Dust in the Wind:

Challenges for Urban Aerodynamics

Jay P. Boris,

Laboratory for Computational Physics and Fluid Dynamics

U.S. Naval Research Laboratory, Code 6400

Washington, D.C. USA 20375-5344

The fluid dynamics of airflow through a city controls the transport and dispersion of airborne contaminants. This is primarily a problem of urban aerodynamics rather than of meteorology. The space scales are short, tens of meters to a few kilometers, so meteorological effects enter primarily through the aerodynamic boundary conditions. The average flow, large-scale fluctuations and turbulence are closely coupled to the building geometry. Buildings create large “rooster-tail” wakes; there are systematic fountain flows up the backs of tall buildings; and dust in the wind also moves perpendicular to or even against the locally prevailing wind. The fact that surfaces are rough and have sharp edges somewhat simplifies the aerodynamics but realistic atmospheric factors also play a role. Requirements for better prediction accuracy demand time-dependent, three-dimensional CFD computations that include solar heating and buoyancy, complete landscape and building geometry specification including foliage, realistic wind fluctuations, and sometimes particle and droplet distributions.

Computing urban aerodynamics accurately is a time-dependent, High-Performance Computing (HPC) problem. On the other hand, using this technology in the emergency assessment of industrial spills, transportation accidents, or terrorist attacks has very tight time requirements that suggest simple approximations that unfortunately produce inaccurate models. The trade-off has been the need to choose either a fast model or to live with accurate results. Using new fluid-dynamic principles, an urban-oriented emergency assessment system called CT-AnalystÒ has been invented to solve this dilemma. It produces accurate results for airborne contaminant scenarios nearly instantly. Designed to predict all airborne contaminants including Chemical, Biological, and Radiological (CBR) threats, CT-Analyst has unique new capabilities and gives HPC accuracy while running much faster than current alternatives.