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Unformatted text preview: This file is licensed to John Murray ([email protected]). Publication Date: 6/1/2017 24.10 2017 ASHRAE Handbook—Fundamentals (SI) 2 1.2 7.8 pfan eff = pfan + [0.8 – (–0.43)] ---------------------2 = pfan + 44.9 Pa This wind-assisted hourly averaged pressure is exceeded only 1% of the time (88 hours per year). When wind direction reverses, the outlet will be on the upwind wall and the inlet on the downwind wall, producing wind-opposed flow, changing the sign from +44.9 to –44.9 Pa. The importance of these pressures depends on their size relative to the fan pressure rise pfan, as shown in Figure 17. Licensed for single user. © 2017 ASHRAE, Inc. Minimizing Wind Effect on System Volume Flow Rate Wind effect can be reduced by careful selection of inlet and exhaust locations. Because wall surfaces are subject to a wide variety of positive and negative pressures, wall openings should be avoided when possible. When they are required, wall openings should be away from corners formed by building wings (see Figure 15). Mechanical ventilation systems should operate at a pressure high enough to minimize wind effect. Low-pressure systems and propeller exhaust fans should not be used with wall openings unless their ventilation rates are small or they are used in noncritical services (e.g., storage areas). Although roof air intakes in flow recirculation zones best minimize wind effect on system flow rates, current and future air quality in these zones must be considered. These locations should be avoided if a contamination source exists or may be added in the future. The best area is near the middle of the roof, because the negative pressure there is small and least affected by changes in wind direction (see Figure 12). Consider avoiding edges of the roof and walls, where large pressure fluctuations occur. Either vertical or horizontal (mushroom) openings can be used. On roofs with large areas, where intake may be outside the roof recirculation zone, mushroom or 180° gooseneck designs minimize impact pressure from wind flow. Vertical louvered openings or 135° goosenecks are undesirable for this purpose or for rain protection. Heated air or contaminants should be exhausted vertically through stacks, above the roof recirculation zone. Horizontal, louvered (45° down), and 135° gooseneck discharges are undesirable, even for heat removal systems, because of their sensitivity to wind effects. A 180° gooseneck for hot-air systems may be undesirable because of air impingement on tar and felt roofs. Vertically discharging stacks in a recirculation region (except near a wall) have the advantage of being subjected only to negative pressure created by wind flow over the tip of the stack. See Chapter 45 of the 2015 ASHRAE Handbook—HVAC Applications for information on stack design. Chemical Hood Operation Wind effects can interfere with safe chemical hood operation. Supply volume flow rate variations can cause both disturbances at hood faces and a lack of adequate hood makeup air. Volume flow rate surges, caused by fluctuating wind pressures acting on the exhaust system, can cause momentary inadequate hood exhaust. If highly toxic contaminants are involved, surging is unacceptable. The system should be designed to eliminate this condition. On low-pressure exhaust systems, it is impossible to test the hoods under wind-induced, surging conditions. These systems should be tested during calm conditions for safe flow into the hood faces, and rechecked by smoke tests during high wind conditions. For more information on chemical hoods, see Chapter 16 of the 2015 ASHRAE Handbook—HVAC Applications. For more information on stack and intake design, see Chapter 45 of that volume. 5. BUILDING PRESSURE BALANCE AND INTERNAL FLOW CONTROL Proper building pressure balance avoids flow conditions that make doors hard to open and cause drafts. In some cases (e.g., office buildings), pressure balance may be used to prevent confinement of contaminants to specific areas. In other cases (e.g., laboratories), the correct internal airflow is towards the contaminated area. Pressure Balance Although supply and exhaust systems in an indoor area may be in nominal balance, wind can upset this balance, not only because of its effects on fan capacity but also by superimposing infiltrated or exfiltrated air (or both). These effects can make it challenging to control environmental conditions. Where building balance and infiltration are important, consider the following: • Design HVAC system with pressure adequate to minimize wind effects • Include controls to regulate flow rate, pressure, or both • Separate supply and exhaust systems to serve each building area requiring control or balance • Use revolving or other self-closing doors or double-door air locks to noncontrolled adjacent areas, particularly exterior doors • Seal windows and other leakage sources • Close natural ventilation openings Internal Flow Control Airflow direction