The Experts below are selected from a list of 10278 Experts worldwide ranked by ideXlab platform
Andy Chan - One of the best experts on this subject based on the ideXlab platform.
-
numerical simulation of atmospheric Pollutant Dispersion in an urban street canyon comparison between rans and les
Journal of Wind Engineering and Industrial Aerodynamics, 2011Co-Authors: Salim Mohamed Salim, Riccardo Buccolieri, Andy Chan, Silvana Di SabatinoAbstract:Prediction accuracy of Pollutant Dispersion within an urban street canyon of width to height ratio W/H=1 is examined using two steady-state Reynolds-averaged Navier–Stokes (RANS) turbulence closure models, the standard k–e and Reynolds Stress Model (RSM), and Large Eddy Simulation (LES) coupled with the advection–diffusion method for species transport. The numerical results, which include the statistical properties of Pollutant Dispersion, e.g. mean concentration distributions, time-evolution and three-dimensional spreads of the Pollutant, are then compared to wind-tunnel (WT) measurements. The accuracy and computational cost of both numerical approaches are evaluated. The time-evolution of the Pollutant concentration (for LES only) and the mean (time-averaged) values are presented. It is observed that amongst the two RANS models, RSM performed better than standard k–e except at the centerline of the canyon walls. However, LES, although computationally more expensive, did better than RANS in predicting the concentration distribution because it was able to capture the unsteady and intermittent fluctuations of the flow field, and hence resolve the transient mixing process within the street canyon.
-
large eddy simulations of wind flow and Pollutant Dispersion in a street canyon
Atmospheric Environment, 2005Co-Authors: Andy Chan, Anton Y T WongAbstract:Abstract The wind flow and Pollutant Dispersion phenomena in urban streets of different aspect ratios (h/w) and relative canyon height ratios (h2/h1) are studied using large-eddy simulations (LES). The concerned large eddies are computed by the filtered Navier–Stokes equations in LES and the unresolved small eddies are modelled using Smagorinsky subgrid scale model. The domain is discretised into uneven staggered grids using marker and cell (MAC) method. The objective of this work is to demonstrate the various flow regimes and their threshold values in urban street canyon using LES for various canyon geometries and Reynolds numbers. All cases are investigated with Reynolds number 400 primarily to obtain information of the three regimes of canyon flow and the Reynolds number is then increased incrementally to 2000 to study the consequent flow fields and Pollutant dilution patterns. In low Reynolds numbers, results presented agree with the generally obtained threshold values for different flow regimes. Increase in Reynolds number has smeared the flow regimes boundary. The ease of Pollutant Dispersion is mainly promoted by better mixings inside the canyon, formation of unstable circulations and higher Reynolds numbers. All results show that the flow regime and pollution pattern demarcations depend on both the varying canyon geometry and Reynolds number.
-
strategic guidelines for street canyon geometry to achieve sustainable street air quality part ii multiple canopies and canyons
Atmospheric Environment, 2003Co-Authors: Andy ChanAbstract:Abstract The flow field and Pollutant Dispersion characteristics in a three-dimensional urban street canyon are investigated for various building array geometries. The street canyon in consideration is located in a multi-canopy building array that is similar to realistic estate situations. The Pollutant Dispersion characteristics are studied for various canopy aspect ratios, namely: the canyon height to width ratio, canyon length to height ratio, canyon breadth ratio and crossroad locations are studied. A three-dimensional field-size canyon has been analysed through numerical simulations using k − e turbulence model. As expected, the wind flow and mode of Pollutant Dispersion is strongly dependent on the various flow geometric configurations and that the results can be different from that of a single canyon system. For example, it is found that the Pollutant retention value is minimum when the canyon height-to-width ratio is approximately 0.8, or that the building height ratio is 0.5. Various rules of thumbs on urban canyon geometry have been established for good Pollutant Dispersion.
