Strouhal number
From CFD-Wiki
The Strouhal number is a dimensionless parameter describing the oscillating frequency of a flow. Usually it is defined as:
where 
is the frequency of the flow (e.g. vortex shedding frequency), 
is a characteristic length scale of the flow and 
is a characteristic velocity.
The Strouhal Number is a function of the Reynolds number, typically between 20 < Re < 20,000.
