An Experimental Study of Free-surface Aeration on Embankment Stepped Chutes 2005

248 Pages·2005·8.14 MB·English

An Experimental Study of Free-surface Aeration on
Embankment Stepped Chutes

Carlos A. Gonzalez, B.Eng.Civil, M.Eng. Hydraulics UNAM

Department of Civil Engineering,
Faculty of Engineering, Physical Sciences and Architecture
The University of Queensland
Brisbane, Australia

Presented as a thesis to the University of Queensland
for the degree of Doctor of Philosophy.

2005
Statement of Originality
Statement of originality
I hereby declare that the work presented in this thesis is, to the best of my
knowledge, original, except as acknowledged in the text. This material has not been
submitted, either in whole or in part, for a degree at any university.

Carlos Gonzalez
Abstract
Abstract
Stepped chutes have been used as hydraulic structures for more than 3.5 millennia
for different purposes: For example, to dissipate energy, to enhance aeration rate in
the flow and to comply with aesthetical functions. They can be found acting as
spillways in dams and weirs, as energy dissipators in artificial channels, gutters and
rivers, and as aeration enhancers in water treatment plants and fountains.
Spillways are used to prevent dam overtopping caused by floodwaters. Their design
has changed through the centuries. In ancient times, some civilizations used steps to
dissipate energy in open channels and dam over-falls in a similar fashion as natural
cascades. However, in the first half of the twentieth century, the use of concrete
became popular and the hydraulic jump was introduced as an efficient energy
dissipator. In turn, the use of a stepped geometry became obsolete and was replaced
with smooth chutes followed by hydraulic jump stilling basins. In recent years, new
construction techniques and materials (Roller Compacted Concrete RCC, rip-rap
gabions, wire-meshed gabions, etc.) together with the development of new
applications (e.g. re-aeration cascades, fish ladders and embankment overtopping
protection or secondary spillways) have allowed cheaper construction of stepped
chutes, increasing the interest in stepped chute design. During the last three
decades, research in the hydraulics of stepped spillways has been very active.
However, studies prior to 1993 neglected the effect of free-surface aeration. A
number of studies since this time have focused on air-water flows in steep chutes (θ
≈ 50o). But experimental data is still scarce, and the hydraulic performance of
stepped cascades with moderate slope is not yet understood.
This study details an experimental investigation of physical air-water flow
characteristics down a stepped spillway conducted in two laboratory models with
moderate slopes: the first model was a 3.15 m long stepped chute with a 15.9o slope
comprising two interchangeable-height steps (h = 0.1 m and h = 0.05 m); the second
model was a 2.5 m long, stepped channel with a 21.8o slope comprising 10 steps (h
= 0.1 m). Different arrangements of turbulence manipulators (vanes) were also
placed throughout the chute in the second model. A broad range of discharges within
transition and skimming flow regimes was investigated to obtain a reliable
representation of the air-water flow properties. Measurements were conducted using
single and double tip conductivity probes at multiple span wise locations and at
streamwise distances along the cavity between step edges to obtain a complete
Abstract
three-dimensional representation of the flow. Although the present study was
conducted for two moderate slope chutes (θ = 15.9º & 21.8o), it is believed that the
outcomes are valid for a wider range of chute geometry and flow conditions.
The purpose of this study is to improve the understanding of turbulent air-water flows
cascading down moderate slope stepped chutes, and gain new understandings of
the interactions between aeration rate, flow turbulence and energy dissipation; scale
effects are also investigated. The study provides new, original insights into air-water
turbulent flows cascading down moderate slope stepped spillways not foreseen in
prior studies, thus contributing to improve criterion designs. It also presents an
extensive experimental database (available in a CD-ROM attached at the end of this
thesis) and a new design criterion that can be used by designers and researchers to
improve the operation of stepped chutes with moderate slopes. The present thesis
work included a twofold approach. Firstly, the study provided a detailed investigation
of the energy dissipative properties of a stepped channel, based upon detailed air-
water flow characteristics measurements conducted with sub-millimetric conductivity
probes. Secondly, the study focused on the microscopic scale properties of the air-
water flow, using the experimental data to quantify the microscopic scale physical
processes (e.g. momentum transfer, shear layer development, vertical mixing, air-
bubbles/water-droplets break-up and coalescence etc.) that are believed to increase
the flow resistance in stepped canals. The study highlighted the tridimensionality of
skimming flows and hinted new means of enhancing flow resistance by manipulating
turbulence in the stepped chute.
