Geometric microcanonical theory of two-dimensional Truncated Euler flows

TL;DR

This paper develops a geometric microcanonical ensemble approach to analyze two-dimensional Truncated Euler flows, accurately predicting energy spectra and large-scale mode statistics, and aligning well with numerical simulations.

Contribution

It introduces a microcanonical ensemble framework for 2D Truncated Euler flows, providing explicit phase space integrals and improved predictions over canonical models.

Findings

Energy spectrum matches Kraichnan's canonical ensemble predictions.

Probability density for large-scale modes shows bimodal behavior.

Microcanonical theory aligns well with numerical simulations, unlike canonical theory.

Abstract

This paper presents a geometric microcanonical ensemble perspective on two-dimensional Truncated Euler flows, which contain a finite number of (Fourier) modes and conserve energy and enstrophy. We explicitly perform phase space volume integrals over shells of constant energy and enstrophy. Two applications are considered. In a first part, we determine the average energy spectrum for highly condensed flow configurations and show that the result is consistent with Kraichnan's canonical ensemble description, despite the fact that no thermodynamic limit is invoked. In a second part, we compute the probability density for the largest-scale mode of a free-slip flow in a square, which displays reversals. We test the results against numerical simulations of a minimal model and find excellent agreement with the microcanonical theory, unlike the canonical theory, which fails to describe the bimodal statistics. This article is part of the theme issue "Mathematical problems in physical fluid dynamics".