Nappe flows on a stepped chute with prototype-scale steps height: Observations of flow patterns, air-water flow properties, energy dissipation and dissolved oxygen

Abstract

Air-water flows occur commonly in stepped spillways including the nappe flow regime for low flow rates. The present laboratory experiments researched the nappe flow regime in a stepped chute with prototype-scale steps height providing unique insights into the evolution of nappe flows along a stepped chute. Detailed visual observations highlighted the varying flow features along the stepped chute including the evolution of flow aeration, of jet properties and of instationarities in form of jump waves and cavity fluctuations with typical frequencies of around 1 Hz. These instationarities were caused by complex flow interactions at the impingement of the jet on the horizontal step face. Detailed air-water flow measurements revealed the complexity of the flows highlighting both S-shape and jet-like void fraction distributions and jet-like interfacial velocity distributions downstream of the jet impact and at step edges. This resulted in a downwards shift of the bubble count rate distributions closer to the step face. The nappe flows showed strong energy dissipation and reaeration performances along the stepped chute. The present study provided a robust and extensive characterisation of nappe flows and due to the large scale of the experiments, the results should provide confidence for the design of stepped chutes with embankment dam slope.