Basic dimensional analysis results emphasized that physical modelling of stepped
chutes is more sensitive to scale effects than classical smooth-invert chute studies
and thus suggested that the extrapolation of results obtained from heavily scaled
experimental models should be avoided. The present study also demonstrated that
alterations of flow recirculation and fluid exchanges between free-stream and cavity
flow affects drastically form losses and in turn the rate of energy dissipation. The
introduction of vanes demonstrated simple turbulence manipulation and form drag
modification that could lead to more efficient designs in terms of energy rate
dissipation without significant structural load on the stepped chute.
Acknowledgements
Acknowledgements
No part of this thesis I have been so eager to write but the acknowledgements and
no other has been so difficult to finish mainly due to the enormous amount of people
whom I am greatly indebted.
In first place I would like to acknowledge all the help provided by my supervisor, Dr.
Hubert Chanson without whose knowledge, support, and exceptional encouragement
and guidance this project would never have been conceived. His boundless
enthusiasm and his contagious love for the physics of nature are a great example to
follow. I never ceased to be amazed by his readiness to discover new things and by
his eagerly desire to excel in every thing he does. I also wish to thank him for his
continuous and unconditional support throughout the project and for his invaluable
advices at the different stages of my stay in Australia. I am greatly indebted with him
and I would like to acknowledge him with the next quote: ‘If I have seen farther than
others, it is because I was standing on the shoulders of giants’. Isaac Newton (1642-
1727). Hubert, Je vous remercie infiniment pour votre soutien, votre aide, vos
conseils ainsi que votre presence et comprehension apportes durant toute la
realisation de cette these. Je vous prie d'agreer mes meilleures salutations et vous
souhaite une bonne continuation. Merci encore enormement pour tout.
I would like to express my gratitude to Graham Illidge, Clive Booth, Rob Stephan and
the rest of the Civil Engineering laboratory Staff for their assistance and expertise
always at the service of my project.
I thankfully acknowledge the help of my friends and colleagues Nick Cartwright, Dave
Callaghan, Ian Teakle, Farshid Homayouni and the rest of the Postgraduate students
at the Department who willingly welcomed me to Australia, shared the good times
and helped me through difficulties. Cheers guys, I had lots of fun and I hope to
welcome you at my home soon. Special thanks to: Luke Toombes for sharing his
programming knowledge, Mark Threvethan for his help printing this thesis and Daniel
Franks for an excellent and constructive proofreading of my thesis.
I also would like to thank: Prof. Iwao Ohtsu, Dr. Youichi Yasuda and Masayuki
Takahashi from Nihon University, for providing me with expert advice and helpful
discussions and for the invitation to collaborate with you at Nihon University in
December 2002. I hope it won’t be the last time. Prof. J. Kongeter, Dr. Paul Kamrath
Acknowledgements
and Jens Torwarth for the invitation to visit RWTH, the expert advices and all the
attentions towards me while in Germany. I hope to be your host soon. Prof. Josep
Dolz, Dr. Marti Sánchez-Juny and Antonio Amador for helpful discussions and for the
opportunity to visit their laboratory at UPC and Dr. J. Matos for helpful discussions
and expert technical advices.
I wish to express my gratitude to the academic and administrative staff of the
department who helped me in one way or another. Especially to Prof. Colin Apelt, Dr.
Peter Nielsen and Dr. Tom Baldock for their questions and interest in my project.
During the duration of my project I enjoyed the friendship of many people who made
my stay in Australia a pleasant, wonderful and rewarding experience. I found new
friends that will stay forever near my heart. Special thanks to: Erick, Vera, Marion,
Daniel, Rocío, Jerome, José, Nick, Nina, James and Phoebe.
I thankfully acknowledge the financial support of the Mexican council for science and
technology (CONACYT), without their support this project would have never been
possible. I wish to go back to Mexico soon and have the honour to serve the people
of my country.
I wish I could write a chapter to thank my parents Tito and Irma, I deeply thank them
for their love and support that has always accompanied me. I hope one day I can
offer them at least a small fraction of what they have offered me throughout my whole
life. I thank their continuous encouragement and all their sacrifices. I extend my
gratitude to my brothers Hugo and Memo and my sister Karla for all their concern and
love.
Lastly, I would like to thank the girl of the emerald eyes whose serene and
conquering sight rode my wild horses, comforted my fears, guided me through storms
and filled my life with love and happiness. I don’t have words to express my deepest
gratitude for her love and patience. This humble work is also hers because of all the
tears we dropped together over it and all the effort that she put into it. Thank you
Carmen.
Dedication
Dedication
I dedicate this humble work of mine to:

