Strouhal number

Builds on Dimensionless numbers in fluid mechanics

Definition

Consider an unsteady flow with a characteristic frequency $f$ , a characteristic length $L$ and a characteristic speed $U$ . The Strouhal number is $\mathrm{St} = \frac{f L}{U}.$ For vortex shedding behind a body, $f$ is the shedding frequency and $L$ is the width of the body. The speed $U$ is that of the oncoming flow.

Remarks

It is a dimensionless number: $[f L] = \mathsf{T}^{-1} \mathsf{L} = [U]$ . Example: a cylinder of diameter $L = 0.1\ \mathrm{m}$ in a flow at $U = 1\ \mathrm{m/s}$ that sheds vortices at $f = 2\ \mathrm{Hz}$ has $\mathrm{St} = 0.2$ . Non-example: $f / U$ is not a Strouhal number; it has dimension $\mathsf{L}^{-1}$ . When the equations of motion are made dimensionless with a time scale $1/f$ , the Strouhal number multiplies the time-derivative term.

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Dimensionless numbers in fluid mechanics concept
Strouhal number definition

Tag	`22DK2F6QNC`
URI	`m2://22DK2F6QNC`
Kind	definition
Writing	W0 stub
MSC	76M55, 76D25
Defines	Strouhal number
Uses	Dimensionless numbers in fluid mechanics
Changed	3 November 2026

Strouhal number

Definition

Remarks

Reading path

Further reading

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