Indoor dispersion

A model of the air flow in a building will give insights into the transportation of agents through the building. That way it can indicate which rooms in the building are “hotzones” of agent. Possible further actions, such as evacuation of people or even decontamination of the specific rooms can be taken, based on the model results.

The model can be used to indicate which locations (rooms or ventilation ducts) are best to deploy detectors and the filters. It can also generate requirements for the filters, such as required protection factors and required throughput.

This indoor model contains the following capabilities:

• The model has to describe the concentration of agent in every room in the building as a function of time.
• The model has to be able to work with airborne Chemical or Biological warfare agents.
• Possibility to easily implement the building structure (floor plan) inside the model, inclusive rooms, corridors, doors and windows and various floor levels.
• Possibility to integrate an HVAC system into this building, with various ventilation rates per room.
• Possibility to use the outdoor air concentration of agent around the building as input for the model, inclusive the incorporation of the outdoor wind direction and wind speed (important for windows etc.).
• Possibility to integrate filtering systems (both chemical and biological) into the HVAC system.
• Prediction of residual filter life.
• Possibility to integrate the model results with detector output.

The indoor dispersion tool can be used in two modes:

• Stand alone:
  • Implementation of a floor plan
  • Calculation of threat scenarios
  • Analyzing treats
  • Localizing agent hotspots in buildings
  • Calculation of Concentrations + Air flows
•  Live:

• Connection to agent sensors
• Compares concentration to threshold level
• Real time calculations
• Updates predictions of dispersion in building by using measurements
• Generates alarms when required

Outdoor dispersion 

To disperse agent in the air, a dispersion source can be used. The most important property of a dispersion source is the mass of the agent which is to be dispersed. This mass will be dispersed in the air with a (not necessarily constant) release rate or can even be dispersed almost instantaneously. The size of the source affects the cloud formation. A cloud can be described by a plume model. The most plume models, which exist, are models, which only describe the average concentration of vapor or aerosol in time. Though the average concentration can be described quite accurately, a time dependent concentration cannot be described with that type of plume model. Because of wind fluctuations in the atmosphere the concentration will change in time. The movement of the plume due to the local wind velocities is called meandering. 

The HAVAC model is able to predict local wind fluctuations, and is thereby able to predict cloud meandering.

Urban dispersion 

Because the HAVAC model is able to incorporate local air fluctuations, it is also able to predict the dispersion in urban areas. In urban areas, an airflow process occurs which is called canyoning. This basically is the effect where the airflow in streets in between buildings can be in a slightly different direction than the overall wind direction.

On top of that, if the wind is perpendicular to the canyon direction, the wind profile will look like shown in the Figure below. 

Wind is blowing over the city in a specific direction, which is perpendicular to the local canyon (the street). Eddies will occur in the canyon. A vertical wind velocity exists and the horizontal wind direction can be opposite to the overall wind direction.