Computational Fluid Dynamics Modelling Applied to Urban Drainage: The Case Study of the Monsanto-Santa Apolónia Tunnel Inlet Structure, in Lisbon

Abstract

Drainage systems are crucial for managing wastewater and rainwater in urban areas and face challenges like urban sprawl, climate change, and aging infrastructure. The conventional methods employed to analyse the performance of drainage systems are inherently limited in their capacity to address the full complexity of these systems. Computational Fluid Dynamics (CFD) emerges as a promising tool by enabling 3D simulation of free-surface flows, incorporating turbulence phenomena. To create a methodology for developing a CFD model for turbulent free-surface flow, key parameters (mesh discretization, turbulence models, geometry constraints, boundary conditions, convergence criteria, scale-related assessments) were systematized in an optimized flowchart by applying it to the inlet infrastructure to the Monsanto-Santa Apolónia tunnel of Lisbon's Drainage Master Plan, employing the commercial software Flow3D. The study demonstrates that CFD simulations of free surface turbulent flows, employing a model development with increasingly refined and different mesh formulations and a validation mechanism, based on traditional hydraulic modelling, literature or real data collection, can produce strong results when employing a convergence criterion based on mass balance, with simulation times compatible with decision-making in civil engineering. It was also evidenced that reduced scale CFD models can offer faster convergence and accurate results of velocity fields, water heights or general flow behaviour. However, phenomena like hydraulic jumps can suffer from scale effects, due to the predominance of the viscosity. Finally, the study proves the CFD capabilities to implement Digital Twins in context of design, operation and maintenance, then offering numerous advantages for management utilities.