Exact formulas for arbitrary order velocity-gradient moments in isotropic turbulence

TL;DR

This paper develops a systematic method to derive exact formulas for statistical moments of any order of velocity gradients in isotropic turbulence, applicable to both compressible and incompressible flows.

Contribution

It introduces a unified framework combining tensor theory and averaging to compute high-order velocity gradient moments exactly.

Findings

Higher-order moments depend on strain self-amplification terms.

Derived formulas are validated against existing theories and simulations.

Method applies to both compressible and incompressible turbulence.

Abstract

Statistical moments of velocity gradients provide fundamental information on the small-scale properties of turbulence. In this work, we propose a systematic method to derive exact expressions for statistical moments of arbitrary order for both longitudinal and transverse velocity gradients in isotropic turbulence. The approach is applicable to both compressible and incompressible flows and expresses the moments in terms of invariants of the velocity gradient tensor. The derivation combines isotropic tensor theory, orientational averaging, and an algorithmic implementation, enabling the computation of high-order moments in a unified framework. We show that longitudinal velocity gradient moments of order higher than three depend not only on $\mathrm{tr}(\boldsymbol{S}^2)$, which is proportional to the dissipation rate, but also on $\mathrm{tr}(\boldsymbol{S}^3)$, which reflects strain self-amplification, where $\boldsymbol{S}$ denotes the strain-rate tensor. The resulting theoretical expressions are validated through comparisons with existing theoretical results and direct numerical simulations.