Edge states as mediators of bypass transition in boundary-layer flows

TL;DR

This paper investigates the role of edge states in boundary-layer flows, revealing their connection to turbulent spot nucleation and suggesting a nonlinear dynamical systems perspective on transition processes.

Contribution

It demonstrates the existence of localized edge states in boundary layers and links them to turbulent spot nucleation, advancing understanding of transition mechanisms.

Findings

Edge states are localized, recurrent structures with low-speed streaks.

Edge states act as mediators in turbulent spot nucleation.

Nonlinear dynamical systems theory explains the transition process.

Abstract

The concept of edge state is investigated in the asymptotic suction boundary layer in relation with the receptivity process to noisy perturbations and the nucleation of turbulent spots. Edge tracking is first performed numerically, without imposing any discrete symmetry, in a large computational domain allowing for full spatial localisation of the perturbation velocity. The edge state is a three-dimensional localised structure recurrently characterised by a single low-speed streak that experiences erratic bursts and planar shifts. This recurrent streaky structure is then compared with predecessors of individual spot nucleation events, triggered by non-localised initial noise. The present results suggest a nonlinear picture, rooted in dynamical systems theory, of the nucleation process of turbulent spots in boundary-layer flows, in which the localised edge state play the role of state-space mediator.