The physics of the dandelion
It’s a masterpiece of flow dynamics, Swiss researchers say.
The floating fluff of a dandelion is a masterpiece of aerodynamic design, a study from the Swiss Federal Institute of Technology has found.
The key, the researchers say, is in the flower’s wispy form: the gaps between its filaments, known collectively as a pappus, allow air to flow through and help to stabilise the wake it generates so the seed can cruise serenely for up to a kilometre.
Pier Giuseppe Ledda and his team modelled the pappus as a collection of rods arranged like the spokes of a bicycle wheel. They found that the porosity of the pappus led to a steady recirculating vortex ring in the wake, which allowed cruising behaviour much more stable than a solid umbrella shape.
“The wake, which would be unsteady if the pappus was completely impermeable, can be stabilised by changing the body structure so as to allow the flow to pass through,” say the authors in a paper in the journal Physical Review Fluids.
The team used standard equations from fluid dynamics and were able to reproduce the experimentally measured wake of the pappus. Tweaking the model to find the optimum arrangement of rods, they found around 100 rods gave steady flight – the same number as in real seeds, suggesting the dandelion has evolved for efficient cruising.
Ledda says the findings could help with the design of lightweight parachutes for objects of similar size and weight.