# Strouhal Number

## An introduction to and a definition of the Strouhal Number

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**The Strouhal Number** is a dimensionless value useful for analyzing oscillating unsteady fluid flow dynamics problems.

The Strouhal Number can be expressed as

St = ω l / v(1)

where

St= Strouhal Number

ω= oscillation frequency

l= characteristic length

v= flow velocity

The Strouhal Number can be important when analyzing unsteady, oscillating flow problems. The Strouhal Number represents a measure of the ratio of inertial forces due to the unsteadiness of the flow or local acceleration to the inertial forces due to changes in velocity from one point to an other in the flow field.

The vortices observed behind a stone in a river, or measured behind the obstruction in a vortex flow meter, illustrates these principles.

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