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    http://ibe.sagepub.com

    Indoor and Built Environment

    DOI: 10.1177/1420326X06067336

    2006; 15; 305Indoor and Built Environment Zhiqiang Zhai

    Application of Computational Fluid Dynamics in Building Design: Aspects and Trends

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    Key Words

    Building modelling · Computational fluid dynamics ·

    Building design · Energy efficiency · Building

    systems · Development trend

    Abstract

    Computational fluid dynamics (CFD), as the most

    sophisticated airflow modelling method, can simultan-

    eously predict airflow, heat transfer and contaminant

    transportation in and around buildings. This paper

    introduces the roles of CFD in building design, demon-

    strating its typical application in designing a thermally-

    conformable, healthy and energy-efficient building.

    The paper discusses the primary challenges of using

    CFD in the building modelling and design practice. Fur-thermore, it analyses the developing trends in applying

    CFD to building design, by thoroughly reviewing the lit-

    eratures in all the proceedings of the International Con-

    ference on Building Simulation, one of the most

    influential symposiums in the building simulation field.

    Introduction

    Building, as one of the largest industries, has signific-

    ant impacts on the environment and natural resources. In

    the United States, buildings account for one-third of theprimary energy usage and two-thirds of all the electricity

    consumption [1]. The construction and operation of 

    buildings generate tremendous pollution that directly

    and indirectly cause urban air quality problems and

    climate change. Poor design of buildings and systems not

    only wastes resources and energy and causes adverse

    impacts to the environment, but also creates uncomfort-

    able and unhealthy indoor environments. Reports of 

    symptoms and other health complaints due to poor

    indoor environments have been increasing in the last

    decade. It was estimated that potential annual savingsand productivity gains could be $15 to $40 billion from

    reduced sick building syndrome symptoms, and $20 to

    $200 billion from direct improvements in worker per-

    formance that are unrelated to health [2].

    In the past few years, CFD has been playing an

    increasingly important role in building design, following

    its continuing development for over a quarter of a

    century. The information provided by CFD can be used

    to analyse the impact of building exhausts to the environ-

    Zhiqiang ZhaiDepartment of Civil, Environmental and Architectural EngineeringUniversity of Colorado at Boulder, UCB 428, ECOT 441Boulder, CO 80309-0428, USATel. 303-492-4699, Fax 303-492-7317, E-mail [email protected]

    © 2006 SAGE PublicationsDOI: 10.1177/1420326X06067336Accessible online at http://ibe.sagepub.comFigures 1 to 8 appear in colour online

    Review Paper

    Indoor Built Environ 2006 15;4:305–313 Accepted: November 20, 2005

    Application of ComputationalFluid Dynamics in BuildingDesign: Aspects and TrendsZhiqiang Zhai

    Department of Civil, Environmental and Architectural Engineering, University of Colorado

    at Boulder

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    ment, to predict smoke and fire risks in buildings, to

    quantify indoor environment quality, and to design

    natural ventilation systems, etc. This paper summarises

    the most important aspects in which CFD can assist in

    achieving a comfortable, healthy, and energy-efficient

    building design. The areas range from building site plan-

    ning to individual room layout design, from activeheating, ventilating and air-conditioning (HVAC) system

    design to passive ventilation study, and from regular

    indoor air quality assessment to critical fire smoke and

    contaminant control. By using the work of the author as

    examples, this paper demonstrates the typical CFD appli-

    cation processes and discusses the primary application

    challenges encountered. It further analyses the potential

    trends in applying CFD for building design by reviewing

    all the CFD-related papers in the proceedings of the

    International Conference on Building Simulation for the

    years 1985–2003. This series of conferences is among the

    most important in the building industry with its focus on

    computer simulation of buildings.

    Assessment Index for Building Performance

    CFD, by numerically solving the governing equations

    for fluid flow, provides spatial- and temporal-distributed

    information of airflow, pressure, temperature, turbulence

    intensity, moisture and contaminant concentration. These

    details can be used to evaluate the levels of thermal

    comfort, indoor air quality (IAQ) and building systemenergy efficiency, which are interesting to architects, build-

    ing HVAC designers, building consultants and researchers.

    Air velocity, temperature and humidity ratio are the

    most important parameters for the determination of the

    predicted percentage dissatisfied (PPD) distribution in a

    building. PPD is a major index for building thermal

    comfort judgement. It can be calculated via [3]:

    PPD

    10095Exp(0.03353PMV40.2179PMV2) [%]. (1)

    The PMV (predicted mean vote) in the equation is deter-

    mined by:

    PMV [0.303Exp(0.036M)0.028]L (2)

    where M is the body metabolism (W·m2) and L is the

    thermal load on the body (W·m2). M and L are the func-

    tions of air velocity, temperature, humidity ratio and

    enclosure temperature.

