“…Mostly dealing with mathematically infinitesimal perturbations and focusing on linear normal-mode dynamics, the conventional stability theory (Lin, 1955) experiences difficulties in accounting for the emergence of turbulent behavior directly out of laminar flow, as anticipated by Reynolds (1883) who understood the importance of finite-amplitude localized disturbances. It was not until the rise of nonlinear dynamics and chaos theory in the 1970's (Aubin & Dalmenico, 2002), the non-modal approach to transient growth (Trefethen et al, 1993;Grossmann, 2000), and the systematic development of numerical simulations of the Navier-Stokes equations in relevant flow configurations (Moin & Mahesh, 1998), that some theoretical understanding has been gained out of the accumulation of phenomenological evidence on the direct transition via turbulent puffs or slugs in pipe flow (Reynolds, 1883;Lindgren, 1951;Wygnanski & Champagne, 1973;etc. ) and, in other flows, spots (Emmons, 1951;Carlson et al, 1982;etc.…”