An investigation of thermal effects on flow and dispersion in an idealized urban area using LES

Air quality within an urban region can be strongly influenced by nearby buildings which perturb the local winds, generate turbulence, and serve as both sources and potentially receptors of pollutants. Buildings also cause differential heating in urban areas, and recent studies have shown that the temperature difference between the buildings and the surrounding air can be as high as 20 oC. As a part of this work, the effect of the temperature differential between the buildings and surrounding air on flow and dispersion has been investigated using large eddy simulations (LES). The buoyancy effects have been incorporated by applying the Boussinesq approximation and solving a scalar transport equation for temperature using a Prandtl number of 0.7. A scalar transport equation is also solved for airborne contaminants using a Schmidt number of 0.9. High resolution LES has been performed on an idealized urban area consisting for two identical cubical buildings separated by a distance equal to the building height, thus forming an ideal street canyon. The walls have been resolved sufficiently so that the use of wall functions is not necessary. Simulations were performed for different cases by alternately keeping the upwind, downwind or the bottom wall surrounding the street canyon at a higher temperature than the ambient air temperature. Each of these cases had a pollutant source at the center of the street canyon. The mean and fluctuating component of velocity, temperature and concentration data from these cases will be presented and the results will be interpreted.