Fully localised nonlinear energy growth optimals in pipe flow

TL;DR

This paper identifies a fully localized nonlinear energy growth optimal in long pipe flows, which closely approximates the minimal seed needed to trigger turbulence, advancing understanding of transition mechanisms.

Contribution

It introduces a new fully localized energy growth optimal in long pipes, improving the approximation of the minimal seed for triggering turbulence in real flows.

Findings

Optimal emerges below a threshold disturbance energy.

Optimal remains the most effective until transition energy is reached.

Form of optimal is similar to that in short pipes but fully localized.

Abstract

A new, fully-localised, energy growth optimal is found over large times and in long pipe domains at a given mass flow rate. This optimal emerges at a threshold disturbance energy below which a nonlinear version of the known (streamwise-independent) linear optimal (Schmid \& Henningson 1994) is selected, and appears to remain the optimal up until the critical energy at which transition is triggered. The form of this optimal is similar to that found in short pipes (Pringle et al.\ 2012) albeit now with full localisation in the streamwise direction. This fully-localised optimal perturbation represents the best approximation yet of the {\em minimal seed} (the smallest perturbation capable of triggering a turbulent episode) for `real' (laboratory) pipe flows.