Critical threshold in pipe flow transition

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 low, the analysis presented here covers autonomous and impulsive scenarios where either the low 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 Re similar to O(10(4)), the present work provides critical amplitudes that decay as Re-3/2 and Re-1 for the autonomous and impulsive scenarios, respectively. For ReZ2875, 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.