-
strategic guidelines for street canyon geometry to achieve sustainable street air quality
Atmospheric Environment, 2001Co-Authors: Andy Chan, Subash C SamadAbstract:This paper is concerned with the motion of air within the urban street canyon and is directed towards a deeper understanding of Pollutant Dispersion with respect to various simple canyon geometries and source positions. Taking into account the present days typical urban configurations, three principal flow regimes “isolated roughness flow”, “skimming flow” and “wake interference flow” (Boundary Layer Climates, 2nd edition, Methuen, London) and their corresponding Pollutant Dispersion characteristics are studied for various canopies aspect ratios, namely relative height (h2/h1), canyon height to width ratio (h/w) and canyon length to height ratio (l/h). A field-size canyon has been analyzed through numerical simulations using the standard k-e turbulence closure model. It is found that the Pollutant transport and diffusion is strongly dependent upon the type of flow regime inside the canyon and exchange between canyon and the above roof air. Some rules of thumbs have been established to get urban canyon geometries for efficient Dispersion of Pollutants.
Leslie K Norford - One of the best experts on this subject based on the ideXlab platform.
-
multilayer urban canopy modelling and mapping for traffic Pollutant Dispersion at high density urban areas
Science of The Total Environment, 2019Co-Authors: Chao Yuan, Jian Hang, Xianxiang Li, Ruiqin Shan, Yangyang Zhang, Leslie K NorfordAbstract:Abstract A semi-empirical multilayer urban canopy model is developed to estimate the vertical Dispersion of traffic emissions in high density urban areas. It is motivated by the heterogeneity of urban morphology in real urban cities and the need of quick urban design and planning. The urban canopy is divided into multiple layers, to include the impact of building height variance on Pollutant Dispersion. The model is derived by mass conservation within each layer through adopting a box model. To validate the model, results in several cases with uniform and non-uniform building height distributions are compared with CFD simulations. The validation study indicates that the assumption of zero Pollutant concentration over the modeled canopy and no horizontal Pollutant transfer has increasingly negligible influence with increasing urban densities. The new multilayer model performs well to model the vertical Pollutant transport, and modelling results can mostly follow the trend of the CFD simulations. The present paper conducts two case studies in metropolitan areas in Singapore and Hong Kong to illustrate how to implement this multilayer urban canopy model in the planning practice. With an in-house GIS team using available data, the multilayer model provides planners a way to understand air Pollutant Dispersion in high-density urban areas.
-
improving air quality in high density cities by understanding the relationship between air Pollutant Dispersion and urban morphologies
Building and Environment, 2014Co-Authors: Chao Yuan, Edward Ng, Leslie K NorfordAbstract:Abstract In high-density megacities, air pollution has a higher impact on public health than cities of lower population density. Apart from higher pollution emissions due to human activities in densely populated street canyons, stagnated air flow due to closely packed tall buildings means lower Dispersion potential. The coupled result leads to frequent reports of high air pollution indexes at street-side stations in Hong Kong. High-density urban morphologies need to be carefully designed to lessen the ill effects of high density urban living. This study addresses the knowledge-gap between planning and design principles and air pollution Dispersion potentials in high density cities. The air ventilation assessment for projects in high-density Hong Kong is advanced to include air Pollutant Dispersion issues. The methods in this study are CFD simulation and parametric study. The SST κ – ω model is adopted after balancing the accuracy and computational cost in the comparative study. Urban-scale parametric studies are conducted to clarify the effects of urban permeability and building geometries on air pollution Dispersion, for both the outdoor pedestrian environment and the indoor environment in the roadside buildings. Given the finite land resources in high-density cities and the numerous planning and design restrictions for development projects, the effectiveness of mitigation strategies is evaluated to optimize the benefits. A real urban case study is finally conducted to demonstrate that the suggested design principles from the parametric study are feasible in the practical high density urban design.
-
Flow and Pollutant Transport in Urban Street Canyons of Different Aspect Ratios with Ground Heating: Large-Eddy Simulation
Boundary-Layer Meteorology, 2012Co-Authors: Rex E. Britter, Leslie K Norford, Tieh-yong Koh, Dara EntekhabiAbstract:A validated large-eddy simulation model was employed to study the effect of the aspect ratio and ground heating on the flow and Pollutant Dispersion in urban street canyons. Three ground-heating intensities (neutral, weak and strong) were imposed in street canyons of aspect ratio 1, 2, and 0.5. The detailed patterns of flow, turbulence, temperature and Pollutant transport were analyzed and compared. Significant changes of flow and scalar patterns were caused by ground heating in the street canyon of aspect ratio 2 and 0.5, while only the street canyon of aspect ratio 0.5 showed a change in flow regime (from wake interference flow to skimming flow). The street canyon of aspect ratio 1 does not show any significant change in the flow field. Ground heating generated strong mixing of heat and Pollutant; the normalized temperature inside street canyons was approximately spatially uniform and somewhat insensitive to the aspect ratio and heating intensity. This study helps elucidate the combined effects of urban geometry and thermal stratification on the urban canyon flow and Pollutant Dispersion.
