Ensemble-statistical approach in the measurement of air–water flow properties in highly unsteady breaking bores

Abstract

Breaking bores are commonly observed in a number of natural processes, often associated with the presence of a transient mixture of air and water, with intense recirculation, air bubble entrainment, and splashing. Two-phase flow measurements in such highly unsteady flows cannot be based on long-duration measurements and require novel ensemble-statistical approaches based on multiple repetitions. Detailed measurements of air–water flow properties were then conducted in a breaking bore with Fr1 = 2.4 using an array of multiple dual-tip phase-detection probes. Based on an extensive experimental program, inclusive of 2000 tests at a single position and 100 tests at multiple elevations, a detailed sensitivity analysis was conducted on the necessary number of repetitions to obtain physically meaningful and statistically reliable air–water flow properties. The results led to a robust methodology to estimate ensemble-statistical values, including confidence intervals and residual error. In addition, these results provided a detailed characterization of the behavior of air–water flow properties in highly unsteady flows, including void fraction, number of interfaces, and bubble chord time/length. Despite the transient nature, all physical processes showed consistent behaviors with theoretical models and other stationary flows, including hydraulic jumps and plunging jets. Overall, this study provided two-phase flow characteristics that go beyond the limitations imposed by the unsteady nature of the flow, proving thoroughly the importance of large datasets for the estimation of air–water flow properties in highly unsteady flows.

Turbulent free-surface in self-aerated flows: superposition of entrapped and entrained air

“…hydraulic jumps. The application to unsteady aerated flows, such as breaking waves, is more involved and requires the hundredfold repetition of experiments, followed by an application of ensemble-averaging techniques, see Blenkinsopp & Chaplin (2007) and Whutrich, Shi & Chanson (2022).…”

Air-Water Flow Properties In Highly Unsteady Flows

“…low conductivity = air; high conductivity = water). The unsteadiness of the flow required a large number of repetitions, to obtain physically meaningful and statistically reliable results (Wüthrich et al, 2022). Therefore, a total of 200 repetitions were conducted for each flow condition, where a reference probe was located at a fixed elevation 2mm above the initial water level to allow for the synchronisation of all tests.…”