Thermal Effects on Flow and Reactive Pollutant Dispersion in an Urban Street Canyon

Thermal effects on mean flow, turbulence, and reactive pollutant dispersion in an urban street canyon with a street aspect ratio of one are numerically investigated using a computational fluid dynamics (CFD) model. The CFD model we developed is a Reynolds-averaged Navier-Stokes equations (RANS) model with the k-å turbulence closure scheme based upon the renormalization group theory. Simple ozone chemistry processes are included in the CFD model with prognostic equations for reactive pollutants O3, NO, and NO2. Four cases with different heating configurations (no heating, upwind building-wall heating, street-bottom heating, and downwind building-wall heating) are considered.

It is shown that the structure and intensity of mean flow in a street canyon can be explained by a combination of mechanically induced flow and thermally induced flow. The degree of the combination at any location of a street canyon depends on the configuration and intensity of heating. Dispersion of the reactive pollutants is largely determined by a canyon-scale vortex (vortices) formed in the presence of heating. Since the photolysis and reaction rate coefficients are temperature-dependent, the inhomogeneous temperature distribution itself in a street canyon affects the dispersion of the reactive pollutants to some extent. This is quantified by analyzing reactive pollutant concentration fields simulated with temperature-dependent photolysis and reaction rate coefficients and constant coefficients.