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.