Couette-Poiseuille flow experiment with zero mean advection velocity: Subcritical transition to turbulence

TL;DR

This study introduces an experimental setup for zero mean advection shear flow, investigates the subcritical transition to turbulence, and characterizes turbulent spots and their dynamics in Couette-Poiseuille flow.

Contribution

First experimental analysis of subcritical turbulence transition in zero mean advection Couette-Poiseuille flow with detailed turbulent spot characterization.

Findings

Turbulent spots are nearly stationary due to zero mean advection velocity.

Two regions of turbulence identified: active core and surrounding region.

Traveling waves move downstream in the flow.

Abstract

We present a new experimental set-up that creates a shear flow with zero mean advection velocity achieved by counterbalancing the nonzero streamwise pressure gradient by moving boundaries, which generates plane Couette-Poiseuille flow. We carry out the first experimental results in the transitional regime for this flow. Using flow visualization we characterize the subcritical transition to turbulence in Couette-Poiseuille flow and show the existence of turbulent spots generated by a permanent perturbation. Due to the zero mean advection velocity of the base profile, these turbulent structures are nearly stationary. We distinguish two regions of the turbulent spot: the active, turbulent core, which is characterized by waviness of the streaks similar to traveling waves, and the surrounding region, which includes in addition the weak undisturbed streaks and oblique waves at the laminar-turbulent interface. We also study the dependence of the size of these two regions on Reynolds number. Finally, we show that the traveling waves move in the downstream (Poiseuille).