Buoyancy-driven flows at the building and city scales

Abstract

Buoyancy-driven flows are important for urban ventilation and pollutant dispersion in calm background conditions. Convective eddy structures of different scales and the role they play in pollutant dispersion and transport under no/weak synoptic wind conditions are investigated theoretically and experimentally in this thesis. Building-scale natural convective wall flows are crucial for air change and pollutant dispersion between the indoor and outdoor environments and between the urban canopy layer and background flow. The wall flow on a 16-story building is considered in this research. The vertical velocity along the building wall presents a diurnal profile, changing with a difference in temperature between the wall surface and ambient air. The boundary layer of the wall flow can be divided into an inner viscous layer and an outer turbulent layer. The velocity profile of the vertical component normal to the wall across the outer turbulent layer can be represented by a Gaussian profile. The air change rate caused by the wall flow in a homogeneous urban canopy layer is estimated: the mean air change rate increases slightly, in the form of H1/30, with an increase in mean building height H. The city-scale natural convective urban dome flow is driven by the urban heat island effect, and is vital for air change and pollutant transport between the urban canopy layer and regional-scale background. The characteristics of the urban dome flow are explored theoretically and experimentally in a stably stratified water tank. The urban dome flow, which is characterized as the convergent inflow at the lower level, divergent outflow at the upper level, and upward flow over the center of an urban area, is formed under an inversion layer in no/weak synoptic wind conditions. The mixing height of that flow is determined by the Froude number and urban diameter, and its horizontal extent is approximately two to four times of the urban diameter. The shape of the urban area has a significant influence on the mean flow structure of the urban dome flow. Unlike the urban dome flow over a circular city, the square city shows a non-uniform flow that the inflow at the lower level is mainly along the diagonal and the outflow at the upper level is mainly perpendicular to the edges. A regional-scale natural convective flow comprises multiple urban dome flows, and is an important factor in air change and pollutant transport between that flow and the mesoscale flow. For two neighboring cities with the same urban heat island intensity and size, the return flow is formed at the point at which the two urban dome flows meet. For two adjacent cities with different urban heat island intensities and sizes, a chain flow is formed between them. The building-scale wall flow, city-scale urban dome flow, and regional-scale multiple urban dome flows and their roles in urban ventilation, the urban thermal environment, and pollutant transport are systematically investigated in this thesis. Further investigation of the interaction of multiple urban dome flows considering the influence of urban shape is recommended in future research.