Inertial-range behaviour of a passive scalar field in a random shear flow
Infrared asymptotic behaviour of a scalar field, passively advected by a random shear flow, is studied by means of the field theoretic renormalization group and the operator product expansion. The advecting velocity is Gaussian, white in time, with correlation function of the form $\propto δ(t-t') / k_{\bot}^{d-1+ξ}$, where $k_{\bot}=|{\bf k}_{\bot}|$ and ${\bf k}_{\bot}$ is the component of the wave vector, perpendicular to the distinguished direction (`direction of the flow') --- the $d$-dimensional generalization of the ensemble introduced by Avellaneda and Majda [{\it Commun. Math. Phys.} {\bf 131}: 381 (1990)]. The structure functions of the scalar field in the infrared range exhibit scaling behaviour with exactly known critical dimensions. It is strongly anisotropic in the sense that the dimensions related to the directions parallel and perpendicular to the flow are essentially different. In contrast to the isotropic Kraichnan's rapid-change model, the structure functions show no anomalous (multi)scaling and have finite limits when the integral turbulence scale tends to infinity. On the contrary, the dependence of the internal scale (or diffusivity coefficient) persists in the infrared range. Generalization to the velocity field with a finite correlation time is also obtained. Depending on the relation between the exponents in the energy spectrum ${\cal E} \propto k_{\bot}^{1-ε}$ and in the dispersion law $ω\propto k_{\bot}^{2-η}$, the infrared behaviour of the model is given by the limits of vanishing or infinite correlation time, with the crossover at the ray $η=0$, $ε>0$ in the $ε$--$η$ plane. The physical (Kolmogorov) point $ε=8/3$, $η=4/3$ lies inside the domain of stability of the rapid-change regime; there is no crossover line going through this point.