Eddys

Eddys are circular or swirling motions in a fluid, formed when a flow encounters obstacles, experiences shear, or becomes unstable. They are common in rivers, oceans, the atmosphere, and industrial systems. Eddies are not a single phenomenon but a family of rotating flow features, ranging from small, short-lived swirls to large, long-lived circulations. In many high-Reynolds-number flows, eddies are approximately two-dimensional in the horizontal plane and have a rotating core surrounded by a shear layer.

In rivers and streams, eddies develop downstream of rocks, bends, or structures, creating zones of recirculating flow that affect sediment transport, nutrient mixing, and aquatic habitats. Oceanic eddies span tens to hundreds of kilometers and can trap and move water masses with distinct temperature and salinity, influencing heat transport, nutrient distribution, and biological activity. Atmospheric eddies occur at a wide range of scales, from small turbulent swirls to large cyclones and anticyclones, contributing to weather patterns and climate variability. Eddies interact with mean flows and can exchange momentum, heat, and tracers across boundaries.

Measurement and modeling approaches for eddies include turbulence research methods and geophysical techniques. The eddy-covariance method estimates turbulent fluxes of gases and heat by analyzing fluctuations in velocity and scalar fields. In fluid dynamics, eddy viscosity is a modeling concept used to represent the momentum transfer by unresolved eddies. Eddies are studied through numerical simulations, remote sensing (such as satellite imagery for oceanic eddies), and in-situ observations, providing insight into transport processes in natural and engineered fluid systems.