Space-time statistics of a linear dynamical energy cascade model

TL;DR

This paper analyzes a linear dynamical model for turbulent energy cascades, deriving explicit temporal correlation predictions and exploring how forcing correlation affects spatial and temporal statistics, revealing limitations in modeling turbulence regularity.

Contribution

It provides explicit asymptotic predictions for the temporal correlation function and examines the effects of forcing correlation on spatial and temporal turbulence statistics.

Findings

Temporal correlation regularity depends on forcing time correlation.

Spatial homogeneity is maintained for small forcing correlation times.

Large forcing correlation causes spatial concentration and breaks homogeneity.

Abstract

A linear dynamical model for the development of the turbulent energy cascade was introduced in Apolin\'ario \emph{et al} (J. Stat. Phys. \textbf{186}, 15 (2022)). This partial differential equation, randomly stirred by a forcing term which is smooth in space and delta-correlated in time, was shown to converge at infinite time towards a state of finite variance, without the aid of viscosity. Furthermore, the spatial profile of its solution gets rough, with the same regularity as a fractional Gaussian field. We here focus on the temporal behavior and derive explicit asymptotic predictions for the correlation function in time of this solution and observe that their regularity is not influenced by the spatial regularity of the problem, only by the correlation in time of the stirring contribution. We also show that the correlation in time of the solution depends on the position, contrary to its correlation in space at fixed times. We then investigate the influence of a forcing which is correlated in time on the spatial and time statistics of this equation. In this situation, while for small correlation times the homogeneous spatial statistics of the white-in-time case are recovered, for large correlation times homogeneity is broken, and a concentration around the origin of the system is observed in the velocity profiles. In other words, this fractional velocity field is a representation in one-dimension, through a linear dynamical model, of the self-similar velocity fields proposed by Kolmogorov in 1941, but only at fixed times, for a delta-correlated forcing, in which case the spatial statistics is homogeneous and rough, as expected of a turbulent velocity field. The regularity in time of turbulence, however, is not captured by this model.