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We'll get to the "why?" later.
The problem is this: how to make a wind-powered
device
to produce very high-speed winds.
A trivial way to do this would be as follows. Take one
large
wind turbine (minus the generator), and couple it to a
smaller fan via a suitable ratio gearbox or belt-drive.
Now
for each revolution of the large wind-driven fan, the
smaller fan turns (say) 50 times, thereby creating a small
stream of very high-speed air.
This, however, is a messy solution. There is another way.
In a conventional wind turbine or a propellor, the blades
twist so that they are steeply-pitched near the hub, but
only shallowly-pitched at their ends. This compensates
for
the fact that he tips are moving faster than the roots of
the blades.
Suppose we take a conventional wind turbine, then, and
make this blade-twist even greater. So now the roots of
the blades have an *extremely* steep pitch, while the
blade tips have an *extremely* shallow pitch.
For any given speed of rotation, now, the air near the
hub
wants to be moving faster past the blades than the air
near the tips.
So, if we build this huge passive fan and face it into the
wind, it will use the low-speed wind passing over the
outer
parts of the blades to drive a high-speed core of air over
the inner part.
In effect, we have created a "wind lever" that might
produce a core of air at 120mph in a breeze of only
30mph.
Now, to come back to the "why?" part...
The why.
Public_20clifftop_20skydiving
[MaxwellBuchanan, Sep 17 2015]
Obligatory XKCD
https://xkcd.com/1378/
wind turbine joke [sninctown, Sep 18 2015]
[link]
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//We'll get to the "why?" later// [marked-for-tagline] |
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Makes Big Wind was the most disgusting Native American warrior. |
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//I was expecting the turbines to be able run in
reverse// Well, they probably could, if the inner
section had a reverse pitch compared to the larger
outer section. |
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Back in the day there were attempts to build all sorts of
wacky gas turbine arrangements. I remember seeing one of
these in which the compression and expansion functions
were carried out on the same disc using a mechanism not
unlike what you have proposed. |
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A fundamental problem is that you just don't have much
blade speed to work with on the inner portion. You can only
camber the blade so much before the air just ignores the
shape of the aerofoil and does its own thing instead. |
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That's true. However, things can work without
relying of aerofoilics. In theory, you could do this
with perfectly flat blades; it would be less efficient
than proper profiles, but would still work. |
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I don't see how a flat plate helps you achieve this. A flat
plate is just a poorly designed aerofoil. The air will just
separate off the leading edge due to the extreme
incidence. |
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Well, the same argument could be made about
normal wind turbines or propellors, but it doesn't
seem to be the case. |
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Take a flat-bladed propellor spinning at a constant
rate, and with no twist in the blades. If the pitch
is zero (ie, the blades are slicing through the air in
a plane at right angles to the axis), it moves no air.
If the pitch is slightly positive, it will move air
slowly. If the pitch is increased, it will move air
more quickly (and will require more power to drive
it, of course). Once the pitch hits 90° (chord of
the blades at right-angles to the axis of rotation),
of course it stops moving air again (except
circularly). |
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Yes, there will be an angle less than 90° where
separation occurs, but that's OK - we don't need to
go all the way to 90°. |
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If you wanted to get the maximum "leverage",
you'd actually have something roughly like a
passive turboprop, with a big set of propellorish
blades directly driving a shaft on which were
mounted several sets of blades with successively
steeper pitches. |
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And here I was thinking that only the "why" would
be controversial. |
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You're also going to have extreme turbulence and vortex
generation in the transitional zone, which is going to
sharply limit the velocity of the central column. |
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Yes, that's certainly true. A possible solution would
be to have an annulus dividing two distinct zones. |
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//don't have much blade speed to work with on the
inner portion. |
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So, don't have an inner portion, just use rods. |
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I have this sneaking sensation that somewhere in
Mother Nature's design set, there's something that's
already the most efficient way... |
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// I have this sneaking sensation that somewhere in Mother
Nature's design set, there's something that's already the most
efficient way... // |
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So far mother nature seems to have done OK without
resorting to anything wheel like or axle based (ready to be
proved wrong, of course). Her systems of wings, diaphragms
and bladders seem to be all that's needed to generate flows
of air, gases and liquids for motion and propulsion when legs
and feet aren't enough. |
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The robotic preparation for the Martian colony should have large assemblies of these, to augment low density wind and enable gas collection / pressurization. |
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A fine use for this scheme would be if it were mounted on a boat with the large collector blades positioned to turn from the wind of the boats forward progress, and the smaller central blades turned to provide a reverse breeze which in turn propelled the sailboat. The sail must be turnable to decrease momentum, as otherwise the boat would tend to accelerate indefinitely. |
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Wouldn't a cone with a hole in the center provide basically
the same effect: slow the wind in the outer area and speed
it up in the center... The only advantage of a turbine for
this purpose seems like it would be to allow for a higher
profit margin on installation and to accelerate natural
selection in birds by eliminating the ones that don't know
to avoid spinning blades. |
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// a cone with a hole in the center// I thought that.
