Near-field Pollutant Dispersion Research Articles

High-Resolution Simulation of the Near-Field Pollutant Dispersion in a Nuclear Power Plant Community with High-Performance Computing

This study aims to employ numerical simulations to understand the dynamics of wind fields and air pollutant dispersion in the proximity of a nuclear plant, situated within a specified urban environment. By leveraging computational fluid dynamics (CFD) combined with geographical information system (GIS) data, the research comprehensively models atmospheric interactions in terms of wind flow patterns, building-induced pressure variances, and pollutant trajectories. The computational domain extends over an area of 8.8km×8.4km\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$8.8\\,\ extrm{km} \ imes 8.4\\,\ extrm{km}$$\\end{document}, vertically stretching to 0.5 km. The wind and pollutant distribution equations are discretized using the finite volume method, providing detailed insights into fluid interactions with urban topographies. Key findings highlight the profound influences of terrain, urban structures, and wind flow behavior on the dispersion of radioactive aerosols, shedding light on potential risks and safety protocols for nuclear plant environments.

Predicting Concentration Fluctuations of Locally Emitted Air Pollutants in Urban-like Geometry Using Deep Learning

The accurate quantification of concentration fluctuations is crucial when evaluating the exposure to toxic, infectious, reactive, flammable, or explosive substances, as well as for the estimation of odor nuisance. However, in the field of Computational Fluid Dynamics (CFD), the industry currently relies predominantly on steady-state RANS turbulence models for simulating near-field pollutant dispersion, which are only capable of producing the time-averaged concentration field. This paper presents a regression relationship for calculating the standard deviation of the local concentration based on the mean concentration and the downstream distance from a point source, over a city-like surface, in the case of the wind direction perpendicular to the streets. The desired peak values and other statistical characteristics can be predicted by assuming a gamma distribution which is fitted based on the average and standard deviation. To obtain the regression function, a deep neural network model was used. The model was trained using time-resolved concentration data obtained from wind tunnel experiments. The validation results show that the concentration fluctuations predicted by the DNN-based model are in satisfactory agreement with the measurement data in terms of the skewness, the kurtosis, the median, and the peak concentrations. Furthermore, the present paper suggests a workflow for estimating the concentration fluctuations based on RANS CFD results, as well as recommendations for generating further training data for specific applications.

Computational wind engineering: 30 years of research progress in building structures and environment

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.

Eulerian RANS simulations of near-field pollutant dispersion around buildings using concentration diffusivity limiter with travel time

To improve the prediction accuracy of pollutant dispersion near a source, a limiter for turbulent concentration diffusivity accounting for pollutant travel time is introduced into the Reynolds-averaged Navier–Stokes equations (RANS) and the Eulerian dispersion model. We predict pollutant dispersion in a two-dimensional street canyon and three-dimensional building arrays via the conventional modeling of turbulent diffusivity based on a simplified gradient diffusion hypothesis and the limiter. The results are validated using data from a wind tunnel experiment. Conventional modeling with Sct=0.3 yields results similar to experimental results. However, the mean concentration near the source is underestimated. The limiter reduces the turbulent diffusivity near the source by adjusting the combination of model parameters and improves the accuracy of concentration prediction. Based on the result with the limiter, we discovered that the turbulent Schmidt number near the source by the conventional definition can exceed three times that in the downwind region. Finally, we discuss the effect of the limiter on the net mass fluxes in pollutant removal. For the building arrays, the limiter increases the advective mass flux near the source and transports more pollutants toward the leeward wall, thereby decreasing the surface-sum spanwise turbulent-mass-flux at the source-located canyon side.

CFD Study on the Influence of Atmospheric Stability on Near-field Pollutant Dispersion from Rooftop Emissions

CFD Study on the Influence of Atmospheric Stability on Near-field Pollutant Dispersion from Rooftop Emissions

CFD simulations of near-field pollutant dispersion with different plume buoyancies

This study performs computational fluid dynamics simulations for flow and dispersion fields around an isolated cubic building model with tracer gases being exhausted from an exit behind the building. The tracer gases have three different buoyancies according to the difference in density with ambient air and, therefore, behave as neutral, light, and heavy gases. The performance of steady Reynolds-averaged Navier–Stokes (RANS) simulations with the Boussinesq approximation is examined herein by comparing the simulation results with the experimental results for different plume buoyancies. The steady RANS computations can generally reproduce the impact of plume buoyancy on the mean concentration in the experimental results even if the model performance for heavy gases is better than that for light gases and worse than that for neutral gases. This tendency is closely related to the prediction accuracy of the mean velocity and turbulent kinetic energy behind the building, which is restricted by the steady RANS simulations. The study also confirmed that the buoyancy modeling in the ε equation shows a negligible influence on the results.

