The growth of turbulent spots in plane Couette flow

Using Particle Image Velocimetry (PIV) in an experimental plane Couette flow, we investigate the growth of turbulent spots invading formerly a laminar flow. We observe the existence of large scale flows appearing as soon as laminar and turbulent domains coexist. Spectral analysis is used to study the dynamical evolution of these large-scale structures as well as that of the small-scale structure associated with turbulence. Visualisations allow a study of the evolution of the spot growth rate and also the velocity of waves which we observe at the spot edges. All these results show that two mechanisms are at work when turbulent spots grow; a growth by destabilization but also in the same proportion a growth by large-scale transport.