Strouhal Number


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Strouhal number

[′strü·əl ‚nəm·bər]
(mechanics)
A dimensionless number used in studying the vibrations of a body past which a fluid is flowing; it is equal to a characteristic dimension of the body times the frequency of vibrations divided by the fluid velocity relative to the body; for a taut wire perpendicular to the fluid flow, with the characteristic dimension taken as the diameter of the wire, it has a value between 0.185 and 0.2 Symbolized Sr . Also known as reduced frequency.

Strouhal Number

 

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

References in periodicals archive ?
The first one was found at a Strouhal number of St = 0.2, based on the wheel diameter.
The flow velocity is adjusted to control the frequency of the Karman vortices (according to the Strouhal number), and static pressure induces cavitation.
Two extra converging points appear on the windward side of the jet and the position varies at different Strouhal numbers. As the Strouhal number decreases, the corresponding positions are going down.
Note that the Strouhal number is applicable for fishes whose swimming is through their body and caudal fins [14].
(2003) Usefulness of the Strouhal number in evaluating human underwater undulatory swimming.
In general, the matter at hand is not about researching the influence of the change in the Strouhal number on the instantaneous values of lift and profile drag coefficients or flow deflection angle and coefficient of total pressure loss that characterise the efficiency of compressor grids but is about researching the influence of the Strouhal number on the display of hysteresis in the performance of compressor grids.
Zhou, Xu had been investigated Reynolds number effects on the structure of flow behind of two side by side cylinder [4] and observed that flow structure was asymmetric in T/d=1.2-1.6 and in T/d>2.2-2.5 two separated vortex paths were created and also observed that same structures remained between distance of two cylinders when Reynolds number changed, and two separated paths created as well, that had same Strouhal number.
is the Strouhal number. F factor defines the separation frequency of the vortexs.
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Finally, the effect of [R.sub.ed] on the Strouhal number will be investigated.
The effect of oscillating flow may be accounted for using appropriate correlations for oscillating flows that are a function of the Strouhal number or the Valensi number as well as the Reynolds number and Prandtl number.
His team's research, published in Nature, identified a quantity called the Strouhal number that measures how efficiently an animal moves.