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

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 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.iii 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.iv Declaration by authorThis 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.

Turbulence and self-similarity in highly-aerated shear flows: the stable hydraulic jump

“…The video signals were analysed with the Farnebaeck method (Appendix A.1). This image-based velocimetry technique belongs to the class of optical flow (OF) methods and has recently attracted interest in the study of airwater flows, proving to produce less errors in bubbly flow images (Bung and Valero, 2016;Zhang, 2017;.…”

“…The Farnebaeck method was previously used for optical flow velocity estimation in highly aerated spillway flows (Zhang, 2017;. Bung and Valero (2017) used synthetic particle images to show that this technique is suitable for turbulence analysis (Bung and Valero, 2017).…”

“…Arrays of wire gauges and phase-detection probes were used to characterise interfacial correlation length scales and transverse motion (Mouazé et al, 2005;Wang and Murzyn, 2017;Wang and Chanson, 2019). Mossa and Tolve (1998) drew attention to the potential use of imaging techniques for the study of hydraulic jumps and recent developments in this area have now enabled the estimation of velocity fluctuations in highlyaerated flows (Lin et al, 2012;Bung and Valero, 2016;Zhang, 2017;. Similarly, pseudo-instantaneous velocity measurements with dual-tip phase-detection probes were recently obtained by , while phase-detection probes have been previously used to study mean velocities only (Chanson and Brattberg, 2000;Murzyn and Chanson, 2007;Chanson, 2009a;Wang, 2014).…”

Transition flow regime on stepped spillways: air–water flow characteristics and step-cavity fluctuations

“…[12]. A physical reasoning for the second peak might be a continuous breakdown of entrained air [25].…”

Air-water flows on stepped spillways with inclined steps

“…The stepped invert profile generates intense turbulent dissipation during the spill associated with a significant reduction of kinetic energy in the free-surface flow, as well as strong self-aeration (Rajaratnam 1990, Chanson 1993, Felder and Chanson 2015. The energy dissipation and aeration characteristics have been extensively studied for stepped chutes with flat horizontal steps (e.g., Chanson 2001, Ohtsu et al 2004, Gonzalez 2005, Felder 2013, Zhang 2017. A few studies showed that the cavity recirculation is strongly three-dimensional (Matos 2001, Gonzalez and.…”