Lift-up and streak waviness drive the self-sustained process in wall-bounded transition to turbulence

TL;DR

This paper provides experimental evidence linking streak waviness and lift-up effects to the self-sustained process in wall-bounded transition to turbulence, using local analysis of turbulent patterns in Couette-Poiseuille flow.

Contribution

It offers the first experimental quantification of the relationship between streak waviness, streamwise rolls, and lift-up effect in the SSP of turbulence transition.

Findings

Streamwise roll amplitude correlates with streak waviness.

Low waviness levels show measurable lift-up effect.

First experimental validation of SSP mechanisms in wall-bounded turbulence.

Abstract

Flow field measurements from a Couette-Poiseuille experiment are used to examine quantitatively certain steps of the self-sustained process (SSP) of wall-bounded transition to turbulence. Although the different parts of the SSP have been discussed at large in the literature, direct measurements from experiment are scarce and, to our knowledge, the present results are the first to show, using a local analysis of the turbulent patterns, that: (1) the amplitude of streamwise rolls is related to streak waviness, bringing a quantitative picture to one of the main physical mechanisms of Waleffe's model of SSP ; and (2), at low waviness, direct measurements of the correlation between the streak and roll amplitudes, respectively probed by the streamwise and wall-normal velocity perturbations, quantify the lift-up effect.