-
large eddy simulation of flow and Pollutant transport in urban street canyons with ground heating
Boundary-Layer Meteorology, 2010Co-Authors: Xianxiang Li, Leslie K Norford, Rex E. Britter, Dara Entekhabi, Dennis Y C LeungAbstract:Our study employed large-eddy simulation (LES) based on a one-equation subgrid-scale model to investigate the flow field and Pollutant Dispersion characteristics inside urban street canyons. Unstable thermal stratification was produced by heating the ground of the street canyon. Using the Boussinesq approximation, thermal buoyancy forces were taken into account in both the Navier-Stokes equations and the transport equation for subgrid-scale turbulent kinetic energy (TKE). The LESs were validated against experimental data obtained in wind-tunnel studies before the model was applied to study the detailed tur- bulence, temperature, and Pollutant Dispersion characteristics in the street canyon of aspect ratio 1. The effects of different Richardson numbers (Ri) were investigated. The ground heat- ing significantly enhanced mean flow, turbulence, and Pollutant flux inside the street canyon, but weakened the shear at the roof level. The mean flow was observed to be no longer isolated
T Stathopoulos - One of the best experts on this subject based on the ideXlab platform.
-
steady and unsteady rans simulations of Pollutant Dispersion around isolated cubical buildings effect of large scale fluctuations on the concentration field
Journal of Wind Engineering and Industrial Aerodynamics, 2017Co-Authors: Yoshihide Tominaga, T StathopoulosAbstract:The performance of unsteady Reynolds-Averaged Navier–Stokes equations (URANS) for simulations of flow and Dispersion fields around isolated cubical buildings has been examined in this study. URANS results were compared with those obtained from steady-RANS (SRANS) computations and experiments. The comparison determines not only the applicability of URANS simulations, but also the contribution of unsteady large-scale fluctuations to Pollutant Dispersion around buildings. Three different source locations, i.e. upwind, rooftop and downwind releases, were considered for Pollutant Dispersion around the building. It was found that the improvement of the predicted concentration field achieved by URANS largely depends on the source location. Although this improvement was not as significant in the upwind and rooftop release cases, the prediction accuracy achieved by URANS was substantially improved for the downwind release case, for which, the unsteady-RANS simulations yielded larger estimates of the momentum and concentration diffusions behind the building than SRANS did, improving the accuracy of the estimation of the mean concentration.
-
ten questions concerning modeling of near field Pollutant Dispersion in the built environment
Building and Environment, 2016Co-Authors: Yoshihide Tominaga, T StathopoulosAbstract:Outdoor air pollution is a major current environmental problem. The precise prediction of Pollutant concentration distributions in the built environment is necessary for building design and urban environmental assessment. Near-field Pollutant Dispersion, involving the interaction of a plume and the flow field perturbed by building obstacles, is an element of outdoor air pollution that is particularly complex to predict. Modeling methodologies have been discussed in a wide range of research fields for many years. The modeling approaches are categorized into field measurements, laboratory (wind and water tunnel) experiments, (semi-) empirical models, and computational fluid dynamics (CFD) models. Each of these approaches has advantages and disadvantages. It is therefore important to use due consideration for the underlying theory and limitations when applying these modeling approaches. This paper considers some of the most common questions confronting researchers and practitioners in the modeling of near-field Pollutant Dispersion in the built environment.