    In addition, CFD results can be used to calculate the

    distribution of percentage dissatisfied (PD) people due to

    draft [4], another major thermal comfort index, through

    the equation:

    PD(34T)(U0.05)0.62(3.140.37U Tu) [%] (3)

    where T is the local air temperature (°C), U is the local

    air speed (m·s1), and Tu is the turbulence intensity (%).

    If the turbulence kinetic energy k (m2·s2) is simulated

    with a turbulence model, the turbulence intensity can be

    estimated as

    Tu100(2k)0.5/U [%]. (4)

    As for the indoor air quality, CFD can directly predict

    the concentration distributions of different contaminants

    in a space with appropriate boundary conditions. These

    concentration distributions can be further used to deter-

    mine the ventilation effectiveness, :

    C

    C

    e

    C

    C

    s

    s (5)

    where Ce, Cs and C are the contaminant concentration

    (ppm) of exhaust air, supply air and room air, respec-

    tively.

    Thermal comfort and indoor air quality status of a

    building are influenced dominantly by installation loca-

    tions, operating conditions and control strategies of the

    HVAC systems used. CFD can examine the effectivenessand efficiency of various HVAC systems by easily chang-

    ing diffuser types and locations, supply air conditions and

    system control schedules. Furthermore, CFD can help

    develop passive heating/cooling/ventilation strategies

    (e.g. natural ventilation) by modelling and optimising

    building site-plans and indoor layouts.

    The following sections demonstrate some typical

    aspects in which CFD can contribute to building and

    system design, by using the projects investigated by the

    author and other collaborators in architecture and engin-

    eering.

    Applications of CFD for Building Design

     Application-1: Site Planning

    Site planning is the first stage of building design. CFD

    can help optimise building sites by predicting the distri-

    butions of air velocity, temperature, moisture, turbulence

    intensity and contaminant concentration around build-

    306 Indoor Built Environ 2006;15:305–313 Zhiqiang Zhai

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    ings. Good site planning can effectively protect building

    groups from adverse impacts of surrounding pollution. It

    can also improve outdoor pedestrian comfort and

    increase energy efficiency of buildings by allowing

    passive HVAC strategies, such as using natural ventila-

    tion for summer and using wind break for winter.

    Figure 1 presents such an example of using CFD for asite planning in Beijing, China. The initial plan intro-

    duces highly imbalanced airflow through the four high-

    rise buildings at the right hand of the figure, which may

    cause the pedestrian discomfort due to the large wind

    speed among some of the high-rise buildings. The non-

    uniform airflow pattern also reduces the chance of using

    natural ventilation for the buildings that confront less

    wind. The new design revised the building shape and ori-

    entation to allow natural wind movement to smoothly

    cross each building so that there is a comfortable outdoor

    environment and the occupants have the opportunity to

    use a natural ventilation strategy. Chen et al. [5] indi-

    cated that natural ventilation can save about 40% of the

    total cooling energy required by buildings in Beijing.

    Applying CFD for building site planning has become

    fairly convenient as most current commercial CFD pro-

    grams can import AutoCAD files of building site models

    into the computational domain of a CFD simulation. The

    major remaining challenge is probably the long comput-

    ing time due to the large number of mesh grids required

    to cover a building site with reasonable resolution. Thecomputing cost may become more significant when

    dynamic wind conditions need to be modelled. Multi-grid

    and locally-refined grid technologies may, to some

    extent, accelerate the simulation; however, substantial

    computing time is still needed even with a multi-

    processor parallel computer.

     Application-2: Natural Ventilation Study

    Natural ventilation is one of the most fundamental

    ways to reduce energy usage in buildings. In principle,

    CFD can simultaneously model indoor and outdoor air-

    flows to achieve an optimal natural ventilation strategy.

    However, because of the scale difference between a

    typical room (1m) and a site plan (100m), a large

    number of numerical grids must be used to meet the res-

    olution requirement. This imposes an undue expense to

    designers by challenging current computer memory and

    speed. Therefore, a practical approach is to decouple the

    outdoor and indoor airflow simulation. Outdoor airflow

    around buildings is first predicted, which provides airflow

    and pressure information at the openings of buildings.

    With these boundary conditions, indoor airflow for each

    space can be simulated independently and natural venti-lation rate can be determined. Designers can then change

    building indoor layouts and window sizes and locations

    to maximise natural ventilation rate.

