Assessment of pollutant dispersion from rooftop stacks: ASHRAE,ADMS and wind tunnel simulation

The prediction of downwind concentration of effluents from stack located on top of buildings is important. Most current dispersion models assess the pollutant concentration at distances away from the building. It is important to study pollutant dispersion within the recirculation zone of the building, since studies have shown that effluents released from rooftop stacks have a tendency to re-enter the building through intakes located on the roof. These effects get more pronounced with the influence of RoofTop Structures (RTS). This paper presents a comparative study of the Atmospheric Dispersion Modelling System (ADMS), American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE 2003 and 2007 versions) and wind tunnel results. Four different cases involving a low-rise and high-rise building for stack heights (h_s) ranging from 1 m to 7 m, exhaust momentum ratios (M) ranging from 1 to 5 and wind direction (θ) of 0° and 45°, have been studied for neutral atmospheric stability conditions. In this regard the effect of RTS has also been examined by using wind tunnel, ADMS and ASHRAE models. ADMS yields higher dilutions near the stack at θ = 0° and cannot model the effect of RTS. Wind tunnel data compare well with ASHRAE 2003 at M = 5 for the low-rise building, but generally predict higher dilutions for the high-rise building. ASHRAE 2003 predicts lower dilutions than ADMS for the high-rise building, while ASHRAE 2007 yields very low dilutions for all cases, suggesting a need to reassess its suitability for practical design.     

A wind tunnel experiment and CFD analysis on airflow performance of enclosed-arcade markets in Korea

Many traditional markets, which had previously formed along the streets, have been remodeled into arcade-type malls in Korea to reduce the inconveniences due to rain. The markets that used to be exposed to outside air have become a semi-open space. Therefore sufficient natural ventilation has become an important environmental factor to make the marketplace more comfortable and healthy.     

Simulation of honeycomb-screen combinations for turbulence management in a subsonic wind tunnel

Honeycomb and screens, mainly used for turbulence reduction, are the key elements of a subsonic wind tunnel. In this paper, design aspects of these elements are addressed for an open-circuit wind tunnel, installed at Indian Institute of Technology Guwahati (IITG), India. The effectiveness of honeycomb and honeycomb-screen combinations, in reducing swirl and turbulence level in the test section, is studied by simulating the flowfield using commercial computational fluid dynamics (CFD) package ANSYS-CFX. RNG k−ε turbulence model, with scalable wall functions, is used for modeling turbulence. Results of these simulations for turbulence management, using honeycombs of different lengths, cell shapes and screens of different open area ratios, are found to be in good agreement with experimental and theoretical results available in open literature. These simulations have confirmed the methodology to be adopted for design of wind tunnel subsections. Thus validated design parameters have been used for fabricating the honeycomb and screens for the IITG wind tunnel.     

被上游建筑遮挡的下游建筑表面风压CFD模拟的可靠性研究

研究城市通风或建筑风环境的主要目的是探索建筑之间和建筑内外的气流运动及污染物扩散规律,主要手段有风洞实验和计算流体动力学（CFD）模拟,后者相对前者的优点是易操作且成本低,但由于流体运动的高度不确定性,其结果的准确性存疑。在城市中,建筑之间的互相遮挡是主要特征之一,提取其中最基本单元,把两个建筑之间的遮挡问题分为上游建筑有孔（如穿堂风）与无孔两种情况,运用CFD中的雷诺平均模型（RANS）模拟下游建筑表面的风压,并与风洞实验进行严格对比,以讨论CFD方法在这一基本单元问题中的可靠性。网格敏感性分析显示,建筑表面最小网格为建筑高度的2%时,可获得可靠的结果。5种常用RANS模型的结果与风洞实验严格对比表明：RANS模型对模拟上游有孔遮挡的可靠性明显高于模拟无孔遮挡;RANS模型对于建筑上部的风压模拟结果普遍好于建筑下部;其中,SST k-ω模型准确性最高,在上游有孔时,平均误差11%,上游无孔遮挡时,则为16%。     

基于CFD技术的城市风环境设计策略研究——以重庆市永川区凤凰湖城市设计为例

风环境是城市环境的一个重要组成部分。本文以重庆市永川区凤凰湖城市设计为例,建立适用于城市复杂环境条件下的风场分析模型。基于概率阈值法,对场地风速超越行人舒适阈值的概率进行数值模拟。通过模拟结果分析了场地风环境的特征及其对行人舒适度的影响,并提出相应的场地风环境品质改善策略。希望对未来的城市规划与设计有所帮助。     

