Critical behaviour of a fluid in a random shear flow: Renormalization   group analysis of a simplified model
  Critical behaviour of a fluid, subjected to strongly anisotropic turbulent mixing, is studied by means of the field theoretic renormalization group. As a simplified model, relaxational stochastic dynamics of a non-conserved scalar order parameter, coupled to a random velocity field with prescribed statistics, is considered. The velocity is taken Gaussian, white in time, with correlation function of the form $\propto \delta(t-t') /|{\bf k}_{\bot}|^{d+\xi}$, where ${\bf k}_{\bot}$ is the component of the wave vector, perpendicular to the distinguished direction (``direction of the flow''). It is shown that, depending on the relation between the exponent $\xi$ and the space dimensionality $d$, the system exhibits various types of large-scale self-similar behaviour, associated with different infrared attractive fixed points of the renormalization-group equations. Existence of a new, non-equilibrium and strongly anisotropic, type of critical behaviour (universality class) is established, and the corresponding critical dimensions are calculated to the second order of the double expansion in $\xi$ and $\epsilon=4-d$ (two-loop approximation). The most realistic values of the model parameters (for example, $d=3$ and the Kolmogorov exponent $\xi=4/3$) belong to this class. The scaling behaviour appears anisotropic in the sense that the critical dimensions related to the directions parallel and perpendicular to the flow are essentially different. 
N. V. Antonov and A. A. Ignatieva