Non-equilibrium Ensembles for the three-dimensional Navier-Stokes Equations

TL;DR

This paper explores a reversible formulation of the 3D Navier-Stokes equations with fluctuating viscosity, investigating conditions for statistical equivalence with traditional models and analyzing viscosity distribution across different flow regimes.

Contribution

It introduces a reversible Navier-Stokes model conserving enstrophy and systematically studies its statistical properties and empirical viscosity distribution.

Findings

Negative viscosity occurrences diminish at higher Reynolds numbers.

Statistical ensembles show conditions for equivalence.

Viscosity distribution depends on flow parameters.

Abstract

At the molecular level fluid motions are, by first principles, described by time reversible laws. On the other hand, the coarse grained macroscopic evolution is suitably described by the Navier-Stokes equations, which are inherently irreversible, due to the dissipation term. Here, a reversible version of three-dimensional Navier-Stokes is studied, by introducing a fluctuating viscosity constructed in such a way that enstrophy is conserved, along the lines of the paradigm of microcanonical versus canonical treatment in equilibrium statistical mechanics. Through systematic simulations we attack two important questions: (a) What are the conditions that must be satisfied in order to have a statistical equivalence between the two non-equilibrium ensembles? (b) What is the empirical distribution of the fluctuating viscosity observed by changing the Reynolds number and the number of modes used in the discretization of the evolution equation? The latter point is important also to establish regularity conditions for the reversible equations. We find that the probability to observe negative values of the fluctuating viscosity becomes very quickly extremely small when increasing the effective Reynolds number of the flow in the fully resolved hydro dynamical regime, at difference from what was observed previously.