h a l f b a k e r yQuis custodiet the custard?
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[edited for clarity]
A regular shaped propeller style turbine sits
horizontally (with vertical axis) in a small
"tower" (say half a meter high)
Air is caught around the tower and diverted down to
the bottom of the tower.
The top of the tower is shaped like a chimney, or
many small chimneys,
creating a Venturi effect,
sucking the air out from the top.
The air exits on these Venturi devices can face
various directions, with the wind having very little
effect on the exits faced towards the wind, but a
very strong effect on the ones facing away.
The turbine does not need to "change directions" and
has no external moving parts other than the internal
turbine. It is shaped and performs like a regular
horizontal axis propeller style turbine does, which is
known for better efficiency and lower cost.
Birds cannot hit the blades, and no need to get special
permits because the whole thing can be very low (but
very wide, taking up a complete flat roof!)
The top can be painted black, heating the air on top
and causing extra draft.
(?) similar
http://phys.org/new...nergy-rooftops.html
But not what I intended. I'll look for a better image of the shape I'm talking about [pashute, Oct 30 2012]
funneling wind
http://www.i2e.org/...ay-to-capture-wind/
[pashute, Oct 30 2012]
Shape of windfront
http://plusmood.com...do_wind_diagram.jpg
Taken from a similar but different idea. No moving parts or vents are needed. [pashute, Oct 30 2012]
VAWT/HAWT
http://en.wikipedia...ine#Horizontal_axis
Turbine Orientation [MechE, Oct 30 2012]
Optimization of the power output from ducted turbines
http://www.google.c...Be-AEXVzVQBrezPB7GA
[spidermother, Oct 31 2012]
(?) Ducted or Augmented Turbines
http://www.wind-wor...mentedTurbines.html
[spidermother, Oct 31 2012]
Bernoulli's Principle
http://en.wikipedia...oulli%27s_principle
There is no free lunch … [8th of 7, Oct 31 2012]
Lunch paid for
https://docs.google...sJXPbsTOSIUse8/edit
[pashute, Nov 03 2012]
Roof Ridge Windmill
Roof_20Ridge_20Windmill
Vaguely similar idea [marklar, Nov 04 2012]
simple wind concentration calculations
http://wind-do.com/...concentrator%20.pdf
[pashute, Jul 27 2013]
with almost no diversion - claimed to work
http://www.getsmartenergy.com/
[pashute, Jul 27 2013]
Why concentrated wind projects wont work
http://www.earthtec...t-wwii-vets-design/
skepticism of non substantiated claims [pashute, Jul 27 2013]
Professor explains why a wind funnel is not a good idea
http://www.youtube....watch?v=vRWOTxspBxs
But what happens if its just diverting the wind a step upwards? [pashute, Jul 27 2013]
Delft University: Wind concentration in built areas - promising idea
http://repository.t...-8754-6c58124f4a10/
at roofs of buildings" configuration and a variation on the "in ducts through buildings" configuration are promising. Lack permissions so I'm not able to read article itself. [pashute, Jul 27 2013]
Wind tower
http://news.discove...nd-tower-120606.htm
using water not wind concentration and tower not flat... [pashute, Jul 13 2015]
Thanks halfbakers for getting me off of this...
http://cleantechnic...long-line-failures/
Finally found this by searching for "ducted wind energy" [pashute, Jul 13 2015]
Someone claiming it would work
https://www.youtube...watch?v=K-rlU91rEOk
- according to the discussion here, its probably a hoax [pashute, Jul 18 2015]
[link]
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Is a ducted turbine going to be more efficient ? |
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On the plus side, this avoids having to articulate the rotor head. |
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On the downside, there's going to need to be a very large cowl-like structure with vanes to keep it pointing into the wind, and the side forces will be high. The "venturi slot" effect proposed will be self-cancelling. |
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A funnel shape would allow a smaller, higher speed rotor which would reduce the need for gearing, and might make it more resistant to overspeed. |
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//diverted down//
//sucking the air out from the top//
Is it just me, or is the description a little confused?
Perhaps a sketch would help.
And is the turbine axis vertical? |
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//diverted down//
//sucking the air out from the top// |
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It starts out level, and then the wind is ducted down
so it comes in under the turbine. The center of the
structure (the turbine support) is a cone or similar
that directs it back up, through the blades. The
Venturi effect aids this. |
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//is the turbine axis vertical?// Yes, [pashute], that's someting I've gently tried to correct in your other ideas. It's conventional (and less confusing) to talk about the orientation of the *axis* of turbines, flywheels etc. |
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//funnel // In general, you can't concentrate wind energy with any funnel-like arrangement. It's a bit like making a wire thinner in the hope of increasing the voltage. It doesn't work like that. |
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[Spidermother] First, thank you for your remarks. But
-
about conventions: the term "Horizontal wind
turbines"
is quite known to anybody interested in wind energy.
