Universality and imaginary potentials in advection-diffusion equations in closed flows

This article addresses the scaling and spectral properties of the advection-diffusion equation in closed two-dimensional steady flows. We show that homogenization dynamics in simple model flows is equivalent to a Schrödinger eigenvalue problem in the presence of an imaginary potential. Several properties follow from this formulation: spectral invariance, eigenfunction localization, and a universal scaling of the dominant eigenvalue with respect to the Péclet number Pe. The latter property means that, in the high-Pe range (in practice Pe ≥ 102-103), the scaling exponent controlling the behaviour of the dominant eigenvalue with the Péclet number depends on the local behaviour of the potential near the critical points (local maxima/minima). A kinematic interpretation of this result is also addressed. © 2004 Cambridge University Press.