Abstract
A universal relationship between the frequency of oscillation induced in a restrained fluid body by an external free flow, the velocity of the latter, and a typical width of the body, is established. Evidence is offered of the accuracy of this “Strouhal law” in such phenomena as wing autorotation, intermittent formation of vortices behind weirs and orifices, jet flapping and puffing, wake vortex shedding, oscillation of shock waves, cavitation bubbles and vortex breakdowns. A corollary of the Strouhal law shows that travelling waves of a well‐defined length may associate with the fluid‐body oscillation. Corroborations are offered concerning the spacing of wind waves, internal waves and dunes and Taylor‐cell waves. The usefulness of the law as a heuristic tool is stressed.

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