Kinetic wave turbulence

We consider a general model of Hamiltonian wave systems with triple resonances, in the standard kinetic limit of a continuum of weakly interacting dispersive waves with random phases. We show in this limit that the leading-order, asymptotically valid dynamical equation for multimode amplitude distributions is not   the well-known equation of Peierls (also, Brout & Prigogine and Zaslavskii & Sagdeev), but is instead a reduced equation containing only a subset of the terms in that equation. Our equations are consistent with the Peierls equation in that the additional terms in the latter vanish as inverse powers of volume in the large-box limit. The equations that we derive are the direct analogue of the Boltzmann hierarchy obtained from the BBGKY hierarchy in the low-density limit for gases. We show that the asymptotic multimode equations possess factorized solutions for factorized initial data, which correspond to preservation in time of the property of “random phases & amplitudes”. The factors satisfy the equations for the 1-mode probability density functions (PDFs) previously derived by Choi et al. and Jakobsen & Newell. Analogous to the Klimontovich density in the kinetic theory of gases, we introduce the concepts of the “empirical spectrum” and the “empirical 1-mode PDF”. We show that the factorization of the hierarchy equations implies that these quantities are self-averaging: they satisfy the wave-kinetic closure equations of the spectrum and 1-mode PDF for almost any selection of phases and amplitudes from the initial ensemble. We show that both of these closure equations satisfy an HH-theorem for an entropy defined by Boltzmann’s prescription S=kBlogWS=kBlogW. We also characterize the general solutions of our multimode distribution equations, for initial conditions with random phases but with no statistical assumptions on the amplitudes. Analogous to a result of Spohn for the Boltzmann hierarchy, these are “super-statistical solutions” that correspond to ensembles of solutions of the wave-kinetic closure equations with random initial conditions or random forces. On the basis of our results, we discuss possible kinetic explanations of intermittency and non-Gaussian statistics in wave turbulence. In particular, we advance the explanation of a “super-turbulence” produced by stochastic or turbulent solutions of the wave kinetic equations themselves.

► We derive equations for multimode distributions in weak 3-wave kinetics.
► All distributions arise from random ensembles of solutions of kinetic equations.
► Such “super-turbulence” can explain intermittency and non-Gaussian statistics.
► The “empirical” spectrum and 1-mode PDF are self-averaging quantities.
► The HH-theorem holds for relative entropy of distribution of wave amplitudes.