Multi-scale dynamics at the turbulent/non-turbulent interface of jets and plumes

Flows in environmental fluid mechanics contain a wide range of scales at which important physical processes are taking place. For instance, in wastewater outfall hydrodynamics, the relevant processes range from large scales e.g. the size of the water body (order of 10^4 m), to the smaller scales of the discharge (order of 10 m) and down to the Kolmogorov scale at which turbulent energy is dissipated (order 10^-3 m). The separation between scales is even more evident in the atmospheric dynamics, where over ten orders of magnitude separate the largest cyclonic scales of several thousand kilometres from the smallest ones.
These examples highlight the challenges engineers and scientists are facing to study such problems. In particular, existing modeling tools should be able to describe outcomes related to large scale dynamics in problems like pollutant dispersion, ocean outfalls or atmospheric plumes, where the smaller scales, though substantially affecting the overall physics, are often approximated rudimentarily. The overarching goals of the research proposed here are to provide a detailed insight into the role that different scales have on the dynamics of jets and buoyant plumes, which occur in a wide variety of natural and man-made situations, and to develop better modeling solutions, based on the physics of the small scales, that might be employed, e.g., in the context of Large Eddy Simulation (LES), to improve the representation of near field processes in existing modeling tools. New models will be built into open source codes and this will increase the impact of the project on environmental science and industrial applications. In fact, understanding the multi-scale interactions between sub-metre scale installations and ocean scales over hundreds of metres is crucial for assessing environmental impacts and optimisation of modeling tools is vital to minimize project costs or maximize profits.

The immediate beneficiary of this work will be the scientific community. The study of jet and plume dynamics is of paramount importance in science and engineering. The tool we propose for the analysis of the Direct Numerical Simulation data sets, i.e. the generalized Kolmogorov and Yaglom equations, has been already successfully applied to wall bounded flows and to natural convection and we are confident that the outcome of the project will lead to a fundamental research advancement in the area of free shear flows.
The development of an improved model for Large Eddy Simulation (LES), which is becoming increasingly popular within the fundamental as well as applied science community, will drastically increase the impact of this research, as demonstrated by the unanimous support given by our (industry) partners. Another benefit of this project will be the training of two postdoctoral research assistants who will become knowledgeable in the state of the art in turbulence modelling and advanced numerical techniques.
A large part of the project will involve the incorporation of a new subgrid scale LES model(s) into two software platforms. One will be Hydra3D, the software developed by one of the investigators and another will be OpenFOAM, a free and open source CFD code having a large user base across most areas of engineering and science. We will collaborate closely with our partners Gexcon and HR Wallingford, who are key consultancies providing solutions that range from risk assessment of pollutant dispersion to environmental engineering. Our project is of great interest to both companies, as they are aware of the invaluable contribution that reliable LES could give to the study of practically-relevant flows, which are still extremely demanding in terms of computational resources.
The project has also a sound social and economic impact, although rather indirect and on a long-term time scale. The accurate knowledge of the dynamics of jet and plume is necessary to predict the quality of air and water in both anthropic and natural environments. The turbulence subgrid scale models currently employed suffer from several limitations, in particular their demands for very fine grids, which makes them uneconomic in applications. The proposed research intends to eliminate, or at least significantly reduce, their weaknesses by addressing the fundamentals of the problem and developing ways of incorporating the gained theoretical and physical insights into applied numerical modeling tools for the prediction of the physics of jets and plumes. This will eventually contribute to a better quality of life, to the preservation of the environment, but also to energy savings, as discharge processes and turbulent mixing in combustion can be optimized based on numerical simulations.