Hairlike filaments help spawn the swirling vortex and create drag
When you’re essentially a little ball of floof, flying is hard.
To ride the wind, dandelion seeds stir up a weird type of whirlpool in the air directly above them. The newly discovered way of moving through the air, described October 17 in Nature, resolves a long-standing question about how the seeds stay aloft.
Dandelion seed flight is not unlike the flight of Mary Poppins: Utterly charming, yet inexplicable when it comes to physics — until now. When a gust of wind plucks a seed from the plant’s fuzzy head, a fluffy structure called the pappus keeps the seed aloft before it ultimately falls to the ground. The structure, which extends from the seed, is made up of tiny hairlike filaments, making it mostly empty space.
“It’s a weird structure,” says coauthor Naomi Nakayama, a biophysicist at the University of Edinburgh. “Nobody really knew how it could fly.”
So Nakayama and her colleagues dug into the weeds. High-speed video and mathematical simulations revealed that the pappus filaments act together like a uniform sheet or a parachute and create drag — a force that counters gravity. Air also flows around the pappus and gets sucked into the area just above it. This air forms a swirling bubble that the researchers call a separated vortex ring, which adds to the drag.
Swirlin’ and whirlin’
Dandelion seeds are attached to a fuzzy structure called a pappus (top-down view at left). The pappus creates a swirling bubble of air in the low-pressure region above the structure (seen at right with a seed in a wind tunnel). The pappus is key to making this work. The spacing of its filaments allows just enough airflow through to keep the vortex stable, helping the seed float.
As far as vortex rings go, the dandelion’s is odd. 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 Cathal Cummins, a postdoctoral researcher in Nakayama’s lab.
The bubble stays in place above the seed because the filament parachute is leaky. Some air gets through the pappus, and a pressure gradient forms with low pressure above the filaments and high pressure below them, keeping the bubble stable. This low pressure is also what sucks in the air flowing around the filaments to form the bubble in the first place.
The way the dandelion moves through air is four times as efficient at generating drag — and therefore fighting against gravity — as a more parachute-like structure would be. Other seeds with fuzzy exteriors or extensions may also use these odd vortex rings to fly through the air.
SEEDS, FLOW AND VIDEOTAPE Researchers used high-speed video to characterize the airflow around a humble dandelion seed.
C. Cummins et al. A separated vortex ring underlies the flight of the dandelion. Nature. Published online October 17, 2018. doi:10.1038/s41586-018-0604-2.