• toroidal vortex
• terminal velocity
• Taraxacum officinale
• Reynolds number
• porous disk
• permeability
• Karman-Cozeny
• Darcy number
• Dandelion
• aerodynamics

Anyone who has ever blown a dandelion clock, will know that its fruit (the feathery, seed-bearing structure) fly off with the slightest breeze. Each fruit is made of a group of stringy hollow tubes called lamellae, which together form the parachute, the pappus. Contrary to a popular belief, the physics of a conventional parachute cannot be applied, because of the parachute's high porosity. This structure gives outstanding flight capability to the fruit while, at the same time, saving weight. In steady descent, the flight of the fruit is characterized by two dimensionless parameters: the Reynolds number and the Darcy number.
We measured the Reynolds number for the dandelion fruit by performing terminal velocity measurements for a variety of fruit weights. We tested previous theories for the flight of the dandelion by artificially weighting the dandelion, and determining the variation in the terminal velocity as a function of the parachute loading. Our experiments reveal that the parachute behaves more like a porous disk than individual independent lamellae. We performed direct numerical simulations of a porous disk of a certain Darcy number (as measured by us), and compared our results to flow visualizations realized in a specially designed vertical wind tunnel.