vortexshedding

Vortex shedding is a fluid dynamics phenomenon in which oscillating vorticity is shed from the sides of a bluff body placed in a fluid flow. The alternating shedding of vortices creates a pair of counter-rotating structures in the wake, forming a von Kármán vortex street that convects downstream and induces periodic forces on the body.

The shedding frequency is typically described by the Strouhal number, St = fD/U, where f is the shedding frequency, D is a characteristic cross-section dimension, and U is the free-stream velocity. For circular cylinders, St is about 0.2 over a broad range of Reynolds numbers (roughly Re from 100 to 100,000), though St can vary with shape, surface roughness, and Reynolds number.

Vortex shedding can cause significant engineering effects. The alternating pressure fluctuations produce transverse forces that may lead to vortex-induced vibrations (VIV) if the shedding frequency locks in with a structural natural frequency. This phenomenon is a concern for slender structures such as bridges, chimneys, smokestacks, towers, offshore platforms, and underwater cables, where fatigue or resonance may occur. Mitigation strategies include streamlined or rounded shapes, surface treatments, helical strakes, fairings, dampers, or tuned mass dampers, and methods to shift natural frequencies away from the shedding range.

Measurement and visualization techniques include flow visualization with smoke or dye, hot-wire anemometry, pressure sensors, particle image velocimetry (PIV), and other turbulence diagnostics.