    The decoupled simulation method is based on the

    assumption that indoor airflow and building openings

    have little impact on outdoor airflow and pressure distri-

    butions; indoor and outdoor flow fields can therefore be

    studied separately. The study [6] verified that room parti-

    tions and windows do not contribute to a major dif-

    ference in outdoor flow patterns and pressure fields. This

    decoupled method logically well matches the generalarchitectural design procedure: from site plan to unit

    design. The decoupled method first studies the outdoor

    airflow around solid building site models during the site

    plan stage when most details about building units are not

    determined yet; then it moves into building interior

    layout and opening design when the site plan is generally

    finalised. As a result, refining the microscopic unit design

    during the second stage of building design does not

    require the recalculation of the macroscopic site plan.

    307Indoor Built Environ 2006;15:305–313CFD in Building Design

    Fig. 1. CFD for site planning.

    (a) Initial plan

    (b) Final plan

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    Hence, the method greatly reduces the unnecessary com-

    puting time for dynamic design modifications, which

    allows designers to more easily refine the site plan and

    apartment and window layouts separately.

    As examples, Figure 2 shows the simulation results of 

    (a) buoyancy-driven natural ventilation in a high-rise

    building with atrium and chimney, and (b) wind-drivennatural ventilation in a typical one-floor apartment.

    These results provide designers with a straightforward

    understanding of the performance of natural ventilation

    design, and thus allow them to refine the plans and reach

    an optimal solution.

    One of the major challenges of using CFD for natural

    ventilation study is the method to extract airflow con-

    ditions at building openings from outdoor simulations

    and specify them for indoor simulations. Because of the

    high sensitivity of CFD results to boundary conditions,

    small changes in airflow conditions at openings may

    result in a significant shift of indoor airflow patterns. In

    addition, simplification methods for indoor heat sources

    (e.g. occupant, equipment, etc.) also challenge indoor

    environment modelling.

     Application-3: HVAC System Design

    CFD is a powerful tool to evaluate indoor air quality

    and thermal comfort provided by diverse HVAC

    systems, leading to an effective and efficient system

    design. It is superior to the conventional design approach

    that typically relies on the use of charts provided by dif-

    fuser manufacturers and jet formulae that were

    developed from laboratory data. The use of such empiri-

    cal data can result in great uncertainties when they are

    applied to large spaces (such as atria, concert halls and

    sports facilities) or applications that are dissimilar from

    those upon which the laboratory data were developed.

    When an innovative HVAC system is used, there are

    inevitably no data or formulae available for the engin-

    eering design.

    Figure 3 illustrates the modelling of an office with new

    displacement ventilation systems. Displacement ventila-

    tion is an advanced indoor ventilation approach. Unlike

    the conventional mixing ventilation, displacement venti-

    lation provides a cleaner indoor environment with less

    energy consumption. A typical displacement ventilation

    system supplies fresh air at or near floor level at a verylow velocity and a temperature slightly below room tem-

    perature. Exhausts are located at or near the ceiling. The

    supply air spreads across the floor and rises as it is heated

    by sources such as people and equipment, removing

    indoor heat and contaminants directly from the occupied

    zone to the upper zone without mixing. Since only the

    occupied zone must be maintained at the room set-point

    temperature while the upper zone may be warmer, the

    supply air flow rate can be significantly reduced due to

    the vertical temperature gradient, resulting in the

    reduced fan energy. The CFD results help to understandthe physics of the displacement ventilation (such as the

    large re-circulation at the lower part of the room). They

    also quantify the vertical temperature stratification that is

    necessary for building energy calculation. Moreover, the

    supply air conditions can be optimised in CFD to reach

    the best comfort for occupants.

    CFD can also be directly used to guide design process

    and optimise ventilation system design. Figures 4 and 5

    demonstrate such an example that uses CFD to design

    308 Indoor Built Environ 2006;15:305–313 Zhiqiang Zhai

    z

    Fig. 2. CFD for natural ventilation study.

    (a) Buoyancy-driven natural ventilation

    (b) Wind-driven natural ventilation

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    HVAC systems for the world’s first large-scale indoor

    auto-racing facility. The facility is primarily a single space

    building with a floor area of over 0.2106 m2 and a

    ceiling height of 46m. It is designed to accommodate up

    to 120,000 spectators – 60,000 in the grandstands and

    60,000 in the infield, as well as a maximum of 45 racing

    cars running simultaneously on the track at an average

    speed of 217km·h1 (135mph). Such a large-scale and

    complicated building with a variety of indoor com-

    ponents strongly challenges the experience and capability

    of ventilation system designers, even with the aid of CFD

    modelling tools. CFD simulation has been used to

    improve the initial HVAC system design, step by step, to

    an optimal design. Figures 4 and 5 compare the air tem-

    perature and lead concentration in the mid-section of the

    facility under the steady design conditions by using dif-ferent HVAC systems. The study concluded that a com-

    bination of an underneath displacement ventilation

    system and a conventional overhead duct system as well

    as a partial air curtain system between the occupied zone

    and the racing zone is the most effective solution for this

    complex to obtain a comfortable and healthy indoor

    environment with less energy consumption [7].

