Extended localized structures and the onset of turbulence in channel flow

TL;DR

This paper demonstrates numerically that localized turbulent bands in channel flow can sustain and extend through different mechanisms before the onset of full turbulence, with invariant properties during extension.

Contribution

It introduces the concept of localized turbulent bands in plane Poiseuille flow and details their sustenance and extension mechanisms near turbulence threshold.

Findings

Localized turbulent bands can sustain and extend in channel flow.

Band width and tilt angle remain statistically invariant during extension.

Different mechanisms sustain turbulence before the transition to equilibrium turbulence.

Abstract

In this letter, it is shown numerically that in plane Poiseuille flow and before the threshold of equilibrium turbulence defined by the directed-percolation universality class, a sparse turbulent state in form of localized turbulent band can sustain either by continuous increase of the turbulence fraction due to band extension when the flow domain is large enough, or by a dynamic balance between the band extension and the band breaking and decay caused by the band interaction in a finite domain. The width and tilt angle of the band keep statistically invariant during its oblique extension, a process which is not sensitive to random disturbances.