The background method: Theory and computations

TL;DR

This paper reviews recent theoretical and computational advances in the background method, a technique for rigorously bounding mean properties of turbulent flows, with applications to Rayleigh-Bénard convection.

Contribution

It introduces a systematic auxiliary function framework, explores symmetry exploitation, and connects semidefinite programming with timestepping approaches for optimizing bounds.

Findings

Unified framework for the background method

Relation between semidefinite programming and timestepping approaches

Application to Rayleigh-Bénard convection

Abstract

The background method is a widely used technique to bound mean properties of turbulent flows rigorously. This work reviews recent advances in the theoretical formulation and numerical implementation of the method. First, we describe how the background method can be formulated systematically within a broader "auxiliary function" framework for bounding mean quantities, and explain how symmetries of the flow and constraints such as maximum principles can be exploited. All ideas are presented in a general setting and are illustrated on Rayleigh-B\'enard convection between stress-free isothermal plates. Second, we review a semidefinite programming approach and a timestepping approach to optimizing bounds computationally, revealing that they are related to each other through convex duality and low-rank matrix factorization. Open questions and promising directions for further numerical analysis of the background method are also outlined.