Shear instability makes vortex timing exquisitely sensitive to perturbation

Evidence

Instability and sensitivity are bedfellows. Fast columns of air flowing though surrounding still air are unstable due to the shear forces—they tend to kink and then roll up on themselves. The flow of a cylinder or sheet of air is not only unstable but also sensitive, just as a pencil balanced on its tip is both unstable and sensitive: any slight push will determine when and in what direction it falls. The timing of the formation of vortices, in other words, is easily influenced. — Eric J. Heller, Why You Hear What You Hear- An…, 1315

Categorization

• CANDY: The pencil-on-tip analogy for fluid instability is vivid and memorable.