, because you are using two different frames of reference so the equation has two positive energy inputs instead of a positive and a negative inputs.
     
  5. gonzo
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    gonzo Senior Member

    There is no "air stream" coming out of the propeller in an opposite direction to the wind. The Betz limits shows that the maximum power possible for a wind turbine to absorb from the wind is about 59%. The "air stream" is flowing in the same direction as the wind, but at a lower velocity. The loss of velocity is caused by the power absorbed by the turbine. You are claiming that the power input in the turbine is creating an output of over 100% the power of the wind.
     
  6. DogCavalry
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    DogCavalry I aim to misbehave.

    You keep talking about a turbine. There's no turbine.
     
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  7. gonzo
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    gonzo Senior Member

    What are you using to get power from the wind?
     
  8. tlouth7
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    tlouth7 Senior Member

    You get power from your fan pushing against that wind, which pushes the vehicle in the other direction (forwards).
     
  9. gonzo
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    gonzo Senior Member

    That is called a turbine when it is used to get power from the wind. However, it does not get power from being pushed, but from the flow of air around a foil (the blades).
     
  10. tlouth7
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    tlouth7 Senior Member

    A turbine is spun by the airflow and power is taken from the shaft and used to do some task. Here power is supplied through the shaft and used to spin the fan. The air acts to slow the fan down.

    A turbine resists airflow and so experiences a force in the direction of airflow (so that would be drag here), a fan adds to the airflow and so experiences a force in the opposite direction (in this case thrust).
     
  11. gonzo
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    gonzo Senior Member

    OK, where is the power to the propeller coming from?
     
  12. tlouth7
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    tlouth7 Senior Member

    From the forward motion of the wheels.

    Clearly this creates a drag on the wheels which acts to slow the vehicle down. Luckily the drag force on the wheels can be less than the forwards thrust on the fan because (to quote an earlier comment):

    A good transmission will transfer almost all power from the wheels to the propeller, and power is force x velocity. So a small force x high velocity at the wheels = large force x low velocity of the airstream being created by the propeller. Even once the cart starts moving faster than the wind downwind the velocity of the ground moving past the wheels will always be faster than the velocity of the air moving past the propeller.
     
  13. gonzo
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    gonzo Senior Member

    OK, we are going full circle. Where is the power coming from to make the wheel turn?
     
  14. BlueBell
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    BlueBell "Whatever..."

    Where do you think it's coming from?
     

  15. tlouth7
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    tlouth7 Senior Member

    From the thrust acting on the propeller.

    Did you read the second bit of my previous comment? Do you see how the argument works?
     
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