Quantitative Evaluation of Forward and Backward Scattering in Isotropic Turbulence via H\"anggi--Klimontovich and It\^o Stochastic Processes

TL;DR

This paper models turbulence scattering phenomena using stochastic processes, providing analytical insights into energy cascade, bifurcation rates, and transport properties, aligning well with numerical data.

Contribution

It introduces a non-diffusive stochastic framework for turbulence analysis, linking bifurcation rates to scattering and transport properties.

Findings

Distribution of Lyapunov exponents is uniform and driven by bifurcation rates.

Analytical closure of von Karman-Howarth and Corrsin equations achieved.

Transport parameters like eddy viscosity emerge from non-diffusive dynamics.

Abstract

This work evaluates the magnitude of the turbulent energy cascade in terms of forward and backward scattering by modeling the "stretch and fold" mechanism through a drift-free Hanggi-Klimontovich stochastic process. Mapping this dynamics onto an equivalent Ito process provides a statistical justification for the uniform distribution of the Lagrangian Lyapunov exponent via the associated Fokker-Planck equation. This continuous distribution is shown to be driven by a Lagrangian bifurcation rate significantly higher than the Lyapunov exponents themselves, reflecting the high frequency with which trajectories encounter the singular surfaces of the velocity gradient. The resulting PDF corresponds to the simultaneous maximization of the information entropy and the Kolmogorov-Sinai entropy. This stochastic formulation, framed within the author's Lyapunov-Liouville analysis, provides a non-diffusive analytical closure of the von Karman-Howarth and Corrsin equations. While forward scattering emerges from trajectory instabilities and bifurcations, backscattering is linked to fluid incompressibility. These phenomena are quantified through the continuously distributed Lyapunov exponents, allowing for an estimation of canonical exponents and fundamental transport properties, such as eddy viscosity, eddy thermal diffusivity, and the turbulent Prandtl number. These parameters, traditionally associated with diffusive models, are shown to emerge naturally from non-diffusive Lagrangian dynamics and bifurcation-driven fluctuations. The analytical results demonstrate close agreement with numerical data available in the literature.