Structures cohérentes et transition vers la turbulence par intermittence spatio-temporelle dans l'écoulement de couette plan...An experimental study of the transition to turbulence in the plane Couette flow, that possesses a linearly stable velocity profile for all Reynolds number, is reported. Finite amplitude perturbation can destabilize the flow and a coexistence between the laminar and turbulent state can be observed. Using a localized and permanent perturbation -a wire in the cross-stream direction- we show that coherent structures identified as streamwise counter-rotating vortices present around the wire are the same as those observed on the edges of the turbulent spots, and they are finite amplitude solutions in the limit of the unmodified plane Couette flow. Introducing a little bead in center of the flow, we spatially freeze the dynamic of these vortices; this dynamic happens to be reminiscent of spatio-temporal chaos for the unmodified system. We then use an instantaneous perturbations and quenches to study the nature of the transition, we observe spatio-temporal intermittency (STI) and characterize a critical Reynolds number. Above $R_{\rm c}$, STI is sustained, a mean turbulent fraction can be well defined that increases with $R$. A unique turbulent attractor coexists with the laminar flow, and we properly identify a critical amplitude perturbation curve. Below $R_{\rm c}$, long transients are observed that relax asymptotically to the laminar state, the attractor turns into a repellor....
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