Free-Surface Aeration, Turbulence, and Energy Dissipation on Stepped Chutes with Triangular ...

361 Pages·2017·11 MB·English

by  Gangfu Zhang

Free-Surface Aeration, Turbulence, and Energy Dissipation
on Stepped Chutes
with Triangular Steps, Chamfered Steps, and Partially Blocked Step Cavities

Gangfu Zhang
BE Civil (Hons. I), University of Queensland

A thesis submitted for the degree of Doctor of Philosophy at
The University of Queensland in 2017
School of Civil Engineering
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Abstract
The present thesis work contains investigations of the two-phase flow properties and energy
dissipation mechanisms in a 45° steep chute lined with various types of stepped macro-roughness,
including one configuration with triangular flat steps, one with chamfered steps, and three with
partial step cavity blockages. The study was conducted in large-size physical models with an
emphasis on the skimming flow regime subject to minimal scale effects. Detailed two-phase flow
patterns were documented using an ultra-high-speed video camera and processed with advanced
computer vision techniques. Both the developing clear-water flow region and the fully-developed
aerated flow region were investigated.
In the clear water flow region, the free-surface curvature was pronounced notably at smaller
discharges and was approximately in-phase with the bottom roughness. Rapid pressure variations
were recorded underneath, defying the hydrostatic pressure assumption and highlighting the rapidly
varied nature of the flow. A further analysis revealed similarities between turbulence properties in
flows in stepped chutes and those over macro-roughness elements.
The experimental findings concluded that the air-water flow properties were broadly similar across
all setups and consistent with previous investigations. The aeration process in the initial rapidly
varied flow region downstream of the inception point may be adequately described by a newly
proposed two-dimensional solution. A number of quantities exhibited self-similar relationships in
the gradually varied flow region downstream of the inception point of aeration, while uniform
equilibrium flow conditions were not achieved in any setup. Further exploration revealed a physical
demarcation about the point where the time averaged void fraction equals 50%, which implied a
statistically discrete water-phase above this location. A new set of scaling variables corresponding
to this location were proposed accordingly, which displayed more physical relevance than the
traditional scaling choices based on a time averaged void fraction of 90%.
A novel setup consisting of a dual-tip phase detection probe and a total pressure transducer was
proposed to enable separation of the water-phase properties from the mixture. The system allowed a
better assessment of the water phase turbulence properties, which displayed similar levels of
fluctuations to those in the clear water flow region. The same setup was used to appraise the energy
dissipation performance of each model, with an average level of 50% energy dissipation found in all
but the chamfered chute at large discharges. Direct total head measurements by the transducer
appeared to be of superior quality compared to those inferred from phase-detection probe data
alone, which were shown to have typically overestimated the energy dissipation potential.
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Comparison between different models revealed relatively larger sensitivities of the free-surface
aeration than of the energy dissipation performance to the roughness density and the step shape.
Sharp edges with a smaller roughness density appeared to improve air-entrainment, which could be
linked to the state of vortex shedding determined by the roughness. The chamfered steps increased
the skin friction contribution to the total drag, which was associated with a delayed onset of aeration
and an energy dissipation performance that scaled poorly with an increasing discharge. Overall, the
flow processes more resembled those over obstacles, which might demand investigations on an
individual basis.
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Declaration by author
This thesis is composed of my original work, and contains no material previously published or
written by another person except where due reference has been made in the text. I have clearly
stated the contribution by others to jointly-authored works that I have included in my thesis.
I have clearly stated the contribution of others to my thesis as a whole, including statistical
assistance, survey design, data analysis, significant technical procedures, professional editorial
advice, and any other original research work used or reported in my thesis. The content of my thesis
is the result of work I have carried out since the commencement of my research higher degree
candidature and does not include a substantial part of work that has been submitted to qualify for
the award of any other degree or diploma in any university or other tertiary institution. I have
clearly stated which parts of my thesis, if any, have been submitted to qualify for another award.
I acknowledge that an electronic copy of my thesis must be lodged with the University Library and,
subject to the policy and procedures of The University of Queensland, the thesis be made available
for research and study in accordance with the Copyright Act 1968 unless a period of embargo has
been approved by the Dean of the Graduate School.
I acknowledge that copyright of all material contained in my thesis resides with the copyright
holder(s) of that material. Where appropriate I have obtained copyright permission from the
copyright holder to reproduce material in this thesis.
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Publications during candidature
Peer-reviewed journal papers
Zhang, G., and Chanson, H. (2016). “Interactions between free-surface aeration and total pressure
on a stepped chute.” Experimental Thermal and Fluid Science, Vol. 74, pp. 368-381 (DOI:
10.1016/j.expthermflusci.2015.12.011);
Zhang, G. and Chanson, H. (2016). “Hydraulics of the developing flow region of stepped spillways.
Part I: physical modelling and boundary layer development.” Journal of Hydraulic
Engineering, ASCE, Vol. 142, No. 7, 8 pages (DOI: 10.1061/(ASCE)HY.1943-
7900.0001138);
Zhang, G. and Chanson, H. (2016). “Hydraulics of the developing flow region of stepped spillways.
Part II: pressure and velocity fields.” Journal of Hydraulic Engineering, ASCE, Vol. 142, No.
7, 9 pages (DOI:10.1061/(ASCE)HY.1943-7900.0001136);
Zhang, G., and Chanson, H. (2016). “Gabion stepped spillway: interactions between free-surface,
cavity and seepage flows.” Journal of Hydraulic Engineering, ASCE, Vol. 142, No. 5, 5
pages (DOI: 10.1061/(ASCE)HY.1943-7900.0001120);
Zhang, G., Chanson, H. and Wang, H. (2016). “Total pressure fluctuations and two-phase flow
turbulence in self-aerated stepped chute flows.” Flow Measurement and Instrumentation, Vol.
51, pp. 8-20 (DOI: 10.1016/j.flowmeasinst.2016.08.007);
Zhang, G., and Chanson, H. (2017). "Self-aeration in the rapidly- and gradually-varying flow
regions of steep smooth and stepped spillways." Environmental Fluid Mechanics, Vol. 17,
No. 2, pp. 27-46 (DOI: 10.1007/s10652-015-9442-z).

