Effects of density differences on lateral mixing in open-channel flows
Creators: name: Prych, Edmund A.
Publisher: name: California Institute of Technology
Year: 1970

description: This study investigates lateral mixing of tracer fluids in turbulent open-channel flows when the tracer and ambient fluids have different densities. Longitudinal dispersion in flows with longitudinal density gradients is investigated also. 

Lateral mixing was studied in a laboratory flume by introducing fluid tracers at the ambient flow velocity continuously and uniformly across a fraction of the flume width and over the entire depth of the ambient flow. Fluid samples were taken to obtain concentration distributions in cross-sections at various distances, x, downstream from the tracer source. The data were used to calculate variances of the lateral distributions of the depth-averaged concentration.When there was a difference in density between the tracer and the ambient fluids, lateral mixing close to the source was enhanced by density-induced secondary flows; however, far downstream where the density gradients were small, lateral mixing rates were independent of the initial density difference. A dimensional analysis of the problem and the data show that the normalized variance is a function of only three dimensionless numbers, which represent: (1) the x-coordinate, (2) the source width, and (3) the buoyancy flux from the source. 

A simplified set of equations of motion for a fluid with a horizontal density gradient was integrated to give an expression for the density-induced velocity distribution. The dispersion coefficient due to this velocity distribution was also obtained. Using this dispersion coefficient in an analysis for predicting lateral mixing rates in the experiments of this investigation gave only qualitative agreement with the data. However, predicted longitudinal salinity distributions in an idealized laboratory estuary agree well with published data.
description: This study, one of a group of investigations at the California Institute of Technology titles "Dispersion in Hydrologic and Coastal Environments," was funded by the Federal Water Pollution Control Administration through Grants No. 16000 DGY and No. 16070 DGY. During the study the writer also received financial assistance in the form of stipends and tuition payments while a U.S. Public Health Service Trainee (1967-1970) and while a National Science Foundation Trainee (1966-1967). The writer thanks each of these agencies for their support. 

To Dr. Norman H. Brooks, adviser and fruitful source of "the right questions," the writer expresses his sincere gratitude. The writer also thanks Dr. Vito A. Vanoni for comments made during the study, and Dr. E. John List for his comments during the writing of this report. 

For their assistance in constructing and modifying laboratory equipment, the writer offers a hearty thanks to Mr. Elton F. Daly, supervisor of the shop and laboratory; to Mr. Robert L. Greenway, his talented assistant; and to Mr. Carl A. Green, Jr., who also prepared most of the drawings in this text. A warm thank you is extended to Mrs. Arvilla F. Krugh, for typing this manuscript; to Mr. Carl T. Eastvedt, for doing the photographic work; and to Mrs. Patricia A. Rankin, for performing many of the secretarial duties for this project. The writer also acknowledges the efforts of the student assistants who performed a variety of tasks during this investigation; they are: Messrs. Raul Basu, George Chan, Yoshiaki Daimon, Edward F. Thompson, W. H. Waggy, Paul T. Wegener, and Mashio Yatsuzuka. 

This report was submitted in May 1970 as a Ph. D. thesis by the author to the California Institute of Technology.
description: Submitted - TR000086.pdf