Transition to turbulence in wall-bounded flows: Where do we stand?

TL;DR

This paper reviews the transition to turbulence in wall-bounded flows, discussing phenomenology, theoretical approaches, dynamical systems, spatiotemporal features, and stochastic modeling, highlighting recent advances and open challenges.

Contribution

It provides a comprehensive overview of recent achievements in understanding the transition to turbulence, emphasizing the role of special solutions, dynamical systems, and stochastic models.

Findings

Identification of nontrivial solutions to Navier-Stokes equations.

Role of dynamical systems theory in analyzing turbulence.

Potential of directed percolation models for turbulence transition.

Abstract

In this essay, we recall the specificities of the transition to turbulence in wall-bounded flows and present recent achievements in the understanding of this problem. The transition is abrupt with laminar-turbulent coexistence over a finite range of Reynolds numbers, the transitional range. The archetypical cases of Poiseuille pipe flow and plane Couette flow are first reviewed at the phenomenological level, together with a few other flow configurations. Theoretical approaches are then examined with particular emphasis on the existence of special nontrivial solutions to the Navier-Stokes equations at finite distance from laminar flow. Dynamical systems theory is most appropriate to analyze their role, in particular with respect to the transient character of turbulence in the lower transitional range. The extensions needed to deal with the prominent spatiotemporal features of the transition are then discussed. Turbulence growth/decay in terms of statistical physics of many-body systems and the relevance of directed percolation as a stochastic process able to account for it are next scrutinized. To conclude, we advocate the recourse to well-designed modeling able to provide us with a conceptually coherent picture of the full transitional range and put forward some open issues.