is maintained by controlling pressure differentials between spaces. In a laboratory building, for example, peripheral rooms such as offices and conference rooms are kept at positive pressure, and laboratories at negative pressure, both with reference to corridor pressure. Pressure differentials between spaces are normally obtained by balancing supply system airflows in the spaces in conjunction with exhaust systems in the laboratories. Differential pressure instrumentation is normally used to control airflow. The pressure differential for a room adjacent to a corridor can be controlled using the corridor pressure as the reference. Outdoor pressure cannot usually control pressure differentials within internal spaces, even during periods of relatively constant wind velocity (wind-induced pressure). A single pressure sensor can measure the outdoor pressure at one point only and may not be representative of pressures elsewhere. Airflow (or pressure) in corridors is sometimes controlled by an outdoor reference probe that senses static pressure at doorways and air intakes. The differential pressure measured between the corridor and the outdoors may then signal a controller to increase or decrease airflow to (or pressure in) the corridor. Unfortunately, it is difficult to locate an external probe where it will sense the proper external static pressure. High wind velocity and resulting pressure changes around entrances can cause great variations in pressure. To measure ambient static pressure, the probe should be located where airflow streamlines are not affected by the building or nearby buildings. One possibility is at a height of 1.5R, as shown in Figure 18. However, this is usually not feasible. If an internal space is to be pressurized relative to ambient conditions, the pressure must be known on each exterior surface in contact with the space. For example, a room at the northeast corner of the building should be pressurized with respect to pressure on both the north and east building faces, and possibly the roof. In some cases, multiple probes on a single building face may be required. Figures 8 to 12 may be used as guides in locating external pressure probes. System volume and pressure control is described in Chapter 45 of the 2015 ASHRAE Handbook—HVAC Applications. This file is licensed to John Murray ([email protected]). Publication Date: 6/1/2017 Airflow Around Buildings 24.11 Licensed for single user. © 2017 ASHRAE, Inc. Fig. 18 Flow Patterns Around Rectangular Block Building (modified from Hosker 1984) 6. ENVIRONMENTAL IMPACTS OF BUILDING EXTERNAL FLOW Pollutant Dispersion and Exhaust Reentrainment Pollutant dispersion around buildings is highly affected by the complex flow field. Contaminants are not always transported along the approaching flow direction and can be advected windward by reverse flows and retained in wake flows (Huber and Snyder 1982; Li and Meroney 1983; Stathopoulos et al. 2002). Intakes and exhausts should be installed carefully, considering wind direction and roof geometry (e.g., stack height, rooftop structures) to avoid air intake contamination and exhaust reentrainment (Gupta et al. 2012; Lazure et al. 2002; Stathopoulos et al. 2004). Empirical guidelines that can be used are given in Chapter 45 of the 2015 ASHRAE Handbook—HVAC Applications. More detailed prediction can be done by physical and numerical modeling, as explained in the section on Physical and Computational Modeling. State-of-the-art reviews on modeling of pollutant dispersion were performed by Canepa (2004), Di Sabatino et al. (2013), Lateb et al. (2016), Meroney (2004), and Tominaga and Stathopoulos (2013). Pedestrian Wind Comfort and Safety Although thermal comfort is also important [e.g., Metje et al. (2008); Stathopoulos (2006)], wind comfort and safety generally only refer to the mechanical effects of wind on people [e.g., Lawson and Penwarden (1975); Willemsen and Wisse (2007)]. Particularly near high-rise buildings, high wind velocities can occur at pedestrian level that can be uncomfortable or even dangerous. For an isolated high-rise building or one amid low rise buildings, high wind speed at pedestrian level can be caused by the downflow that creates the standing vortex and the corner streams (see Figure 1). For building groups, amplified wind speed can occur in passages through and between buildings. Uncomfortable wind conditions can be detrimental to the success of new buildings. Wise (1970) reported shops that were left untenanted because of the windy environment that discouraged shoppers. Lawson and Penwarden (1975) reported dangerous wind conditions to be responsible for the death of two elderly women who were blown over by sudden wind gusts near a high-rise building. Many current urban authorities recognize the importance of pedestrian wind comfort and wind safety, and require studies before granting building permits for new buildings or new urban areas. ASCE (2004) documented the state of the art of outdoor human comfort and its