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1.1.5.3 Procedures Used to Account for the Effects of Building Wakes on Effluent Dispersion.

     The procedures used by the ISC models to account for the effects of the aerodynamic wakes and eddies produced by plant buildings and structures on plume dispersion originally followed the suggestions of Huber (1977) and Snyder (1976).  Their suggestions are principally based on the results of wind‑tunnel experiments using a model building with a crosswind dimension double that of the building height.  The atmospheric turbulence simulated in the wind‑tunnel experiments was intermediate between the turbulence intensity associated with the slightly unstable Pasquill C category and the turbulence intensity associated with the neutral D category.  Thus, the data reported by Huber and Snyder reflect a specific stability, building shape and building orientation with respect to the mean wind direction.  It follows that the ISC wake‑effects evaluation procedures may not be strictly applicable to all situations.  The ISC models also provide for the revised treatment of building wake effects for certain sources, which uses modified plume rise algorithms, following the suggestions of Schulman and Hanna (1986).  This treatment is largely based on the work of Scire and Schulman (1980).  When the stack height is less than the building height plus half the lesser of the building height or width, the methods of Schulman and Scire are followed.  Otherwise, the methods of Huber and Snyder are followed.  In the ISC models, direction‑specific building dimensions may be used with either the Huber‑Snyder or Schulman‑Scire downwash algorithms.

     The wake‑effects evaluation procedures may be applied by the user to any stack on or adjacent to a building.  For regulatory application, a building is considered sufficiently close to a stack to cause wake effects when the distance between the stack and the nearest part of the building is less than or equal to five times the lesser of the height or the projected width of the building.  For downwash analyses with direction‑specific building dimensions, wake effects are assumed to occur if the stack is within a rectangle composed of two lines perpendicular to the wind direction, one at 5Lb downwind of the building and the other at 2Lb upwind of the building, and by two lines parallel to the wind direction, each at 0.5Lb away from each side of the building, as shown below:

 

 

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