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

Gonzalez, Carlos A. (2005). An Experimental Study of Free-surface Aeration on Embankment Stepped Chutes PhD Thesis, Engineering, University of Queensland.

       
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Author Gonzalez, Carlos A.
Thesis Title An Experimental Study of Free-surface Aeration on Embankment Stepped Chutes
School, Centre or Institute Engineering
Institution University of Queensland
Publication date 2005
Thesis type PhD Thesis
Supervisor Hubert Chanson
Abstract/Summary 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 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 airwater flow characteristics measurements conducted with sub-millimetric conductivity probes. Secondly, the study focused on the microscopic scale properties of the airwater flow, using the experimental data to quantify the microscopic scale physical processes (e.g. momentum transfer, shear layer development, vertical mixing, airbubbles/ 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.
Keyword stepped spillways
air-water turbulent flows
energy dissipation
recirculating vortices

 
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Created: Fri, 21 Nov 2008, 16:55:49 EST