Effects of cooling near building roofs on flow and pollutant dispersion in urban street canyons

Effects of cooling near building roofs on flow and pollutant dispersion in idealized urban street canyons are numerically investigated using a computational fluid dynamics (CFD) model. The CFD model used is a Reynolds-averaged Navier-Stokes equations (RANS) model with the RNG (renormalization group) k-e turbulence closure scheme. Extensive numerical experiments are performed with various cooling intensities and different building configurations. In the presence of cooling near the building roof, the in-canyon temperature decreases and the vortex center in the street canyon moves upward and toward the upwind building. As the cooling intensity increases, the vortex becomes strengthened. For strong cooling, the vertical heat flux by mean flow averaged over the canyon-top height is dominant over the vertical turbulent heat flux averaged over that height. It is found that the cooling reduces in-canyon pollutant concentration. It is further found that the residue pollutant concentration ratio decreases as the cooling intensity increases. For weak (strong) cooling, the vertical turbulent flux of pollutants averaged over the canyon-top height is much larger (smaller) than the vertical flux of pollutants by mean flow. These results have important implications to urban heat island mitigation and air-quality improvement by planting short trees on building roofs in urban areas.