Multi-scale analysis of air–water properties and free-surface dynamics in dam-break waves

Abstract

Aeration plays a key role in the breaking roller of dam-break waves, however, their multiphase behavior remains insufficiently understood due to the complexity of turbulent air–water interactions in unsteady aerated flows. Laboratory experiments are typically designed under Froude similitude to preserve the balance between inertial and gravitational forces. In aerated free-surface flows, incomplete dynamic similarity leads to scale effects as viscous and surface tension forces become increasingly influential at smaller scales. While scaling behavior has been extensively investigated for steady aerated flows, corresponding insights for unsteady flows remain scarce. This study experimentally investigates the scaling behavior of unsteady dam-break wave rollers using geometrically similar experiments at two different scales, each with four flow conditions. A comprehensive dataset was obtained combining free-surface measurements, video-based analysis, and intrusive phase-detection probes, enabling detailed characterization of free-surface dynamics and air–water flow properties. Fluctuations of the roller-toe perimeter agree well between scales, suggesting Froude-dependence. In contrast, free-surface fluctuations along the roller exhibit scale dependence, reflecting the influence of aeration and large recirculating structures. Bubble characteristics showed strong scale effects, underscoring the role of turbulence dissipation and interfacial forces that are not dynamically similar across scales, whereas void-fraction profiles are comparatively less sensitive. Overall, the results demonstrate that many multiphase flow properties cannot be directly extrapolated solely based on Froude similarity. While highlighting the need for prototype measurements, this study provides new insight for improving the extrapolation of laboratory-scale findings to natural unsteady phenomena.