-
near field Pollutant Dispersion in an actual urban area analysis of the mass transport mechanism by high resolution large eddy simulations
Computers & Fluids, 2015Co-Authors: Pierre P Gousseau, T Stathopoulos, Bje Bert Blocken, G J F Van HeijstAbstract:Large-Eddy Simulation of near-field Pollutant Dispersion from stacks on the roof of a low-rise building in downtown Montreal is performed. Two wind directions are considered, with different wind-flow patterns and plume behavior. The computed mean concentration field is analyzed by means of the convective and turbulent (including subgrid-scale) mass fluxes. This decomposition provides insight into the Dispersion process and allows an evaluation of common turbulent transport models used with the Reynolds-Averaged Navier–Stokes approach, such as the standard gradient-diffusion hypothesis. Despite the specific character of the flow and Dispersion patterns due to the complex geometry of the urban area under study, some similarities are found with the generic case of Dispersion around an isolated simple building. Moreover, the analysis of Dispersion in downtown Montreal is facilitated by the physical insight gained by the study of the generic case. In this sense, the present study supports the use of generic, simplified cases to investigate and understand environmental processes as they occur in real and more complex situations. Reciprocally, the results of this applied study show the influence on the Dispersion process of the rooftop structures and of the orientation of the emitting building with respect to the incoming wind flow, providing directions for further research on generic cases.
-
cfd simulation of near field Pollutant Dispersion in the urban environment a review of current modeling techniques
Atmospheric Environment, 2013Co-Authors: Yoshihide Tominaga, T StathopoulosAbstract:Near-field Pollutant Dispersion in the urban environment involves the interaction of a plume and the flow field perturbed by building obstacles. In the past two decades, micro-scale Computational Fluid Dynamics (CFD) simulation of Pollutant Dispersion around buildings and in urban areas has been widely used, sometimes in lieu of wind tunnel testing. This paper reviews current modeling techniques in CFD simulation of near-field Pollutant Dispersion in urban environments and discusses the findings to give insight into future applications. Key features of near-field Pollutant Dispersion around buildings from previous studies, i.e., three-dimensionality of mean flow, unsteadiness of large-scale flow structure, and anisotropy of turbulent scalar fluxes, are identified and discussed. This review highlights that it is important to choose appropriate numerical models and boundary conditions by understanding their inherent strengths and limitations. Furthermore, the importance of model evaluation was emphasized. Because Pollutant concentrations around buildings can vary by orders of magnitudes in time and space, the model evaluation should be performed carefully, while paying attention to their uncertainty. Although CFD has significant potential, it is important to understand the underlying theory and limitations of a model in order to appropriately investigate the Dispersion phenomena in question.
-
cfd simulation of near field Pollutant Dispersion on a high resolution grid a case study by les and rans for a building group in downtown montreal
Atmospheric Environment, 2011Co-Authors: Pierre P Gousseau, T Stathopoulos, Bert Blocken, G J F Van HeijstAbstract:Turbulence modeling and validation by experiments are key issues in the simulation of micro-scale atmospheric Dispersion. This study evaluates the performance of two different modeling approaches (RANS standard k-e and LES) applied to Pollutant Dispersion in an actual urban environment: downtown Montreal. The focus of the study is on near-field Dispersion, i.e. both on the prediction of Pollutant concentrations in the surrounding streets (for pedestrian outdoor air quality) and on building surfaces (for ventilation system inlets and indoor air quality). The high-resolution CFD simulations are performed for neutral atmospheric conditions and are validated by detailed wind-tunnel experiments. A suitable resolution of the computational grid is determined by grid-sensitivity analysis. It is shown that the performance of the standard k-e model strongly depends on the turbulent Schmidt number, whose optimum value is case-dependent and a priori unknown. In contrast, LES with the dynamic subgrid-scale model shows a better performance without requiring any parameter input to solve the Dispersion equation.
Dennis Y C Leung - One of the best experts on this subject based on the ideXlab platform.
