Gradual development of perturbations toward a turbulent puff in pipe flows

Development of perturbations toward a turbulent puff in pipe flows was examined experimentally at moderate Reynolds numbers around 2000. Controlling amplitude of cross-flow jet as perturbations represented complex threshold for the transition that was also reported in Tasaka et al. Phys. Rev. Lett. (2010). Detailed measurements of local velocity fluctuations at pipe center by LDV elucidated two different routes toward a turbulent puff along the complex threshold. With relatively small amplitudes, transitions are accompanied by formation of hair-pin like vortices, and the transition process progresses gradually; a primary short puff grows up in both spatial extent and velocity defect. With relatively stronger perturbations, a short slug-like structure is formed, and relaminalization at the front of structure leads it toward a developed puff. These different routes were described as trajectories on phase space of magnitude of the velocity defect and intensity of the velocity fluctuations.