Isothermal and non-isothermal flow in street canyons: A review from theoretical, experimental and numerical perspectives

Abstract

Urban street canyon flows play a central role in microclimate control, from street canyon to neighbourhood and city scale, which affect pollutant dispersion, thermal comfort of residents and building energy consumption for various indoor and outdoor flow conditioning systems. Extensive studies have been devoted to this field, from theoretical modelling to experimental measurements and numerical simulations. While some important characteristics of street canyon flows, such as the presence of a primary standing vortex and the mechanism for its establishment, have been well revealed, a holistic understanding consolidating theoretical, experimental and numerical research has not been achieved. This review is therefore aimed to holistically articulate these approaches and research outcomes in the field. Some critical points overlooked in the literature are identified and commented on. These are: distinguishing gradient and bulk Richardson numbers; identifying dominant similarity characteristic numbers for scaling down models; predicting the condition in which thermal effects come into play; modelling the convective heat transfer in non-equilibrium mixed convection conditions using wall functions; and exploring the influence of neutral and non-neutral urban boundary layers on street canyon flows and associated passive scalar dispersion. The review is concluded with an outlook to most challenging research questions towards understanding of microclimate of full-scale realistic street canyons. © 2020 Elsevier Ltd