Scale effects on air-water flow properties and oxygen transfer in hydraulic structures experimental modelling

Scale effects on air-water flow properties and oxygen transfer in hydraulic structures experimental modelling.

© @ULiège

Air-water flows can be found both in nature and in hydraulic structures, including spillways, free-falling jets, and plunge pools. These flows are characterized by a nonhomogeneous mixture of air and water, which leads to a deformed free surface and a bubbly flow layer. Air entrainment or entrapment occurs when the kinetic energy of surface eddies exceeds surface tension—conditions typically observed in full-scale structures. The aeration of flow not only improves the safety of hydraulic structures by mitigating cavitation risks but also creates extensive air-water interfaces that enhance oxygen transfer. However, the transport of air increases the bulk of the water flow, resulting in a greater mixture flow depth that must be carefully considered in engineering designs, and reduces flow resistance. Accurate prediction of air-water flow properties is thus crucial to enable sustainable design of hydraulic structures.

Given the size of typical hydraulic structures and the challenges in controlling flow conditions in the field, studies are conducted in laboratory setups under controlled conditions and at reduced scale. The latter leads to scale effects, possibly affecting upscaling of the experimental results to the prototype scale. In the field of air-water flow research, instrumentation is another challenge.

In this framework, the HydrO2Scale project, funded by FNRS (Belgium) and DFG (Germany), aims to achieve two primary objectives: first, to obtain an accurate characterization of microscopic flow features in highly turbulent, aerated flows, and second, to evaluate the influence of measurement methods and scale effects on estimating gas transfer efficiency and air-water properties. To achieve these goals, three scale models (1:1, 1:5, and 1:10) of a stepped spillway—known for high energy dissipation and strong air-water interaction due to their macro-roughness step elements and significant turbulence—will be operated alongside two measurement techniques: intrusive phase detection probes and direct dissolved oxygen (DO) concentration measure. These methodologies will leverage on facilities available at FH Aachen and the University of Liège.

Through this research, we aim to advance our understanding of air-water flows, ultimately contributing to more sustainable hydraulic structures design.