    CFD results are much more informative and accurate

    than those that could be obtained via empirical-formu-

    lae-based hand calculation for HVAC design. However,

    CFD for HVAC system design still presents various chal-

    lenges, especially in the simplification of sophisticated

    building system components, such as diffusers [8], fans,

    evaporators and diverse heat and contaminant sources

    (e.g. moving cars, breathing occupants).

     Application-4: Pollution Dispersion and Control 

    Wide use of CFD has demonstrated its capability in

    modelling the transportation of contaminants, with its

    309Indoor Built Environ 2006;15:305–313CFD in Building Design

    6 2

    8

    53 (2)

    7

    8

    7

    1

    3 (1)

    4

    2.43 m

    4.16 m

    3.65 m

    xz

    Fig. 3. Simulation of displacement ventilation in an office: (a)CFD model; (b) velocity and temperature distribution in themiddle plane of the room; (c) velocity and temperature distributionin the plane across an occupant.

    (a)

    (b)

    (c)

    Fig. 4. Air temperature distribution in the middle section of anindoor auto-racing complex (°C).

    (a) Base case

    (b) Optimal case

    inlet-1, outlet-2, person-3, table-4, window-5, fluorescent lamps-6,cabinet-7, computer-8

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    low costs, high efficiency and flexibility. It is particularly

    useful for predictive studies in extreme conditions, for

    instance, extreme-hot or toxic scenarios, and it can be

    easily employed to investigate the impact of a particular

    flow parameter, such as wind speed or air temperature,on the dispersion of a certain contaminant. Both indoor

    and outdoor contaminant dispersions can be simulated,

    while indoor scenarios are more complicated and haz-

    ardous because people spend over 90% of their time

    indoors and more factors can affect the dispersion of 

    indoor contaminants. The geometry and structure of a

    building, as well as the HVAC system used in the build-

    ing, have a dominant influence on the dispersion of 

    indoor contaminants. Partitions, furniture and passage-

    ways between indoor spaces can also distort the airflow

    and the contaminant distributions. Importance of indoorcontaminant study is also reflected by the fact that the

    indoor pollution is controllable by using good sensor and

    response systems. CFD prediction can be used to locate

    the best sensor positions in a building, to indicate the safe

    paths for evacuating occupants, and to develop the

    effective emergency response strategies to isolate and

    clean the contaminated air.

    Figure 6 presents a realistic office complex as an

    example, in which CFD has been used to predict the dis-

    persion of contaminants from different locations in the

    offices. The study showed that the contaminant disper-

    sion is very fast and strongly depends on the indoor

    airflow pattern. It also indicated that early warning from

    the sensors is possible if they are placed properly. The

    investigation proposed and tested several response strat-

    egies by supplying or exhausting emergency air through

    three ceiling-mounted air devices. It found that the con-

    taminant dispersion can be effectively controlled by

    simply pressurising or vacuuming the indoor spaces.

    Figure 7 illustrates another example of using CFD todesign exhaust hoods for chemical and biological laborato-

    ries. The simulated results showed that, without particular

    design cares, hoods with standard/enhanced ventilation

    rate may still leak toxic materials from operating zone to

    occupied zone due to the local turbulent vortices at hood

    openings. Hence, central air system and hood air system

    should be designed as a comprehensive system.

    In general, the study of indoor contamination is not an

    easy task. Zhai [9] previously discussed the primary

    310 Indoor Built Environ 2006;15:305–313 Zhiqiang Zhai

    Fig. 5. Lead concentration distribution in the middle section of anindoor auto-racing complex (gLead per kgAir).

    EASE 1 EASE 2 EASE 3

    S7

    S3

    S2

    S1

    C1

    S1S8

    Office 2

    C2

    Corridor

    O2

    S6

    S5

    S4

    C3 from

    diffuser S10

    O1

    C1–C3 represent three different types of airborne contaminants from three

    locations – under a desk in office 1 (C1), in the corridor (C2) and from the supply

    air in office 1 (C3). O1 and O2 are two occupants’ nose locations (0.9 m above the

    floor) and S1–S10 are ten different sensor locations to be tested in office 1.

