separated vortex ring

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The Flight of the Dandelion

The common dandelion (Taraxacum officinale) comes with a collection of traits that make it a very successful weed. Nearly everything about it screams success, from its asexually produced seeds to its ability to resprout from a root fragment. Evolution has been kind to this plant, and up until the recent chemical warfare we’ve subjected it to, humans have treated it pretty well too (both intentionally and unintentionally).

One feature that has served the dandelion particularly well is its wind-dispersed seeds. Dandelions have a highly-evolved pappus – a parachute-like bristle of hairs attached to its fruit by a thin stalk. The slightest breath or puff of wind will send this apparatus flying. Once airborne, a dandelion’s seed can travel up to a kilometer or more away from its mother plant, thereby expanding its territory with ease.

Such a low-growing plant achieving this kind of distance is impressive. Even more impressive is that it manages to do this with a pappus that is 90% empty space. Would you leap from a plane with only 10% of a parachute?

Dandelion flight was investigated by researchers at the University of Edinburgh, who used a wind tunnel along with long-exposure photography and high-speed imaging to observe the floating pappus. Their research was presented in a letter published in an issue of Nature in October 2018. Upon close examination, they observed a stable air bubble floating above the pappus as it flew. This ring-shaped air bubble – or vortex – which is unattached to the pappus is known as a separated vortex ring. While this type of vortex ring had been considered theoretically, this marked the first time one had been observed in nature.

Seeing this type of air bubble associated with the dandelion’s pappus intrigued the researchers. About a 100 filaments make up the parachute portion of the pappus. They are arranged around the stalk, leaving heaps of blank space in between. The air bubble observed was not what was expected for such a porous object. However, the researchers found that the filaments were interacting with each other in flight, reducing the porosity of the pappus. In their words, “Neighboring filaments interact strongly with one another because of the thick boundary layer around each filament, which causes a considerable reduction in air flow through the pappus.”

The pappus acts as a circular disk even though it is not one, and its limited porosity allows just enough air movement through the filaments that it maintains this unique vortex. “This suggests,” the researchers write, “that evolution has tuned the pappus porosity to eliminate vortex shedding as the seed flies.” Fine-tuned porosity and the resultant unattached air bubble stabilizes the floating fruit “into an equilibrium orientation that minimizes [its] terminal velocity, allowing [it] to make maximal use of updrafts.” The result is stable, long distance flight.

Wind-dispersed seeds come in two main forms: winged and plumed. Winged seeds are common in trees and large shrubs. They benefit from the height of the tree which allows them to attain stable flight. While such seeds have the ability to travel long distances, their success is limited on shorter plants. In this case, plumed seeds, like those of the dandelion, are the way to go. As the researchers demonstrated, successful flight can be achieved by bristles in place of wings. The tiny seeds of dandelions seen floating by on a summer breeze are not tumbling through the air haphazardly; rather, they are flying steadily, on their way to spoil the dreams of a perfect lawn.

Further Reading (and Watching):