42540_24 - CHAPTER 24 AIRFLOW AROUND BUILDINGS Flow...

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24.1 CHAPTER 24 AIRFLOW AROUND BUILDINGS Flow Patterns ................................................................................................................................ 24.1 Wind Pressure on Buildings .......................................................................................................... 24.3 Wind Effects on System Operation ................................................................................................ 24.7 Building Pressure Balance and Internal Flow Control ................................................................. 24.9 Physical and Computational Modeling ......................................................................................... 24.9 Symbols ....................................................................................................................................... 24.12 IRFLOW around buildings affects worker safety, process and A building equipment operation, weather and pollution protec- tion at inlets, and the ability to control indoor temperature, humid- ity, air motion, and contaminants. Wind causes variable surface pressures on buildings that change intake and exhaust system flow rates, natural ventilation, infiltration and exfiltration, and interior pressures. The mean flow patterns and turbulence of wind passing over a building can recirculate exhaust gases to air intakes. This chapter provides basic information for evaluating windflow pat- terns, estimating wind pressures, and identifying problems caused by the effects of wind on intakes, exhausts, and equipment. In most cases, detailed solutions are addressed in other chapters. Related information can be found in Chapters 11, 14, 16, and 36 of this volume; in Chapters 29, 30, 44, 46, and 52 of the 2007 ASHRAE Handbook—HVAC Applications ; and in Chapters 29, 34, and 39 of the 2008 ASHRAE Handbook—HVAC Systems and Equipment . FLOW PATTERNS Buildings having even moderately complex shapes, such as L- or U-shaped structures, can generate flow patterns too complex to gen- eralize for design. To determine flow conditions influenced by sur- rounding buildings or topography, wind tunnel or water channel tests of physical scale models, tests of existing buildings, or careful computational modeling efforts are required (see the section on Physical and Computational Modeling). Only isolated, rectangular block buildings are discussed here. English and Fricke (1997), Hosker (1984, 1985), Khanduri et al. (1998), Saunders and Mel- bourne (1979), and Walker et al. (1996) review the effects of nearby buildings. As wind impinges on a building, airflow separates at the building edges, generating recirculation zones over downwind surfaces (roof, side and downwind walls) and extending into the downwind wake (Figure 1). On the upwind wall, surface flow patterns are largely influenced by approach wind characteristics. Figure 1 shows that the mean speed of wind U H approaching a building increases with height H above the ground. Higher wind speed at roof level causes a larger pressure on the upper part of the wall than near the ground, which leads to downwash on the lower one-half to two- thirds of the building. On the upper one-quarter to one-third of the building, windflow is directed upward over the roof ( upwash ). For a building with height H three or more times width W of the upwind face, an intermediate stagnation zone can exist between the upwash and downwash regions, where surface streamlines pass horizontally around the building, as shown in Figures 1 (inset) and 2. (In Figure 2, the upwind building surface is “folded out” to illustrate upwash, downwash, and stagnation zones.) Downwash on the lower surface of the upwind face separates from the building before it reaches
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