# Eddy (fluid dynamics) An eddy is a swirl of fluid that has separated, at least temporarily, from the mean flow — the recirculating pocket behind a rock in a stream, the rotating parcel in a [[Turbulence|turbulent]] boundary layer, the ocean ring pinched off from a current. The word is deliberately loose. Unlike a [[Vortex]], which implies a defined core and a measurable [[Circulation_(physics)|circulation]], an eddy is identified by behaviour rather than structure: it is the scale at which fluid is being turned over and mixed. Eddies are the [[Fluid_dynamics| fluid]]'s memory — they hold [[Vorticity]] generated upstream and release it downstream, sometimes seconds later, sometimes a season later in the case of an ocean ring. ## Separation makes them Eddies form where a flow is asked to turn a corner it cannot follow. As fluid moves into a region of rising pressure, the slow layer nearest the wall runs out of momentum first, stalls, and reverses; the main flow detaches and the space behind it fills with recirculating fluid. That is separation, and it is the single most consequential event in applied [[Fluid_dynamics]]: it sets the [[Drag_(physics)|drag]] of a bluff body, ends the useful range of an aerofoil at stall, and decides whether a diffuser recovers pressure or wastes it. Whether separation happens depends on the state of the boundary layer, which is why tripping a layer into [[Turbulence]] deliberately — golf-ball dimples, a [[Vortex_generator]] on a wing — can *reduce* drag by delaying separation, a result that surprises everyone the first time. ## The cascade in one word Richardson's 1922 formulation of the [[Turbulence|turbulent]] cascade is written entirely in eddies: large ones break into smaller ones, which break into smaller ones still, until [[Viscosity]] ends the sequence. Kolmogorov's 1941 theory made it quantitative, predicting that in the inertial range the [[Energy]] spectrum follows a −5/3 power law in wavenumber, a result confirmed in flows from wind tunnels to the [[Atmosphere_of_Earth|atmospheric]] boundary layer. Eddy size therefore functions as a coordinate: quoting "the energy-containing eddies" or "the dissipative eddies" locates a phenomenon in scale as precisely as quoting a frequency locates a tone in [[Acoustics|acoustics]]. The whole apparatus of [[Fourier_analysis|spectral]] description in [[Signal_processing|signal processing]] carries over intact. ## Eddy viscosity — the useful lie Because resolving every eddy is unaffordable, engineering practice models their aggregate effect as an enhanced [[Viscosity]]. The eddy-viscosity hypothesis treats turbulent momentum transport as if it were molecular [[Diffusion]] with a much larger coefficient, and it underpins the k–ε and k–ω closures inside nearly every code in [[List_of_computational_fluid_dynamics_software|the CFD catalogue]]. The assumption is known to be wrong in detail — real turbulent transport is not local, not isotropic, and not instantaneous — and it is retained because it is calibratable and cheap. Large-eddy [[Simulation]] splits the difference by resolving the big eddies and modelling only the small ones, on the reasoning that the large scales carry the geometry-specific behaviour and the small scales are closer to universal. ## Eddies at every Center Scour holes below a culvert on a [[WT!Southern_Agricultural_Center_of_Excellence|southern]] farm ditch are eddies excavating a bed. Recirculation in a [[WT!IT_Center_of_Excellence|server-room]] cold aisle is an eddy starving a rack of cool air. The wake behind a [[WT!Transportation_Center_of_Excellence|truck]] is a pair of eddies costing fuel at every kilometre. Blood recirculating in an aneurysm is an eddy with clinical significance for the [[WT!HealthForce_Center_of_Excellence|HealthForce]] bridge. And in a [[WT!Energy_Center_of_Excellence|wind farm]], the eddy wake of one turbine is the inflow of the next, which is why spacing is a [[Mathematical_optimization|optimisation]] problem rather than a rule of thumb. The [[WT!Thury_Hydrodynamics_Compendium]] keeps them on one page for that reason. **On the spine:** [[Turbulence]] · [[Vortex]] · [[Vorticity]] · [[Vortex_shedding]] · [[Kármán_vortex_street]] · [[WT!Thury_Hydrodynamics_Compendium]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Eddy_(fluid_dynamics)) : [Wikitube](https://en.wikitube.io/wiki/Eddy_(fluid_dynamics)) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_WT!Thury_Hydrodynamics_Compendium]], [[PORTAL_Physics]], [[PORTAL_Dynamical_system]], [[PORTAL_Complex_system]]. --- *Vorticity wave · 2026-09-10 · original prose · microsim layer deferred to the next pass.*