Laminar-turbulent transition in a slowly pulsating pipe flow

Transitional pulsating flow in a pipe is investigated experimentally in the frequency region where its behavior is assumed to be quasisteady. Instantaneous velocity measurements were performed at several radial locations at the exit plane of the pipe. The output of flush-mounted hot wires at two upstream positions was also recorded simultaneously. The results indicate that the quasisteady assumption is valid in general. The flow behavior is the function primarily of the instantaneous Reynolds number, although the laminarization process and retransition to turbulence are qualitatively different. The laminarization at subcritical instantaneous Reynolds numbers in a time-dependent flow is a gradual process, similar to the observed turbulence decay in flow geometries where Re is varied spatially. The retransition to turbulence occurs by means of the generation of turbulent slugs at various locations along the pipe. These slugs are convected downstream and coalesce, eventually resulting in fully developed turbulent flow in the whole pipe at higher instantaneous Reynolds numbers.