__Strouhal number__

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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