Experimental investigation of turbulent flow properties leading to self-aeration in high-speed water flows

• Applicant: Professor Dr.-Ing. Daniel Bung
• Subject Area: Fluid Mechanics ; Geotechnics, Hydraulic Engineering
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 558217342

Entrainment of air bubbles into water flows may occur in natural rivers as well as at hydraulic structures. This two-phase flow phenomenon is of utmost relevance as it inevitably modifies basic flow properties, e.g. flow depth and friction factors. In high-velocity spillway flows, a continuous self-aeration is often observed which is commonly considered by means of empirical design equations, which are valid only for constrained conditions. The underlying physics, however, are still not fully understood. Traditionally, it was assumed that the growth of a turbulent boundary layer, developing from the spillway crest, and its intersection with the free-surface leads to the ejection of droplets and entrainment of bubbles. Observations on free-surface roughening further upstream of this intersection, indicating additional contribution of free-surface instabilities, were discarded over some decades and first studied analytically very recently to develop a general, physics-based equation for prediction of self-aeration. When this theory was formulated, many assumptions were made that require further quantification to obtain a better understanding of the underlying mechanisms. In this project, new experimental developments will provide the insights needed to validate and render this theory useful. The air and the water phase flow fields as well as their interface will be studied using camera-based measuring techniques, complemented by the use of conventional instrumentation. In detail, the flow fields will be studied by means of highly-resolved Particle Tracking Velocimetry measurements and linked to turbulent properties of the free-surface, which is captured with stereoscopic cameras. Experimental data on turbulent stresses and pressures at the interface between air and water and new observations on the perturbation growth will be analyzed in depth, including a detailed evaluation of slope and roughness effects to obtain an accurate characterization of the water and air flow in the non-aerated region of supercritical flows down steep chutes and to bring together a functional theory able to predict the onset of air entrainment based on physics – therefore being universal across scales characteristics and flow conditions in nature and at hydraulic structures.

• DFG Programme: Research Grants
• International Connection: United Kingdom
• Cooperation Partner: Daniel Valero Huerta, Ph.D.