What Is Strouhal Number (St)?
The Strouhal number is a dimensionless quantity that relates oscillation frequency to flow velocity and a characteristic body size. In practice, it is used to describe repeating unsteady phenomena such as vortex shedding, where a body in moving fluid produces periodic pressure fluctuations at a frequency tied to its geometry.
In vortex-shedding device tuning, the Strouhal number helps engineers estimate resonant operating frequency before a prototype exists. For cylinders over a useful range of Reynolds numbers, the value stays near a narrow band, which makes it practical for turning wind measurements into a target device diameter.
The defining relation is St = f x D / V, where f is oscillation frequency, D is characteristic dimension, and V is flow speed. Why it matters is that a stable dimensionless ratio lets designers scale from wind tunnel data to real hardware while predicting sound, vibration, and shedding behavior.
Used in devices include vortex flow meters, aeroacoustic test rigs, and vibration energy harvesters. Engineers compare Strouhal number across shapes and operating regimes because even small shifts in geometry can move a system away from resonance or into an undesirable vibration band.
Example:
A designer can estimate the shedding frequency of a 25 mm cylinder in wind by combining the local velocity with an appropriate Strouhal number.
Related Terms:
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