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Entrainment of Sediment Particles from a Flat Mobile Bed with the Influence of Near-Wall Turbulence

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Entrainment of Sediment Particles from a Flat Mobile Bed with the Influence of Near-Wall Turbulence School of Civil Engineering The University of New South Wales a ? Sydney, Australia Entrainment of Sediment Particles from a Flat Mobile Bed with the Influence of Near-wall Turbulence by Alireza Keshavarzy B.Eng., Shiraz University, Shiraz, Iran M.Eng. Sc., The University of New South Wales, Sydney A thesis submitted in partial fulfilment of the requirement for the degree of Doctor of Philosophy 1997 L S W 18 APS 1303 Li UR ARY CERTIFICATE OF ORIGINALITY I hereby declare that this submission is my own work and that, to the best of my knowledge and belief, it contains no material previously published or written by another person nor material which to a substantial extent has been accepted for the award of any other degree or diploma of a university or other institute of higher learning, except where due acknowledgment is made in the text. (Signed) CERTIFICATE OF ORIGINALITY 3 hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, nor material which to a substantial extent has been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to die research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project’s design and conception or in style, presentation and linguistic expression is acknowledged. (Signed) Abstract This study examines the influence of turbulence on the entrainment of sediment particles from a mobile bed. The structure of the turbulent boundary layer is very complicated and, as yet, not fully understood. Consequently, the influence of the boundary layer and the turbulence within the boundary layer on the entrainment of sediment particles is not fully understood. Turbulence has been defined through a quadrant analysis of velocity fluctuations, which are known as bursting process. Four types of events have been recognised on the basis of a quadrant analysis; these events are sweep, ejection, outward interaction and inward interaction. Of these events, the sweep event has been recognised as the most important event for particle entrainment from the bed. In this study, the turbulent characteristics of bursting events in an open channel flow was investigated experimentally and applied to the problem of the initiation of sediment particles. The turbulent velocity fluctuations of the flow were measured in two dimensions and analysed. A statistical analysis of the experimental data was undertaken. In this analysis, a Box-Cox transformation was used to convert shear stresses and the angle of inclination to a normally distributed sample. From the statistical analyses of shear stresses in bursting events it was found that, close to the bed, the average magnitude of the shear stress in a sweep event was 140 percent of the time averaged shear stress, that the frequency of occurrence was 30 percent of the time, and that the average inclination angle for the sweep events was 22°. A stochastic-deterministic mathematical model was developed to define particle entrainment from the bed under the influence of turbulence. This model combines Bagnold’s energy concepts with Einstein’s probability concepts through a combination of the two approaches as postulated in this study. The use of these two concepts was considered necessary in the model as the forces applied to the sediment particles are temporally variable due to the occurrence of turbulence and the associated bursting processes. The model was solved numerically for two alternative cases; these were an instantaneous shear stress and a time-averaged shear stress. When comparing the predicted sediment motion, it was found, for a particular particle diameter, that initiation of particle motion would commence at lower flow rate with consideration of the instantaneous shear stress than would be the case for a time-averaged shear stress using Shields criteria. An inherent assumption in the model is that sweep events are the primary processes inducing particles into motion. The validity of this assumption was tested by comparing i the statistics of particles initiated into motion with statistics of the instantaneous shear stresses during sweep events. In order to find the number of particles initiated into motion at any increment of time, a series of sequential video images of particle motion were analysed. Good agreement was found between the statistics of area entrained and the instantaneous shear stresses during sweep events. The stochastic-deterministic mathematical model was also verified using image processing techniques. The use of