Wind tunnel tests on the relationship between building density and pedestrian-level wind velocity: Development of guidelines for realizing acceptable wind environment in residential neighborhoods

The main purpose of this study is to reveal the relationship between the building density and the average wind velocity at pedestrian level in residential neighborhoods. This paper firstly presents the results of wind tunnel tests on 22 residential neighborhoods selected from actual Japanese cities. The results show that there is a strong relationship between the gross building coverage ratio and the mean wind velocity ratio. Secondly, the wind environment evaluation for case study areas is performed by using the wind tunnel results and the climatic conditions of several major Japanese cities. The development method of guidelines for realizing acceptable wind environment in residential neighborhoods using the gross building coverage ratio is proposed.     

基于计算流体动力学方法的风荷载对高层钢建筑影响的数值评价

进行了风对联邦航空咨询委员会(CAARC)标准高层建筑影响的综合数值研究。采用计算流体动力学(CFD)技术,如大涡流模型(LES)、RANS方程模型等,用于预测风荷载的作用以及风在建筑物周围流动的规律。本研究的主要目的是探索有效可靠的方法来利用CFD技术评估风对高层建筑的影响。计算结果与大量的试验数据进行了比较(这些试验数据来自7个风洞试验),并对引起数值预测与试验数据误差的原因进行了识别和讨论。通过比较发现,采用LES和SGS模型可以得到满意的作用在高层建筑上的平均或脉动风载预测值;修正的RANS模型在大多数情况下可以得到较好的结果而且具有快速解决问题的优势。此外,表面有起伏的建筑物的空气边界层气流场的典型特征可以用数字表达。分析发现,入射风的速度分布主要影响建筑物的平均压力系数,而入射风的紊流强度分布对风力的变化有显著影响。因此,需要正确模拟入射风风速分布及紊流强度分布,采用CFD方法精确预测风对高层建筑的影响。CFD技术和相关的数字处理方法为设计者提供了一个有效的方法来估计风对高层建筑的影响,并满足详细的风洞试验的要求。     

Verification of the accuracy of CFD simulations in small-scale tunnel and atrium fire configurations

In preparation for the use of computational fluid dynamics (CFD) simulation results as ‘numerical experiments’ in fire research, the agreement with experimental data for two different small-scale set-ups is discussed. The first configuration concerns the position of smoke-free height in case of fire with vertical ventilation in an atrium. The second set-up deals with the critical velocity for smoke backlayering in case of fire in a horizontally ventilated tunnel. An N-percent rule is introduced for the determination of the presence of smoke in the simulation results, based on the local temperature rise. The CFD package FDS is used for the numerical simulations. The paper does not scrutinize the detailed accuracy of the results, as this is hardly possible with any state-of-the-art experimental data at hand. Rather, the global accuracy is discussed with current numerical implementation and models in FDS, considering continuous evolution over different version releases with time. The agreement between the experiments and numerical simulations is very promising. Even when quantitative agreement with experimental data is not perfect, the trends are very well reproduced in the simulations. While much additional work is required, both in CFD as in ‘real’ experiments, the results are encouraging for the potential of state-of-the-art CFD to be used as numerical experiments.     

从风环境试验看建筑群对自身风环境的影响——深圳福田商城建筑风洞风环境试验研究

本文作者从建筑风环境的试验目的和评价标准出发 ,以试验原始数据为基础 ,对自成一个建筑群的深圳福田商城的风洞风环境试验结果进行了分析。     

Numerical modeling of the flow conditions in a closed-circuit low-speed wind tunnel

A methodology for numerically simulating the flow conditions in closed-circuit wind tunnels is developed as a contribution to the general philosophy of incorporating Computational Fluid Dynamics (CFD) in wind tunnel design and testing and to CFD validation studies. The methodology is applied to the full-scale Jules Verne climatic wind tunnel in which experimental data have been obtained. Due to the specific features of this closed-circuit wind tunnel, the conventional CFD modeling approach, in which only the flow in the wind tunnel test section is modeled, is inadequate. To obtain accurate results the entire wind tunnel has to be modeled. In the numerical closed-circuit wind tunnel, the conventional flow inlet and outlet are replaced by a single “fan boundary condition”. Special attention is given to the theoretical background and the practical implementation of this type of boundary condition in the CFD model. The numerical model is validated for the case of an empty wind tunnel and for the case in which a block-type building is placed in the test section. It will be shown that this methodology can generally reproduce the wind tunnel measurements of mean velocities with an error equal to or less than 10% despite the occurrence of multiple flow separations upstream of the test section. This provides perspectives for the future use of this methodology as a tool for wind tunnel design and testing and for CFD validation purposes.     

