The growth mechanism of turbulent bands in channel flow at low Reynolds numbers

TL;DR

This study uses direct numerical simulations to analyze the growth and dynamics of turbulent bands in low Reynolds number channel flow, revealing an asymmetrical growth mechanism driven by spanwise inflectional instability.

Contribution

It identifies the spanwise inflectional instability as the key mechanism behind streak generation and band growth, providing a new understanding of turbulence self-sustenance at low Reynolds numbers.

Findings

Downstream end features fast streak generation and growth.

Upstream streaks decay more slowly, causing asymmetry.

Spanwise inflectional instability drives streak generation.

Abstract

In this work, we carried out direct numerical simulations in large channel domains and studied the kinematics and dynamics of fully localised turbulent bands at Reynolds number Re = 750. Our results show that the downstream end of the band features fast streak generation and travels into the adjacent laminar flow, whereas streaks at the upstream end continually and relatively more slowly decay. This asymmetry is responsible for the transverse growth of the band. We particularly investigated the mechanism of streak generation at the downstream end, which engines the growth of the band. We identified a spanwise inflectional instability associated with the local mean flow near the downstream end and our results strongly suggest that this instability is responsible for the streak generation and ultimately for the growth of the band. Based on our study, we propose a possible self-sustaining mechanism of fully localised turbulent bands at low Reynolds numbers in channel flow.