Glossary term
Vortex Shedding
The periodic release of alternating vortices behind a bluff body, often causing oscillating forces and flow-induced vibration.
Definition
phenomenonVortex shedding is the periodic formation and release of alternating vortices behind a bluff body in a fluid flow.
Vortex shedding creates unsteady lift and drag forces that can excite structural vibration, acoustic noise, fatigue, and flow-induced instability. It affects chimneys, bridge cables, risers, heat-exchanger tubes, aircraft components, probes, masts, towers, and bluff bodies in air or water.
For many bluff bodies, vortex shedding frequency is estimated with the Strouhal relation:
where St is Strouhal number, f_s is shedding frequency, D is characteristic body width, and U is flow velocity. The applicable Strouhal number depends on shape, Reynolds number, turbulence, surface roughness, blockage, and compressibility.
Engineering use
Alternating vortices create periodic cross-flow forces. If the shedding frequency approaches a structural natural frequency, the structure can vibrate strongly. In some cases, lock-in occurs: the shedding frequency synchronizes with structural motion across a range of velocities, increasing fatigue risk.
Engineers manage vortex shedding by changing natural frequency, adding damping, changing cross-section, adding helical strakes, fairings, shrouds, spoilers, tuned mass dampers, or altering flow speed and support conditions. Testing or simulation is often needed for high-consequence structures because geometry, end effects, and turbulence intensity strongly influence response.
Design review
Vortex-shedding checks are most important when a structure is slender, lightly damped, exposed to a wide velocity range, or expected to accumulate many cycles. Stacks, bridge members, risers, antenna masts, heat-exchanger tubes, aircraft probes, and cables can all be vulnerable. A practical review compares shedding frequency with natural frequencies across operating conditions and then checks stress range, damping assumptions, fatigue life, and mitigation effectiveness.
Common mistakes
A common mistake is checking only static drag while ignoring unsteady lift forces. Another is using one Strouhal number across all Reynolds numbers and geometries. A strong vortex-shedding review states body geometry, characteristic length, velocity range, Reynolds number, Strouhal assumption, natural frequencies, damping, fatigue limit, lock-in risk, and validation method.