Maria Grazia Giordano

To minimize the environmental impact of outfall systems, through which wastewater is discharged in receiving water bodies after having undergone treatment, a careful design process is of the utmost importance. It is paramount to study the processes that take place inside the mixing zone, where contaminants are reduced by more than 99% due to the turbulent behaviour of jets discharged from diffusers and where entrainment plays a crucial role in diluting wastewater. This phenomenon can be analyzed with the help of mathematical and numerical models, both using pre-existing software and developing ad-hoc code. Most of these numerical tools are based on solving the Navier-Stokes Equation. In the last few decades the Lattice Boltzmann Methods (LBM) have become more and more widespread as they are, at their core, very simple to implement, versatile and easy to parallelise. They have been employed in several different fields, but not yet in the study of wastewater outfalls.

This project aims to employ LBM to investigate the dilution and dispersion processes of wastewater discharged in the water bodies, in order to provide an open source tool that can be used as a support for decision making when planning interventions on existing outfalls or when building new ones.

LBM are based on a mesoscopic approach that relies on solving the Boltzmann equation, discretised both in physical space, velocity space and time. Macroscopic quantities, like the mass density and the flow velocity, are recovered as moments of the discrete velocity distribution functions. LBM can be used in conjunction with Large Eddy Simulations (LES) in order to take turbulence into account, and reduce computational costs. Specifically, in this project a Smagorinsky model is employed.

Simulations based on a hybrid LBM-LES approach show promising results as they are able to describe the fundamental characteristics of dense jets in crossflows. Further simulations are being carried out in order to validate the numerical results against experimental data.