Late stage of transition in a circular pipe flow

Transition in fully developed circular pipe flow was investigated experimentally by introducing periodic perturbations. The simultaneous excitation of helical modes having indices m = +/-1, +/-2, +/-3 and a "single-peak" excitation were 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 disturbance amplitude. The breakdown to turbulence starts with appearance of spikes in the temporal traces of velocity. Spectral characteristics of these spikes are similar to those observed in a boundary layer. A high-shear layer and a low-speed "bump" were also observed in both flows. It is concluded that the transition process in a pipe flow is similar to that in a boundary layer in many aspects.