Background

Urban airflow that is accompanied by contaminant transport presents new, extremely challenging modeling requirements (e.g., Britter and Hanna 2003). Reducing health risks from the accidental or deliberate release of chemical, biological, or radiological (CBR) agents and pollutants from industrial leaks, spills, and fires motivates this work. Configurations with very complex geometries and unsteady buoyant flow physics are involved. The widely varying temporal and spatial scales exhaust current modeling capacities. Crucial technical issues include turbulent fluid transport and boundary condition modeling, and post processing of the simulation results for practical use by responders to actual emergencies.

Relevant physical processes to be simulated include complex building vortex shedding, flows in recirculation zones, and approximating the dynamic subgrid-scale (SGS) turbulent and stochastic backscatter. The model must also incorporate a consistent stratified urban boundary layer with realistic wind fluctuations; solar heating, including shadows from buildings and trees; aerodynamic drag and heat losses that are due to the presence of trees; surface heat variations; and turbulent heat transport. Because of the short time spans and large air volumes involved, modeling a pollutant as well mixed globally is typically not appropriate. It is important to capture the effects of unsteady, buoyant flowon the evolving pollutant-concentration distributions. In typical urban scenarios, both particulate and gaseous contaminants behave similarly insofar as transport and dispersion are concerned, so that the contaminant spread can usually be simulated effectively on the basis of appropriate pollutant tracers with suitable sources and sinks. In some cases, the full details of multigroup particle distributions are required. Additional physics includes the deposition, resuspension, and evaporation of contaminants.