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_τ, turbulence occurs in the form of intermittent localised structures. Different types of localisation are identified as the aspect ratio is varied from η = 0:8 to 0:1. These coherent structures vary from helical turbulence at η = 0:8 to streamwise-localised puffs at η = 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 η = 0:3, accompanied by a monotonic reduction of the pitch angle. For η = 0:3 and marginally low Re_τ, 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.