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 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. 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.