# Strouhal Number

## Introduction to 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 the *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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