Transition to turbulence in starting pipe flows

An experimental investigation was performed to study the transition to turbulence in pipe flow started impulsively with a supercritical Reynolds number. The results show that a turbulent-non-turbulent interface propagates downstream by consuming the non-turbulent region where laminar boundary-layer flow is developing with time. For the startup of a completely quieted fluid, the propagation velocity is equal to the maximum velocity of the steady-state turbulent flow. For restarting the fluid flow after a brief shutoff, however, the propagation velocity is larger than the maximum velocity, and the difference increases with shorter quieting times. In the process of interface propagation, a wave of very low frequency appears ahead of the interface centring around the radial position where the smallest change in velocity is observed at the instant of propagation. A study of the mechanism of interface propagation shows that the final break of the above-mentioned wave continuously triggers downstream propagation of the turbulent-flow region. In addition, random jumping of the interface in turbulence-decaying flow is concluded to be the main cause for the increase in propagation velocity for short quieting times.