On the transition to turbulence of wall-bounded flows in general, and plane Couette flow in particular

• Reviews recent achievements about transition to turbulence in wall-bounded flows.
• Highlights laminar–turbulent oblique band coexistence in plane Couette flow.
• Analyzes respective relevance of temporal chaos and spatiotemporal chaos.
• Introduces account of transition using the tools of statistical physics.
• Presents new results and a model for band patterning in Couette flow.

The main part of this contribution to the special issue of EJM-B/Fluids dedicated to Patrick Huerre outlines the problem of the subcritical transition to turbulence in wall-bounded flows in its historical perspective with emphasis on plane Couette flow, the flow generated between counter-translating parallel planes. Subcritical here means discontinuous and direct, with strong hysteresis. This is due to the existence of nontrivial flow regimes between the global stability   threshold Reg, the upper bound for unconditional return to the base flow, and the linear instability   threshold Rec characterized by unconditional departure from the base flow.The transitional range   around Reg is first discussed from an empirical viewpoint (Section  1). The recent determination of Reg for pipe flow by Avila et al. (2011) is recalled. Plane Couette flow is next examined. In laboratory conditions, its transitional range displays an oblique pattern made of alternately laminar and turbulent bands, up to a third threshold Ret beyond which turbulence is uniform.Our current theoretical understanding of the problem is next reviewed (Section  2): linear theory and non-normal amplification of perturbations; nonlinear approaches and dynamical systems, basin boundaries and chaotic transients in minimal flow units; spatiotemporal chaos in extended systems and the use of concepts from statistical physics, spatiotemporal intermittency and directed percolation, large deviations and extreme values. Two appendices present some recent personal results obtained in plane Couette flow about patterning from numerical simulations and modeling attempts.