assessment. The first standard on wind comfort and wind safety was developed in the Netherlands and published in 2006 (NEN Standard 8100; Willemsen and Wisse 2007) and applied in several published case studies [e.g., Blocken et al. (2012)]. Reviews on studies of pedestrian wind comfort and safety were provided by Blocken (2014), Blocken and Stathopoulos (2013), Blocken et al. (2016), Mochida and Lun (2008), and Stathopoulos (2006). Although laser Doppler anemometry, particle image velocimetry, and large-eddy simulation (LES) are inherently more accurate techniques, Blocken et al. (2016) recommended faster and less expensive techniques for pedestrian-level wind (PLW) studies, such as hot-wire anemometry, Irwin probes, or steady Reynoldsaveraged Navier-Stokes computational fluid dynamics (RANS CFD) simulations. The reason is that their lower accuracy at lower amplification factors does not necessarily compromise the accuracy of PLW comfort assessment, because the higher amplification factors provide the largest contribution to the discomfort exceedance probability in the comfort criterion. Wind-Driven Rain on Buildings Wind-driven rain (WDR), also called driving rain, is one of the most important moisture sources for building facades. It is an essential boundary condition for the analysis of the hygrothermal behavior and durability of historical and contemporary building facade components (Blocken and Carmeliet 2004; Dalgliesh and Surry 2003; Masters et al. 2008; Sanders 1996; Tang et al. 2004). Wind-driven rain can be assessed by full-scale measurements, wind tunnel measurements, semiempirical formulas, or numerical simulation with CFD. The experimental methods consist of measuring WDR with WDR gages. However, for practical purposes, measurements are generally time consuming, expensive, and often impractical. Blocken and Carmeliet (2006) and Högberg et al. (1999) found that WDR measurements are very prone to error. In addition, measurements made on facades of a particular building at a particular site have limited applicability to facades of other buildings at other sites. This awareness has led researchers to develop calculation models, which have been progressively improved throughout the years. Today, the most advanced and most frequently used models are the semiempirical model in ISO Standard 15927-3 (ISO model), the semiempirical model by Straube (1998) and This file is licensed to John Murray ([email protected]). Publication Date: 6/1/2017 24.12 2017 ASHRAE Handbook—Fundamentals (SI) Straube and Burnett (2000) (SB model), and the CFD model by Choi (1991, 1993, 1994) extended into the time domain by Blocken and Carmeliet (2002). State-of-the-art reviews on the assessment of WDR on building facades were provided by ASCE (2014) and Blocken and Carmeliet (2004, 2010). 7. PHYSICAL AND COMPUTATIONAL MODELING For many routine design applications, flow patterns and wind pressures can be estimated using the data and equations presented in the previous sections. Exhaust dilution for simple building geometries in homogeneous terrain environments (e.g., no larger buildings or terrain features nearby) can be estimated using the data and equations presented in the previous sections and in Chapter 45 of the 2015 ASHRAE Handbook—HVAC Applications. However, in critical applications, such as where health and safety are of concern, more accurate estimates may be required. Licensed for single user. © 2017 ASHRAE, Inc. Physical Modeling Measurements on small-scale models in wind tunnels or water channels can provide information for design before construction. These measurements can also be used as an economical method of performance evaluation for existing facilities. Full-scale testing is not generally useful in the initial design phase because of the time and expense required to obtain meaningful information, but it is useful for verifying data derived from physical modeling and for planning remedial changes to improve existing facilities (Cochran 2006). Detailed accounts of physical modeling, field measurements and applications, and engineering problems resulting from atmospheric flow around buildings are available in international journals, proceedings of conferences, and research reports on wind engineering (see the Bibliography). The wind tunnel is the main tool used to assess and understand airflow around buildings. Water channels or tanks can also be used, but are more difficult to implement and give only qualitative results for some cases. Models of buildings, complexes, and the local surrounding topography are constructed and tested in a simulated turbulent atmospheric boundary layer. Airflow, wind pressures, snow loads, structural response, or pollutant concentrations can then be measured directly by properly scaling wind, building geometry, and exhaust flow characteristics. Wind tunnel studies of natural ventilation are particularly suitable for buildings with large openings that provide a strong coupling between outdoor wind flow and indoor airflow (Karava et al. 2011; Kato et al. 1992). Dalgliesh (1975) and Petersen (1987a) found generally good agreement between the results of wind tunnel simulations and corresponding full-scale data. Cochran (1992) and Cochran and Cermak (1992) found good agreement between model and full-scale measurements of low-rise architectural aerodynamics and cladding pressures, respectively. Stathopoulos et al. (1999, 2002, 2004) obtained good agreement between model and full-scale measurements of the dispersion of gaseous pollutants from rooftop stacks on two different buildings in an urban environment. Similarity Requirements Physical modeling is most appropriate for applications involving small-scale atmospheric motions, such as recirculation of exhaust downwind of a laboratory, wind loads on structures, wind speeds around building clusters, snow loads on roofs, and airflow over hills or other terrain features. Winds associated with tornadoes, thunderstorms, and large-scale atmospheric motion cannot currently be physically modeled accurately, although the physical modeling of tornadoes and thunderstorm downbursts is a current topic of significant research. Snyder (1981) gives guidelines for fluid modeling of atmospheric diffusion. This report contains explicit directions and should be used whenever designing wind tunnel studies to assess concentration levels of air pollutants. ASCE Standard 7, ASCE Manual of Practice 67 (ASCE 1999), and AWES Quality Assurance Manual (AWES 2001) also provide guidance when wind tunnels are used for evaluating wind effects on structures. A complete and exact simulation of airflow over buildings and the resulting concentration or pressure distributions cannot be achieved in a physical model. However, this is not a serious limitation. Cermak (1971, 1975, 1976a, 1976b), Petersen (1987a, 1987b), and Snyder (1981) found that transport and dispersion of laboratory exhaust can be modeled accurately if the following criteria are met in the model and full scale: 1. Match exhaust velocity to wind speed ratios, Ve /UH. 2. Match exhaust to ambient air density ratios, e /a. 3. Match exhaust Froude numbers. Fr 2 = a V e2 /[(e – a)gd], where d is effective exhaust stack diameter. 4. Ensure fully turbulent stack gas flow by ensuring stack flow Reynolds number (Res = Ve d/) is greater than 2000 [where is the kinematic viscosity of ambient (outdoor) air], or by placing an obstruction inside the stack to enhance turbulence. 5. Ensure fully turbulent wind flow. 6. Scale all dimensions and roughness by a common factor. 7. Match atmospheric stability by the bulk Richardson number (Cermak 1975). For most applications related to airflow around buildings, neutral stratification is assumed, and no Richardson number matching is required. 8. Match mean velocity and turbulence distributions in the wind. 9. Ensure building wind Reynolds number (Reb = UH R/) is greater than 11 000 for sharp-edged structures, or greater than 90 000 for round-edged structures. 10. Ensure less than 5% blockage of wind tunnel cross section. For wind speeds, flow patterns, or pressure distributions around buildings, only conditions 5 to 10 are necessary. Usually, each wind tunnel study requires a detailed assessment to determine the appropriate parameters to match in the model and full scale. In wind tunnel simulations of exhaust gas recirculation, buoyancy of the exhaust gas (condition 3) is often not modeled. This allows using a high wind tunnel speed or a smaller model to achieve high enough Reynolds numbers (conditions 4, 5, and 9). Neglecting buoyancy is justified if density of building exhaust air is within 10% of the ambient (outdoor) air. Also, critical minimum dilution Dcrit occurs generally at wind speeds high enough to produce a wellmixed, neutrally stable atmosphere, allowing stability matching (condition 7) to be neglected (see Chapter 45 of the 2015 ASHRAE Handbook—HVAC Applications for discussion of Dcrit). However, in some cases and depending on emission sources, calm conditions may produce critical dilution. Nevertheless, omission of conditions 3 and 7 simplifies the test procedure considerably, reducing both testing time and cost. Buoyancy should be properly simulated for high-temperature exhausts such as boilers and diesel generators. Equality of model and prototype Froude numbers (condition 3) requires tunnel speeds of less than 0.5 m/s for testing. However, greater tunnel speeds may be needed to meet the minimum building Reynolds number requirement (condition 4). Wind Simulation Facilities Boundary-layer wind tunnels are required for conducting most wind studies. The wind tunnel test section should be long enough to establish, upwind of the model building, a deep boundary layer that slowly changes with downwind distance. Other important wind tunnel characteristics include width and height of the test section, range of wind speeds, roof adjustability, This file is licensed to John Murray ([email protected]
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