Coherent structures in the form of nearly periodic streamwise vortices appear in many 3D linear hydrodynamical instabilities, such as Taylor-Couette instabilities. They are also present in wall-bounded turbulence and are the subject of detailed studies in the subcritical transition to turbulence in channels and other confined shear flows.
In this talk, we present various characteristics of these structures, such as the presence of streaks and rolls observed in our experiments on Couette-Poiseuille flow, where we measure the velocity components of the flow using Particle Image Velocimetry (PIV).
We discuss the large-scale flow that appears when spatial inhomogeneities exist in the basic periodic streak structures. We track the dynamics of the small-scale flow using the separation between streaks and rolls, and by analyzing the streak undulations in the transition to turbulence in our experiment [1]
Through this, we identify the nonlinear mechanisms of Self-Sustaining Process (SSP), proposed by Waleffe[2], as the main scenario for the dynamic evolution of the flow during the transition.

[1] Liu T., Semin B., Godoy-Diana R., Wesfreid JE., Lift-up and streaks wavines drive the self-sustained process in wall-bounded transition to turbulence. Physical Review Fluids 9, 033901, 2024

[2] Fabian Waleffe, On a self-sustaining process in shear flows. Physics of Fluids 9, 883, 1997