J. Fluid Mech. (2013), vol. 731, R1, doi:10.1017/jfm.2013.388 Turbulent spots in a channel: large-scale flow and self-sustainability Grégoire Lemoult†, Jean-Luc Aider and José Eduardo Wesfreid Laboratoire de Physique et M´ ecanique des Milieux H´ et´ erog` enes (PMMH), UMR CNRS 7636, ESPCI, UPMC, Paris Diderot, 10 rue Vauquelin, 75005 Paris, France (Received 22 May 2013; revised 2 July 2013; accepted 24 July 2013; first published online 14 August 2013) Using a large-time-resolved particle image velocimetry field of view, a developing turbulent spot is followed in space and time in a rectangular channel flow for more than 100 advective time units. We show that the flow can be decomposed into a large-scale motion consisting of an asymmetric quadrupole centred on the spot and a small-scale part consisting of streamwise streaks. From the temporal evolution of the energy of the streamwise and spanwise velocity perturbations, it is suggested that a self-sustaining process can occur in a turbulent spot above a given Reynolds number. Key words: channel flow, nonlinear instability, transition to turbulence 1. Introduction Transition to turbulence in wall-bounded shear flows like plane Couette (PCF), Hagen–Poiseuille (HPF) or plane Poiseuille (PPF) flow and boundary layer flow occurs in the presence of localized coherent structures, known as turbulent spots. These objects, first discovered by Emmons (1951), are not composed of a single large-scale homogeneous structure, but of an assemblage of small-scale longitudinal vortices, separated from laminar flow by sharp fronts. For reviews of the subject see Riley & Gad-el Hak (1985), Henningson, Johansson & Alfredsson (1994) and Mathew & Das (2000). The dynamics of a subcritical transition to turbulence, first observed in pipes by Reynolds (1883), is strongly related to the existence and interplay of these localized structures. Other isolated structures have been observed in a great variety of physical systems, such as thermal convection, magnetic liquids, granular material or buckling instabilities. Localized structures are related to subcritical instabilities, i.e. due to finite-amplitude perturbations (Knobloch 2008). In the present study, we present an exhaustive description of turbulent spots in a rectangular channel, in a range of Reynolds numbers in which transition occurs. The † Email address for correspondence: gregoire.lemoult@espci.fr c  Cambridge University Press 2013 731 R1-1