Effects of differential wall heating in street canyons on dispersion and ventilation characteristics of a passive scalar

Abstract

Abstract This paper presents the first large-eddy simulation (LES) study of transfer and dispersion of a scalar released from a rough urban facet, either the street surface, the upstream-wall, or the downstream-wall, under the thermo-dynamical conditions of either the upstream-wall or the downstream-wall, plus the roof, heated by solar radiation. The boundary condition of a constant value is adopted for the scalars on the rough urban facets and a wall function is proposed for the scalars. The LES results demonstrate that dispersion inside the street canyon possesses distinctive characteristics for two conditions: the assisting condition in which the thermal-driven flow has the same direction as that of the wind-driven vortex and the opposing condition in which the thermal-driven flow has the opposite direction as that of the wind-driven vortex. For the street-released scalar under the opposing condition, the concentration fluctuations relative to the mean concentration can reach 50% and in general they are much larger than those for the assisting cases which are in the range of 25–30%. The exchange velocity of a scalar between the street canyon air and the urban boundary layer (UBL), w C B ( c ) , is one order of magnitude larger than the exchange velocity between a facet and the urban boundary layer, UBL, w 0 B ( c ) , indicating quantitatively that the resistance to the transfer of a facet-released scalar is dominated by the near-facet processes. As the temperature difference between the wall and the UBL, ΔT, increases, the total resistance to street canyon ventilation becomes more dominated by the near-facet resistance. The assisting conditions are favourable to ventilating the scalars from both walls, whereas the opposing conditions are only favourable to the ventilation of the downstream-wall released scalar. In the range of ΔT tested in this study, the exchange velocity, w C B ( c ) , linearly increases with ΔT and can be well parameterised. For the assisting cases, the advective (or dispersive) flux dominates the turbulent flux in most part of the canyon and the partition between the two alters dramatically across the roof level with turbulent processes dominating above the roof level. This study also suggests that for the street-released scalar, the resistance between the bottom canyon and the upper canyon is significant compared with the one between the upper canyon and the UBL. It is therefore necessary to adopt a two-box model in order to reasonably model such a case. The results of this study provide the guidance of improving the parameterisation schemes of transfer and dispersion for street canyons which currently do not consider the influence of wall heating.