Flow, turbulence, and pollutant dispersion in urban atmospheres

Abstract

The past half century has seen an unprecedented growth of the world's urban population. While urban areas proffer the highest quality of life, they also inflict environmental degradation that pervades a multitude of space-time scales. In the atmospheric context, stressors of human ͑anthropogenic͒ origin are mainly imparted on the lower urban atmosphere and communicated to regional, global, and smaller scales via transport and turbulence processes. Conversely, changes in all scales are transmitted to urban regions through the atmosphere. The fluid dynamics of the urban atmospheric boundary layer and its prediction is the theme of this overview paper, where it is advocated that decision and policymaking in urban atmospheric management must be based on integrated models that incorporate cumulative effects of anthropogenic forcing, atmospheric dynamics, and social implications ͑e.g., health outcomes͒. An integrated modeling system juxtaposes a suite of submodels, each covering a particular range of scales while communicating with models of neighboring scales. Unresolved scales of these models need to be parametrized based on flow physics, for which developments in fluid dynamics play an indispensible role. Illustrations of how controlled laboratory, outdoor ͑field͒, and numerical experiments can be used to understand and parametrize urban atmospheric processes are presented, and the utility of predictive models is exemplified. Field experiments in real urban areas are central to urban atmospheric research, as validation of predictive models requires data that encapsulate four-dimensional complexities of nature.

Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation

“…However, a simulation of a selected area in downtown Phoenix performed by Fernando et al [4] showed that buoyancy effects were appreciable in the middle of the street canyon; but near the ground (say z < 10 m) the buoyancy effects are hardly felt due to shear production. This also seems at odds with Louka et al [51] observations and conclusions that a thin thermal layer develops locally within a few 10's centimetres from the heated wall.…”

“…There are, of course, substantial contributions to this general area of study from laboratory (water flume and wind tunnel, e.g., [4,5]) and from the direct use of RANS (Reynods-averaged Navier Stokes equations) models, though it is often difficult to extract results as detailed as those from LES (large-eddy simulation) modeling, from such studies. We might also consider whether it is an appropriate time to reconsider our overall view of pollutant dispersion in urban areas.…”

Modelling Street-Scale Flow and Dispersion in Realistic Winds—Towards Coupling with Mesoscale Meteorological Models

“…It is also known that the urban boundary layer, the rural boundary layer, the city scale flow and the regional weather are coupled aerodynamically and thermodynamically (Britter and Hanna, 2003;Fernando et al, 2010). This again suggests that it is of great importance to consider the wind-direction effects on urban flows and dispersion.…”

“…Given the strong gradients caused by the presence of buildings and street scales, a resolution down to one metre is required (Xie and Castro, 2006;Fernando et al, 2010). Direct numerical simulation (DNS) is the most accurate computational fluid dynamics (CFD) approach (Coceal et al, 2006), but it requires enormous computer resources for high Reynolds number (Re) flows and is not suitable for urban flows in full scale.…”

Small wind turbines in turbulent (urban) environments: A consideration of normal and Weibull distributions for power prediction

“…The factitious nature of the urban topography is discussed by Fernando in [17] and fluid dynamic analyses performed in [18] describes the complexity associated with the urban topography as being the rule governing the wind resource. Indeed, this work further describes how the flow through urban RSL is highly sensitive to building morphology.…”