ASHRAE FUNDAMENTALS SI CH 24-2017 Airflow Around Buildings.pdf

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1、24.1CHAPTER 24AIRFLOW AROUND BUILDINGSFlow Patterns. 24.1Wind Pressure on Buildings 24.4Sources of Wind Data 24.7Wind Effects on System Operation. 24.8Building Pressure Balance and Internal Flow Control . 24.10Environmental Impacts of Building External Flow . 24.11Physical and Computational Modeling

2、 24.12Symbols 24.14IRFLOW around buildings affects worker safety, process andA building equipment operation, pollution infiltration at buildinginlets, and the ability to control indoor environmental parameterssuch as temperature, humidity, air motion, and contaminants. Windcauses variable surface pr

3、essures on buildings that can change intakeand exhaust system flow rates, natural ventilation, infiltration andexfiltration, and interior pressures. The mean flow patterns and tur-bulence of wind passing over a building can also lead to recirculationof exhaust gases into air intakes.This chapter pro

4、vides basic information for evaluating wind flowpatterns, estimating wind pressures, and identifying problemscaused by the effects of wind on pedestrians and buildings, includingventilation intakes, exhausts, and equipment. In most cases, detailedsolutions are addressed in other chapters. Related in

5、formation can befound in Chapters 11, 14, 16, and 37 of this volume; in Chapters 31,32, 45, 47, and 53 of the 2015 ASHRAE HandbookHVAC Appli-cations; and in Chapters 30, 35, and 40 of the 2016 ASHRAE Hand-bookHVAC Systems and Equipment.1. FLOW PATTERNSFlow Patterns Around Isolated, Rectangular Block

6、-Type BuildingsBuildings with even moderately complex shapes, such as L- or U-shaped structures, can generate flow patterns too complex to gener-alize for design. To determine flow conditions for such buildings,wind tunnel or water channel tests of physical scale models, full-scaletests of existing

7、buildings, or appropriate computational modelingefforts are required (see the section on Physical and ComputationalModeling). Thus, only isolated, rectangular block-type buildings arediscussed here.Figure 1 shows the wind flow pattern around a single, wide, high-rise building slab, with the main flo

8、w features indicated by numbers.The following description of the flow pattern is adapted fromBlocken et al. (2011). As wind impinges on the building, part of theflow is deviated over the building (point 1) and part flows around it(2, 9). A stagnation point is present at the windward faade at about70

9、% of the building height. From this point, part of the flow is devi-ated upward (upwash) (3), part is deviated sideways (4), and a largepart is directed downwards (downwash) (5). This downflow devel-ops into a ground-level vortex (6) called standing vortex, frontal vor-tex, or horseshoe vortex. The

10、main flow direction of this vortex nearground level is opposite to the direction of the approach flow. Bothflows collide at the stagnation point at ground level in front of thebuilding (7). The standing vortex subsequently wraps around thebuilding corners, yielding the concentrated corner streams, c

11、harac-terized by very high wind speed amplification (8). These cornerstreams are further amplified by the general ground-level flowaround the building (9). At the buildings leeward side, the under-pressure zone results in recirculation flow (10, 13). A stagnationzone is also present downstream of th

12、e building at ground level,where the flow directions are opposite and wind speeds are low (11).Further downstream, the wind speed remains low for a considerabledistance behind the building (i.e., the far wake) (12). Backflow isalso responsible for creating slow-rotating vortices behind the build-ing

13、 (13). Between these vortices and the corner streams (9) is a zonewith a high velocity gradient (shear layer) that comprises small,fast-rotating vortices (16).Figure 2 provides a more detailed illustration of the wind flowpattern around an isolated building. It more clearly shows the vorti-cal natur

14、e of the corner streams, and it indicates the areas of flowseparation and reattachment and the flow in the near wake. It isimportant to note that Figures 1 and 2 only show the mean wind flowpattern, and that the actual flow pattern exhibits pronounced tran-sient features, such as the build-up and co

15、llapse of the separation/recirculation bubbles and periodic vortex shedding in the wake(Murakami 1993; Tominaga et al. 2008a).For a building with height H that is three or more times the widthW of the upwind face, an intermediate stagnation zone can existbetween the upwash and downwash regions, wher

16、e surface stream-lines pass horizontally around the building (Figure 3A). (In Figure 3,the upwind building surface is “folded out” to illustrate upwash,downwash, and stagnation zones.) Downwash on the lower surface ofthe upwind face separates from the building before it reaches groundlevel and moves

17、 upwind to form the standing vortex. Figure 3B showsthe near-surface flow patterns for oblique approach flow. Strong vor-tices develop from the upwind edges of the roof, causing strongdownwash onto the roof. High speeds in these vortices (vorticity)cause large negative pressures near roof corners th

