= Separated vortex ring
A \b[separated vortex ring] is a ring-shaped recirculating flow structure that remains detached from the body producing it while occupying a relatively stable downstream position in the body's frame. It differs from a ring attached to the body and from a ring that is shed and then travels away. The stable pattern does not require the same parcel of fluid to remain permanently in one place.
The term was used for the flow behind a <dandelion> seed's <pappus>. Flow visualization showed a detached recirculation ring downstream of the porous bristles. Air passing through the structure, rather than simply being excluded by a solid disk, was important to the observed pattern.<A separated vortex ring underlies the flight of the dandelion by Cathal Cummins and others>[Abstract and flow-visualization results: detached recirculation owing to through-flow.]{ref}
To test the role of structure, the investigators compared microfabricated disks with different <porosities>. A disk with a pappus-like porosity reproduced the characteristic flow behavior, supporting a mechanism involving permeability and geometry rather than a special biological material alone.<A separated vortex ring underlies the flight of the dandelion by Cathal Cummins and others>[Porous-disk comparisons and stabilization of the separated ring.]{ref}
For an invented geometric example, a screen with total projected area $10\,\mathrm{mm^2}$ and solid area $1\,\mathrm{mm^2}$ has open-area fraction $0.9$. That fraction alone does not fully specify its aerodynamic behavior: the distribution and shape of openings, fluid properties and flow speed also matter. This is not the measured porosity of the paper's seeds.
The example shows why an apparently empty part of a structure can control its <drag>. A <vortex ring> is a property of the surrounding <velocity field>, so visualizing that field can reveal mechanisms invisible in a static outline of the object.
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