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 simulations of near-field dispersion in the urban environment are reviewed.•Key features of near-field pollutant dispersion are identified and discussed.•To understand inherent strengths and limitations of numerical models is important.•Careful model evaluation while paying attention to their uncertainty is necessary.
CFD modeling using RANS and LES of pollutant dispersion in a three-dimensional street canyon is investigated by comparison with measurements. The purpose of this study is to confirm the accuracy of ...LES in modeling plume dispersion in a simple street canyon model and to clarify the mechanism of the discrepancy in relation to RANS computation. Simple LES modeling is shown by comparison with wind tunnel experiments to give better results than conventional RANS computation (RNG) modeling of the distribution of mean concentration. The horizontal diffusion of concentration is well reproduced by LES, mainly due to the reproduction of unsteady concentration fluctuations in the street canyon.
Several studies have been carried out on CFD prediction based on a RANS (Reynolds Averaged Navier–Stokes equations) model for dispersion around buildings, but it was reported that a RANS computation ...often provides extremely high concentration, which are not observed in usual measurements. These results suggest that transient simulations such as the large-eddy simulation (LES) might be required to achieve more accurate results. Nevertheless, very few studies have evaluated the basic performance of LES in modeling the dispersion field for a simple configuration in comparison with the RANS model. Therefore, relative performance of these simulation methods for dispersion problem around buildings should be clarified in order to make it possible to choose a suitable numerical method for its purpose. The purpose of this study is to confirm the accuracy of LES in modeling plume dispersion near and around a simple building model and to clarify the mechanism for the discrepancy in relation to the RANS computation. Simple LES modeling gives better results than RNG modeling of the distribution of concentration, although the difference for mean velocity is not so large. The horizontal diffusion of concentration is well reproduced by LES. This tendency is closely related to the reproduction of unsteady periodic fluctuation around cubical forms in LES.
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.
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•Performance of URANS for the flow and dispersion around buildings was examined.•Modified ε-equation was introduced into URANS with the RNG k-ε model and validated.•Contribution of unsteady organized fluctuations to pollutant dispersion was clarified.•Prediction accuracy of URANS was largely improved for the downwind release case.
Accurate Computational Fluid Dynamics (CFD) simulations of atmospheric boundary layer (ABL) flow are essential for a wide variety of atmospheric studies including pollutant dispersion and deposition. ...The accuracy of such simulations can be seriously compromised when wall-function roughness modifications based on experimental data for sand-grain roughened pipes and channels are applied at the bottom of the computational domain. This type of roughness modification is currently present in many CFD codes including Fluent 6.2 and Ansys CFX 10.0, previously called CFX-5. The problems typically manifest themselves as unintended streamwise gradients in the vertical mean wind speed and turbulence profiles as they travel through the computational domain. These gradients can be held responsible—at least partly—for the discrepancies that are sometimes found between seemingly identical CFD simulations performed with different CFD codes and between CFD simulations and measurements. This paper discusses the problem by focusing on the simulation of a neutrally stratified, fully developed, horizontally homogeneous ABL over uniformly rough, flat terrain. The problem and its negative consequences are discussed and suggestions to improve the CFD simulations are made.
The paper reviews the evolution of computational wind engineering from environmental and structural perspectives, since the inaugural conference of computational wind engineering held in Tokyo 30 ...years ago (CWE 92). The progress in computational methodologies and important aspects for accurate analysis are discussed. As a groundwork for the application of computational fluid dynamics (CFD) to various environmental issues, the importance of accurate modeling of atmospheric boundary layer, urban boundary layer, and urban canopy layer is pronounced. Environmental applications refer to urban micro-climate, pedestrian level wind, near-field pollutant dispersion, natural and urban ventilation, urban wind energy and snow/sand erosion and accumulation. Structural applications refer to wind loading on low- and high-rise buildings, including wind directionality. The most seminal contributions are examined, and their results are presented. It becomes clear that the engineering community has gained more benefits from environmental than structural computational wind engineering applications, mainly due to the usually less demanding computational needs for the former. Future challenges in CFD applications are thoroughly discussed and the need to bridge the gap between environmental and structural applications is highlighted.
•Computational wind engineering progress since the seminal conference of CWE 1992, in environmental and structural applications.•Turbulence models, verification, validation, and best practice guidelines in computational wind engineering.•Progress is environmental research field of pedestrian level wind, near-field pollutant dispersion, natural ventilation, urban ventilation, urban wind energy, snow/sand erosion and accumulation.•Progress in structural research field for design of low- and high-rise buildings.•Future perspective of CFD application to environmental and structural problems.
