Structure function of passive scalars in two-dimensional turbulence

Abstract

The structure function of a scalar $θ({\bf x},t)$, passively advected in a two-dimensional turbulent flow ${\bf u}({\bf x},t)$, is discussed by means of the fractal dimension $δ^{(1)}_g$ of the passive scalar graph. A relation between $δ^{(1)}_g$, the scaling exponent $ζ_1^{(θ)}$ of the scalar structure function $D_1^{(θ)}(r)$, and the structure function D_2(r) of the underlying flow field is derived. Different from the 3-d case, the 2-d structure function also depends on an additional parameter, characteristic of the driving of the passive scalar. In the enstrophy inertial subrange a mean field approximation for the velocity structure function gives a scaling of the passive scalar graph with $δ^{(1)}_g<2$ for intermediate and large values of the Prandtl number Pr. In the energy inertial subrange a model for the energy spectrum and thus D_2(r) gives a passive scalar graph scaling with exponent $δ^{(1)}_g={5/3}$. Finally, we discuss an application to recent observations of scalar dispersion in non-universal 2-d flows.