PhD Studentship – Numerical Simulation of Buoyant Plumes and Bubble Screens for Engineering Applications

This problem attempts to do a CFD study of different technical solutions for the destratification of lakes and reservoirs and preventing salt intrusion in waterways that involve the use of bubble plume/screen technology. The stratification of water in lakes, reservoirs or closed bays is an increasingly common and challenging environmental problem. Dissolved Oxygen (DO), which is critical to many forms of life, enters water through the air (diffusing across the surface or through quick aeration) or as a plant by-product. In deep waters DO levels tend to drop due to the respiration of aquatic organisms and microbial decomposition. The increasing levels of eutrophication of lakes and reservoirs due to human pollution may force the DO to very low values or even zero. On top of that many organisms are adapted to a tight range of temperatures, and their metabolisms are strongly affected by colder waters and sudden changes. When considering reservoirs, the stratification is also a threat for the aquatic biodiversity downstream of the dam, due to the cold-water discharges.

Several technical solutions have been proposed along the years to tackle this problem: multi-level outlets, surface pumps, submerged curtains, trunnions, bubble plumes, or stilling basins are the more widely tested. Gas injectors strategically located at the bottom of the reservoir can generate buoyant bubble plumes which will interact with the stratified water column, breaking and mixing its layers and generating convective cells of motion. This can constitute a cheap and effective solution to connect the upper layers of water subjected to the solar radiation and the aeration with the bottom, hypoxic layers. As side effects, the mixing also reduces evaporation rates in summer and algae blooms. The lack of an overall design tool which allows an initial prediction of costs vs. benefits has prevented it from being a standard solution. As an example, engineers still refer today to the ’rule of thumb’ proposed by Lorenzen and Fast (1977) of approx. flow rate of 150 l/s of compressed air per 100 ha of reservoir surface area. This may be a good general suggestion, but it ignores key aspects as the water depth, the number and distribution of injectors, the bubble size or the previous biochemical conditions in the reservoir.

At the Department of Civil Engineering we are looking for a self-motivated candidate who will extend existing work on the numerical simulation of buoyant plumes. The successful candidate would use our in-house CFD code based on a novel Eulerian-Lagrangian approach (Fraga et al., 2016) to provide new insights on the capabilities of the bubble plume technology to break stratification in lakes and reservoirs and to quantify the energy consumptions that it would require. Candidates interested in Fluid Dynamics and Environmental Engineering are encouraged to apply. Knowledge of FORTRAN coding would be a plus. Experimental modelling could be part of the project if the candidate is interested on that side of the research.