Dandelion seeds stay afloat with the help of vortex pairs
Dandelion seeds stay afloat with the help of vortex pairs lead image
A child picks up a dandelion and blows, sending its seeds flying into the breeze. Watching the seeds’ seemingly gravity-defying journey is almost magical. To help demystify this, Dong et al. used simulations to study how the physical characteristics of dandelion seeds’ bristles impact their diffusion and wake formation.
They looked at the effects of the spacing between the bristles and the Reynolds number of the seeds – the ratio between their inertial force and the viscous force of the surrounding air – when subject to incoming airflow, like a child’s breath. By varying the number of bristles in the simulations, the researchers monitored the impact of gaps on delaying the formation of unsteady vortices and enhancing the loading capacity.
According to their results, an imbalance between the convection of the surrounding air and the vorticity generation of the seed’s wake leads to the formation of a pair of steady vortex rings. At higher Reynolds numbers or bristle densities, the vortex elongates and strengthens, but lower Reynolds number or a smaller number of bristles is not sufficient for the generation of the steady vortex.
In other words, the more bristles a dandelion seed has, the easier it is to make the seed fly by blowing on it.
In addition to impacting the vortex, the gaps in the seeds’ bristles also increase the seeds’ drag coefficient, making them more resilient to leeward pressure while reinforcing the loading capacity.
The authors said additional research on higher Reynolds number situations is required to fully understand the stability of dandelion seeds. Once this is achieved, the flowers can inspire the design of next-generation aircraft wings resembling the shape of a dandelion that can cut down on material requirements, while sustaining the same loading capacity.
Source: “The steady vortex and enhanced drag effects of dandelion seeds immersed in low-Reynolds-number flow,” by Yangyang Dong, Kexin Hu, Yongbin Wang, and Zijian Zhang, AIP Advances (2021). The article can be accessed at https://doi.org/10.1063/5.0057589 .
