Turbulence spreading and anomalous diffusion on combs

TL;DR

This paper introduces a comb-based model to describe turbulence spreading and anomalous diffusion, revealing universal subdiffusive behavior driven by inelastic wave interactions and connecting it to fractional-derivative equations.

Contribution

It develops a novel comb model framework for turbulence spreading, deriving asymptotic transport equations and fractional indexes, and links turbulence self-organization to a unified mathematical formalism.

Findings

Turbulence spreading follows a subdiffusive pattern modeled as a continuous-time random walk.

The spreading process depends on the type of wave interaction (three- or four-wave).

The model explains turbulence band formation and staircasing phenomena.

Abstract

This paper presents a simple model for such processes as chaos spreading or turbulence spillover into stable regions. In this simple model the essential transport occurs via inelastic resonant interactions of waves on a lattice. The process is shown to result universally in a subdiffusive spreading of the wave field. The dispersion of this spreading process is found to depend exclusively on the type of the interaction process (three- or four-wave), but not on a particular instability behind. The asymptotic transport equations for field spreading are derived with the aid of a specific geometric construction in the form of a comb. The results can be summarized by stating that the asymptotic spreading pursues as a continuous-time random walk (CTRW) and corresponds to a kinetic description in terms of fractional-derivative equations. The fractional indexes pertaining to these equations are obtained exactly using the comb model. A special case of the above theory is a situation when two waves with oppositely directed wave vectors couple together to form a bound state with zero momentum. This situation is considered separately and associated with the self-organization of wave-like turbulence into banded flows or staircases. Overall, we find that turbulence spreading and staircasing could be described based on the same mathematical formalism, using the Hamiltonian of inelastic wave-wave interactions and a mapping procedure into the comb space. Theoretically, the comb approach is regarded as a substitute for a more common description based on quasilinear theory. Some implications of the present theory for the fusion plasma studies are discussed and a comparison with the available observational and numerical evidence is given.