-
large eddy simulation of flow and Pollutant transport in urban street canyons with ground heating
Boundary-Layer Meteorology, 2010Co-Authors: Xianxiang Li, Leslie K Norford, Rex E. Britter, Dara Entekhabi, Dennis Y C LeungAbstract:Our study employed large-eddy simulation (LES) based on a one-equation subgrid-scale model to investigate the flow field and Pollutant Dispersion characteristics inside urban street canyons. Unstable thermal stratification was produced by heating the ground of the street canyon. Using the Boussinesq approximation, thermal buoyancy forces were taken into account in both the Navier-Stokes equations and the transport equation for subgrid-scale turbulent kinetic energy (TKE). The LESs were validated against experimental data obtained in wind-tunnel studies before the model was applied to study the detailed tur- bulence, temperature, and Pollutant Dispersion characteristics in the street canyon of aspect ratio 1. The effects of different Richardson numbers (Ri) were investigated. The ground heat- ing significantly enhanced mean flow, turbulence, and Pollutant flux inside the street canyon, but weakened the shear at the roof level. The mean flow was observed to be no longer isolated
-
large eddy simulation of flow and Pollutant Dispersion in high aspect ratio urban street canyons with wall model
Boundary-Layer Meteorology, 2008Co-Authors: Chunho Liu, Dennis Y C LeungAbstract:A large-eddy simulation (LES) with a one-equation subgrid-scale (SGS) model was developed to investigate the flow field and Pollutant Dispersion inside street canyons of high aspect ratio (AR). A 1/7th power-law wall model was implemented near rigid walls to mitigate the demanding near-wall resolution requirements in LES. This LES model had been extensively validated against experimental results for street canyons of AR = 1 and 2 before it was applied to the cases of AR = 3 and 5. A ground-level passive Pollutant line source, located in the middle of the street, was used to simulate vehicular emissions. Three and five vertically aligned primary recirculations were developed in the street canyons of AR 3 and 5, respectively. The ground-level mean wind speed was less than 0.5% of the free stream value, which makes it difficult for the Pollutant to be transported upward for removal. High Pollutant concentration and variance were found near the buildings where the air flow is upwards. It was found that the velocity fluctuation, Pollutant concentration and variance were all closely related to the interactions between the primary recirculations and/or the free surface layer. Several quantities, which are non-linear functions of AR, were introduced to quantify the air quality in street canyons of different configurations.
-
Pollutant Dispersion in urban street canopies
Atmospheric Environment, 2001Co-Authors: Jiyang Xia, Dennis Y C LeungAbstract:Abstract The effects of building configurations on Pollutant Dispersion around street canopies were studied numerically. The Dispersion of Pollutants emitted from ground sources was simulated by continuously discharging large number of particles into the computation domain. The mean wind velocities at each time-step were firstly computed by solving the time-dependent incompressible Navier–Stokes equations, while the fluctuated velocities were determined using a statistical procedure. The trajectories of the discharged particles were obtained from a Lagrangian particle model. Three categories of numerical simulation were conducted to study the effect of different canopy geometries on the Pollutant Dispersion. The computed wind field data were consistent with the wind field characteristics described in the previous wind tunnel studies. A counter-clockwise vortex was found resulting in high Pollutant concentration at the windward side of the downstream building of the street canopy and low Pollutant concentration at the leeward side of the upstream building. The increase in height of the urban roughness buildings would facilitate the Pollutant Dispersion in urban street canopy under certain building configurations. Two or more vortices stacked vertically in a street canopy were found when height of the upstream and downstream buildings of a street canopy was increased, preventing Pollutants from escaping out of the canopy.
Tingzhen Ming - One of the best experts on this subject based on the ideXlab platform.
-
review on Pollutant Dispersion in urban areas part b local mitigation strategies optimization framework and evaluation theory
Building and Environment, 2021Co-Authors: Tingzhen Ming, Renaud De Richter, Hao Zhang, Chihyung Wen, Tianhao Shi, Xu Dong, Chong PengAbstract:Abstract Outdoor air pollution is a significant global issue because it poses a major long-term health risk. A growing number of studies are conducted to develop local mitigation strategies for improving air quality. This review paper critically evaluates the available literature to provide a better understanding of potential local mitigation strategies and ascertain the methods for reducing local air pollution exposure. For these purposes, the first part of the review is categorized into three groups: (i) improving urban ventilation and turbulence level for Pollutant Dispersion. (ii) controlling source-receptor pathways by constructing barriers. (iii) capturing and mitigating air pollution by introducing Pollutant sinks. Subsequently, a series of studies on optimization frameworks are summarized. It is found that surrogate model-based optimization frameworks efficiently handle multi-objective optimizations at a low computational cost. Finally, this review examines publications on the evaluation theory for Pollutant Dispersion to determine feasible methods for the removal of Pollutants from urban areas. This study is useful for urban planners and architects responsible for decision-making.
