CFD simulations within complex urban building environments

There is a need to properly develop the application of Computational Fluid Dynamics (CFD) methods in support of air quality studies involving pollution sources within complex urban building environments. CFD models are emerging as a promising technology for such assessments, in part due to the advancing power of computational hardware and software. CFD simulations have the potential to yield more accurate solutions than other modeling methodologies because it is a solution of the fundamental physics equations and includes the effects of detailed three-dimensional geometry and local environmental conditions. However, the tools are not well validated for environmental modeling and best-practice methodologies have not been established. The results of CFD simulations can both be directly used to better understand specific case studies as well as be used to support the development of better-simplified algorithms that may be generally applied. Boundary layer turbulence is being simulated as characterized by surface roughness (characterized by u*) and surface heat flux (characterized by Monin-Obukhov length L) in absence of urban buildings.

This presentation summarizes ongoing developments and applications of CFD simulations through case studies using Fluent CFD software for simulating air pollutant concentrations from sources near buildings. Comparisons of CFD simulations to reference wind tunnel data and field measurement studies are presented to assess model performance. In addition to the need for model evaluation/validation there is a need to identify the most efficient set-up for CFD simulations characterized by selection of grid/mesh size, turbulence models and other critical operational parameters. Building geometry has been to date based on photogrammetry applied by Vexcel Corporation. Photogrametry derived buildings are based on analsyes of a series of photographs. While starting with a good ArcView file of basic building geometry there is work required to clean up and refinement to the geometry before it is ready for CFD applications. Improved methods for creating working building geometry and mesh is a key element for improvement in order to minimize the overall time required to develop a CFD application in real urban areas. The process is much smoother for idealized building shapes. Tests are conducted to determine what level of building details is necessary. Once good building geometry is applicable then the set up turns to defining mesh refinement and defining the surface boundary conditions, the turbulence model, and the options for selecting code parameters. Some of this selection is critically determine by the software being used and the available hardware or supporting the computations. May of the parameters use within CFD code for the turbulence models and other settings have been developed based on experiences gained from a history of development and application in the aerospace and automotive industrials. New tuning of these parameters may be necessary for appropriate application to simulating flows in urban environments. Future wind tunnel and field studies should play a very critical role to supporting these developments.

In particular, examples developed for areas of Manhattan including simulations in support of EPA’s study of the smoke/dust plume from the NY World Trade Center site following the events of September 11, 2001 are summarized. An example of the horizontal winds through a complex array of buildings in lower Manhattan is shown in the following figure. In this figure only 10 percent of the calculated wind vectors are displayed for one horizontal level due to visual limitations. The texture of the calculated mesh can be seen underlying the displayed vector winds. While setting up a working model of the extremely complex building environment for lower Manhattan has been a challenging exercise there have been many lessons learned that should make it easier to setup similarly complex urban environments in the future. Understanding the pathway of toxic air pollutants from source to human exposure in urban areas finds immediate application for both routine air pollution assessments and in support of Homeland Security. The collapse of the New York World Trade Center Towers demonstrated some of the shortcomings in conducting rapid exposure and risk analyses in urban areas where the understanding of airflow around large buildings is poor. While problem specific applications of CFD may not be feasible in “real-time” support, it does seem that there is a major role for CFD simulations to be run for developing archives that could be tabularized for supporting real-time applications. Also, CFD simulations should have a significant role in supporting field studies in urban environments, which should then be used to develop performance verification. Future research and development including CFD simulations should lead to the development of reliable simplified models (or databases) as needed to support emergency responders. In any case, CFD simulations can be used to support necessary post event analyses as is being done in support of post 9/11 studies for the US EPA, as summarized by this presentation. A report on progress for generally developing CFD applications in support of urban studies involving complex building environments is summarized by this presentation.

Disclaimer - Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy

Figure. Example of wind vectors calculated for one horizontal plan.