FIG. 5.8.11 Stack-tip downwash. (Reprinted, with permission, from Trinity Consultants, Inc., 1989, Atmospheric diffusion notes, Issue no. 13, Dallas, Tex. [June].)
downwash may occur depending on the ambient conditions at the time.
Downwash can also be caused by local topography. Large hills or mountains can change the normal wind patterns of an area. If the stack is located closely downwind of a hill above stack height, the air flowing off the hill can cause the plume to impact closer to the stack than normal as shown in part (c) of Figure 5.8.10. Modeling of these situations often employs physical models in wind tunnels. A recently developed model, the complex terrain dispersion model plus algorithms for unstable situations (CTDM-PLUS), employs a critical hill height calculation that determines if the plume impacts the hill or follows the uninterrupted laminar flow around it (U.S. EPA 1989).
(c) Stack-Tip Downwash
(b) Building Downwash
(c) Terrain Downwash
FIG. 5.8.10 Physical conditions that cause downwash. Reprinted from G.A. Briggs, 1969. Plume rise, U.S. Atomic Energy Commission Critical Review Series TID-25075, Clearinghouse for Federal Scientific and Technical Information.
Large structures surrounding the stack also affect ambient wind conditions. The boundaries of the wake region resulting from surrounding structures are not sharply defined. They depend on the three-dimensional characteristics of the structure and are time dependent. The extent of distortion depends extensively upon building structure geometry and wind direction. Generally, a single cylindrical structure (e.g., a free-standing silo) has little influence on the wind flow compared to a rectangular structure.
Part (b) in Figure 5.8.10 shows the building downwash that occurs when the plume is drawn into a wake from a nearby structure. Two zones exist within the downwash area of a structure. The first zone, which extends approximately three building heights downwind, is the cavity region where plume entrapment can occur. The second zone, which extends from the cavity region to about ten times the lesser dimension of the height or projected width, is the wake region where turbulent eddies exist as a result of structure disturbance to the wind flow. Figure 5.8.12 shows an example of these zones. Generally, the cavity region concentration is higher than the wake region concentration due to plume entrapment. Bittle and Borowsky
(1985) examined the impact of pollutants in cavity zones and found several calculations apply depending on the building and stack geometry. Beyond these zones, wind flow is unaffected by the structure.
The first downwash calculations were developed as the result of studies in a wind tunnel by Snyder and Lawson (1976) and Huber (1977). However, these studies were limited to a specific stability, structure shape, and orientation to the wind. Additional work by Hosker (1984), Schulman and Hanna (1986), and Schulman and Scire (1980) refined these calculations. Figure 5.8.13 shows the areas where the Huber-Snyder and Schulman-Scire down-wash calculations apply. The following equation determines whether the Huber-Snyder or Schulman-Scire downwash calculations apply:
hs = the physical stack height
H = the structure height
L = the lesser dimension of the height or projected width
The adjustments are made to the dispersion parameters.
To avoid building downwash, the EPA has developed a general method for designing the minimum stack height needed to prevent emissions from being entrained into any wake created by the surrounding buildings. In this way, emissions from a stack do not result in an excessive concentration of the pollutant close to the stack. This ap-
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