Pedestrian wind comfort around a large football stadium in an urban environment: CFD simulation, validation and application of the new Dutch wind nuisance standard

Three-dimensional steady Reynolds-averaged Navier–Stokes (RANS) Computational Fluid Dynamics (CFD) simulations are used in combination with the new Dutch wind nuisance standard to assess pedestrian wind comfort around a large football stadium in Amsterdam, before and after the addition of new high-rise buildings. The focus of the study is on the elevated circulation deck and the surrounding streets and squares. CFD validation is performed by comparison of the simulated mean wind speed at the deck with full-scale measurements. The important effect of local ground roughness specification on the simulated wind speed values is indicated. Application of the Dutch wind nuisance standard shows that wind comfort at the elevated circulation deck is only slightly influenced by the new buildings. Wind comfort at the surrounding streets and squares however significantly deteriorates. Finally, the results obtained by the Dutch wind nuisance standard are compared to those obtained by a more simplified procedure for the transformation of wind statistics to the building site, as used in earlier studies. The more sophisticated transformation procedure in the Dutch standard was successfully validated based on full-scale measurements in earlier research. Comparison of the Dutch standard results in this study with those of the simplified procedure shows that the latter provides overestimations by up to 25% for the highest discomfort probabilities. This type of large discrepancies can significantly change the outcome of wind comfort studies.     

CFD study of the thermal environment in an air-conditioned train station building

This study used the computational fluid dynamics (CFD) method to evaluate the indoor thermal environment of an air-conditioned train station building under three types of air-conditioning design schemes. The impacts of air-conditioning design parameters such as supply air temperature, velocity, altitude and angle of incidence were also investigated. The numerical results showed that if the waiting hall and entrance hall of the train station building were connected to each other and served with the cooling air respectively, when the cooling loads in the two halls were fixed and air-conditioning systems were designed properly, altering largely the cooling air supply scheme in the waiting hall while keeping the cooling air supply scheme in the entrance hall unchanged would have significant effects on the air distribution and thermal comfort in the occupied region of the waiting hall but may have some minor effects on those in the occupied region of the entrance hall. The uniformities of velocity and temperature distributions in the occupied region of waiting hall were satisfactory when side supply scheme was applied. Changing supply air temperature, velocity, altitude and angle of incidence would yield great effects on the thermal environment in the train station building. For the stratified air-conditioning design in the train station building, in order to obtain the satisfactory thermal comfort in the occupied region, the mid-height of the building was found to be a good position for the cooling air supply and the supply angle of 0° from the horizontal could be recommendable. The results also indicated that analyzing the effects of air-conditioning design parameters on the building environment with CFD was an effective method to find the way to optimize the air-conditioning design scheme.     

CFD模拟课程在建环专业本科生中的教学实践

随着CFD技术在暖通空调领域应用的普及,在建环专业本科生中开展CFD相关教学有助于提高学生的就业竞争力和科研能力。作者通过自身的教学实践,就CFD教学用软件的选择、教学内容和教学方法的设置及考核方式等问题进行了探讨。CFD模拟教学实践表明,通过选择合适的CFD软件和适当的教学内容,使本科生在较短的时间内掌握该工具解决不太复杂的专业问题是可行的。     

An experimental investigation of the wind environment and air quality within a densely populated urban street canyon

This paper presents an experimental investigation of wind flow characteristics and air quality along a street canyon located within a dense urban area. Four typical models of a highly populated urban area are studied and wind tunnel experiments are carried out over an extended range of the applied wind directions. The building patterns are represented by 1:100 scale models, where wind velocity and tracer gas concentrations are measured along the two sides of the street. The study results provide evidence that building configurations and wind directions are very important factors in determining both wind flow and pollutant dispersion characteristics within urban domains. Also, the results demonstrate that gaps between buildings are a very important factor to be considered by urban planners and designers, because, for a given building height, larger gaps induce more wind in urban canyons, thus improving the ventilation process.     

