The influence of thermal effects on flow and dispersion in street canyons

The thermal effects due to the variable heating of the building walls by solar radiation and the heating produced by the vehicles on the airflow within a street canyon were studied by this group. The full-scale and wind tunnel measurements suggest that the overall effect of the heated walls on the street canyon flow dynamics is smaller than in 2-D numerical simulations. Thermal effects may generate a thin thermal convective flow close to the heated wall. As the flow in the wall boundaries carries air from the street level upwards, while normally cleaner air is transported from above, thermal effects may still be important for the air quality at pedestrian level and for the pollution transfers indoor. The heated walls affect the three components of the wind close to the wall and, therefore, this topic must be dealt with 3-D numerical calculations.The heat flux close to the heated wall is an important issue of these studies. In numerical calculations the use of heat flux boundary conditions is certainly more appropriate than the use of temperature boundary conditions. The results of the Nantes’99 campaign together with the new experiment Nantes 2000 documenting temperature and wind speed close to the wall are expected to give further input to the refinement of the treatment of wall heating effects and to the formulation of the heat flux boundary condition in CFD codes. Traffic heat-induced turbulence can be comparable in magnitude to that due to traffic mechanically-induced turbulence (TPT), especially for low vehicle speeds, and so needs to be considered for inclusion in models.