Details on the intermittent transition to turbulence of a locally forced plane Couette flow

Abstract : Experimental results are presented on the transition to turbulence of a plane Couette flow locally and permanently forced by a small bead. The intermittent aspect of this transition is investigated in a detailed analysis of the transition periods from coherent to turbulent and vice versa. A maximum number of transitions are achieved at a Reynolds number of about 310. It is shown that the breakdown of the flow coherence results from a complex succession of events occurring as follows. A secondary instability takes place leading to a drift of at least one vortex pair away from the excitation source. Consequently, the flow in the bead vicinity is laminar and a vortex pair is generated. Turbulence results from the interaction between the newly born vortex and the one already active. As the fluctuation intensity decreases all over the flow, the laminar state is recovered in the bead vicinity from which two vortex pairs are regenerated. The second main contribution of this paper is the study of the turbulent state using spatio-temporal correlation. The role of lateral streaks and their properties in the turbulent state are investigated. It is demonstrated that the turbulent spot is sustained by a vortex generation near the source followed by convection towards the boundaries. In both coherent and turbulent states, the flow near the bead is found to determine the plane Couette flow evolution.