[pf] Windturbines: why they work

· As you know, I like boasting, and the following letter to a 900-member
home wind energy list was, /I/ think (!), a good answer to Laurie Forbes'
question. Yet the only comment on my "insight" was three people starting a
discussion { http://groups.yahoo.com/group/awea-wind-home/message/4656 } of
what direction the lift and drag forces were, that produce what I called
"the shifting sideways caused by the blade".

· Nothing about my "turning-straight-through-to-spiral" explanation!

· Any comment? /I/ think it's not hard to understand. Maybe it'e too
obvious for comment/criticism. (Or maybe it's wrong, but that list is quick
to pick up errors.)

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At 10:51 9/5/2001 -0600, Laurie Forbes wrote to the a-w-h list {at:
http://groups.yahoo.com/group/awea-wind-home/message/4637 } :-

>.. concept I'm having trouble grasping is how a two or three blade turbine
manages to "intercept", and derive energy from, the mass of air flowing
through its swept area i.e. the surface area of the blades is far less than
the swept area so one would think the bulk of the air would simply flow
through unaffected, especially at lower speeds (or so it seems to me).
>
[David Mac: {at:
http://groups.yahoo.com/group/awea-wind-home/message/4644 } ]

· The reason it works is that the blades are moving through the air with
the tips going about 5-12 times faster than the wind speed. {I've included
some [a-w-h] references to this ratio at the bottom of this, below the
_X_X_X_ }

· Together with everything moving, blades and air, being a long way below
the speed of sound (the speed of any pressure wave through the air), this
means that the whole "tube" of air flowing through the spinning-blade-disc
is affected by the shifting sideways caused by the blade. (Since the
disturbance propagates molecule-to-molecule at the speed of sound.)

· The whole purpose of the blades as transducer from wind speed to
rotational torque and power, is to turn the straight-ahead wind flow into a
spiral flow (downwind of the turbine blades).

· It's an essentially-3-dimensional version of the 2-dimensional effect of
the sails of a tall-masted America's Cup yacht: the moving air is pulled
towards the stern of the yacht by the sails: they exert a force towards the
rear _on_the_air_, and so the air exerts an equal and opposite force
forwards _on_the_boat_ (via _sails_ and rigging).

· For an aircraft, the carefully shaped wings push the air downward, so the
air necessarily pushes the wings upward (to balance the weight, if
straight-and-level). Newton's third law.

· For a turbine getting energy from a flowing fluid (gas or liquid), to get
the turbine to turn (say) clockwise requires the wind to be turned
anti-clockwise, resulting in an anticlockwise spiral behind the blade disc.

· I think I remember someone (Hugh) saying that you can't get more than
about half the raw energy from the wind, using a turbine for transducer.
The explanation I've just given explains such a limitation: you'd have to
stop the flow completely, to get 100% of the energy in that flow. Can't do
it: the air has to go somewhere, and changing it from straight-through to
spiral (actually helical) looks like a good method (the least drag in a
storm), to me.

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At 00:34 10/1/2001 EST, John Parsons (windpower4jp) wrote:

Marcellus Jacobs claimed he and his brother experimented with the
multi-bladed water pumper rotors in their early days -- even to the extent
of making a wheel where the angle on all of the blades were made to
"feather". Mr Jacobs stated "all those blades just get in each other's
way".  The water pumper rotors are considered a "drag" device, not an
airfoil. It is unusual for a drag device to run a tip speed ratio above
1:1, whereas airfoil blades will run typically 4:1 - 10:1  They do produce
lots of torque, but at quite slow speed, compared to an airfoil rotor.

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At 14:29 18/1/2001 -0800, Robert W. Preus wrote, with Subject: Re: [a-w-h]
Article in Home Power #65, June/July 1998 :-

The energy efficiency conversion of a stopped rotor is zero.  The energy
efficiency of a low solidity rotor will not rise to a reasonable number
until it gets close to its design tip speed ratio, which could be anywhere
from 5 to 14.  
There is torque even with zero power (stopped rotor).

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 [full e-mails at:
http://davd.tripod.com/TipSpeedRatio.html#top ]
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_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_X_

sent to Pos Fut by David.
(David MacClement) davd@ihug.co.nz 
http://www.geocities.com/davd.geo/index.html#top
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