18、at can be a hazardto roof-mounted equipment during high winds. In some extremecases, the negative pressures can be strong enough to lift heavyobjects such as roof pavers, which can result in a projectile hazard.When the angle between the wind direction and the upwind faceof the building is less than

19、 about 70, the upwash/downwash patternson the upwind face of the building are less pronounced, as is theground-level vortex shown in Figure 1 and 2. Figure 3B shows that,The preparation of this chapter is assigned to TC 4.3, Ventilation Require-ments and Infiltration.Fig. 1 Wind Flow Pattern Around

20、High-Rise Building SlabAdapted from Blocken et al. (2016) and Beranek and Van Koten (1979)24.2 2017 ASHRAE HandbookFundamentals (SI)for an approach flow angle of 45, streamlines remain close to thehorizontal in their passage around the sides of the building, exceptnear roof level, where the flow is

21、drawn upwards into the roof edgevortices (Cochran 1992).Both the upwind velocity profile shape and its turbulence inten-sity strongly influence flow patterns and surface pressures (Mel-bourne 1979).The downwind wall of a building exhibits a region of low averagevelocity and high turbulence (i.e., a

22、flow recirculation region) ex-tending a distanceLr downwind. If the building has sufficient lengthL in the windward direction, the flow reattaches to the building andmay generate two distinct regions of separated recirculation flow, onthe roof of the building and in its wake, as shown in Figure 4. F

23、igure4 also shows a rooftop recirculation cavity of length Lcat the upwindroof edge and a recirculation zone of lengthLr downwind of the roof-top penthouse. Velocities near the downwind wall are typically one-quarter of those at the corresponding upwind wall location. Figures2 and 3 show that an upw

24、ard flow exists over most of the downwindwalls.Streamline patterns and the size of the wake(s) are generally inde-pendent of wind speed and depend mainly on building shape andupwind conditions. Because of the three-dimensional flow around aFig. 2 Wind Flow Pattern Around Isolated Building(Hunt et al

25、. 1978)Fig. 3 Surface Flow Patterns for Normal and Oblique Winds(Wilson 1979)Airflow Around Buildings 24.3building, the shape and size of the recirculation airflow are not con-stant over the surface. Airflow reattaches closer to the upwind build-ing face along the edges of the building than it does

26、near the middleof the roof and sidewalls (Figure 3). Recirculation cavity height Hc(Figure 4) also decreases near roof edges. Calculating characteristicdimensions for recirculation zones Hc, Lc, and Lris discussed inChapter 45 of the 2015 ASHRAE HandbookHVAC Applications.For wind perpendicular to a

27、building wall, the height H and widthW of the upwind building face determine the scaling length R thatcharacterizes the buildings influence on wind flow. According toWilson (1979),R = (1)whereBs= smaller of upwind building face dimensions H and WBL= larger of upwind building face dimensions H and WW

28、hen BLis larger than 8Bs, use BL= 8Bsin Equation (1). For build-ings with varying roof levels or with wings separated by at least adistance of Bs, only the height and width of the building face belowthe portion of the roof in question should be used to calculate R.Flow accelerates as the streamlines

29、 compress over the roof anddecelerates as they spread downward over the wake on the down-wind side of the building. The height above roof level where a build-ing influences the flow is approximately 1.5R. In addition, roofpitch also begins to affect flow when it exceeds about 15 (1:4).When roof pitc

30、h reaches 20 (1:3), flow remains attached to theupwind pitched roof and produces a recirculation region downwindof the roof ridge that is larger than that for a flat roof.Flow Patterns Around Building GroupsIn building groups, the flow patterns can interact, yielding ahigher complexity. To determine

31、 flow conditions around buildinggroups, wind tunnel or water channel tests of physical scale mod-els, full-scale tests of existing buildings, or careful computationalmodeling efforts are required (see the section on Physical andComputational Modeling). English and Fricke (1997), Hosker(1984, 1985),

32、Khanduri et al. (1998), Saunders and Melbourne(1979), and Walker et al. (1996) review the effects of nearby build-ings, whereas Blocken et al. (2007a, 2008), Stathopoulos andStorms (1986), Yoshie et al. (2007), and others assess the effects ofnearby buildings by wind tunnel testing and computational

33、 model-ing. To illustrate the complexity of wind flow patterns induced bynearby buildings, Figure 5 provides a top view of two high risebuildings in V shape. Depending on the wind direction, thisconfiguration is labeled as converging or diverging. Although thehighest wind speed in the passage betwee

