Air concentrations in plunge pools due to aerated plunging high-velocity jets and dynamic pressures in underlying fissures
High-velocity plunging water jets, such as those formed by spillways of high-head dams, cause scour on the rock foundations. The scour process is the result of complex physical phenomena concerning the three media involved, namely, water, rock and air. Air is entrained in the jet during the travel through the air and at the impinging point into the plunge pool. The air bubbles flow in the plunge pool influence energy dissipation and dynamic pressures. The bubbles may also enter rock fissures, where they will completely change properties of the propagation of pressure waves. The detailed knowledge of air bubbles behavior in plunge pools and of their influence in pressure fluctuations in the water-rock interface and inside fissures under prototype conditions is still lacking. Froude, Weber and Reynolds dependent phenomenon, observations on near-prototype scale models and consideration of fully transient pressures are necessary to appropriately assess the effect of jet impingement and air entrainment in the development of rock scour. The objective of this study is to assess experimentally air concentrations at different positions throughout the shear layer of impinging aerated jets and relate them with the dynamic pressures occurring around a confined block located at the bottom of the plunge pool. The experimental facility can reproduce vertical near-prototype jets, whose velocities vary between 4.9 and 22.1 m/s, impinging in an 80 cm deep plunge pool. Corresponding pool depth - jet diameter ratio is 11.1, creating developed jet impact on the bottom. Compressed air is added to the jet by means of 6 small openings in the nozzle. Air-to-water ratios of the jets vary from 0 to 23%. A double fiber-optic probe is used in different depths and radial distances from the jet axis in the plunge pool. Thus, air concentration, bubble rate and the vertical component of flow velocity are obtained. A metallic cube, with a edge of 200 mm, is inserted into the bottom of the plunge pool representing a confined rock block, allowing to measure dynamic pressures at 12 points around it. Using a systematic experimental approach, for the first time dynamic pressures occurring around a confined block could be related to air concentrations in a plunge pool impacted by aerated near-prototype jets. Air concentrations, bubble rate and flow velocity at each position of a plunge pool impacted by aerated water jets are clearly influenced by two opposing features, namely, the kinetic energy reduction of the jet and the jet aeration itself. Furthermore, kinetic energy reduction governs the lowering of mean pressures and their oscillations on the water-rock interface and underlying fissures. These results are part of an on-going research that aims at a physically-based description of the influence of air entrainment for scour assessment.