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Experimental Study of Air-Water Flow Properties on Low-Gradient Stepped Cascades
Toombes, Luke (2002-01-01). Experimental Study of Air-Water Flow Properties on Low-Gradient Stepped Cascades PhD thesis, Department of Civil Engineering, The University of Queensland.
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| Name |
Description |
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APPENDICES_.pdf
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APPENDICES_.pdf |
application/pdf |
1.78MB |
475 |
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_THESIS_.pdf
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_THESIS_.pdf |
application/pdf |
39.40MB |
549 |
| Author(s) |
Toombes, Luke
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| Thesis Title |
Experimental Study of Air-Water Flow Properties on Low-Gradient Stepped Cascades
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| School, Centre or Institute |
Department of Civil Engineering
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| Institution |
The University of Queensland
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| Publication date |
2002-01-01
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| Thesis type |
PhD thesis
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| Subjects |
290699 Chemical Engineering not elsewhere classified
290105 Other Industrial Biotechnology
260502 Surfacewater Hydrology
240502 Fluid Physics
291100 Environmental Engineering
290000 Engineering and Technology
290600 Chemical Engineering
290601 Chemical Engineering Design
300100 Soil and Water Sciences
300000 Agricultural, Veterinary and Environmental Sciences
290100 Industrial Biotechnology and Food Sciences
290802 Water and Sanitary Engineering
300105 Applied Hydrology (Drainage, Flooding, Irrigation, Quality, etc.)
290800 Civil Engineering
291103 Environmental Engineering Design
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| Abstract/Summary |
Stepped cascades are recognised for both aeration potential and energy dissipation, and have been employed in hydraulic structures for over 3,500 years. Yet little detailed
information exists on their performance, especially pertaining to low-gradient cascades. This study presents a detailed investigation of both the macro and micro-scale flow properties on a
low-gradient cascade (3.4 degree slope). Research is conducted on two large-size physical models: a 24m long multi-step cascade (10-2.4m long steps), and a single-step model with identical step
height and length. The large size of the model allows near full-scale data acquisition under controlled flow conditions, minimising potential scale effects. The study comprises three distinct
components: 1. A global investigation of the general flow properties of nappe flow on a low-gradient, multi-step cascade. Unforeseen three-dimensional characteristics of the flow, including
supercritical shockwaves and sidewall standing-waves downstream of nappe impact, are identified and examined by the study. Although comparable to similar phenomena at channel bends and expansions,
these have not been previously described on stepped cascades. Energy dissipation on the cascade is investigated, and is found to be over twice that observed for a smooth chute of similar gradient.
2. A complete characterisation of the air-water structure of flow in a nappe regime. Significant outcomes of the analysis include: - Air-concentration Distribution: The air-concentration
distribution at the lower nappe of the free-falling jet shows good agreement with an analytical solution of the diffusion equation. The experimental results from the study, and a reanalysis of
existing data, indicate a distinct relationship between the turbulent diffusivity in the shear layer and distance from the step brink. This contradicts earlier investigations that assumed constant
diffusivity. Strong aeration of the flow, with a large volume of spray, occurs downstream of the nappe impact. Depth-averaged air concentrations of 40% to 50% are observed within the spray region,
decreasing towards the downstream end of the step. -Velocity Distribution: A theoretical analysis of the momentum transfer process imparts an improved understanding of the momentum transfer and
velocity redistribution within the free-falling jet. An analytical solution based on twodimensional wake flow is developed, superseding existing solutions based upon a monophase free-mixing layer.
- Bubble-frequency Distribution: A quasi-parabolic relationship between bubble frequency and time-average air concentration across a cross-section is observed. A theoretical explanation for the
parabolic relationship is developed, and two correction factors are introduced to provide a better representation of the experimental data. - Air-bubble and Water-droplet Size Distributions:
Chord-length distributions are compared with standard probability distributions, showing good agreement with standard Weibull, gamma and log-normal probability distributions within various regions
of the flow on the step. A computer model is developed to model interaction between a bubbly transition from water to air and fluctuations of the free surface. 3. A parallel investigation of the
oxygen aeration efficiency of a stepped cascade. Measured air-water property data is used to calculate the air-water interface area in bubbly flow, and to estimate the theoretical aeration
efficiency of the stepped cascade based upon the integration of the mass transfer equation. The aeration performance of the stepped cascade model is also measured experimentally in terms of
dissolved oxygen content. This analysis allows a unique, successful comparison of experimental dissolved oxygen measurements with the numerical integration of the mass transfer
equation.
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| Keyword(s) |
stepped cascades
spillways
aeration cascades
air-water mass transfer
re-oxygenation
energy dissipation
physical modelling
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| Additional Notes |
Full bibliographic details: TOOMBES, L. (2002). "Experimental Study of Air-Water Flow Properties on Low-Gradient Stepped Cascades." Ph.D. thesis, Dept of Civil Engineering, The
University of Queensland.
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