Dispersion Analysis for VA Hospitals
By Kevin Linfield, Ph.D., P.Eng., P.E. and Jeff Everett
The US Department of Veteran Affairs Office of Construction and Facilities Management publishes the HVAC Design Manual, in which it is specified that architectural and engineering firms shall perform an analysis using Computational Fluid Dynamics modeling or wind tunnel analysis to ensure odors and hazardous exhausts do not enter into outdoor air intakes and open windows of VA facilities and adjoining properties.
These guidelines also reference the ASHRAE Handbook of Fundamentals chapter on Airflow Around Buildings:
For all required projects the A/E shall perform an analysis using either Computational Fluid Dynamics (CFD) modeling or via wind tunnel analysis. The CFD modeling must be performed by qualified practitioner using an appropriate turbulence simulation algorithm. The objective of both CFD modeling and wind tunnel is to ensure through quantification that odors and hazardous exhaust do not enter into outdoor air intakes and open windows of VA facilities and adjoining properties. See Chapter 24 Airflow Around Buildings in ASHRAE Handbook of Fundamentals – latest edition. Any contamination problems indicated by the simulation shall be corrected prior to proceeding with any additional design development.
The analysis must assess all wind directions that might pose a risk, at different wind speeds and at the range of anticipated exhaust velocities. Mitigation might require changing the height of the release which would required additional iteration through wind directions, speeds and release velocities.
The modeling requirements set forth through this directive specifies that the analysis needs to assess all wind directions that might pose a risk while taking in account different wind speeds and a range of exhaust velocities. Any contamination problems indicated by the simulation should be corrected and re-simulated prior to proceeding with any additional design development.
CFD modeling can provide analysis of the plume behavior under a range of different ambient wind conditions and emission flow rates. A three-dimensional CFD model is constructed of the exterior of the buildings, and the surrounded community, with exhaust gasses emitted being tracked.
A visualization of the gas dispersion using color contours and path lines are superimposed on the geometry of the buildings to communicate the flow patterns and gas concentration levels at key locations.
Wind tunnels are also used for environmental flow testing around scale models of buildings. Shown here is a 100th scale model featuring a portion of downtown Detroit.
This tunnel includes a 360-degree turntable to allow any incoming wind direction to be analyzed, along with appropriate spires and surface roughness generators to represent the Earth boundary layer for wind approaching the buildings.
Airflow Sciences Corporation (ASC) is a fluid dynamics solutions company, specializing in the design and optimization of equipment and processes involving flow, heat transfer, combustion, and mass transfer. Now celebrating 50 years as an industry leader, ASC has focused on testing and simulation of air, gas, liquid, and particulate flows since 1975.
ASC also manufactures standard and custom test equipment, including probes and wind tunnels, enabling customers to collect data accurately and efficiently. ASC’s primary CFD software, Azore®, is also available for customers with in-house CFD personnel. ASC offers comprehensive flow solutions and optimization and serves a wide range of industries including HVAC, power, auto, rail, and food processing.