# @stem_antics on Instagram

- **Type:** Video
- **Original URL:** https://www.instagram.com/p/DaT0XDPPG-9
- **Gondola URL:** https://gondola.cc/posts/67391856-stem-antics-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/b6e1e50fed.jpg
- **Posted:** 2026-07-02T23:55:32.000+00:00
- **Account Owner:** Stem Antics (@stem_antics) — https://gondola.cc/stem_antics

## Caption

That elegant sinusoidal waviness rippling through a high-altitude contrail is not turbulence, and it is not wind. It is a fluid dynamic instability with a name: the Crow instability, first characterised by S. C. Crow in 1970.

Every finite wing generating lift sheds two counter-rotating vortices from its tips. The pressure differential that keeps the aircraft aloft spills around each tip and rolls up into a concentrated trailing pair, separated by roughly (pi/4) of the wingspan for elliptical loading. Left alone, these vortices would descend together, serene, under their mutual induction.

They are not left alone. Each vortex sits inside the velocity field of its partner, and any faint perturbation along its length gets amplified by the strain that field imposes. The outcome is a slow, symmetric, long-wavelength oscillation:

- The most amplified wavelength is around 8.6 times the vortex separation, producing broad regular loops rather than fine ripples
- Growth is fastest in a plane inclined roughly 48 degrees to the horizontal, a truce between the self-induced rotation of a bent vortex and the external strain of its neighbour
- Amplitude grows exponentially, not linearly, so the waviness seems to arrive all at once rather than creeping in

As the two sinusoids swell toward one another, they eventually touch. The vortices reconnect, pinching off into a train of near-circular vortex rings that drift apart, and the contrail dissolves into a chain of quiet puffs. This same linking process is what finally breaks up the hazardous wake behind a heavy jet, which is why the phenomenon is studied as closely on runways as it is admired from the ground.

It is one of the rare cases where an instability is not a failure mode but a signature, written across the sky in water vapour.

#stemantics #aerodynamics #fluiddynamics #aerospace #physics

## Stats

- **Views:** 0
- **Likes:** 593
- **Shares:** 0
- **Comments:** 1

## Tags

fluiddynamics, aerodynamics, physics, stemantics, aerospace

---
Copyright (c) Gondola