56aises brittle star ~ external surfaces

( 56aises humpback wind turbine, revision 1 )

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27 January 2011 .

This model is adapted from the 56aises humpback wind turbine, (posted 13 January 2011),

and the 55 humpback wind turbine, (posted 21 January 2011) .

The design is intended to sync with the wind when its outermost lift-producing surfaces are traveling at about 6.2 times windspeed .

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It differs from the 55 humpback in its longer and serrated wings,

the whole assembly resembling the deep-sea relative of the star-fish .

Apart from expanding upon the 56aises humpback's previous post to include all five wings and their hub area,

this model has longer and reshaped wingtip tails .

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These tails attempt to better address the issue of wingtip vortices ; (Wikipedia has a good page) .

On a wing, surface air will seek to flow from the high pressure, 'ventral', side to the low pressure, 'dorsal', side by way of the tip's, (pressure neutral), outside edge .

The result is a steady vortex which trails behind the wingtip like a small tornado .

Surprisingly strong and long lived, these drain energy from the system, and carry with them potential for noise and hazard .

To the extent that wingtip vortices are not resolved in a wind turbine's design, one can expect them to hit the tower .

Under some conditions, (if i understand correctly), these impacts can be loud ;

and a cumulative source of wear to such components as mounts .

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I believe that four basic strategies for their control have evolved .

First is to taper the wing to a very fine tip which offers little outside edge for the pressure differential to seek ;

( this is preferred in wind turbines ) .

A second would be to interrupt the 'siphon effect', (my imperfect analogy), acting across the tip's outer edge ;

( this is preferred in aircraft ) .

Third is to rake the wingtip, forcing the siphon to work at a less advantageous angle .

And fourth is to allow the vortex, while structuring the wing's outer edge so that it forms

~ and leaves ~ cleanly ; (this is applied in some fighter aircraft) .

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The solution i would apply here includes extending the #aises designs' wingtip tails .

I hope this will have the effect of (further) stretching the siphon ;

and as above, by tuning it downwind, of placing it at an unfavorable angle .

This aspect is similar to the 'rake & taper' approach, already in use .

However, the siphon might respond by snaking around the outside edge of the turn .

Therefore, a second aspect of the solution here proposed is to change the geometry of the tail from a lifting profile,

(where it meets the last regular segment of the wing), to a lift-neutral profile, (where it meets the tip bulb) .

This should equalize pressure on the upper and lower surfaces of the wing at the end, giving the siphon less foothold .

The measure also causes a lip to form on the underside of the turn's outer edge,

which may help prevent the siphon from getting around it, as mentioned above .

This aspect has similarity to the tip-tank approach .

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Notably : in many wings, the areas of low and high pressure forming on the dorsal and ventral sides are contiguous ;

thus the pressure differential available to drive the tip-siphon and its vortex is cumulative across the entire wing .

It seems possible that in brittle-star-type turbines, because of the serration,

the dorsal and ventral pressure zones may be subcompartmentalized .

This may result in less powerful tip vortices, but a number of inner ones .

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Full views are in the column to the left ;

close-up views of the hub area, (with truncated wings), are in that to the right ;

and close-up views of the last regular sections of a wing, with the wingtip tail and bulb, are below .

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