Plume Modeling in the Urban Environment – Challenges and Issues (Invited Presentation)

Transport and dispersion (T&D) of airborne contaminants in urban environments is extremely complicated. Buildings alter the flow fields and deflect the wind, causing updrafts and downdrafts, channeling between buildings, areas of calm winds adjacent to strong winds, and horizontally and vertically rotating-eddies between buildings, at street corners, and at the leading edge of buildings. Accounting for the impacts of buildings on the flow field is crucial in estimating the travel direction, the areal extent, the residence time, and the toxicity levels of the contaminant plume, and ultimately for calculating exposures to the population. Our understanding of urban transport and dispersion has made significant advances in the past twenty years, as a handful of comprehensive urban-scale field experiments have been conducted and increased computational power has allowed for unprecedented numerical modeling studies. Twenty years ago, the majority of “urban” CFD modeling efforts were of single buildings or idealized street canyons. Today, entire city centers are being modeled down to scales of meters. However, there are still significant deficiencies in our understanding and disagreement on the best approach(es) forward. In real-world applications (e.g., air quality, emergency response, homeland security) there has been a troubling reluctance to give up “flat earth” models and slow adoption of new methods. In this presentation, the complex phenomenology of flow around buildings and plume transport through cities will be described and different modeling approaches will be discussed. The variety of applications in which urban plume modeling plays a role will be highlighted and the interdisciplinary aspects featured. Some of the major challenges and issues facing the urban transport and dispersion modeling community will be presented, as well as thoughts on where investment in research dollars could result in major capability enhancements.