Quantifying exchange processes in the urban canopy layers of dense neighborhoods

Abstract

There is a global trend towards urbanization, particularly in developing regions that are home to new and rapidly growing cities. In the center of large, dense urban areas, weak exchange between the urban canopy layer (UCL) and the urban boundary layer (UBL) above it results in insufficient removal of heat and pollutants. The magnitude of the exchange processes between the UCL and the UBL is directly related to the built environment that is devised by urban designers and influenced by policy makers. In this work, representative urban forms are defined and the vertical mixing potential is quantified for each case using computational fluid dynamics ( CFD). Insight is gained into the flow phenomena responsible for these results and the findings are presented in the context of urban design. Dense, repetitive, orthogonally-gridded neighborhood layouts are common in urban areas and are selected as the subject of analysis. Using dimensional reasoning, the relevant geometric parameters, including the dimensions of the city block as well as the spacing between blocks, are determined. These quantities are parametrically varied and the resulting flow fields are calculated using CFD for each case. Next, simulated passive scalars are released into each neighborhood from a near-ground volumetric source, and the average steady state non-dimensional concentrations are calculated in the UCL and at pedestrian height. Using these concentrations as metrics, the vertical mixing potential is quantified as a function of each of these building geometry parameters. Transient results, fluid mechanical reasoning and flow visualizations are used to develop a physical understanding of the flow phenomena responsible for these quantitative results allowing them to be interpolated and extrapolated with confidence. Analytical models are developed where applicable. The results are presented at both a neighborhood scale, of interest to policy makers and urban planners, and at a street canyon scale, of interest to urban designers and architects. In addition to the repetitive neighborhood, common urban planning elements are evaluated to determine how changes in these forms affect mixing within the UCL and between the UCL and the UBL. These include variations in building height, the introduction of parks or open areas to a gridded neighborhood and building clustering. In gridded neighborhoods with buildings of uniform height and one grid axis aligned with the wind, street canyons perpendicular to the wind have larger vertical mixing potentials than street canyons aligned with the wind. Decreasing street canyon aspect ratios significantly increases the vertical mixing potential in these gridded neighborhoods. Introducing height variation to a neighborhood with uniform height buildings increases the vertical mixing potential at a neighborhood scale and reduces the difference in vertical mixing potential between the street canyons perpendicular to and aligned with the wind. The addition of parks and open areas also increases the vertical mixing potential in gridded neighborhoods. Finally, preliminary work on building clustering suggests these urban forms have greater vertical mixing potentials than gridded neighborhoods, in general. Guidelines and quantitative methods are developed for practitioners to use in the assessment of the impact of a neighborhood scale design on the exchange processes between the UCL and the UBL, information that might otherwise be inaccessible or cost prohibitive. Examples that show how to apply these methods are presented throughout this work, and the implications of urban design choices on pedestrians are discussed. A supplemental summary of the most relevant findings for practitioners is also provided.