Transition sous-critique à la turbulence : étude de la croissance de poches turbulentes dans l’écoulement de Couette plan

Abstract : This thesis focuses on the study of the laminar-turbulent coexistence in plane Couette flow, the flow sheared between two walls where a subcritical transition to turbulence occurs. We jointly use an experimental setup and direct numerical simulations which allow us to have access to visualizations and velocity fields when both laminar and turbulent states coexist. We show for the first time in a plane Couette experiment the presence of large scale flows in the vicinity of turbulent fronts during the turbulence expansion. We quantify them and show a correlation between their dynamics and the one of turbulent spot. We then focus on the turbulent spot growth in an initially laminar flow. A new global mechanism is identified which complements the usually evoked local mechanism. The study of the growing rate of both mechanisms allows one to get a more comprehensive view of the spot expansion along the spanwise direction which occurs via nucleations of velocity streaks. We also establish a link between the dynamics of the spot fronts, the one of the edge vortices and the one of large scale flows. We eventually study the turbulent spot growth along the streamwise direction by way of spot front velocities analysis as a function of time and Reynolds number. The turbulent spot shape can then be understood as the result of the effect of the mechanisms acting along both directions.
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Submitted on : Friday, January 8, 2016 - 2:33:16 PM
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Marie Couliou. Transition sous-critique à la turbulence : étude de la croissance de poches turbulentes dans l’écoulement de Couette plan. Mécanique des fluides [physics.class-ph]. ENSTA PARISTECH UNIVERSITÉ PARIS SACLAY, 2015. Français. ⟨tel-01252947⟩

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