34、n both buildings mightbe expected to occur for the converging arrangement, wind tunneltests and numerical simulations indicate that the divergingarrangement actually has higher wind speed. This is shown by theamplification factors in Figure 6, which are defined as the ratio ofthe local wind speed to

35、 the wind speed that would occur at the sameheight in absence of the buildings. The higher amplification factorin the diverging arrangement is caused by the lesser flow resistancein this configuration. The Venturi effect does not apply in this case:the Venturi effect refers to confined flows, wherea

36、s wind flow inthe atmosphere is unconfined. Further information on this studycan be found in Blocken et al. (2008).Figure 7 shows the flow over building arrays with increasing H/W.The description of these flow patterns is adopted from Oke (1988). Ifthe buildings are sufficiently apart (H/W 0.05), th

37、eir flow fields doFig. 4 Flow Recirculation RegionsBs0.67BL0.33Fig. 5 Buildings in (A) Converging and (B) Diverging ConfigurationFig. 6 Amplification Factor K in Horizontal Plane at y = 2 m above Ground for Converging and Diverging Arrangement with H = 30 m and w = 75 m and 20 m(Blocken et al. 2008)

38、24.4 2017 ASHRAE HandbookFundamentals (SI)not interact, and the flow is called isolated roughness flow. Whenthe buildings are positioned closer together, their flow patterns showsome degree of interaction, mainly manifested as a disturbance of thewake structure (wake interference flow). When the rat

39、io H/W in-creases further, a stable circulatory vortex is formed in the canyonand the bulk of the flow does not enter the canyon (skimming flow).2. WIND PRESSURE ON BUILDINGSIn addition to flow patterns described previously, the turbulenceor gustiness of approaching wind and the unsteady character o

40、f sep-arated flows cause surface pressures to fluctuate. Pressures dis-cussed here are time-averaged values, with a full-scale averagingperiod of about 600 s. This is approximately the shortest time periodconsidered to be a “steady-state” condition when considering atmo-spheric winds; the longest is

41、 typically 3600 s. Instantaneous pres-sures may vary significantly above and below these averages, andpeak pressures two or three times the mean values are possible. Peakpressures are important with regard to structural loads, and meanvalues are more appropriate for computing infiltration and ventil

42、a-tion rates. Time-averaged surface pressures are proportional to windvelocity pressure pvgiven by Bernoullis equation:pv= (2)wherepv= wind velocity pressure at roof level, PaUH= approach wind speed at upwind wall height H, m/s see Equation (4)a= ambient (outdoor) air density, kg/m3The proportional

43、relationship is shown in the following equation,in which the difference psbetween the pressure on the building sur-face and the local outdoor atmospheric pressure at the same level inan undisturbed wind approaching the building isps= Cppv(3)where Cpis the local wind pressure coefficient at a point o

44、n thebuilding surface.Approach Wind SpeedThe local wind speed UHat the top of the wall required for Equa-tion (2) is estimated by applying terrain and height corrections to thehourly wind speed Umetfrom a nearby meteorological station.Umetis generally measured in flat, open terrain (i.e., category 3

45、 inTable 1). The anemometer that records Umetis located at heightHmet, usually 10 m above ground level. The hourly average windspeed UHin the undisturbed wind approaching a building in its localterrain can be calculated from Umetas follows:UH= Umet(4)The atmospheric boundary layer thickness and expo

46、nent a forthe local building terrain and ametand metfor the meteorologicalstation are determined from Table 1. Typical values for meteorolog-ical stations (category 3 in Table 1) are amet= 0.14 and met= 270 m.The values and terrain categories in Table 1 are consistent with thoseadopted in other engi

47、neering applications (e.g., ASCE Standard 7).Equation (4) gives the wind speed that occurs at a certain height Habove the average height of local obstacles, such as buildings andvegetation, weighted by the plan area. At heights at or below thisaverage obstacle height (e.g., at roof height in densely

48、 built-up sub-urbs), speed depends on the geometrical arrangement of the build-ings, and Equation (4) is less reliable.An alternative mathematical description of the atmosphericboundary layer, which uses a logarithmic function, is given byDeaves and Harris (1978). Although their model is more compli

49、-cated than the power law used in Equation (4), it more closely mod-els the real physics of the atmosphere and has been adopted byseveral codes around the world (e.g., SA/SNZ 2002).Example 1. Assuming a 10 m/s anemometer wind speed for a height Hmetof 10 m at a nearby airport, determine the wind speed UHat roof levelH = 15 m above grade for a building located in a city suburb.Solution: From Table 1, the atmospheric boundary layer properties forthe anemometer are amet= 0.14 and met= 270 m. The atmosphericboundary layer properties at the building site are a = 0.22 and