My parents

To the memory of uncle Mario

Carmen

The people of México

Table of contents
Table of Contents
Page
STATEMENT OF ORIGINALITY
ABSTRACT
ACKNOWLEDGMENTS
DEDICATION
TABLE OF CONTENTS.................................................................................................I
NOTATION..................................................................................................................VI
Symbols.....................................................................................................................X
Subscripts................................................................................................................XI
Abbreviations...........................................................................................................XI
LIST OF FIGURES.....................................................................................................XII
LIST OF TABLES.....................................................................................................XIX
1 INTRODUCTION....................................................................................................1
1.1 Historical development: From antiquity to the present...............................1
1.2 Literature review.............................................................................................10
1.2.1 Description of basic terms.........................................................................10
1.2.2 Flow structure............................................................................................12
1.2.3 Flow regimes.............................................................................................13
1.3 Objectives.......................................................................................................19
1.4 Outline.............................................................................................................21
2 EXPERIMENTAL SET-UP AND INSTRUMENTATION......................................22
2.1 Experimental Setup........................................................................................22
2.2 Instrumentation..............................................................................................25
2.2.1 Point gauges.............................................................................................26
2.2.2 Pitot-Prandtl Tube.....................................................................................27
2.2.3 Conductivity probes...................................................................................28
2.2.4 Air bubble detector....................................................................................31
2.3 Data analysis...................................................................................................33
2.3.1 Single-Tip Conductivity Probe signal........................................................33
Experimental Study of Free-surface Aeration on Embankment Stepped Chutes I
Table of contents
2.3.2 Double-tip conductivity probe signal.........................................................35
2.3.3 Sampling frequency..................................................................................46
2.4 Measurement accuracy.................................................................................46
2.5 Non-representative samples.........................................................................48
3 FLOW PROPERTIES IN MODERATE SLOPE CHUTES COMPRISING
HORIZONTAL STEPS................................................................................................50
3.1 Introduction....................................................................................................50
3.1.1 Visualized flow patterns............................................................................50
3.2 Experimental flow conditions.......................................................................53
3.2.1 Crest observations and water discharge measurements.........................53
3.2.2 Double-tip probe measurements..............................................................55
3.3 Experimental Results....................................................................................56
3.3.1 Air concentration measurements..............................................................56
3.3.2 Air-water velocity measurements.............................................................60
3.3.3 Turbulent shear stress..............................................................................66
3.3.4 Turbulence intensity measurements.........................................................67
3.3.5 Flow resistance in skimming flows...........................................................69
3.3.6 Flow resistance in transition flows............................................................72
3.3.7 Bubble count rate data..............................................................................73
4 PHYSICAL MODELLING AND SCALE EFFECTS OF STEPPED SPILLWAYS
WITH MODERATE SLOPES.....................................................................................78
4.1 Introduction....................................................................................................78
4.1.1 Dimensional analysis and similitude.........................................................79
4.1.2 Previous studies on scale effects.............................................................81
4.2 Experimental study with geometrically-similar physical models.............82
4.3 Experimental Results....................................................................................85
4.3.1 Air–water flow characteristics in moderate slope stepped spillways
operating with transition flow...................................................................................85
4.3.2 Air–water flow scale effects for moderate slope stepped spillways
operating with skimming flow..................................................................................90
4.4 Dynamic similarity in terms of flow resistance on moderate slope
stepped spillways...................................................................................................94
4.4.1 Scale effects in terms of flow resistance in skimming flows.....................95
4.4.2 Scale effects in terms of flow resistance in transition flows.....................97
4.5 Discussion and outcomes............................................................................99
5 ENERGY DISSIPATION ENHANCEMENT AND TURBULENCE
MANIPULATION......................................................................................................101
5.1 Introduction..................................................................................................101
Experimental Study of Free-surface Aeration on Embankment Stepped Chutes II