a convolution technique with cross-correlation enabled the determination of particle displacement in a given small time increment and consequently the instantaneous velocity of particles. Using these data, the exceedance probabilities of the particle velocities were able to be determined and compared with that predicted by the stochastic-deterministic model. The measured and predicted particle velocities were similar and within the 95% confidence limits. Finally, using the stochastic-deterministic force balance model, the area entrained from the image processing techniques and the measured turbulent shear stresses of the flow, a modification to Shields diagram was proposed. This modification indicates the probability of a particle being induced into motion. ii Acknowledgments I would like to expresss my appreciation to my supervisor Dr. James Ball, for his support, encouragement, comments, discussion and guidance towards the completion of this dissertation. Particularly, his encouragement to express this idea and to publish some papers in this field of study are acknowledged. The suggestions given by Dr. David Luketina and review of dissertation draft is also acknowledged. During the period of this work, the writer has received assistance and suggestions from many people to whom he would like to express appreciation. Special thanks are due to Prof. H.W. Shen for his encouragement and comments during the Stochastic Hydraulics Symposium held in Mackay, Queensland in 1996. Also thanks to Prof. William Dunsmir, Head of the Dept of Statistics at the University of NSW, for his statistical suggestions, consulting and review of statistical analysis, Prof. Trinder J. Head of School of Geomatic Engineering, UNSW, for his suggestions in image processing and review of the final draft of the parts of this dissertation related to image processing and Dr. Wayne Erskine for his suggestion and review some parts of the draft dissertation. The writer also is grateful to Assoc. Prof. Ron Cox, Director of the Water Research Lab. for his support in the experimental parts of this study. Also special thanks to WRL staff; James Carley, John Hart, John Baird, Ross Mathews and Margaret Titterton. Also thanks to Mr. Ken Higgs for his encouragement, introduction to the C language and its application in image processing which proved to be a valuable contribution. During study in UNSW, I benefited from some very close friends, specially Mahmoud Bina, Saied Saiedi, Hamid Rahimipour, Jafar Nazemosadat, Saied Eslamian and Paul Hogan. Finally, I would like to express my thanks to my family; to my wife Mina, for her patience, inspiration and teaching my children during our stay in Australia, as well as to my children, Reza and Zahra for their tolerance in living with a student father. Also I debt my study to my mother, late father, my sisters, brothers, and brother-in-law Mr Safar Keshavarzy for iii their supporting and encouragement during my study. Thanks to God for giving me wisdom, success and strength. The writer also gives thanks the Ministry of Culture and Higher Education, Iranian Government, for the sponsoring my scholarship during study in Australia. iv Table of Contents Contents Pages Abstract i Acknowledgments iii Table of Contents v List of Figures xi List of Tables xvi Notation xvii CHAPTER 1: Introduction 2 1.1 Environmental impact of sediment transport 2 1.2 The problem investigated 4 1.3 Objective of the study 6 1.4 Layout of the dissertation 10 CHAPTER 2: Review of Previous Studies 12 2.1 Extent of review 12 2.2 Some basic concepts of sediment transport and Shields diagram 13 2.3 Particle entrainment and instantaneous turbulent shear stress 23 2.4 The turbulent structure of the flow and bursting phenomena 25 2.4.1 Basic concepts and relevant parameters 25 2.4.2 Quadrant analysis of the bursting process 27 2.4.3 Structure of the turbulent boundary layer 30 2.5 Initiation of sediment motion and force balance model 37 2.6 The entrainment function and intensity of particle entrainment 43 2.7 Summary 47 v CHAPTER 3: Experimental Apparatus and Procedure 49 3.1 Introduction 49 3.2 The flume 50 3.2.1 General description 50 3.2.2 Water supply of the flume 54 3.2.3 Bed roughness characteristics 54 3.2.4 Roughness estimation of the flume and F.C. sheet at the bed 56 3.3 Sediment characteristics 58 3.3.1 Definitions 58 3.3.2 Size distribution of sediment particles 58 3.4 Flow velocity measurement 59 3.4.1 Equipment and procedure 60 3.4.2 Determination of shear velocity 65 3.5 Particle movement measurement 66 3.5.1 Equipment 66 3.5.2 Light illumination 66 3.5.3 Method of capturing video images 70 3.5.4 Analysis of the particle motion images 72 3.6 Experimental
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