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

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.