Understanding the sub-critical transition to turbulence in wall flows

TL;DR

This paper reviews the transition to turbulence in wall flows, highlighting the complex coexistence of laminar and turbulent regions at sub-critical Reynolds numbers and proposing a new spatio-temporal intermittence framework based on phase transition theory.

Contribution

It introduces an alternative spatio-temporal perspective on turbulence transition in wall flows, contrasting with traditional temporal chaos models.

Findings

Wall flows exhibit coexistence of laminar and turbulent domains at sub-critical Reynolds numbers.

Transition mechanisms are better understood, but statistical properties remain inadequately characterized.

A new phase transition-based framework is proposed for understanding turbulence onset.

Abstract

Contrasting with free shear flows presenting velocity profiles with inflection points which cascade to turbulence in a relatively mild way, wall bounded flows are deprived of (inertial) instability modes at low Reynolds numbers and become turbulent in a much wilder way, most often marked by the coexistence of laminar and turbulent domains at intermediate Reynolds numbers, well below the range where (viscous) instabilities can show up. There can even be no unstable mode at all, as for plane Couette flow (pCf) or for Poiseuille pipe flow (Ppf) that currently are the subject of intense research. Though the mechanisms involved in the transition to turbulence in wall flows are now better understood, statistical properties of the transition itself are yet unsatisfactorily assessed. A review of the situation is given. An alternative to the temporal theory of the transition to turbulence in terms of chaotic transients in such globally subcritical flows is proposed, which invokes spatio-temporal intermittence and the theory of first order (thermodynamic) phase transitions.