How the dandelion stabilises a drag-enhancing vortex ring*

We previously reported the extraordinary vortex produced by dandelion seeds, showing that the dandelion’s parachute design stabilizes the vortex as the seed flies. In this talk, we consider the slow viscous flow (100 < Re < 600) past dandelion seeds and thin porous disks approximating dandelion seeds using flow visualisation techniques and particle image velocimetry. We show that the flow around porous disks exhibits striking similarities to that around impervious disks, even at very high porosities. We show that the vorticity is produced at the edges of the porous disks, even though there is virtually no material there.  The critical Re for the onset of vortex shedding behind the disks changes as a function of porosity, as reported previously; here, we show that the Strouhal number is similar for porous disks as it is for solid ones.