---
title: "The Dandelion and the Ring in the Air"
lang: en-GB
canonical: "https://mariuscomper.uk/inelul-din-aer/en/"
alternate: "https://mariuscomper.uk/inelul-din-aer/"
published: 2026-08-23
author: Marius Comper
---

# The Dandelion and the Ring in the Air

The seed’s pappus is barely there: **91.6%** of its enclosing disk is empty projected area. Flow through those gaps maintains a stable, separated downstream vortex ring that slows the fall.

The 91.6% porosity is the empty projected area divided by the plan area of the disk enclosing the pappus. It is not the percentage of air by volume. The mean and its 95% confidence interval, 90.7% to 92.3%, come from ten samples.

## What the slider does

In the HTML edition, one slider gradually turns a solid disk into the pappus filament crown. Stops at 0%, 33%, 55%, 75% and 89% correspond to porous disks made and tested in the study. The 91.6% value is the mean of the biological samples.

Intermediate ring positions are schematic interpolations. The study does not establish a single porosity at which the ring detaches, and flow state also depends on Reynolds number. At 89%, particle image velocimetry (PIV) shows a separated ring. All ten biological samples produced such a ring in the vertical wind tunnel, both fixed and in free flight.

## How the gaps work

Air passes between the filaments. A region of slowed air forms around each one, and neighbouring filaments sit close enough for those regions to interact. The spread-out crown therefore behaves as a coherent aerodynamic object.

At filament scale, the Reynolds number was **0.422**. Mean filament spacing was about 41 filament diameters, below the estimated spacing of more than 47 diameters at which neighbour effects might become negligible.

Means across the ten samples:

- about 100 filaments, 95% CI 95 to 106;
- pappus diameter 13.8 mm, 95% CI 13.2 to 14.3 mm;
- terminal speed 39.1 cm/s, 95% CI 34.9 to 43.0 cm/s;
- filament projected area 12.6 mm², 95% CI 11.5 to 13.5 mm².

The speed was measured in still air and does not predict how far a seed will travel in wind.

## The same force, far less material

In the study’s comparison, the pappus produced **more than four times as much drag per unit projected area as a solid disk**. The reference disk was defined to produce the same total drag at the same speed; its calculated diameter was 8.6 mm.

Substituting the authors’ values in SI units, a reproducible calculation gives a disk diameter of 8.57 mm and an area of 57.67 mm². The filament area, calculated from pappus diameter and porosity, is 12.56 mm². Their ratio is 4.59; the paper states “more than four times”. Rounded values in the prose do not support extra precision.

## Scope

The team combined free-fall tests, a vertical wind tunnel, particle image velocimetry and microfabricated porous disks. The result applies to the tested seeds and models in a small-scale flow regime. A human parachute operates at a different scale and in a different aerodynamic regime, so this ratio cannot be transferred directly.

The original illustration in the HTML edition shows the mechanism schematically. It does not reconstruct the authors’ velocity field, identify a detachment threshold or predict a seed’s trajectory.

## Sources

1. Cummins et al., [*A separated vortex ring underlies the flight of the dandelion*](https://doi.org/10.1038/s41586-018-0604-2), Nature 562 (2018).
2. [Full accepted manuscript](https://strathprints.strath.ac.uk/79669/1/Cummins_etal_Nature_2018_A_separated_vortex_ring_underlies_the_flight.pdf), including methods and confidence intervals.
3. [The authors’ MATLAB scripts](https://doi.org/10.7488/ds/2362), Edinburgh DataShare, CC BY 4.0.
4. [Videos of the 75% porous disk](https://doi.org/10.7488/ds/2363), Edinburgh DataShare, CC BY 4.0.

Sources checked on 23 August 2026. Illustrations and calculations are original; experimental values are attributed to the study’s authors.