Detailed modeling of air flow around a complex building

The threat of a chemical and/or biological contaminant release in an urban environment is increasingly becoming a concern to the military as well as the civilian sector. Accurate and detailed modeling of contaminant transport in urban environments that accounts for complex physical processes and varying meteorological conditions is central to any consequence management technology aimed at providing a timely, effective response to a chemical or biological threat. Existing contaminant transport (CT) models require more than evolutionary improvements to enable the fluid dynamic resolution of such complex configurations as the downtown area of a city, the complex driven flow patterns within a large building complex, the partially vented flows in a subway tunnel system, and the complex flows in mountainous or forested terrain. To facilitate these improvements, it is important to first assess the ability of a numerical model to predict atmospheric transport and dispersion around single and multiple complex buildings. These assessments can be derived from calibration and validation studies that rely on experimental data, and numerical sensitivity studies of computational parameters.

The Naval Research Laboratory has recently extended its scalable, FAST3D Computational Fluid Dynamics model to contaminant transport problems for urban and environmental hazard assessment. FAST3D is flow solver for three-dimensional, time-dependent, compressible reactive flow problems. The underlying fluid dynamics algorithm is the Flux-Corrected Transport algorithm, which is a high resolution, direction split, monotone, conservative, positivity-preserving algorithm. Turbulence is modeled through the Large Eddy Simulation method MILES, in which subgrid effects are accounted for implicitly by the non-linear flux limiting of the algorithm. Of particular interest to unsteady numerical applications is understanding how to realistically simulate atmospheric wind so that the resulting flow patterns around buildings are accurately described. In previous work, computational modeling of such effects was investigated in a validation study of flow over a surface-mounted cube. In this paper, we investigate the effects of inflow velocity characteristics on flow patterns around a complex full-scale building through a series of calibration studies using field test data and numerical sensitivity studies. Computational velocity fields are compared with both steady (i.e., statistically averaged) and high-frequency field measurements to help calibrate the model. Sensitivity studies are performed to assess the relative effects of wind input patterns and grid resolution on simulation accuracy. Simulations with and without neighboring trees are contrasted to highlight the importance of modeling their dissipative effects.