Transition in a fully developed circular pipe flow was investigated experimentally by introducing periodic perturbations. The simultaneous excitation of helical modes having indices m = ±1, ±2 and ±3 was chosen. The experiments revealed that the late stage of transition is accompanied by the formation of streaky structures that are associated with peaks and valleys in the azimuthal distribution of the streamwise velocity disturbance. The breakdown to turbulence starts with the appearance of spikes in the temporal traces of the velocity. Spectral characteristics of these spikes and the direction of their propagation relative to the wall are similar to those in boundary layers. Analysis of the data suggests the existence of a high-shear layer in the instantaneous velocity profile.

Additional experiments in which a very weak, steady flow was added locally to the periodic axisymmetric perturbation were also carried out. These experiments resulted in the generation of a single peak in the azimuthal distribution of the disturbance amplitude. The characteristics of the transition process (spikes, vortical patterns etc.) within this peak were similar to ones observed in the helical excitation experiments. Based on these results one may conclude that late stages of transition in a pipe flow and in a boundary layer are similar. The present report is part of an ongoing investigation that was initiated by Eliahou, Tumin & Wygnanski (1998a).