Full-scale experiment and CFD simulation on smoke movement and smoke control in a metro tunnel with one opening portal

Three full-scale model experiments were conducted in a unidirectional tube, which is a part of a metro tunnel with one end connected to an underground metro station and the other end opened to outside in Chongqing, PR China. Three fire HRRs, 1.35 MW, 3 MW and 3.8 MW were produced by pool fires with different oil pan sizes in the experiments. Temperature distributions under the tunnel ceiling along the longitudinal direction were measured. At the same time, CFD simulations were conducted under the same boundary conditions with the experiments by FDS 5.5. In addition, more FDS simulation cases were conducted after the FDS simulation results agreed with the experimental results. The simulation results show that the smoke temperature and the decay rate of the temperature distribution under the tunnel ceiling along the longitudinal direction increase as HRR increases. The smoke exhausts effectively from the tunnel under mechanical ventilation system, whether the emergency vent is activated as a smoke exhaust or an air supply vent. The operation mode of the mechanical ventilation system depends on the evacuation route.     

数值模拟在重庆大剧院刚性模型风洞试验中的应用

采用数值模拟方法对重庆大剧院在近地风作用下的表面风压进行了计算.通过数值模拟实现测压孔的优化布置.同时利用数值方法辅助模型风洞试验,在边界层风洞中对大剧院的表面风压进行了测定.将计算得到的风压系数值与风洞试验值作了比较,结果表明,数值模拟较好地反映了重庆大剧院表面风压的分布情况,由其得到的风压系数与风洞试验数据有较好的吻合,而数值模拟的结果可以更加直观全面.     

Investigation of plate-type windborne debris. Part I. Experiments in wind tunnel and full scale

Modeling of the trajectory of windborne debris for incorporation in wind hazard risk assessment requires the knowledge of debris aerodynamics. On-going experiments to determine the flight characteristics of various types of debris are being carried out in the Texas Tech University (TTU) wind tunnel. This paper investigates the aerodynamic characteristics of plate-type debris. Useful data are presented in dimensionless form. Empirical expressions for estimating the horizontal flight speed and distance are derived. Results from wind-tunnel experiments are in reasonable agreement with those from full-scale tests. These results can be used to validate numerical calculations of trajectories of plate-type windborne debris.     

CFD boundary conditions for contaminant dispersion,heat transfer and airflow simulations around human occupants in indoor environments

Indoor computational fluid dynamics (CFD) simulations can predict contaminant dispersion around human occupants and provide valuable information in resolving indoor air quality or homeland security problems. The accuracy of CFD simulations strongly depends on the appropriate setting of boundary conditions and numerical simulation parameters. The present study explores influence of the following three key boundary condition settings on the simulation accuracy: (1) contaminant source area size, (2) convective/radiative heat fluxes, and (3) shape/size of human simulators. For each of the boundary conditions, numerical simulations were validated with experimental data obtained in two different environmental chambers. In CFD simulations, a small release area of a contaminant point source causes locally high concentration gradients that require a very fine local grid system. This fine grid system can slow down the simulations substantially. The convergence speed of calculation is greatly increased by the source area enlargement. This method will not influence the simulation accuracy of passive point source within well-predicted airflow field. However, for active point source located within complicated airflow filed, such an enlargement should be carried out cautiously because simulation inaccuracy might be introduced. For setting thermal boundary conditions, convection to radiation heat flux ratio is critical for accurate CFD computations of temperature profiles around human simulators. The recommended convection to radiation (C:R) ratio is 30:70 for human simulators. Finally, simplified human simulators can provide accurate temperature profiles within the whole domain of interest. However, velocity and contaminant concentration simulations require further work in establishing the influence of simplifications on the simulation accuracy in the vicinity of the human simulator.     

风洞试验及其在结构风工程中的应用

通过试验仪器、试验模型的种类及特点对风洞试验作了简要介绍，并论述了各种模型进行风洞试验研究的主要内容，阐明了通过各类模型风洞试验分析得到的结果在设计和研究中的作用。