A separated vortex ring underlies the flight of the dandelion

Abstract

Wind-dispersed plants have evolved ingenious ways to lift their seeds 1,2. The common dandelion uses a bundle of drag-enhancing bristles (pappus) to help keep their seeds aloft. This passive flight mechanism is highly effective, enabling seed dispersal over formidable distances 3,4 ; however, the engineering underpinning pappus-mediated flight remains unresolved. Here, we have visualized the flow around dandelion seeds, uncovering an extraordinary type of vortex. This vortex is a ring of recirculating fluid, which is detached due 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 able to recapitulate the flow behaviour of the real pappus. The porosity of the dandelion's pappus appears to be tuned precisely to stabilize the vortex, while maximizing the aerodynamic loading and minimizing the material requirement. The discovery of the separated vortex ring signals the existence of a new class of fluid behaviour around fluid-immersed bodies that may underlie locomotion, weight reduction, and particle retention of biological and manmade structures. Dandelions (Taraxacum officinale agg.) are highly successful perennial herbs, which can be found in temperate zones all over the world 5. Dandelions, like many other members of the Asteraceae family, disperse their bristly seeds using the wind and convective updrafts 6,7. Most dandelion seeds likely land within 2 m 8,9 ; however, in warmer, drier and windier conditions, some may fly further (up to 20,000 seeds per hectare travelling more than 1 km by one estimate) 6,10. Asteraceae seeds routinely disperse over 30 km and occasionally even 150 km 3,4. Plumed seeds comprise a major class of dispersal strategies used by numerous and diverse groups of flowering plants, of which the common dandelion is a representative example. Plumed seeds contain a bundle of bristly filaments, called a pappus, which are presumed to function in drag enhancement (Fig. 1a-c). The pappus prolongs the descent of the seed, so that it may be carried farther by horizontal winds 11 , and it may also serve to orientate the seed as it falls 7,12. Dandelion seeds fall stably at a constant speed in quiescent conditions 2,13-15. For wind-dispersed seeds, maintaining stability while maximizing descent time in turbulent winds may be useful for long-distance dispersal 16,17. It is not clear, however, why plumed seeds have opted for a bristly pappus rather than a wing-like membrane, which is known to enhance lift in some other species (e.g., maples 1). Here, we uncover the flight mechanism of the dandelion, characterizing the fluid dynamics of the pappus and identifying the key structural features enabling its stable flight. To examine the flow behaviour around the pappus, we built a vertical wind tunnel (Fig. 1d, and M1), designed so that the seed ca...

Toy model for vortex-ring-assisted particle drag in superfluid counterflow

“…To conclude, we note that there is an appealing connection between the dynamics of VRP states and recent findings for the slow fall of seeds from wind-dispersed plants [41]. It has been found that the passive flight of dandelion seeds actually relies on the existence of approximate axisymmetric VRP states, with vertical symmetry axes.…”

The Mathematics Underlying Eeg Oscillations Propagation

“…It is also worth to be underlined that HBT holds not just for spherical manifolds, but also for every manifold with positive-curvature, provided its genus is zero. For example, Cummins et al (2018) analyzed the motion of wind dispersed plants, reporting a vortex of recirculating air, which is detaching owing the flow passing through the pappus ( Figure 1E). The occurrence of stagnation points (in this case two instead of one) can be noticed where the air velocity at the vortex surface is zero.…”

“…The two prohibition signs depict the stagnation points dictated by HBT. Modified from Cummins et al (2018). Figure 1F.…”

ROS production by localized SCHENGEN receptor module drives lignification at subcellular precision

“…Nakayama presented her work on the aerodynamics of dandelion diaspore flight. Using computational fluid dynamics and wind tunnel experiments with dandelion pappi, as well as fabricated replicas, she found that the particular geometry and porosity of the pappus allows the generation of a separated vortex ring that keeps them flying (Cummins et al, 2018). Furthermore, she observed differences in New Phytologist pappus detachment depending on environmental conditions (Seale et al, 2019), and is currently exploring the biological significance of these findings.…”