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Strouhal number[′strü·əl ‚nəm·bər]
a similarity criterion, or parameter, in the unsteady-state flow of liquids or gases. Symbolized Sr, Sh, or S, it expresses the similarity in the course of processes in time: Sr = l/vt = ωl/v, where v is the characteristic flow velocity, l is a characteristic linear dimension, t is a time interval characteristic of the unsteady-state flow, and w is the characteristic frequency. The inverse quantity vt/l is called the Thomson number NTh
The Strouhal number has been found to be constant (Sr ≈ 0.2–0.3) over a wide range of Reynolds numbers. This empirical relation is used in calculating vibrations of elastic bodies, such as airplane wings and periscopes, in a liquid or gas flow and in determining pressure fluctuations in regions of flow separation—for example, behind a body past which a fluid is flowing, as at the tail of a rocket.
A similar number, H0 = vt/l, is encountered in mechanical, thermal, and electromagnetic processes and is called the homochronous number. The Strouhal number is a special case, pertaining to hydroaeromechanics, of the inverse of the homochronous number.
The Strouhal number was named for the Czech scientist V. Strouhal (1850–1923).
S. L. VISHNEVETSKII