High-speed flows are common in safety-relevant hydraulic infrastructures such as spillways and low-level outlets of reservoir dams. Air entrainment is a typical flow feature caused by turbulent interactions at the air-water interface leading to a breakup of the free surface. This means that the usually smooth water surface dissolves into a mixture of air and water. The entrained air induces strong changes in the flow properties of the air-water mixture, which engineers must consider when designing hydraulic infrastructures.
Challenging scale effects
Experiments that measure high-speed air-water flows at prototype scale are challenging (e.g. due to access difficulties and high costs) and therefore scarce. Hence, science has been studying air-water flows mostly at laboratory-scale.
Laboratory-scale studies showed that scale effects influence several air-water flow properties such as the bubble and droplet size distribution. Consequently, an extrapolation of these properties to prototype dimensions might not be possible. Novel studies including prototype data from Swiss dams indicate that scale effects might also significantly affect key design parameters such as the air demand of low-level outlets. This demonstrates the clear need for detailed prototype data to validate existing physical and numerical modelling approaches. Therefore, this SCCER-SoE project at ETH Zurich aims at providing the missing validation of air-water flow research and at giving design recommendations by measuring air-water flow properties and air demand at prototype scale.
The setup to detect the effects
In spring 2019, researchers at VAW together with partners from the Water Research Laboratory of the University of New South Wales (Sydney, Australia) developed a two-phase flow instrumentation for measurements at prototype outlets and spillways. They installed this system in the Luzzone tunnel spillway in Ticino, Switzerland. The system is capable to measure air-water flow properties including flow velocity, air concentration and bubble or droplet sizes for conditions that prevail at up to ~40 m/s (see picture on the right side). In addition, they installed so-called Kiel probes and commercially available pressure sensors in the air vents of the middle and bottom outlets of the Luzzone dam to monitor air demand and pressure. The setup required the installation of several hundred meters of cables, which had to resist water velocities up to 45 m/s and air velocities of roughly 100 m/s.