Same for "Vertical". It is NOT the axis, but the
direction
of the blades. |
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And about the funnel: you definitely can funnel
wind.
In fact most windy areas are locations where the
airflow has been funneled by mountains or buildings,
directing the airflow into a fast stream. |
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[8th] No vanes needed, no moving parts except the
internal turbine. The shape is that of ... wait, I found
a link to something I DID NOT THINK OF, but it seems
someone baked a similar idea. Mine is simpler. I'll find
a link and post it. |
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I can't find a single reference to vertical or horizontal referring to anything other than the axis of a wind turbine (except right here). Some use 'vertical wind turbine' as shorthand for 'vertical axis wind turbine' - which is sloppy, but still follows the convention. |
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As to funnelling, there are arrangements that direct air from outside the rotor area towards the rotors, making the effective swept area greater than the rotor area. But AFAIK, a simple funnel can't effectively concentrate harvestable energy (except at supersonic wind speeds). (I may be wrong on this one; I'll do some more reading.) |
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Found the links for shape. I'll link to vertical and
horizontal in a sec. |
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[Spider] is correct about direction. Horizontal is one
that spins around the horizontal axis, Vertical,
around the vertical axis. That's absolutely standard. |
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However, [Pashute] is correct about funneling wind.
That's why you can get higher wind speeds
through mountain passes. You do need to play with
the trade off against surface friction. |
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//I may be wrong on this one// Dammit, I'm always right. I thought I was wrong then, but I was mistaken. |
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If you could concentrate wind using a funnel, then you would have the same mass per time traversing a smaller area, and therefore travelling at a greater velocity¹, and thus having more power - clearly impossible. |
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¹ Ignoring compression, which is insignificant at subsonic speeds. |
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Thanks [MechE] and [SpiderMother] for the correction.
I even contradicted myself. I meant "propeller style"
turbine common in the horizontal axis wind turbines,
and mostly not used in VAWT. I edited the idea name
and all to convey the corrections. Thanks again. |
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//mountain passes// That is a rather different situation. A mountain range is like a dam - the energy is available in part as a pressure difference. An object (such as a wind turbine) in a free flow is only able to harvest the velocity of the fluid intercepted by its swept area. |
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//The dome of the mushroom (with a hole in the top) maximising the venturi effect.// No. In fact, Count Rumford recommended a similar structure (a squat pyramid with an opening at its apex) instead of traditional chimney pots, precisely because it creates very little up-draught or down-draught, so the burning of the fire is not greatly affected by wind conditions. |
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Ducted or shrouded turbines do work, there is no
argument about that. What they don't do is work
better than a conventional turbine of the
diameter of the duct. (Ignoring compression,
which is small, but not insignificant). |
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If wind couldn't be accelerated by ducting, then
you get no wind in any path where there is any
obstruction for an infinite distance, since the
wind would back up against it. |
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However, the more I think about this, the less I
think it will work. The air will tend to prefer to
sweep around the stalk of the mushroom (to use
[big]'s analogy), rather than climb it. |
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The downward direction is to intercept the wind
that would otherwise be hitting the turbine edge
on. I think. |
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[SpiderMother]:
//...and thus having more power? |
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There is no law of conservation of power. There is
the law of conservation of energy. You can
concentrate power, formerly distributed in a large
area, now concentrated in a small area. A high-heel
shoe concentrates the power of your weight in a
small area. |
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When concentrating wind, there will be some loss of
energy to heat, but the overall kinetic energy of the
system, which is mass times speed, is preserved,
because although inside the funnel the mass is
moving faster, outside the funnel the mass in most of
the area has been slowed down to almost a total
stop, or converted into pressure (which is a
translation of the kinetic energy between the
molecules and the funnel surface, similar somewhat
to heat). |
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answering MechE and BigSleep, I gave a link to the
shape of the windront (all around this contraption).
