Editing Strouhal number (section)

===Metrology===
In [[metrology]], specifically [[Flow meter#Turbine flowmeter|axial-flow turbine meters]], the Strouhal number is used in combination with the [[Roshko number]] to give a correlation between flow rate and frequency.  The advantage of this method over the frequency/viscosity versus K-factor method is that it takes into account temperature effects on the meter.

:<math> \text{St}=\frac{f}{U} C^3, </math>
where,
: ''f'' = meter frequency,
: ''U'' = flow rate,
: ''C'' = linear coefficient of expansion for the meter housing material.

This relationship leaves Strouhal dimensionless, although a dimensionless approximation is often used for ''C''<sup>3</sup>, resulting in units of pulses/volume (same as K-factor).

This relationship between flow and frequency can also be found in the aeronautical field. Considering pulsating methane-air coflow jet diffusion flames, we get

:<math> \text{St} = \dfrac{aw_j}{U_j}</math>,

where,
: ''a'' = fuel jet radius
: ''w'' = the modulation frequency
: ''U'' = exit velocity of the fuel jet

For a small Strouhal number (St=0.1) the modulation forms a deviation in the flow that travels very far downstream. As the Strouhal number grows, the non-dimensional frequency approaches the [[natural frequency]] of a flickering flame, and eventually will have greater pulsation than the flame.<ref name="Sanchez-Sanz, M." />