-
review on Pollutant Dispersion in urban areas part a effects of mechanical factors and urban morphology
Building and Environment, 2021Co-Authors: Tingzhen Ming, Chong Peng, Shurong Liu, Renaud De Richter, Hao Zhang, Chihyung WenAbstract:Abstract Cities are facing significant challenges due to low outdoor air quality. Hence, a growing number of studies have been conducted to develop strategies for improving air quality. This paper provides a comprehensive and systematic review of the mechanisms and effects of various mechanical measures and urban morphology on Pollutant Dispersion in an urban context. Moreover, a detailed quantification of the reduction potential of the mechanical measures and urban morphology is provided, followed by the identification of the critical urban factors for the practical urban optimizing strategies. Additionally, the two fundamental processes (mean flow and turbulence) affecting the Pollutant Dispersion are clarified to understand the dilution mechanism of Pollutants. For these purposes, the reviewed papers are categorized into two groups: (i) Utilizing mechanical measures. (ii) Designing appropriate urban morphology. Generally, this study is useful for urban planners and architects who are responsible for decision-making.
-
assessment of Pollutant Dispersion in urban street canyons based on field synergy theory
Atmospheric Pollution Research, 2021Co-Authors: Tingzhen Ming, Shurong Liu, Tianhao Shi, Huina Han, Chong PengAbstract:Abstract Vehicle emissions are an important factor in the deterioration of air quality in urban street canyons. To improve the air quality in urban street canyons, it is necessary to have a deep understanding of the mechanism of Pollutant Dispersion within the urban boundary layer. The transport process of Pollutants in street canyons is essentially a convective mass transfer process. In this paper, a computational fluid dynamics (CFD) simulation is conducted to investigate the effects of wind speed, the setting of the viaduct, and the roof shapes of the street canyons on Pollutant Dispersion. According to the field synergy theory, Sherwood number and field synergy number are used to evaluate the three factors on the diffusion of Pollutants in street canyons quantitatively. The results show that the Sherwood number decreases with increased viaduct construction height and decreased wind speed, which is compatible with the observed growth of Pollutant fraction in the street canyon. When the pollution source on the viaduct is considered, a less substantial influence of construction height on Pollutant diffusion is observed. Among the four types of roofs, upward pitched roofs resulted in the highest pollution, with a pollution concentration degree of 1.6–2.3 times that of flat roofs, followed by double pitched roofs, with a pollution concentration degree of 1.2–2 times that of a flat roof. By the method of analogy analysis and numerical simulation, the field synergy theory has been extended from applications in heat transfer in small-scale confined spaces to those in urban street canyon pollution simulation in large-scale open spaces.
-
Impacts of Traffic Tidal Flow on Pollutant Dispersion in a Non-Uniform Urban Street Canyon
Atmosphere, 2018Co-Authors: Tingzhen Ming, Weijie Fang, Renaud Kiesgen De Richter, Chong Peng, Mohammad Hossein AhmadiAbstract:A three-dimensional geometrical model was established based on a section of street canyons in the 2nd Ring Road of Wuhan, China, and a mathematical model describing the fluid flow and Pollutant Dispersion characteristics in the street canyon was developed. The effect of traffic tidal flow was investigated based on the measurement results of the passing vehicles as the pollution source of the CFD method and on the spatial distribution of Pollutants under various ambient crosswinds. Numerical investigation results indicated that: (i) in this three-dimensional asymmetrical shallow street canyon, if the pollution source followed a non-uniform distribution due to the traffic tidal flow and the wind flow was perpendicular to the street, a leeward side source intensity stronger than the windward side intensity would cause an expansion of the pollution space even if the total source in the street is equal. When the ambient wind speed is 3 m/s, the Pollutant source intensity near the leeward side that is stronger than that near the windward side (R = 2, R = 3, and R = 5) leads to an increased average concentration of CO at pedestrian breathing height by 26%, 37%, and 41%, respectively. (R is the ratio parameter of the left side pollution source and the right side pollution source); (ii) However, this feature will become less significant with increasing wind speeds and changes of wind direction; (iii) the pollution source intensity exerted a decisive influence on the Pollutant level in the street canyon. With the decrease of the pollution source intensity, the Pollutant concentration decreased proportionally.