# Vortex shedding Vortex shedding is the periodic release of [[Vortex|vortices]] from alternating sides of a body in a flow. Above a modest [[Reynolds_number]] the wake behind a cylinder, a cable, a chimney, or a bridge deck stops being steady and starts oscillating: a vortex grows on one side, detaches, and its departure triggers the growth of an opposite-signed vortex on the other. The result is a periodic side force on the body at a frequency set by the flow speed and the body's width. That regular forcing is the reason vortex shedding is a structural concern rather than a curiosity — it is a metronome attached to anything cylindrical left standing in wind or [[Water]] — a [[Vibration]] source with a [[Frequency_domain|frequency]] you can predict. ## Strouhal's number The shedding frequency f collapses onto a single dimensionless group, the Strouhal number St = fD/U, where D is the body's cross-stream dimension and U the freestream [[Velocity]]. For a circular cylinder St sits near 0.2 across a remarkably wide [[Reynolds_number]] range — roughly 10³ to 10⁵ — which means the frequency is predictable from geometry and speed alone without solving the flow. Vincenc Strouhal identified the relationship in 1878 while investigating why wires sing in the wind, and the singing is literal: shedding at audible frequencies is the source of the Aeolian tone, which puts this squarely on the [[PORTAL_Acoustics|acoustics]] spine as well as the fluid one. [[Sound]] from a vortex street is the same physics as [[Drag_(physics)|drag]] on it, read through a different [[Sensor|instrument]]. ## Lock-in and the failure mode The danger is resonance. If the shedding frequency drifts near a structural natural frequency, the body begins to move, and its motion synchronises the shedding to itself — lock-in — so the forcing frequency stops tracking the wind speed and stays locked to the structure over a band of conditions. Amplitude grows, [[Fatigue_(material)|fatigue]] cycles accumulate rapidly, and what should have been a small alternating load becomes the design case. Vortex-induced vibration has taken down stacks, cracked [[Heat_transfer|heat-exchanger]] tubes, oscillated marine risers, and set [[Electrical_grid|power lines]] galloping. It is a standard item in [[Reliability_engineering]] and [[Failure_mode_and_effects_analysis]] for any slender structure, and the [[Vibration]] literature treats it as the archetype of self-excited motion. ## Suppression Because shedding needs a coherent spanwise line to organise along, the countermeasures all attack coherence. Helical strakes wound around a chimney force the separation point to vary continuously along the height so no single frequency dominates. Shrouds, fairings, and splitter plates interrupt the [[Feedback|feedback]] between the two sides of the [[Eddy_(fluid_dynamics)|wake]]. Adding [[Damping]] raises the wind speed at which lock-in becomes destructive without touching the fluid mechanics at all. Each of these is a deliberate injection of three-dimensionality into a flow that wants to be two-dimensional — the same logic as a [[Vortex_generator]] on a wing, applied to a different problem, and a recurring theme in the [[WT!Advanced_Manufacturing_Center_of_Excellence|manufacturing]] bridge of this spine. ## The useful side The regularity that threatens structures also makes an instrument. A vortex flowmeter places a bluff body in a pipe and counts shedding events; because St is nearly constant, the count is proportional to volumetric flow, with no moving parts and no wear — a workhorse in [[WT!Energy_Center_of_Excellence|energy]] and process plants. Shedding also drives useful [[Diffusion|mixing]] in a heat exchanger, and energy-harvesting devices have been built to extract power from vortex-induced [[Oscillation|oscillation]] rather than suppress it. And in nature the wake is a resource: fish hold station in the low-pressure cores of a [[Kármán_vortex_street]] behind a rock, spending less [[Energy]] than in still water. See [[WT!Thury_Hydrodynamics_Compendium]]. **On the spine:** [[Kármán_vortex_street]] · [[Vortex]] · [[Vorticity]] · [[Eddy_(fluid_dynamics)]] · [[Vortex_generator]] · [[WT!Thury_Hydrodynamics_Compendium]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Vortex_shedding) : [Wikitube](https://en.wikitube.io/wiki/Vortex_shedding) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_WT!Thury_Hydrodynamics_Compendium]], [[PORTAL_Physics]], [[PORTAL_Reliability_engineering]], [[PORTAL_Acoustics]]. --- *Vorticity wave · 2026-09-10 · original prose · microsim layer deferred to the next pass.*