The vanes cause the wind to enter, regardless of
wind direction. |
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The air entering the tower is directed to the bottom
of the tower, it then moves UP thru the turbine in a
twirling motion, and out from the top, assisted by the
wind movement over the chimney(s), the Venturi
effect and the heat effect. (My wife
made a small solar fruit drier with a tiny solar heated
chimney, it has quite a nice draft going in their daily. |
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Again, it doesn't work like that. Ramjets don't work at all below about half the speed of sound, and are not efficient until well over the speed of sound, for the same reasons. I've said this before here, but low-speed aerodynamics is modelled as incompressible flow, because compression is regarded as insignificant. That's why you can test a low-speed aircraft's aerodynamics in either water or air. |
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I suspect that ducts / cowlings on wind turbines are to reduce blade tip losses. Concentration of the flow to a smaller area would be very limited at ordinary windspeeds. |
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There _is_ a limit to power per unit area in an unconfined fluid flow, and anything that would seem to increase it is perpetual motion. Look up Betz' law, for instance. |
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//When concentrating wind, there will be some loss of energy to heat, but the overall kinetic energy of the system, which is mass times speed, is preserved, because although inside the funnel the mass is moving faster, outside the funnel the mass in most of the area has been slowed down to almost a total stop, or converted into pressure (which is a translation of the kinetic energy between the molecules and the funnel surface, similar somewhat to heat).// |
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Sorry, that's just nonsense. The air must move faster while passing through the constriction than while passing through the (wider) entrance - all of it. That represents additional energy, which must come from somewhere. //outside the funnel the mass in most of the area has been slowed down to almost a total stop, or converted into pressure// is something that simply doesn't happen, and cannot happen in a device that harvests energy from a (subsonic) flow. Again, see Betz' law. I think you're making all this up. |
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Betz law applies to the total swept area at the
mouth of the duct, or to the wind, at speed, at
the constriction. It still applies. |
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And one of the major advantages of a shrouded
turbine, which this one wouldn't gain, is the wider
shroud on the outlet, which allows the
downstream air to expand more coming off the
turbine. |
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Air is flowing top to bottom. You end up with still
air between the slashes, and slower air going
around the unit, in exchange for faster air flow
through the center. Total energy is conserved. |
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If not, what does happen to the air hitting the
slashes? It has to go somewhere. |
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//what does happen to the air hitting the slashes?// At low speeds, there is a (slight) pressure rise within the region defined by the entrance slashes, which causes some of the air that would flow through a straight tube to flow around the outside of the constricted tube. The flow rate within the (constricted) tube is reduced, and total energy is conserved. But _less_ power is available inside the constricted tube than would be available inside a straight tube of the same widest diameter. |
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(At supersonic speeds, the situation is very different. All air entering the mouth of the funnel must pass through, since the (subsonic) pressure wave cannot travel upstream. A shock wave forms, and there is substantial heating and compression of the air. Energy is still conserved, but in a radically different way.) |
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And [pashute] in a steady state system, of course there is conservation of power. It follows directly from conservation of energy. |
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Added a couple of links, explaining how ducted turbines work (the naive version, a funnel-like arrangement somehow 'concentrating' the wind, is incorrect - or at least, over simplified), and why they are mostly not a good idea. |
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//why they are mostly not a good idea.// |
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Which is not the same thing as "do not work". |
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If they allow you to intercept wind you otherwise could not, there is an advantage. Admittedly, as I've said above, this set-up misses the major advantage (ducted expansion, not ducted compression), and will suffer from air diverting around, but the ducting does work. |
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// the same mass per time then you would
have the same mass per time traversing a
smaller area, and therefore travelling at a
greater velocity¹, and thus having more
power - clearly impossible // |
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Bernoulli's principle <link> |
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I'm not really disagreeing with you, [MechE]. The article in my first link says that 'efficient controlled diffusion at outlet can lead to power enhancements of more than 30%', and 'Clearly, relative to this optimum unshrouded case, the
flow downstream of the turbine can be diffused to a greater
extent with a shroud by expanding the area of flow by
more than a factor of 2. This reduces the pressure at the
turbine and hence increases the mass flow and power.' but that 'there is little to be gained from controlled diffusion at the inlet'. |
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The naive idea of greatly concentrating the air flow using a funnel-like arrangement (open towards the wind) is bunkum. The effect is very slight, and is largely offset by the need for lower turbine resistance (to maintain total mass flow). |
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//Bernoulli's principle// Good point. I stand corrected. |
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Nonetheless, it seems that you can only concentrate a free air flow a little bit using a funnel-like arrangement. (I'm largely trusting the one scholarly article I've been able to find on the subject). |
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OK, give me a week to study this... |
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Just a quick question to mother - why are you not
taking into consideration the air moving outside the
