The turbulent transfer of momentum by eddies giving rise to an internal fluid friction, in a manner analogous to the action of molecular viscosity in laminar flow, but taking place on a much larger scale
An approximation to turbulent flow whereby the net effect of molecular diffusion enhanced by strain flows between eddies is parametrized by an eddy viscosity acting on large scale motion Eddy viscosity may be taken as constant or dependent on the length scale of motion The former case is equivalent to assuming that the Reynolds stresses are proportional to the gradients of the large scale flow velocity