Overview

Integrated simulation tools work across different modeling regimes, in order to predict complex interactions between them. This page describes the following integrated simulation tools:

• Residential models in NARAC.
• COMIS in HPAC.
• CFD in COMIS.

The Airflow and Pollutant Transport Group both develops and applies these tools. The following table summarizes each integrated simulation tool, the modeling regimes they couple, and some applications of interest:

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Residential Models in NARAC

The National Atmospheric Release Advisory Center (NARAC) at Lawrence Livermore National Laboratory provides assessments of the consequences of release of nuclear, radiological, chemical, and biological hazardous materials. Currently they only predict the situation outdoors. LBNL is helping to extend the capability of their modeling system, by providing the ability to predict indoor concentrations in both residential and commercial buildings.

Due to their small volume and relatively fast mixing time, houses can be modeled as a simple well-mixed box, in which the indoor concentration is assumed to be spatially uniform. With this assumption, only the air infiltration rate is needed to calculate the indoor concentration profile.

The air infiltration rate can be calculated with an infiltration model that was developed at LBNL in the 1980s. Inputs to the model include the local weather conditions (provided by NARAC) and the "leakiness" of the building, summarized by its Effective Leakage Area (ELA).

The distribution of ELA values for housing can be estimated based on housing characteristics, such as ages and sizes, which are available from the U.S. 2000 Census and American Housing Surveys. In order to extract the geographically-coded information for the vicinity of the release, Geographical Information System (GIS) software is used to overlay Census boundaries onto the concentration grid generated by NARAC.

The result is an integrated tool that predicts indoor exposures due to an outdoor release of chemical or biological agents. The tool can be used for a range of purposes, including planning, emergency response (such as where to send help and what areas to evacuate), and casualty assessment. It also can support research on assessing effectiveness of various defensive strategies, such as creating safe rooms or safe buildings for a community.

The simple residential model can be refined to include other processes that affect indoor-outdoor transport, such as sorption and deposition. However, large commercial buildings cannot be accurately modeled as a single well-mixed zone, so more sophisticated models are being developed.

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COMIS in HPAC

The Hazard Prediction and Assessment Capability (HPAC) toolset, from the Defense Threat Reduction Agency (DTRA), predicts the outdoor transport of hazardous materials, and assesses consequences to human health. The program includes weather forecasts and observations, models of pollutant sources, terrain and population databases, airflow models, pollutant transport algorithms, and the like. It provides the user with probabilistic estimates of the downwind concentrations, and health consequences, for chemical, biological, radiological, and other hazardous agents.

Bringing the COMIS multizone airflow and pollutant transport program to HPAC provides the ability to predict indoor transport for large, complex multizone buildings. The exchange of airborne pollutants between a building and outdoors, and between zones of a building, affects urban residents in a number of important ways:

• Outdoor-to-indoor transport. Infiltration due to wind and thermal effects, and air intake for ventilation purposes, expose building occupants to outdoor pollutants. Typically the building delays and prolongs the exposure to an outdoor release, so that occupants face longer exposure to lower concentrations than do people outdoors.
   
• Building as source. Exfiltration from a building, and air exhausted by a building's ventilation system, expose people downwind to contaminants generated within the building. Since the building tends to contain the agent, and to distribute it to the outdoors over a large surface area, occupants typically sustain higher exposures than do those downwind.
   
• Deposition. For some agents, deposition on building surfaces, and in the cracks and ducts that transport pollutants through a building, reduces the exposure. Thus for these agents, a building occupant will typically suffer less total exposure to an outdoor release than would somebody standing just outside the building. Deposition is relatively important in buildings, since they have more surface area per unit volume than an outdoor setting.
   
• Sorption. For some agents, sorption on building surfaces reduces the peak exposure to both indoor and outdoor releases. However, if the material later desorbs back into the room air, the total exposure may be the same over time. Again, the high ratio of surface area to volume in a building makes sorption relatively important compared to outdoors.

The Airflow and Pollutant Transport Group has ported a version of COMIS to HPAC. Within HPAC, COMIS combines with a second interior model, one from the Science Applications International Corporation (SAIC). Taken together, the two interior models are known as BINEX, short for Building Interior and Exfiltration. Thus the BINEX module gives the user access to COMIS via the HPAC graphical user interface.

In the current version of BINEX, the interface to COMIS carries the following information:

• the building geometry, describing the zones, outdoor nodes, and connections between them;
• details of the pollutant release, including its amount, duration, and location;
• variable building data, such as the zone temperatures, fan speeds, and degree of door or window opening; and
• weather conditions.

In return, COMIS produces the following output:

• the zone concentrations over time; and
• the pollutant mass released from the building over time.

Note that the interface does not currently include infiltration.

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CFD in COMIS

We are developing a general algorithm for coupling the COMIS multizone airflow and pollutant transport program to a Computational Fluid Dynamics (CFD) capability. As with COMIS, the resulting tool finds steady-state airflows and transient pollutant transport in a building. Adding a CFD model allows the simulation of large spaces, where the multizone assumption of instantaneous perfect mixing of pollutants no longer applies.

The presence of a large space, for example an atrium, convention hall, theater, or auditorium, significantly challenges a multizone model of a building. Multizone models do not predict the airflow patterns in a room. Therefore they do not capture interior effects due to thermal plumes, jets from ventilation ducts, partitions, and so forth. These effects can significantly change the mixing of pollutant within a large space, and the transport of pollutant out of the space. Thus, the multizone assumption of instantaneous perfect mixing can lead to:

• over-predictions and under-predictions of the exposure of occupants in the large space;
• over-predictions of the speed at which pollutant propagates from the large space to other rooms; and
• over-predictions and under-predictions of the amount of pollutant that enters adjoining rooms.

When fully operational, the coupled CFD-COMIS simulation tool will allow modeling a large, complex multizone building that contains one or more large indoor spaces. The resulting integrated simulation tool will substantially improve the fidelity of predictions compared to a pure multizone approach, without imposing an unacceptable computational burden as would a pure CFD approach.

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Area Lead:

David Lorenzetti, , (510) 486-4562
 

Rengie Chan, , (510) 495-2459
 
Ashok Gadgil, , (510) 486-4651
 
Buvana Jayaraman, , (510) 486-6587

Publications