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October 21, 2018

The "impossible" flight of the dandelion

Consider the headline of an article published in the October 17, 2018 issue of Nature:

Dandelion seeds fly using "impossible" method never before seen in nature

Below, the story.

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Dandelion seeds fly using a method that researchers thought couldn't work in the real world, according to a study published on 17 October in Nature.

When some animals, aeroplanes or seeds fly, rings of circulating air called vortices form in contact with their wings or wing-like surfaces. These vortices can help to maintain the forces that lift the animal, machine or seed into the air

Researchers thought that an unattached vortex would be too unstable to persist in nature. Yet the light, puffy seeds of dandelions use vortices that materialize just above their surfaces (below)

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and lift the seed into the air.

Dandelion seeds bear filaments that radiate out from a central stalk like the spokes on a bicycle wheel, a feature that seems to be the key to their flight. Many insects harbour such filter-like structures on their wings or legs, suggesting that the use of detached vortices for flight or swimming might be relatively common, says study co-author Naomi Nakayama, a plant scientist at the University of Edinburgh, UK.

Researchers were curious about how these bristly seeds stayed in the air because they looked so different from the wing-like seeds of other plants, such as maples. Those structures act like the wings of a bird or aeroplane, generating pressure differences above and below the wing to fly. To find the answer, Nakayama and her colleagues put dandelion seeds in a vertical wind tunnel and used a laser to illuminate particles that helped to visualize the airflow around the seed.

That's when they saw the vortex floating above the seeds. The amount of open space between the seed’s spokes seems to be the key to the stability of these detached vortices, says study co-author Cathal Cummins, an applied mathematician at the University of Edinburgh. Pressure differences between the air moving through the spokes and the air moving around the seed creates the vortex ring.

Previous studies have found that dandelion seeds always have between 90 and 110 bristles, says Nakayama. It's "scary consistent", and that consistency turns out to be very important.

When the team designed small silicon discs to imitate these spokes, they produced models with a range of openings: from solid discs to ones that were 92% air, like the structures on the dandelion seeds. When the researchers tested these model seeds in their wind tunnel, they found that only the discs that best approximated dandelion seeds could maintain the detached vortex.

If the number of openings in the discs was even 10% off of those in dandelion seeds, the vortex destabilized. The seed looks inefficient for flight because it has so much open space, says Nakayama, but these openings are what allow the unattached vortex ring to remain stable.

It's great to see an analysis of something we see every day but didn't fully understand, says Richard Bomphrey, a comparative biomechanist at the Royal Veterinary College in Hatfield, UK. "To discover that there were aerodynamic mechanisms that we didn't already know — despite the fact that we can fly things at Mach 9 — is always exciting."

Below, a Nature editorial expanding on the discovery.

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Revealed: the extraordinary flight of the dandelion

The English poet and artist William Blake was no fan of the reductionism of Isaac Newton. True discovery, and therefore knowledge, Blake insisted in his poem "Auguries of Innocence", was to be found in the everyday, where a world could be seen in a grain of sand and "heaven in a wild flower".

Today, we know of exotic states of matter that can slow the vast speed of light to a mere sprint. And astronomers have spotted more than 3,800 planets in more than 2,800 distant stellar systems: a staggering rate of discovery, given that the first confirmed detection of a planet orbiting another star similar to the Sun was as recent as 1995.

None of this should blind us to the fact that — as Blake suggested — some of the most surprising discoveries come from the world of the familiar. No one has visited an exoplanet, but most people know what a dandelion looks like. This flower (Taraxacum officinale) is found worldwide. And, as many a child discovers to their delight, when a dandelion sets seed, the flower (actually, hundreds of tiny florets) turns into a mass of seeds known as a dandelion clock. Each seed is suspended from a parachute-like stalk — easily released by a puff of breath.

The parachute is a bunch of bristles called a pappus. Each pappus carries around 100 filaments, each attached to a central point, rather like the head of a chimney sweep’s brush. Just like a parachute, it increases aerodynamic drag, slowing the descent of each seed and allowing it, once aloft, to be wafted kilometres from the parent plant. So much we know.

Here’s the surprising part — the mechanism of this dispersal was unknown until now. As researchers write in Nature this week, the bristles are arranged so that when the pappus falls, air flows between them and creates a low-pressure vortex, like a smoke ring. This vortex travels above the pappus and yet is not attached to it, an invisible yet faithful familiar that generates lift and prolongs the seed’s descent.

The key lies not in the bristles of the pappus, but in the spaces between them. If projected onto a disc, the bristles together occupy just under 10% of the pappus's area, and yet create four times the drag that would be generated by a solid disc of the same radius. The study shows that air currents entrained by each bristle interact with pockets of air held by its neighbours, creating maximum drag for minimum expenditure of mass. The pappus's porosity — a measure of the proportion of air that it lets pass — determines the shape and nature of the low-pressure vortex.

All falling objects, from feathers to cannon balls, create turbulence in their wake. But it takes a rare combination of size, mass, shape and, crucially, porosity for the pappus to generate this vortex ring. Size is also particularly important, because from the point of view of something as small as a pappus, the air is appreciably viscous. At such a scale, a parachute consisting of a bunch of bristles is as effective as the aerofoil found in larger seeds that disperse from taller plants — such as the winged seeds of the maple. In the same way, the tiniest insects do not fly with solid wings, but swim through the air using "paddles" made of bristles.

It's an example of how evolution can produce ingenious solutions to the most finicky problems, such as seed dispersal. There are many things unknown that are smaller than atoms, or larger than galaxies, or billions of years away in time. But there are secrets held by things that we take for granted — things on a human or near-human scale — that seem all the more precious for it. Heaven in a wild flower, even.

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Still with me?

Good.

Now you're ready for the main event: the abstract of the actual scientific report.

It follows below.

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A separated vortex ring underlies the flight of the dandelion

Wind-dispersed plants have evolved ingenious ways to lift their seeds. The common dandelion uses a bundle of drag-enhancing bristles (the pappus) that helps to keep their seeds aloft. This passive flight mechanism is highly effective, enabling seed dispersal over formidable distances; however, the physics underpinning pappus-mediated flight remains unresolved. Here we visualized the flow around dandelion seeds, uncovering an extraordinary type of vortex. This vortex is a ring of recirculating fluid, which is detached owing to the flow passing through the pappus. We hypothesized that the circular disk-like geometry and the porosity of the pappus are the key design features that enable the formation of the separated vortex ring. The porosity gradient was surveyed using microfabricated disks, and a disk with a similar porosity was found to be able to recapitulate the flow behaviour of the pappus. The porosity of the dandelion pappus appears to be tuned precisely to stabilize the vortex, while maximizing aerodynamic loading and minimizing material requirements. The discovery of the separated vortex ring provides evidence of the existence of a new class of fluid behaviour around fluid-immersed bodies that may underlie locomotion, weight reduction and particle retention in biological and manmade structures.

October 21, 2018 at 12:01 PM | Permalink


Comments

"Hundreds of millions of years of natural selection produces excellent results"

Shocker!

Posted by: Fred | Oct 23, 2018 12:15:47 AM

Hah! That outtake at the end of the video -- I was anticipating that the whole time.

Posted by: Luke | Oct 22, 2018 9:13:29 AM

Absolutely fascinating! They are my most favorite flower on earth and now even more so. To me they are 2 flowers in one...the vibrant yellow flower and the fluffy white "flower". Thanks for sharing this...utterly fascinating !

Posted by: Susan Ishida | Oct 21, 2018 5:10:10 PM

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