Microscopic air-water flow properties and impact on oxygen transfer in hydraulic structures scale modelling

• Applicant: Professor Dr.-Ing. Daniel Bung
• Subject Area: Geotechnics, Hydraulic Engineering ; Fluid Mechanics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 511289281

While decline in dissolved oxygen (DO) is widespread in lakes and reservoirs, a minimum level of DO is required to maintain good ecological conditions in downstream rivers and streams. Oxygen transfer may be triggered in aerated flows at hydraulic structures, for instance in self-aerated spillway flows where large air contents are transported with the flow and thus, lead to an increased air-water interface across which gas transfer can take place. Installation of measuring equipment in the field and conducting prototype measurements to determine the gas transfer potential is challenging because of structures dimensions and associated flows, and also because of the lack of control on the flow conditions. Consequently, experiments in laboratories at small-scale are commonly conducted. However, contrary to other flow features such as velocity, pressure and water depth, the size of entrained air bubbles cannot be properly scaled in physical models due to bubble deformation, breakup and clustering at critical conditions. Also, gas transfer estimation is not straightforward. It may be evaluated by direct DO measurements or by estimation of the air-water interface in combination with empirical approaches for the mass transfer coefficient. In this context, the proposed project aims i) at obtaining an accurate characterization of the microscopic flow features in highly turbulent, aerated flows and ii) at evaluating the influence of measurement methods and scale effects on the estimation of gas transfer efficiency. The project focusses on moderately sloped stepped spillways, as an exemplary type of hydraulic structure involving highly-turbulent flows with large void fractions. It has been built on the unique complementary expertise, measurement techniques and facilities available at FH Aachen and Liege University. It is based on the development of a large experimental campaign considering three physical models of the same stepped spillway at different scales ranging from 1:1 to 1:10. Three measurement techniques will be applied to these models: classical intrusive direct DO concentration measurement with oximeters, air bubbles characterization using intrusive conductivity probe, and a new robust computer-vision based toolbox developed in the framework of the project and allowing for the identification of individual bubbles and estimation of their size, deformation and interface.

• DFG Programme: Research Grants
• International Connection: Belgium
• Partner Organisation: Fonds National de la Recherche Scientifique - FNRS
• Cooperation Partner: Dr. Sébastien Erpicum