ducts, not able to enter the duct because of the
limited amount of mass passing there, do to pressure
and speed dynamics? Supposedly I lose some of the
energy compensated by concentrating it from a large
area. |
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I'm not sure I follow you... The air that doesn't enter the duct is wasted (not harvested). You can't concentrate the wind from a large area, because attempting to do so would increase the resistance, decreasing the swallowing power, and reducing the amount of wind harvested. There is little to gain, because ordinary wind speeds do not generate large static pressures when intercepted. |
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You can concentrate the wind (i.e. make the entrance to the duct larger than the turbine) a rather small amount, but you need to reduce the resistance of the turbine to compensate, and the overall increase in power is very small. [8th] was correct, in that a high speed, low resistance turbine is a natural fit for a convergent-divergent shroud, but the effect is small. |
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Thanks spider for your patience, |
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So, if I have a 10 meter wide and 2 meter tall duct
entry, funneled first into 10 meters wide but only 1
meter tall (that's the "under") section and then
upwards into a cylinder of 1.5 meter diameter,
assisted by power from the air NOT able to enter the
ducts because of pressure, passing OVER the
contraption and "pulling" the air out of that
cylinder... |
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Are you saying that the additional power received
would be negligible compared to : |
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a. a simple 1.5 meter diameter turbine (horizontal
axis). |
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b. The same contraption, with only 1.5 meter wide
duct for air entry. |
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OK I only noticed the new links now. I'll read and get
back. (somehow I thought the last link was MechE's
VAWT HAWT, I didn't see spiderMother's). |
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My question is valid. The IMeche article discusses the
diffusion as the limiting factor for speed through the
"funnel". Of course I'm not talking about a "free
lunch".
This has nothing to do with over unity. It has to do
with
practicality, ease of construction, less moving parts,
and, yes, comparison with any other wind system that
is
only 2-3 meters high (but can be as wide as we wish). |
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If anybody has solidworks maybe they can check it
out and give us the results. It should be fairly easy to
set up. |
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I'm partly learning this as I go along too. I can't believe my awful electrical analogy^ hasn't been torn to shreds! Current in a constricted wire behaves like water in a constricted pipe from a pressure source, which is exactly the way wind encountering a highly constricted tube doesn't. A slightly better analogy is an impedance mismatch. |
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I think that the power output will be a > b > your idea, sorry. As the song goes, you might as well try and catch the wind. |
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The trick to harvesting wind is to cover a large area, and to dial in the impedance (pressure difference for a given flow rate) so the Betz limit is approximated. That optimal impedance is rather low. That's why the turbine is generally just placed in the airflow. A rather short, straight, not-much-wider-than-the turbine, well designed duct/shroud increases the turbine's output - but not by much, and probably not enough to justify the extra weight and cost. Anything more convoluted is probably a step backwards. |
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If I might shamelessly draw your attention to my idea
[linked], which had a similar goal and used a vaguely
similar principle, what I suggested was that the large
area of an apex roof would provide additional
windspeed to a 10cm radius turbine, although I didn't
think it would be anywhere near as effective as a
turbine the same area as the roof. |
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but instead of just ruling it off, a simple simulation
could check the worthiness of this idea, and confirm or
"bust" this idea. |
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No one mentioned that a very similar concept has been
used for thousands of years as a "humidifier"?
I think the idea may work, though possibly not with great
efficiency. |
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As stated above (see last two new links) spider and
Meche are probably correct. |
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So question now is if a short diagonal deflection
outwards allowing the continued wind flow at the
exit - directed through a flat horizontal turbine
(vertical axis) but with no funneling would be terribly
inefficient? |
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all ram air designs present an identical problem: that the differential of pressure is not large enough between one side of the system and the other. a giant round disk in the air would make a very good airfoil, acting just as a sail, but it does so because it causes a differential of pressure, that the structure is not so much (pushed) as (pulled) by the relatively lower pressure caused on the side facing away from the wind. All wind energy is contingent on causing this differential of pressure, then extracting the energy from it. When we add an elaborate ram air action we produce a higher pressure at the front of the turbine (or what have you) but do not account for the need to dissipate that higher pressure on the aft side of the turbine. without an equivalently effective extractor to replicate the action of the ram the system will fail. Also it illustrates the fact that the most efficient system will use the absolute minimum rise in pressure with the steepest fall back to the ambient pressure in the outflow. |
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4 new links: (minimal concentration claimed to
work, skepticism of new wind projects, prof.
explaining why funnels won't work, university
study of promising wind concentration techniques
in built areas) |
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But that is exactly what I'm proposing now. I
realize
the funnels won't work, so discarded them. I'm
talking now only about deflecting the wind. |
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Its almost like a wing, except that the wind does
not
push on a solid but rather on the turbine/rotor.
The
air above is pulling away. |
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Perhaps analogous to the rotor of a gyrocopter. |
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