reststrøm

Restström is a term used primarily in hydrodynamics and environmental engineering to describe the portion of a flow that remains after the primary, or main, stream has been identified and characterized. The restström corresponds to residual or secondary currents that persist due to shear, boundary effects, buoyancy, or incomplete mixing. In practice, Restström is used to analyze transport processes where the dominant stream alone does not capture all mass, momentum, or constituents transported by the system.

Mathematically, a flow field u can be conceptually decomposed into a principal component u_p and a restström u_r, with u = u_p + u_r. Classification of restström depends on criteria such as velocity magnitude, coherence, or residence-time thresholds. Restström often exhibits different dispersion properties and can influence sediment transport, dissolved tracer distribution, and ecological conditions in aquatic systems.

Applications of Restström include river and estuarine hydraulics, where the restström helps assess how lingering flows affect habitat connectivity and sediment scouring. In industrial pipe networks, it is used to evaluate mixing efficiency and residence-time distributions in reactors or heat exchangers. In atmospheric or urban hydrology, residual streams may describe secondary circulations or drainage patterns that persist after the main event.

Origin and usage of the term appear in regional literature, with variations in definition across disciplines; it is not a universally standardized term. Some authors use alternative expressions such as residual flow or secondary stream. See also residual flow, secondary flow, mixing, and residence time distribution.