A jet’s natural instability produces noise unless vortices periodicize

Evidence

When a fast jet of air from an orifice or a slit enters calm air, it does not go quietly. This is quite different from flow of air past an obstacle. The jet wriggles and breaks up into vortices, which may not form periodically. If the vortices do form periodically, a pitched tone is the result if the frequency is in the audio range. — Eric J. Heller, Why You Hear What You Hear- An…, 1306

Categorization

• DEFINE: Establishes the fundamental contrast between steady flow past an obstacle and the unstable jet.