Fire modeling is more and more frequently used to support fire origin and cause determinations in the forensic environment, as well as to conduct performance-based evaluations of industrial applications. The most widely used tools in fire modeling are developed and distributed by the National Institute of Standards and Technology. The Fire Dynamics Simulator (FDS) is a computational fluid dynamics (CFD) model of fire-driven fluid flow, with an emphasis on smoke and heat transport from fires. The Consolidated Model of Fire and Smoke Transport (CFAST) is a two-zone fire model used to calculate the evolving distribution of smoke, fire gases, and temperature throughout compartments of a building during a fire. Exponent’s engineers use fire modeling and other CFD modeling tools to supplement our analytical, experimental, and field-based activities.
While FDS was originally developed for fire-driven flows, it is applicable to non-fire-related scenarios, with the limitation of low-speed, thermally-driven flows. Our engineers have used FDS to model scenarios such as fan-induced air flows, purging of large tanks, and the atmospheric dispersion of smoke and particulates over part of a town.
We frequently use commercial CFD models for our analyses. Our staff has been applying CFD models to determine the hazard areas associated with the dispersion of LNG vapor clouds and other dense-vapor releases. The application of CFD models to these types of analyses allows a more realistic representation of the release and dispersion scenarios (for example, accounting for terrain features and other obstructions), resulting in a more accurate understanding of the hazards and a more effective use of mitigation techniques.
The atmospheric dispersion of pollutants, for example as a result of an industrial accident, can also be investigated using CFD tools in the short range (a few hundred feet). For dispersion over longer distances, however, Gaussian or puff-based dispersion models are necessary. Exponent routinely applies EPA-approved models such as AERMOD, CALPUFF, and others to the analysis of atmospheric dispersion.
Simpler modeling tools remain a viable option, and an effective one, in many situations—whether as a screening tool to assess the need for more refined modeling, or as a conservative analysis when that is sufficient to meet the client’s needs. Exponent engineers have experience with tools such as CFAST and ALOFT (fire modeling); SLAB, DEGADIS, and PHAST (dense gas dispersion modeling); and SCREEN and SCIPUFF (air dispersion modeling).