Computational fluid dynamics design and forecasting of urban drainage unit operation behaviour

Abstract

A primary and historical consideration of urban drainage unit operation behavior is the transport/fate (control) of particulate matter (PM). This consideration has a strong foundation given that PM is the primary source and sink for chemicals (metal elements, organics, nutrients…) and a primary habitat as well as vehicle for pathogens. However the control of PM is challenging; in part due to the heterodisperse and temporally variable size gradation, complex geometries of many unit operations and highly variable and episodic flow rates. The control of PM has led to a spectrum of unit operations from hydrodynamic separation (HS) for coarse particulate and gross detritus separation that do not provide hydrologic control, to large clarification systems such as wet or dry storage systems that provide a much wider clarification response and potential hydrologic control. This study applies the principles of computational fluid dynamics (CFD) to predict PM clarification behavior of these systems subject to dilute multiphase flows (< 2% PM), typical of urban drainage. A standard turbulence model is used to resolve flow, and a discrete phase model (DPM) is utilized to examine PM clarification response. CFD results closely followed physical model results across the entire range of flow rates. Results demonstrate that for smaller unit operations such as an HS that scour can result in significant degradation of behavior that are not maintained. Comparisons are made between measured and CFD modeled results and mass balances are identified. CFD have become one of the most powerful tool for design and forecasting of urban drainage unit operation behavior since continuous simulation modelling.