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Wind-generating wind turbine

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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...

MaxwellBuchanan, Sep 13 2015

The why. Public_20clifftop_20skydiving
[MaxwellBuchanan, Sep 17 2015]

Obligatory XKCD https://xkcd.com/1378/
wind turbine joke [sninctown, Sep 18 2015]

[link]






       //We'll get to the "why?" later// [marked-for-tagline]
FlyingToaster, Sep 13 2015
  

       Makes Big Wind was the most disgusting Native American warrior.
4and20, Sep 13 2015
  

       //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.
MaxwellBuchanan, Sep 13 2015
  

       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.   

       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.
EnochLives, Sep 14 2015
  

       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.
MaxwellBuchanan, Sep 14 2015
  

       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.
EnochLives, Sep 14 2015
  

       Well, the same argument could be made about normal wind turbines or propellors, but it doesn't seem to be the case.   

       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).   

       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°.   

       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.   

       And here I was thinking that only the "why" would be controversial.
MaxwellBuchanan, Sep 14 2015
  

       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.
MechE, Sep 14 2015
  

       Yes, that's certainly true. A possible solution would be to have an annulus dividing two distinct zones.
MaxwellBuchanan, Sep 14 2015
  

       //don't have much blade speed to work with on the inner portion.   

       So, don't have an inner portion, just use rods.   

       I have this sneaking sensation that somewhere in Mother Nature's design set, there's something that's already the most efficient way...
not_morrison_rm, Sep 15 2015
  

       // I have this sneaking sensation that somewhere in Mother Nature's design set, there's something that's already the most efficient way... //   

       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.
Tulaine, Sep 15 2015
  

       /wind lever/ excellent.   

       The robotic preparation for the Martian colony should have large assemblies of these, to augment low density wind and enable gas collection / pressurization.
bungston, Sep 15 2015
  

       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.
bungston, Sep 15 2015
  

       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.
scad mientist, Sep 15 2015
  

       // 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.
MaxwellBuchanan, Sep 15 2015
  

       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...
not_morrison_rm, Sep 15 2015
  

       Many bacteria do that, but I don't see how it applies here...
MaxwellBuchanan, Sep 15 2015
  

       // 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.   

       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.   

       Of course maybe you're imagining something quite different than I am since we are still avoiding the pesky question of "Why?"
scad mientist, Sep 15 2015
  

       /how it applies/   

       probably referring to Tulaine's assertion that Ma Nature does not like axles.
bungston, Sep 16 2015
  

       Ah, good point.
MaxwellBuchanan, Sep 16 2015
  

       Why not?
pashute, Sep 16 2015
  

       NIH syndrome.
Ian Tindale, Sep 17 2015
  

       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.
hippo, Sep 17 2015
  

       Time to hear the reason for this large and very inefficient air compressor?
cudgel, Sep 17 2015
  

       <linky>
MaxwellBuchanan, Sep 17 2015
  

       //Well, the same argument could be made about normal wind turbines or propellors, but it doesn't seem to be the case.//   

       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.   

       Also, funnels can't increase windspeed, but I think we're agreed on that.
EnochLives, Sep 17 2015
  

       //Near the axle, the blades will be slow-moving and moving through the air in the least aerodynamic orientation possible...   

       <repeats self> So, don't have an inner portion, just use rods.   

       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....
not_morrison_rm, Sep 18 2015
  

       Alternate design:   

       Aerofoil propeller near the center, centrifugal fan around the outside.   

       Not sure it would work any better, but it takes advantage of the extra speed at the tips, and separates the air streams.
MechE, Sep 18 2015
  

       // separates the air streams.   

       <obligatory mention of "Don't cross the streams">
not_morrison_rm, Sep 18 2015
  

       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.
bs0u0155, Sep 18 2015
  

       ...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.
bs0u0155, Sep 18 2015
  

       //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.
MaxwellBuchanan, Sep 20 2015
  
      
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