188.8.131.52.1 Huber and Snyder building downwash procedures.
The first step in the wake‑effects evaluation procedures used by the ISC model programs is to calculate the gradual plume rise due to momentum alone at a distance of two building heights using Equation (1‑23) or Equation (1‑25). If the plume height, he, given by the sum of the stack height (with no stack‑tip downwash adjustment) and the momentum rise is greater than either 2.5 building heights (2.5 hb) or the sum of the building height and 1.5 times the building width (hb + 1.5 hw), the plume is assumed to be unaffected by the building wake. Otherwise the plume is assumed to be affected by the building wake.
The ISC model programs account for the effects of building wakes by modifying both σy and σz for plumes with plume height to building height ratios less than or equal to 1.2 and by modifying only σz for plumes from stacks with plume height to building height ratios greater than 1.2 (but less than 2.5). The plume height used in the plume height to stack height ratios is the same plume height used to determine if the plume is affected by the building wake. The ISC models define buildings as squat (hw ³ hb) or tall (hw < hb). The ISC models include a general procedure for modifying σz and σy at distances greater than or equal to 3hb for squat buildings or 3hw for tall buildings. The air flow in the building cavity region is both highly turbulent and generally recirculating. The ISC models are not appropriate for estimating concentrations within such regions. The ISC assumption that this recirculating cavity region extends to a downwind distance of 3hb for a squat building or 3hw for a tall building is most appropriate for a building whose width is not much greater than its height. The ISC user is cautioned that, for other types of buildings, receptors located at downwind distances of 3hb (squat buildings) or 3hw (tall buildings) may be within the recirculating region.
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