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Peer-reviewed conference papers
Zhang, G., and Chanson, H. (2014). “Step cavity and gabion aeration on a gabion stepped
spillway.” Proc. of 5th IAHR International Symposium on Hydraulic Structures, Brisbane,
Australia, 25-27 June 2014, 8 pages;
Zhang, G., and Chanson, H. (2014). “Two-phase flow on a gabion stepped spillway: cavity and
seepage air-water motion.” Proc. of 19th Australasian Fluid Mechanics Conference,
Melbourne, Australia, Paper 119, 4 pages;
Zhang, G., and Chanson, H. (2015). “Free-surface and seepage bubbly flows on a gabion stepped
spillway weir: experimental observations.” Proc. 36th IAHR World Congress, The Hague,
The Netherlands, 27 June-3 July, Paper 79661, 11 pages;
Zhang, G., and Chanson, H. (2015). “Broad-crested weir operation upstream of a steep stepped
spillway.” Proc. 36th IAHR World Congress, The Hague, The Netherlands, 27 June – 3 July,
Paper 79662, 11 pages;
Zhang, G., and Chanson, H. (2016). “Turbulence and energy dissipation in the developing non-
aerated and the fully-developed aerated flows on a stepped spillway.” Proc. 6th International
Symposium on Hydraulic Structures, Portland, United States, 27-30 June, Paper 1001;
Zhang, G., and Chanson, H. (2016). “Free-surface aeration in a steep stepped spillway.” Proc. 6th
International Symposium on Hydraulic Structures, Portland, United States, 27-30 June, Paper
1003.

Peer-reviewed research reports
Zhang, G., and Chanson, H. (2015). “Hydraulics of the developing flow region of stepped cascades:
an experimental investigation.” Hydraulic Model Report No. CH97/15, School of Civil
Engineering, The University of Queensland, Brisbane, Australia, 76 pages (ISBN 978 1 74272 134
7).
Zhang, G., and Chanson, H. (2017). “Application of local optical flow methods to high-velocity air-
water flows: validation and application to skimming flows on stepped chutes.” Hydraulic Model
Report No. CH105/17, School of Civil Engineering, The University of Queensland, Brisbane,
Australia, 59 pages (ISBN 978-1-74272-182-8).

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Publications included in this thesis
No publications included.

Contributions by others to the thesis
No contribution by others.

Statement of parts of the thesis submitted to qualify for the award of another degree
None.
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Acknowledgements
I would like to express my deepest gratitude and sincere thanks to my principal advisor Prof. Hubert
Chanson, without whom this thesis would not have been possible. You have been a tremendous
mentor, and I was profoundly motivated by your enthusiasm on the topic and the continued support
and encouragement that you have provided me throughout my years to grow as a young researcher.
I would also like to express my warmest gratitude to my associate supervisors, Dr. John Macintosh
of Water Solutions, and Dr. Hang Wang of the University of Queensland. Your advice on my
research as well as my career has been priceless.
My sincerest thanks also go to my other committee members Prof. William Clarke, Dr. Badin
Gibbs, and Dr. Matthew Mason. Thank you for the stimulating discussions and insightful comments
that led to many improvements to this project.
I am deeply grateful to Prof. Daniel Bung and his research assistant Mr. Daniel Valero at FH
Aachen Hydraulics Division. Thank you for accommodating me during my visit to Germany and
offering many opportunities of brilliant discussions.
I would like to thank Mr. Jason van der Gevel, Mr. Stewart Matthews, Mr. Fraser Raid, and all
other technical staff of the UQ Hydraulics Laboratory for providing timely assistance in the
laboratory.
I am indebted to my friends and colleagues at UQ for your friendship and making this journey a
memorable one. Amongst them are Mr. Numa Bertola, Ms. Xinqian Leng, and Mr. Youkai Li, with
whom I have shared many fun moments and discussed life’s ambitions. I thank my fellow labmates
Mr. Peter Ware and Mr. Alex Atkinson for the countless days of stifling heat we have endured
together. I would also like to thank my office colleagues especially those sitting in the same row,
Ms. Svitlana Pyrohova and Mrs. Bibhuti Sharma, for saying hi every morning and insightful
lunchtime discussions on culinary, cultural, and existential matters.
I would like to acknowledge the scholarship supports of Australian Department of Education and
Training (APA), UQ School of Civil Engineering (APA Top-up, UQ Civil Travel Grant), and UQ
Graduate School (GSITA), which contributed to different parts of the present work and the author’s
personal life.
I am indebted to my parents, Mr. Huailing Zhang and Mrs. Jinfeng Wang, for their understanding
and support during these few tumultuous years.
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Last but not least I want to express my special thanks to my partner, Ms. Mengyao Yang, for her
patience, endurance, and support.
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Keywords
stepped spillways, air-water properties, turbulence, energy dissipation, two-phase flow, chamfer,
turbulence modification, physical modelling, macro-roughness, open channel flow

Australian and New Zealand Standard Research Classifications (ANZSRC)
ANZSRC code: 090509, Water Resources Engineering, 50%
ANZSRC code: 091508, Turbulent Flows, 50%

Fields of Research (FoR) Classification
FoR code: 0905, Civil Engineering, 50%
FoR code: 0915, Interdisciplinary Engineering, 50%

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