advectiondominated

Advection-dominated refers to a regime in which the bulk transport of a quantity by fluid motion (advection) is the primary mechanism over diffusion, radiation, or other transport processes. In fluid dynamics and heat transfer, advection dominates when the Peclet number is large, so that gradients are carried by the flow rather than smoothed locally. In such regimes, energy carried inward by the moving gas is not efficiently radiated away, leading to high temperatures and departures from local thermal equilibrium. This gives rise to optically thin, geometrically thick flows that can be hot and radiatively inefficient.

A well-known application is advection-dominated accretion flow (ADAF), a model for accretion onto compact objects (black holes, neutron stars) at relatively low accretion rates. In an ADAF, viscous heating of the gas is largely balanced by advection toward the central object rather than by radiation. The resulting flow is hot (virial temperatures), optically thin, and geometrically thick, with a significant fraction of the energy being swallowed by the accretor. Radiative efficiency is low, so luminosities are subdued compared with the accretion rate. ADAFs are invoked to explain low-luminosity active galactic nuclei and certain states of X-ray binaries, especially when emission lines and spectra indicate hard X-ray components and radio jets.

ADAFs coexist with other accretion modes: thin, radiatively efficient disks at higher accretion rates, and slim disks at super-Eddington rates. The transition between regimes depends on mass accretion rate, viscosity, and cooling processes. In many systems, a hot, advection-dominated inner flow overlays a cooler outer disk.