Increasing urbanization and population growth have brought attention to urban microclimates in recent years. The study on urban microclimate and its impact on the built environment is gaining ...momentum. A growing number of researchers have examined the relationship between human activity and the immediate surroundings to reduce adverse impacts on the environment and climate. This paper presents the latest progress in urban microclimate research on urban wind and thermal environment, covering traditional methods, including field measurements, wind tunnel modeling, and CFD simulations, as well as emerging methods, such as artificial intelligence or data-driven models. Among the publications reviewed, the topics include isothermal scenarios that neglected thermal aspects (e.g., urban wind energy, wind comfort), as well as thermal scenarios (e.g., urban heat islands and outdoor thermal comfort). In the review, it was found that CFD has been widely applied due to its well-developed nature. In addition to field measurements, new techniques (such as satellite and thermal imaging) provide valuable validation data for CFD and training data for artificial intelligence applications. In isothermal scenarios, wind tunnel modeling has been successfully applied. However, thermal scenarios present significant challenges. In addition, urban data-driven models have emerged with promising results, but systematic investigations have been limited. In this paper, we identify future research needs for urban microclimates based on an overview of recent progress.
•563 publications on urban microclimate from 2010 to 2020 were reviewed.•Research on urban microclimate increased fivefold over the past decade.•Numerical simulation is the most common approach, and data-driven models become promising.•Past findings, existing challenges, and future research needs are presented.
The paper reviews the wind loading of buildings from a code perspective. The Canadian wind load provisions for buildings, due primarily to the ingenuity of Alan G. Davenport, have gained an ...international reputation after recognition for their innovative and pioneering character by researchers and practitioners across the globe. In this regard, these provisions have been influential in the development and evolution of various national and international wind load standards, including the ASCE 7, the ISO wind load standard, the Eurocode, the China standard for wind loads on roof structures and others. The paper provides first an overview of ASCE 7 (USA), NBCC (Canada) and GB 50009 (China) to gain some insight into the extent to which the external pressures, internal pressures, exposure issues and topography - among others - are currently being addressed through these provisions. The current similarities and differences among wind load provisions for buildings are outlined and attempts are made to resolve some of the apparent discrepancies leading to possibly non-conservative results. Ultimately, innovative codification approaches and trends currently under discussion, development and consideration are also presented.
•Design wind loading procedures for buildings and evolution of provisions of wind codes and standards.•Most notable recent developments on wind design procedures for buildings and related factors.•Research efforts needed and discussion to further strengthen the credibility of current wind codes and standards.
Modeling of turbulent scalar flux in CFD (Computational Fluid Dynamics) for near-field dispersion around buildings is examined by investigating both velocity and concentration fields obtained by two ...modeling approaches, i.e., RANS RNG k-ε and LES. A building array model with a point source located in between the central buildings is adopted as a target configuration. First, the prediction accuracy of LES is confirmed by comparing with the RNGcomputation and the results from an experiment conducted by the authors. LES gives better results than RNG, in terms of time-averaged velocity and concentration distribution in comparison with the wind tunnel experimental results. Next, the eddy viscosity and the eddy diffusivity are determined by LES data using a least square approach as suggested in the dynamic sub-grid scale model. Large differences can be observed between the distributions of the estimated eddy viscosity by using LES data and the eddy diffusivity obtained by RNG, since the eddy diffusivity is not always proportional to the eddy viscosity.
► CFD Modeling of turbulent scalar flux for dispersion around buildings is examined. ► Prediction accuracy of LES is confirmed by comparing with RANS and experiment. ► Eddy viscosity, eddy diffusivity and Sct are estimated by LES database.
The paper presents an innovative methodology for the simulation of incoming wind conditions in computational domains that use large eddy simulation (LES) for the evaluation of wind loads on low-rise ...buildings. Simulating the atmospheric boundary layer has proven to be a challenging process for computational wind engineering, especially to correctly introduce the fluctuations in the high-frequency spectral domain. Experiments in the Concordia University Building Aerodynamics Laboratory were carried out to estimate the evolution of turbulence features in the along-wind direction of a generic profile. Virtual probes were placed inside the computational domain to match the location of those in the wind tunnel and compare mean speed values, turbulence intensity, integral length scales, and spectral content. Synthetic methods were applied for the incoming wind velocity to estimate their capacity to capture the evolution of these turbulence features in the streamwise direction. The inability of these synthetic methods to represent the spectral domain, particularly for heights of interest for low-rise buildings, was examined, and the so-called dynamic terrain method in which velocity time histories are extracted from wind tunnel measurements and reconstructed to fit certain statistical parameters was implemented. The proposed method represents the frequency domain’s spectral content with good agreement with the theoretical von Karman spectrum and the experimental results. Advantages of the method include time efficiency and relative simplicity, which makes it attractive to the practitioners for the design of a neutral atmospheric boundary layer respecting the turbulent wind characteristics.
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