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!

Eat, Prey, Swim: Baby Starfish Spin Miniature Whirlpools To Scoop Up Food

      Baby starfish scoop up food by spinning miniature whirlpools. These vortices catch algae and draw them close so the larva can slurp them up, researchers, including Dr. William Gilpin et al from Stanford University report in Nature Physics (12/19/16).

      Before starfish, which are not fish but echinoderms, take on their familiar shape, they freely swim ocean waters as millimeter-sized larvae. 

      To swim around on the hunt for food, the larvae paddle the water with hair-like appendages called cilia. 

     Starfish larvae also adjust the orientation of these cilia to fine-tune their food-grabbing vortices.

      Researchers studied larvae of the bat star (Patiria miniata), a starfish found on the U.S. Pacific coast, 

by observing their activities in seawater suffused with tiny beads that traced the flow of liquid. (Does this remind you a bit of Obi, the parrotlet, and observing the air around him that was suffused with aerosol droplets as he flew?) 

Too many swirls can slow a larva down, the scientists found, so the baby starfish adapts to the task at hand, creating fewer vortices while swimming and whipping up more of them when stopping to feed.

          A video of the  experiment is linked here.

        And I thought we were supposed to wait an hour after eating before swimming. . .

        A post on starfish the week we lost two stars, Carrie Fisher and her mom, Debbie Reynolds? Twin stars/spirits, born of an instant. Rest easy together, ladies.

Starrily,
Steph 

My artist friend, Judith's, words to Don T. on a clay tablet:

        

Obi, the Parrotlet, Laser Goggles, and Air Vortices

         A parrotlet named Obi has his own set of custom-made safety goggles (made from human-sized ones) to protect his eyes when he flies through a laser sheet, as he has been trained to do. Researchers at Stanford University are studying how air moves in the wake of Obi's flight.

  

     This 46-second video of Obi's flight shows the vortices swirling around the bird's flightpath, thanks to the lasers he is flying through.

        Researchers at Stanford University are studying how the air moves in the wake of a bird's flight. Thank to Obi, along with graduate students Eric Gutierrez and Diana Chin, and mechanical engineer David Letink, we now know that there may be some faults in many flight models.

       "The goal of our study was to compare very commonly used models in the literature to figure out how much lift a bird, or other flying animal, generates based off its wake," Chin said. "What we found was that all three models we tried out were very inaccurate because they make assumptions that aren't necessarily true."

     To test the models, the team trained Obi, a parrotlet or pocket parrot, to wear the goggles and fly from one perch to another. Then a laser sheet was seeded with non-toxic, aerosol-sized particles. As Obi flew through this laser sheet, the disturbed particles swirled into vortices left in his wake.

     The tests showed something unexpected. Computer models predicted that once the whirling air patterns or vortices were created by a bird's wings, they would remain relatively stable in the air. But the patterns Obi traced began to disintegrate after the bird flapped its wings just a few times.

      "We were surprised to find the vortices that are usually drawn in papers and text books as beautiful doughnut rings turned out to break up dramatically after two to three wing beats," Lentink said. He explained that this meant the models, which are widely used in animal flight studies to calculate an animal's lift based on the wake it produced, were likely inaccurate.


      "Whereas vortex breakup happens far away behind the aircraft (more than a thousand meters) in birds, it can happen very close to the bird, within two or three wingbeats , and it is much more violent," said Letink.

      The team also found that the models they tested did not accurately predict the lift generated by Obi's wings. This research joins other studies conducted by the lab on many different animals, including different bird species, bats, and insects. The team hopes the findings can be used to help develop flying robots or drones that flap wings, rather than rely on rotors.

Wonder if Obi knows Obi-wan Kenobi,
Steph