title: Critical threshold in pipe flow transition.
abstract: This study provides a numerical characterization of the basin of attraction of the laminar Hagen-Poiseuille flow by measuring the minimal amplitude of a perturbation required to trigger transition. For pressure-driven pipe flow, the analysis presented here covers autonomous and impulsive scenarios where either the flow is perturbed with an initial disturbance with a well-defined norm or perturbed by means of local impulsive forcing that mimics injections through the pipe wall. In both the cases, the exploration is carried out for a wide range of Reynolds numbers by means of a computational method that numerically resolves the transitional dynamics. For , the present work provides critical amplitudes that decay as Re(-3/2) and Re(-1) for the autonomous and impulsive scenarios, respectively. For Re=2875, accurate threshold amplitudes are found for constant mass-flux pipe by means of a shooting method that provides critical trajectories that never relaminarize or trigger transition. These transient states are used as initial guesses in a damped Newton-Krylov method formulated to find periodic travelling wave solutions that either travel downstream or exhibit a helicoidal advection.
authors.affiliation: Department of Applied Physics, Universitat Politècnica de Catalunya, Barcelona, Spain.
authors.forename: Fernando
authors.initials: F
authors.lastname: Mellibovsky
authors.forename: Alvaro
authors.initials: A
authors.lastname: Meseguer