Modeling flow and pollutant dispersion in an urban cubical cavity

Flow and pollutant dispersion in an urban cubical cavity are numerically investigated using a computational fluid dynamics (CFD) model. Some interesting flow features in the cubical cavity are found through trajectory analysis. These include connection between a primary vortex and secondary vortices, which is not present in an open urban street canyon, and a distinct region of the primary vortex core where any particle cannot enter that region. The end walls play important roles in characterizing mean flow and turbulence in the cubical cavity and hence pollutant dispersion therein. The time constant, defined as a time taken for cavity-averaged pollutant concentration to decay to e^-1 of the initial pollutant concentration, is calculated to examine the effects of ambient wind speed and direction, ambient turbulence intensity, and street bottom heating on pollutant ventilation. The dependency of the time constant on the ambient wind speed is quite significant. Thermal effects on the time constant are large for strong street bottom heating. The time constant calculation helps to understand the mechanism of pollutant escape from the cubical cavity, estimate an escape time under certain kind of meteorological conditions, and parameterize the time constant in terms of nondimensional parameters for use in air quality models.