Clock this: Dandelion Seeds Have a Vortex Floating Above Their Filaments

     "Dandelion seeds fly using a method that researchers thought would not work in the real world, according to a study published on October 17, 2018, in Nature." Here is a dandelion head, also called a dandelion clock:

     "When some animals, airplanes, 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 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 harbor 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 Dr. Naomi Nakayama, a plant scientist at the U. of Edinburgh.

      As far as vortex rings go, the dandelion's is unusual. Normally, such air bubbles stay attached to an object or totally separate and disappear. But the dandelion’s bubble separates and hangs out above the seed. “When you show it to a fluid dynamicist, it blows their mind,” says study coauthor Dr. Cathal Cummins.

     Researchers were curious about how these bristly seeds of the pappus (the seed plus the filaments) stayed in the air because they looked so different from the wing-like seeds of other plants, such as maple trees. Those structures act like the wings of a bird or airplane, generating pressure differences above and below the wing to fly. To find the answer, Dr. 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 spokes of the seeds seems to be the key to the stability of these detached vortices, says Dr. Cummins. 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. She described it as “extremely 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 Dr. 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 Dr. Richard Bomphrey, a comparative biomechanist at the Royal Veterinary College. “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.”

        Wow, hanging vortices in a common flower. Nature sure is dandy!
Steph

Zoë had her Peace Corps gong out service, hitting the gong thrice to represent three years of service. Proud of her and looking ahead to what she'll do next!