We study effects of turbulent mixing on the critical behaviour of a nonequilibrium system near its second-order phase transition between the absorbing and fluctuating states. The model describes the spreading of an agent (e.g., infectious disease) in a reaction-diffusion system and belongs to the universality class of the directed bond percolation process, also known as simple epidemic process, and is equivalent to the Reggeon field theory. The turbulent advecting velocity field is modelled by the Obukhov-Kraichnan's rapid-change ensemble: Gaussian statistics with the correlation function vvwhere k is the wave number and 0 < ξ < 2 is a free parameter. Using the field theoretic renormalization group we show that, depending on the relation between the exponent ξ and the spatial dimension d, the system reveals different types of large-scale asymptotic behaviour, associated with four possible fixed points of the renormalization group equations. In addition to known regimes (ordinary diffusion, ordinary directed percolation process, and passively advected scalar field), existence of a new nonequilibrium universality class is established, and the corresponding critical dimensions are calculated to first order of the double expansion in ξ and ε = 4 − d (one-loop approximation). It turns out, however, that the most realistic values ξ = 4/3 (Kolmogorov's fully developed turbulence) and d = 2 or 3 correspond to the case of passive scalar field, when the nonlinearity of the Reggeon model is irrelevant and the spreading of the agent is completely determined by the turbulent transfer.

Abstract. Critical behaviour of two systems, subjected to the turbulent mixing, is studied by means of the field theoretic renormalization group. The first system, described by the equilibrium model A, corresponds to relaxational dynamics of a non-conserved order parameter. The second one is the strongly non-equilibrium reaction-diffusion system, known as Gribov process and equivalent to the Reggeon field theory. The turbulent mixing is modelled by the Kazantsev-Kraichnan "rapid-change" ensemble: time-decorrelated Gaussian velocity field with the power-like spectrumEffects of compressibility of the fluid are studied. It is shown that, depending on the relation between the exponent ξ and the spatial dimension d, the both systems exhibit four different types of critical behaviour, associated with four possible fixed points of the renormalization group equations. Three fixed points correspond to known regimes: Gaussian fixed point, original model without mixing and passively advected scalar field. The most interesting fourth point corresponds to a new type of critical behaviour, in which the nonlinearity and turbulent mixing are both relevant, and the critical exponents depend on d, ξ and the degree of compressibility. The critical exponents and regions of stability for all the regimes are calculated in the leading order of the double expansion in two parameters ξ and ε = 4 − d. For the both models, compressibility enhances the role of the nonlinear terms in the dynamical equations: the region in the ε-ξ plane, where the new nontrivial regime is stable, is getting much wider as the degree of compressibility increases. For the incompressible fluid, the most realistic values d = 3 and ξ = 4/3 (Kolmogorov turbulence) lie in the region of stability of the passive scalar regime. If the compressibility becomes strong enough, the crossover in the critical behaviour occurs, and these values of d and ξ fall into the region of stability of the new regime, where the advection and the nonlinearity are both important. In its turn, turbulent transfer becomes more efficient due to combined effects of the mixing and the nonlinear terms.

Using the field theory renormalization group, we study the critical behavior of two systems subjected to turbulent mixing. The first system, described by the equilibrium model A, corresponds to the relaxational dynamics of a nonconserved order parameter. The second system is the strongly nonequilibrium reactiondiffusion system, known as the Gribov process or directed percolation process. The turbulent mixing is modeled by the stochastic Navier-Stokes equation with a random stirring force with the correlator ∝ δ(t − t )p 4−d−y , where p is the wave number, d is the space dimension, and y is an arbitrary exponent. We show that the systems exhibit various types of critical behavior depending on the relation between y and d. In addition to known regimes (original systems without mixing and a passively advected scalar field), we establish the existence of new strongly nonequilibrium universality classes and calculate the corresponding critical dimensions to the first order of the double expansion in y and ε = 4 − d (one-loop approximation).

The direct bond percolation process (Gribov process) is studied in the presence of random velocity fluctuations generated by the Gaussian self-similar ensemble with finite correlation time. We employ the renormalization group in order to analyze a combined effect of the compressibility and finite correlation time on the long-time behavior of the phase transition between an active and an absorbing state. The renormalization procedure is performed to the one-loop order. Stable fixed points of the renormalization group and their regions of stability are calculated in the one-loop approximation within the three-parameter (ɛ,y,η) expansion. Different regimes corresponding to the rapid-change limit and frozen velocity field are discussed, and their fixed points' structure is determined in numerical fashion.

Critical behaviour of a nearly critical system, subjected to vivid turbulent mixing, is studied by means of the field theoretic renormalization group. Namely, relaxational stochastic dynamics of a non-conserved order parameter of the Ashkin-Teller-Potts model, coupled to a random velocity field with prescribed statistics, is considered. The mixing is modelled by Kraichnan's rapid-change ensemble: timedecorrelated Gaussian velocity field with the power-like spectrum ∝ k −d−ξ . It is shown that, depending on the symmetry group of the underlying Potts model, the degree of compressibility and the relation between the exponent ξ and the space dimension d, the system exhibits various types of infrared (long-time, large-scale) scaling behaviour, associated with four different infrared attractors of the renormalization group equations. In addition to known asymptotic regimes (equilibrium dynamics of the Potts model and the passively advected scalar field), existence of a new, strongly non-equilibrium type of critical behaviour is established. That "full-scale" regime corresponds to the novel type of critical behaviour (universality class), where the selfinteraction of the order parameter and the turbulent mixing are equally important. The corresponding critical dimensions depend on d, ξ, the symmetry group and the degree of compressibility. The dimensions and the regions of stability for all the regimes are calculated in the leading order of the double expansion in two parameters ξ and ε = 6 − d. Special attention is paid to the effects of compressibility of the fluid, because they lead to nontrivial qualitative crossover phenomena.