In dimensional analysis, the Strouhal number is a dimensionless number describing oscillating flow mechanisms. The parameter is named after Vincenc Strouhal, a Czech physicist who experimented in 1878 with wires experiencing vortex shedding and singing in the wind.^{[1]} The Strouhal number is an integral part of the fundamentals of fluid mechanics.
The Strouhal number is often given as
where St is the dimensionless Strouhal number, f is the frequency of vortex shedding, L is the characteristic length (for example hydraulic diameter) and V is the velocity of the fluid.
For large Strouhal numbers (order of 1), viscosity dominates fluid flow, resulting in a collective oscillating movement of the fluid "plug". For low Strouhal numbers (order of 10^{−4} and below), the highspeed, quasi steady state portion of the movement dominates the oscillation. Oscillation at intermediate Strouhal numbers is characterized by the buildup and rapidly subsequent shedding of vortices.^{[2]}
For spheres in uniform flow in the Reynolds number range of 800 < Re < 200,000 there coexist two values of the Strouhal number. The lower frequency is attributed to the largescale instability of the wake and is independent of the Reynolds number Re and is approximately equal to 0.2. The higher frequency Strouhal number is caused by smallscale instabilities from the separation of the shear layer (Kim and Durbin, 1988 and Sakamoto and Haniu, 1990).
In metrology, specifically axialflow turbine meters, the Strouhal number is used in combination with the Roshko number to give a correlation between flow rate and frequency. The advantage of this method over the freq/viscosity versus Kfactor method is that it takes into account temperature effects on the meter.
f = meter frequency, U = flow rate, C = linear coefficient of expansion for the meter housing material
This relationship leaves Strouhal dimensionless, although a dimensionless approximation is often used for C^{3}, resulting in units of pulses/volume (same as Kfactor).
See also
References
 Kim, K. J. and Durbin, P. A. (1988) "Observations of the frequencies in a sphere wake and drag increase by acoustic excitation," Physics of Fluids, 31, pp. 32603265.
 Sakamoto, H. and Haniu, H. (1990) "A study on vortex shedding from spheres in uniform flow," Journal of Fluids Engineering, 112(December), pp. 386392.
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