Transitional structures in annular Poiseuille flow depending on radius ratio

The transitional regime of incompressible pressure-driven flows inside an annular pipe is investigated using accurate direct numerical simulation in long computational domains. At marginally low friction Reynolds number Re, turbulence occurs in the form of intermittent localised structures. Different types of localisation are identified as the aspect ratio is varied from =0. 8 to 0. 1. These coherent structures vary from helical turbulence at =0. 8 to streamwise-localised puffs at =0. 1. They are respectively analogous to the stripe patterns and puffs formerly identified in plane channel flow and cylindrical pipe flow. Helical turbulence has been tracked down to =0. 3, accompanied by a monotonic reduction of the pitch angle. For =0. 3 and marginally low Re, these turbulence structures localise in the streamwise direction, giving rise to a new regime of helical puffs with chirality. The present results suggest that helical puffs mediate the transition from globally axisymmetric puffs to helical stripe patterns.