# Strouhal number

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The Strouhal number is a dimensionless parameter describing the oscillating frequency of a flow. Usually it is defined as:

$St := \frac{fL}{U}$

where $f$ is the frequency of the flow (e.g. vortex shedding frequency), $L$ is a characteristic length scale of the flow and $U$ is a characteristic velocity.

The Strouhal Number is a function of the Reynolds number, typically between 20 < Re < 20,000.