A study of the entrainment and turbulence in a plane buoyant jet

Creators: Kotsovinos, Nikolas E.

Abstract

The entrainment and mixing processes in a two-dimensional vertical turbulent buoyant (heated) jet in its transition state from a pure jet to a pure plume have been studied. The ambient fluid is of uniform density and non-flowing except for the flow induced by the jet. Density variations are assumed small. The equations of motion integrated across the jet have been carefully examined and it has been found that the kinematic buoyancy flux of a heated plume and the kinematic momentum flux of a pure jet are not in general conserved. It has been proven that the flow in a two-dimensional pure jet is not self-preserving. A systematic set of experiments was carried out to examine turbulent buoyant jet behavior for a wide range of initial Richardson numbers (or densimetric Froude numbers). Values of the Richardson number, which describes the relative importance of buoyancy in a jet, extended from the value appropriate for a pure jet (zero) to that appropriate for a plume (approximately 0.6). The buoyant jet temperature and velocity fields were measured using calibrated fast response thermistors and a laser Doppler velocimeter respectively. The velocity and temperature data obtained were recorded magnetically in digital form and subsequently processed to extract both mean and fluctuating values of temperature and velocity. The structure of the mean flow (including the spreading rate of the mean velocity and temperature profiles, velocity and temperature distribution along jet axis, and the heat flux profile), the turbulence structure (including the profile of turbulence intensity and turbulent heat transfer, probability density distribution of temperature and velocity, skewness and flatness factor of temperature fluctuations) and the large scale motions (intermittency, profile of maximum and minimum temperature, frequency of crossing of hot/cold, cold/hot interface) of a buoyant jet were investigated as a function of the jet Richardson number. It was determined that the turbulent heat transfer and the turbulent intensity increase with increasing the Richardson number. The spreading rate of the transverse mean velocity and temperature profiles were found to be independent of the Richardson number. The turbulent buoyancy flux in a fully developed buoyant jet has been found to be a significant fraction (38%) of the axial buoyancy flux.

Additional Information

I wish to express my deepest appreciation and gratitude to my advisor, Professor Ericson John List, whose inspired guidance, insight, perspicacious criticism and enthusiasm were invaluable to this research. I am greatly indebted to Professors Norman H. Brooks and Donald Coles, whose advice and interest in this research continuously stimulated and inspired me. I am grateful to Dr. Robert C. Y. Koh for numerous discussions, and to Dr. Jörg Imberger for his personal interest, encouragement and friendship during the incipient period of my graduate studies. I wish also to express my deepest respect and appreciation to Professor Vito Vanoni for his kind interest and encouragement throughout my research. I gratefully acknowledge that the enthusiastic teaching of Professor Themis Xanthopoulos provided me with the necessary stimulation to pursue graduate studies. I thank the California Institute of Technology for providing me with financial support through Graduate Teaching and Research Assistantships, and for its superb research environment facilities. Financial assistance provided by the National Science Foundation, under Grant ENG75-02985, is greatly appreciated. I am indebted to Mr. Elton F. Daly and Mr. Joseph Fontana for their invaluable assistance in the construction of the laboratory equipment, and Mr. Bob Shultz for the preparation of the graphs. I thank also Mrs. Joan Mathews for her patience in typing this manuscript. Finally, I thank my wife, Kathy, whose love, support and understanding were essential during the course of this work. This report was submitted by the writer in May 1975 as a thesis with the same title to the California Institute of Technology in partial fulfillment of the requirements for the degree of Doctor of Philosophy.

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