Entrainment of fine sediments by turbulent flows
Creators: name: Sutherland, Alexander James
Publisher: name: California Institute of Technology
Year: 1966

description: A study was made of the means by which turbulent flows entrain sediment grains from alluvial stream beds. Entrainment was considered to include both the initiation of sediment motion and the suspension of grains by the flow. Observations of grain motion induced by turbulent flows led to the formulation of an entrainment hypothesis. It was based on the concept of turbulent eddies disrupting the viscous sublayer and impinging directly onto the grain surface. It is suggested that entrainment results from the interaction between fluid elements within an eddy and the sediment grains.

A pulsating jet was used to simulate the flow conditions in a turbulent boundary layer. Evidence is presented to establish the validity of this representation. Experiments were made to determine the dependence of jet strength, defined below, upon sediment and fluid properties. For a given sediment and fluid, and fixed jet geometry there were two critical values of jet strength: one at which grains started to roll across the bed, and one at which grains were projected up from the bed. The jet strength, K, is a function of the pulse frequency, [omega] , and the pulse amplitude, A, defined by

K=A[omega]^-s

where s is the slope of a plot of log A against log [omega]. Pulse amplitude is equal to the volume of fluid ejected at each pulse divided by the cross sectional area of the jet tube.

Dimensional analysis was used to determine the parameters by which the data from the experiments could be correlated. Based on this, a method was devised for computing the pulse amplitude and frequency necessary either to move or project grains from the bed for any specified fluid and sediment combination.

Experiments made in a laboratory flume with a turbulent flow over a sediment bed are described. Dye injection was used to show the presence, in a turbulent boundary layer, of two important aspects of the pulsating jet model and the impinging eddy hypothesis. These were the intermittent nature of the sublayer and the presence of velocities with vertical components adjacent to the sediment bed.

A discussion of flow conditions, and the resultant grain motion, that occurred over sediment beds of different form is given. The observed effects of the sediment and fluid interaction are explained, in each case, in terms of the entrainment hypothesis.

The study does not suggest that the proposed entrainment mechanism is the only one by which grains can be entrained. However, in the writer's opinion, the evidence presented strongly suggests that the impingement of turbulent eddies onto a sediment bed plays a dominant role in the process.
description: © 1966 W. M. Keck Laboratory of Hydraulics and Water Resources. California Institute of Technology.

For his encouragement and invaluable advice the author expresses his sincere appreciation to Professor Vito A. Vanoni, who served as advisor during the course of this research. The author is also indebted to Professor Norman H. Brooks for his continuing interest in the work and for his willingness to discuss all aspects of it. For advice and assistance with all phases of the photographic work during the experimental program the author is indebted to Mr. Carl Eastvedt. For their assistance in the preparation of the final draft of this report the author extends his gratitude to Patricia Rankin, who so ably typed the manuscript, and to Ron Handy who prepared many of the
figures. The research was performed with the support of National Science Foundation Grants GI9194 and GK89. Financial assistance provided by the California Institute of Technology in the form of Graduate Teaching and Research Assistantships is also gratefully acknowledged. This report was submitted by the writer, in May 1966, 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 in Civil Engineering.
description: Submitted - KH_R_13.pdf