Strouhal Number

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Strouhal number

[′strü·əl ‚nəm·bər]
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).


References in periodicals archive ?
2007) How are Strouhal number, drag, and efficiency adjusted in high level underwater monofin-swimming?
r] = [omega]d/W is greater than kinematic Strouhal number [S.
The first flow regime is the single massive object that produce a vortex and one Strouhal number in distances between two cylinders at 1<T/d<1.
From Equation (10), it was concluded that the Strouhal number of the fully developed oscillation is only dependent on the values of the dimensionless diameter of the chamber and the jet Reynolds number.
Thomas, "Flying and Swimming Animals Cruise at a Strouhal Number Tuned for High Power Efficiency," Nature, 425 (2003), pp.
14] reported that the Strouhal number at which unsteady aerodynamic forces take the maxima was in the vicinity of 0.
Mahbub Alam and Zhou investigated the Strouhal number, hydrodynamic forces and flow structures, and vortex shedding frequency of flow past two cylinders in tandem with different diameters in wind tunnel [19].
In particular, Sahin and Owens (19) have provided extensive results on the stability of flow, Strouhal number, hydrodynamic forces experienced by the submerged cylinder and wake structure for a cylinder over the ranges of conditions as ~1.
The GVT results are a complete set of the measured normal vibration with frequencies up to an upper frequency that can be determined based on maximal dive speed, length of aerodynamic chords, and the critical value of the Strouhal number.
The Strouhal number (= reduced frequency parameter) is another dimensionless index that characterizes the flow.
Here, the Strouhal number St is defined by the following formula:
The values of Reynolds number (Re) and Strouhal number (Sr) were found to be negligibly small in experiments, respectively ~0.