The Strouhal number St is a function of the Reynolds number Re (although a sufficiently varying one that it may be said that it is typically equal to 0.2, e.g. see figure below) and is proportional to the reciprocal of vortex spacing expressed as a number of obstacle diameter. It is used in the momentum transfer in general, and in both Von Karmann vortex streets and unsteady flow calculations in particular. It is normally defined in the following form :
where : - n is the frequency of the observed phenomenon,
- d is the characteristic length (which is the diameter of the cylinder in the case of vortex streets),
- U is the velocity of the fluid.
Strouhal number as a function of the Reynolds number
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