Flói

Flæði is the movement of a fluid or other substance and the rate at which it passes through a boundary. In physics and engineering, flæði covers the transport of mass, momentum and energy by liquids and gases, and involves quantities such as the velocity field and the volumetric flow rate. In Icelandic usage the term is general and appears in compounds such as blóðflæði (blood flow) and loftflæði (air flow).

Fundamental principles include the conservation of mass, which leads to the continuity equation, and the motion of a viscous fluid as described by the Navier–Stokes equations. For ideal, non-viscous fluids, Bernoulli’s principle relates pressure and velocity along a streamline. The Reynolds number, Re = ρ v L / μ, characterizes whether flow tends to be laminar or turbulent. Flows can be incompressible (density constant, common for liquids) or compressible (density varies, important for gases at high speeds).

Types and measurements of flæði include steady versus unsteady flow, and laminar versus turbulent flow. In many applications, fully developed flow in pipes is a common idealization. The volumetric flow rate, Q, measures how much fluid passes a surface per unit time and is given by Q = ∫ v · n dA. Units are typically cubic meters per second (m3/s).

Applications span engineering, such as piping, hydraulics and aerodynamics; geophysical and meteorological contexts like ocean and air currents; physiology, including blood flow; and environmental science, where flow determines transport of heat, contaminants and nutrients. Flæði underpins the design, analysis and prediction of systems and natural phenomena involving moving fluids.