Bifurcations to turbulence in transitional channel flow

TL;DR

This paper investigates the transition to turbulence in channel flow, revealing how localized turbulent bands evolve and interact, leading to symmetry-breaking bifurcations and a return to directed-percolation behavior at higher Reynolds numbers.

Contribution

It introduces a detailed numerical study of turbulent band dynamics and proposes a mean-field model explaining symmetry restoration in transitional channel flow.

Findings

LTBs appear above Re ~700 and break spanwise symmetry below Re ~1000.

Transversal splitting increases turbulence spreading, reaching a critical rate at Re2.

2D directed-percolation behavior is observed only above Re2.

Abstract

In wall-bounded parallel flows, sustained turbulence can occur even while laminar flow is still stable. Channel flow is one of such flows and displays spatio-temporal fluctuating patterns of localized turbulence along its way from/to featureless turbulence. By direct numerical simulation, we study the observed inconsistency between turbulence decay according to a two-dimensional directed-percolation (2D-DP) scenario and the presence of sustained oblique localized turbulent bands (LTBs) below the DP critical point. Above Reynolds number Reg \sim 700 sustained LTBs are observed; most LTBs have the same orientation so that the spanwise symmetry of the LTB pattern is broken below Re2 \sim 1000. The frequency of transversal splitting, by which an LTB generates another one with opposite obliqueness, so that turbulence spreading becomes intrinsically two dimensional, increases in the range Reg < Re < Re2. It reaches a critical rate at Re2 beyond which symmetry is restored. 2D-DP behavior is retrieved only above Re2. A mean-field model is proposed which qualitatively accounts for the above symmetry-restoring bifurcation by considering interactions between space-averaged densities of LTBs propagating in either direction.