|M.Sc Student||Vyomesh Mehta|
|Subject||Experimental Investigation of Mechanisms Leading to|
Transition in Pipe Flow
|Department||Department of Aerospace Engineering||Supervisor||Professor Jacob Cohen|
|Full Thesis text|
The issue of transition to turbulence in pipe flows is both of deep scientific interest and of significant engineering importance. Flows in oil and gas pipelines are sometimes run inefficiently turbulent to avoid the large pressure fluctuations found in the transitional regime. Although the problem deals with simple geometry of pipe flow, its implications are far beyond: the control of turbulence is an industrial ultimate goal, whereas an understanding of turbulence is a long standing scientific desire.
In this work an experimental investigation of the transition process in water pipe flow is carried out. The objective is to try and follow carefully the sequence of transitional events while varying gradually the disturbance level from a laminar state to a fully developed turbulent state, using flow visualization, pressure measurements, hot-wire and PIV measurements providing spectral content, mean and r.m.s (root mean square) velocity distributions. The artificial disturbance is introduced in a region where the flow is approximately fully developed. Two modes of water-jet injections are used to generate the disturbance: a single and an opposite double injections, perpendicular to the main stream. The ratio between the jet and pipe diameters is about 0.04 for which transition is obtained for a jet to main stream mass flow ratio of about 0.01% to 0.1%.
As the disturbance level is increased, counter-rotating vortex pairs followed by the formation of a packet of hairpin vortices are developed. A theoretical model explaining this stage of evolution is validated experimentally. The shedding frequency of the hairpins matches a phase velocity of their travelling heads to be 0.33 of the center line velocity. As the disturbance level is further increased, the flow next to the injection location is mixed and the dominance frequency of the hairpins disappears. The evolution of fluctuations patterns at 44 diameters downstream the injection location show that as the intensity of the jet is increased a jet-like pattern is first observed followed by a wake-like pattern, the latter indicating a turbulent state.