Intrusive and non-Intrusive air-water measurements on stepped spillways with inclined steps: a physical study on air entrainment and energy dissipation

Abstract

Stepped spillways have been used worldwide as both primary and secondary spillway systems. Some key features of prototype operations include high Reynolds number overflows, high rate of energy dissipation, highly turbulent flows, and intense free-surface aeration. Fundamentally, the stepped invert profile generates intense turbulent dissipation during the spill associated with a significant reduction of kinetic energy in the free-surface flow, as well as strong self-aeration. The present study focused on the effects of inclined downward steps on the air-water flow properties, flow resistance, rate of energy dissipation and aeration efficiency of a 45° stepped chute (1V:1H). some physical modelling was conducted in a relatively large facility operating with Reynolds numbers 2.8103 < Re < 1106. Detailed air-water flow properties were systematically investigated with Reynolds numbers within 2.3105 < Re < 8.0105 with three inclined downward step slopes, i.e. horizontal (0°), 1V:5H (11.3°), and 1V:2.33H (23.3°). The introduction of downward step slope changed the cavity shape, impacting onto the cavity recirculation motion. While the recirculation was three-dimensional for all step geometries, the motion appeared to be more irregular with the elongated cavity shape, compared to that observed with flat horizontal steps (/k = 2.0). In high-velocity self-aerated chute flows, detailed air-water flow measurements constitute a basic requirement for reliable estimates of flow resistance and head losses, and a complete theoretical development was proposed for the total drag force and rate of energy dissipation. The total drag may be derived from momentum and energy considerations, although both approaches necessitate to take into count the non-uniform air-water velocity and pressure distributions. The systematic observations on stepped chute with horizontal and inclined downward steps presented very close air-water flow properties. Despite negligible differences at the millimetric and macro-scales, the performances of the whole stepped chute was substantially impacted by the downward step slope. The inclined downward stepped chute design was less efficient in terms of rate of energy dissipation and dissolved oxygen aeration efficiency. Importantly, the study demonstrated the importance and significance of detailed air-water flow measurements, using a panoply of complementary metrologies to obtain a full description of the physical multiphase processes.