  Additive noise in Partial Differential equations, in particular those of
fluid mechanics, has relatively natural motivations. The aim of this work is
showing that suitable multiscale arguments lead rigorously, from a model of
fluid with additive noise, to transport type noise. The arguments apply both to
small-scale random perturbations of the fluid acting on a large-scale passive
scalar and to the action of the former on the large scales of the fluid itself.
Our approach consists in studying the (stochastic) characteristics associated
to small-scale random perturbations of the fluid, here modelled by stochastic
2D Euler equations with additive noise, and their convergence in the infinite
scale separation limit.
