Air-water flow characteristics downstream of piano key weirs

[EN] Increasing flood magnitudes and the growing hydraulic uncertainty associated with climate change have intensified the need to rehabilitate ageing linear weir structures that no longer satisfy current discharge and water level control demands. In this context, nonlinear weirs, particularly piano key weirs (PKWs), have emerged as efficient retrofit solutions owing to their ability to substantially increase discharge capacity within limited channel widths and existing infrastructure constraints. Fundamental progress has been made in optimizing the geometric configuration of PKWs to improve discharge efficiency and overall hydraulic performance, while limited attention has been devoted to the hydrodynamics of the downstream aerated region generated in the vicinity of the weir. This region is characterized by intense jet impingement and turbulent air entrainment, generating significant hydrodynamic loading on the downstream channel and surrounding structures. Understanding these prominent two-phase flow processes is therefore essential to strengthen downstream hydraulic safety criteria. This thesis addresses this caveat through a comprehensive experimental investigation of air-water flows downstream of PKWs installed in horizontal channels. A systematic series of laboratory experiments was conducted for a wide range of hydraulic conditions using multiple PKW models, along with reference labyrinth and linear weirs prepared through 3D-printing technique. High-resolution measurements were obtained using phase-detection probes to quantify key two-phase flow characteristics such as void fraction (air concentration by volume), particle (bubble) count rate, interfacial velocity, turbulence intensity, and bubble size distributions. The influences of geometry, crest configuration, downstream submergence, and inflow conditions are briefly examined, with particular emphasis on Reynolds number effects under Froude-similar cases. The findings demonstrate that the air entrainment process of PKWs in horizontal channels is governed by plunging jet impingement and nappe interactions, with outlet key cavities functioning as principal sources of self-aeration. The effect of plan-form geometry and volumetric space (within inlet and outlet keys) on the downstream air concentrations was revealed through the findings obtained based on systematic evaluation of a PKW and labyrinth weir. The evidence depict notable differences in discharge congestion that lead to chain effects on plunging jet characteristics and the resulting downstream entrained air transport. Meanwhile, the choice of crest shape over the parapet walls does not impose any significant influence on the overall air-water flow structure. In terms of spatial distributions, the highest depth-averaged void fractions past PKWs consistently reside along the outlet key centerlines rather than the inlet keys, leading to highly nonuniform trends across the channel. Simultaneously, the longitudinal development of the aerated flow exhibits distinct regimes, transitioning from a turbulence-dominated quasi-wall-jet region near the jet impact zone to a gradually varied open-channel flow further downstream. This transition point correlates with the flow depth above the weir crest, which governs jet momentum and aeration for a particular tailwater level. In addition to the inherent volumetric expansion due to air entrainment, the plunging-jet-induced shock waves (at inlet key centerlines) and the resulting lateral standing waves (at outlet key centerlines) dictate the pattern of the undular free-surface and flow bulking near impingement zones. This pattern resembles a damped sinusoidal wave that stabilizes into the mean free-surface as it progresses along the flow. At high inflows, the highest peak of these undular waves consistently appears at a distance from the crest closely equivalent to the weir height. Air-water flow properties such as particle count rate, interfacial velocity, and bubble-chord size showed pronounced Reynolds dependence in the near-field region (near jet impact zones), which influences the turbulence level and bubble breakup process, while progressively converging downstream as turbulence decays and detrainment dominates. Comparatively, void fraction remains less affected relative to the remaining parameters across the range of inflow Reynolds numbers investigated. Bubble size distributions and interfacial area remain closely linked to turbulence intensity, highlighting the role of plunging jet momentum (Reynolds-dependent) in controlling the air-water interaction processes. Based on the interfacial area distributions within the extent of the aerated region, the results also showed that a balance of inflow Reynolds number and the corresponding available drop height dictates the aeration potential rather than the Reynolds number alone. The findings further reveal that Froude similarity alone is insufficient to reproduce downstream aeration characteristics of PKWs, as key parameters such as bubble size, interfacial area, and characteristic aeration length exhibit clear scale dependence. In summary, this thesis unveils the intricate two-phase flow processes downstream of PKWs to advance the current understanding and provide a basis for improving hydraulic modeling and design involving these contemporary hydraulic structures.