But when I posted such a device, as a means of
producing skydiveable vertical wind velocities, I was
told that it can't happen - there's a limit to how much
a funnel can increase wind speeds. |
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I do seem to recollect one bacteria that has
sort of axle and a corkscrew tail, which it spins
on its axis to do the propulsionly bit... |
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Many bacteria do that, but I don't see how it applies
here... |
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// there's a limit to how much a funnel can increase
wind speeds. // Quite true, but it WILL increase the
wind speed somewhat. I also suspect that it would
increase the wind speed somewhat more than this idea. |
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Your initial suggestion of using a 50:1 gearbox actually
could create a significantly higher speed wind, but I think
your final simplified design is going to severely limit
performance. The wind is already moving through the
turbine at some speed. Since the part of the turbine in
question is going much slower than the tips of the whole
turbine, it seems to me it would be going no faster in
circles than the air is traveling through it. In that
situation, adjusting the angle can make it so that you are
adding some energy to the wind, but I don't think it would
be very significant. |
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Of course maybe you're imagining something quite
different than I am since we are still avoiding the pesky
question of "Why?" |
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probably referring to Tulaine's assertion that Ma Nature does not like axles. |
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Near the axle, the blades will be slow-moving and moving through the air in the least aerodynamic orientation possible, so the air will just flow round them - just like gravy in a pan flows around a slowly stirring spoon. |
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Time to hear the reason for this large and very inefficient
air compressor? |
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//Well, the same argument could be made about normal
wind turbines or propellors, but it doesn't seem to be the
case.// |
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Not really. A properly designed aerofoil is cambered such
that the trailing edge is set to deliver the correct air-
deflection angle whilst the leading edge points
approximately into the oncoming flow, i.e. At near-zero
incidence. A flat plate will inevitably be at extremely high
incidence to the oncoming flow. The boundary layer will
not cope with the extreme level of diffusion being imposed
on it and will immediately separate. Hence an inclined flat
plate will operate stalled whilst a well designed aerofoil
will deliver the intended level of air deflection, up to a
point. |
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Also, funnels can't increase windspeed, but I think we're
agreed on that. |
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//Near the axle, the blades will be slow-moving and
moving through the air in the least aerodynamic
orientation possible... |
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<repeats self> So, don't have an inner portion, just
use rods. |
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But anyway, I'm thinking vortex stylee would be most
efficient, like them vortex drives they keep trying to
make for submarines, air's just another fluid.... |
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Aerofoil propeller near the center, centrifugal fan around
the outside. |
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Not sure it would work any better, but it takes advantage of
the extra speed at the tips, and separates the air streams. |
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// separates the air streams. |
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<obligatory mention of "Don't cross the streams"> |
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This gives me an idea, I reckon you could tune the opposing
flows from the inside and outside of the blades to be equal
quite easily. You could then use this to pump air through a
radiator... though the middle, back through the edges (with
appropriate flow design for ideal temp gradients). Sort of a
single-fan single-sided two pass rad-fan design. I'll pay $5
more for such tech in my next car. |
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...or you could switch the blades twice... 3 pass single fan
single direction.... the diameter increase will help with the
temperature change expanding the air. |
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//single-sided two pass rad-fan // But this would
less effective, shirley, since the returning air will
already have been heated in passing over the
radiator the first time. |
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