Interaction between free-surface aeration and total pressure on a stepped chute

• Strong interplay between air bubble entrainment and turbulence on stepped chutes.
• Complicated interactions between mainstream flow and cavity recirculation motion.
• Bottom roughness as a determining factor on energy dissipation performances.
• Stepped cavity aspect ratio has a marked effect on the residual energy.
• Theoretical relationship between bubble count rate and interfacial turbulence is derived.

Stepped chutes have been used as flood release facilities for several centuries. Key features are the intense free-surface aeration of both prototype and laboratory systems and the macro-roughness caused by the stepped cavities. Herein the air bubble entrainment and turbulence were investigated in a stepped spillway model, to characterise the interplay between air bubble entrainment and turbulence, and the complicated interactions between mainstream flow and cavity recirculation motion. New experiments were conducted in a large steep stepped chute (θ = 45°, h = 0.10 m, W = 0.985 m). Detailed two-phase flow measurements were conducted for a range of discharges corresponding to Reynolds numbers between 2 × 105 and 9 × 105. The total pressure, air–water flow and turbulence properties were documented systematically in the mainstream and cavity flows. Energy calculations showed an overall energy dissipation of about 50% regardless of the discharge. Overall the data indicated that the bottom roughness (i.e. stepped profile) was a determining factor on the energy dissipation performance of the stepped structure, as well as on the longitudinal changes in air–water flow properties. Comparative results showed that the cavity aspect ratio, hence the slope, has a marked effect on the residual energy.

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