    EASE1–3 stand for three emergency air supply and exhaust (EASE) outlets.

    Fig. 6. Simulation of indoor contaminant dispersion and control: (a)CFD model; (b) concentration contour of C1 at occupant head levelat t5min after contaminant release (without emergency response);(c) concentration contour of C1 at occupant head level at t5minafter contaminant release (with air pressurising for corridor and office2 and vacuuming for office 1 starting from t2min).

    (a)

    (b)

    (c)

    (a) Base case

    (b) Optimal case

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    Trend-1: Integration of CFD with other Building

    Modelling Programs

    Building is such a sophisticated product that building

    components can rarely be designed and evaluated sepa-

    rately. For instance, building energy consumption is

    determined by many factors. Besides the traditional

    factors such as building size and materials and schedules,room air distribution and lighting design may result in

    significant heating/cooling load change. The integration

    of energy simulation and CFD can eliminate many

    assumptions involved in separate applications [17]. Many

    efforts (e.g. [18–22]) have been made to develop an

    integrated building design tool by coupling airflow,

    energy, lighting, acoustics, materials, environment and

    life cycle analysis programs. Such an integrated design

    tool can improve the prediction accuracy of building per-

    formance, as well as providing possibilities of developing

    a general graphic user interface. In order to facilitate the

    data exchange between different simulation engines, a

    new file format – Industry Foundation Classes (IFC) –

    has been developed and used [23–24]). Ultimately, an

    integrated and virtual simulation environment for build-

    ing design is highly anticipated [25].

    Trend-2: Simplification and Intelligence of CFD Tools

    Since the majority of CFD users in the building indus-

    try are building designers and engineers who have

    limited knowledge of fluid flow they would undoubtedly

    welcome an intelligent CFD program. A good graphical

    user interface (GUI) can dramatically reduce the inputeffort and errors, which will attract more designers to

    apply CFD programs for their designs. Simple and intelli-

    gent interfaces [26] can minimise the need to understand

    the underlying flow physics and numerical methods but

    still obtain meaningful results, in which automatic gener-

    ation and adjustment of CFD grids are the first necessi-

    ties.

    In addition, the development of the Internet enables

    remote collaborative partnerships and the imminent

    opportunities for “e-simulation”. The Web facilitates

    new forms of data sharing and distributed engineeringthrough web hosted services, as well as new forms of 

    teaching and training solutions, as presented by Lam

    [27].

    Trend-3: Improvement of CFD Computing Cost 

    Due to demanding numerical iteration and necessary

    spatial resolution requirement, most CFD simulations

    require considerable computing time ranging from hours

    to days. It is unacceptable for most design tasks, which

    need rapid evaluation of alternative plans. Extensive

    studies have been conducted to develop approaches that

    can accelerate CFD computation, such as locally-refined

    grid, multi-grid and adaptive grid techniques [28]. In

    addition, the trade-off between speed and accuracy has

    been noticed since most building designers have lessaccuracy requirements than aerospace engineers, espe-

    cially at the early stage of building design when most

    design details have not been determined and architec-

    tural plans may change rapidly. Hence, various simple

    airflow models, such as coarse grid CFD [29] and zonal

    air-flow models [30], may be more suitable for these pur-

    poses. Reliable and simple turbulence models (e.g. con-

    stant viscosity model) for indoor and outdoor airflows

    should be developed.

    Trend-4: Development of Critical Modelling Methods

    Although most building phenomena can be simulated

    by current CFD programs, advances in building continue

    to impose new challenges to CFD, for example, to model

    innovative diffusers [31], new window designs [32], and

    advanced heating/cooling systems [33]. The problem may

    become more complicated when involved with mass

    transfer, phase changes and multi-phase interactions,

    such as air condensation and combustion. Special

    methods and models need to be developed [34–36].

    Moreover, advanced CFD techniques such as Large

    Eddy Simulation (LES) may be needed to study critical

    problems in buildings which can not be solved withregular CFD approaches. Such examples are (1) natural

    ventilation studies that are heavily dependent on instan-

    taneous airflows [37], and (2) particle transport studies

    and their interactions with human bodies [38].

    Summary

    This paper has introduced the applications of CFD for

    building design. CFD can provide important information

    to assist in the design of energy-efficient, user-comfort-able and environmentally friendly buildings. The paper

    discusses the typical aspects that CFD can contribute to a

    successful building design, along with brief comments on

    the application challenges. By reviewing the papers pre-

    sented on one premier building simulation conference in

    the past 20 years, the paper analyses the potential trends

    of using CFD for building design in the next few years.

    312 Indoor Built Environ 2006;15:305–313 Zhiqiang Zhai

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    References

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