Whistling arises from vortex formation in a sheared jet boundary layer

Evidence

Whistling is different, in that the air is being steadily injected—there is no predetermined time when vortices should form. Lip whistling involves a jet of air coming through an approximately circular opening, from a source that is itself a resonator with more than one mode available. A shear develops as the outer part of the cylindrical air jet is retarded by friction with the surrounding still air. The boundary layer becomes unstable against vortex formation for high-enough Reynolds number. — Eric J. Heller, Why You Hear What You Hear- An…, 1322

Categorization

• XREF: Connects to the Helmholtz resonator discussion in the same chapter and to vorticity/sound-generating mechanisms from earlier chapters on turbulence.