Ten questions concerning modeling of near-field pollutant dispersion in the built environment

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

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.

Performance of ASHRAE models in assessing pollutant dispersion from rooftop emissions

The performance of different ASHRAE models besides their general development since 1997 forms the basis of this article. The experimental results of a few recent near-field pollutant dispersion studies are compared to those produced by ASHRAE models. These cases include isolated buildings and adjacent building configurations. In fact, ASHRAE can only be used to estimate rooftop dilutions on an emitting building and does not provide formulations to estimate dilutions on adjacent building surfaces. The results from this study show that ASHRAE models provide reasonable dilution estimates for low exhaust momentum ratios (M), while previous ASHRAE models predict lower dilutions than wind tunnel data for all cases. Furthermore, ASHRAE 2011 predicts reasonable dilutions on the leeward wall of the emitting building, which is an important contribution of the current ASHRAE model. It is suggested that future ASHRAE model versions should be capable of estimating reasonable dilutions on adjacent building surfaces for realistic urban scenarios, by taking into account the spacing between buildings.

A wind tunnel study of the effects of adjacent buildings on near-field pollutant dispersion from rooftop emissions in an urban environment

This paper presents results from a wind tunnel study of near-field pollutant dispersion from rooftop emissions of two multiple building configurations. The configurations mainly consisted of an emitting building in the presence of an upstream and a downstream building. The various parameters that were varied include: stack height (hs), stack location (Xs), spacing between upstream and emitting building (S1), spacing between downstream and emitting building (S2) and exhaust momentum ratio (M). Gas concentrations were measured at various building surfaces using a gas chromatograph. The wind tunnel dilutions were also compared to ASHRAE, 2007 and 2011 models. Results show that a taller upstream and a taller downstream building inhibit the plume from dispersing, thereby increasing the pollutant concentrations on the roof of the emitting building and leeward wall of the upstream building. In general, the spacing between the upstream and emitting buildings, besides the heights of each building were found to be critical parameters influencing the plume characteristics. ASHRAE, 2007 predictions were found to be overly conservative for the isolated building, while ASHRAE, 2011 estimates compared well with experimental data for a few cases. Safe placement of stack and intake on various building surfaces to avoid plume re-ingestion are suggested based on this study.

Performance assessment of Large Eddy Simulation (LES) for modeling dispersion in an urban street canyon with tree planting

The potential of a Large Eddy Simulation (LES) model to reliably predict near-field pollutant dispersion is assessed. To that extent, detailed time-resolved numerical simulations of coupled flow and dispersion are conducted for a street canyon with tree planting. Different crown porosities are considered. The model performance is assessed in several steps, ranging from a qualitative comparison to measured concentrations, over statistical data analysis by means of scatter plots and box plots, up to the calculation of objective validation metrics. The extensive validation effort highlights and quantifies notable features and shortcomings of the model, which would otherwise remain unnoticed. The model performance is found to be spatially non-uniform. Closer agreement with measurement data is achieved near the canyon ends than for the central part of the canyon, and typical model acceptance criteria are satisfied more easily for the leeward than for the windward canyon wall. This demonstrates the need for rigorous model evaluation. Only quality-assured models can be used with confidence to support assessment, planning and implementation of pollutant mitigation strategies.

Assessment of near-field pollutant dispersion: Effect of upstream buildings

The prediction of pollutant dispersion in urban environment is an extremely complex phenomenon, particularly in the vicinity of a cluster of buildings. Dispersion of effluents released from stacks located on building roofs are severely affected by adjacent surroundings. This paper investigates the impact of an upstream building on the near field of a pollutant source in terms of dilution distribution on the roof of an emitting building. The study was carried out using Computational Fluid Dynamics (CFD) approach with Realizable k–ε for turbulent flow modeling. A limited number of cases were also modeled in a wind tunnel for validation purposes. The study shows that when the source is located within the recirculation zone, dilution is highly sensitive to the height of the upstream building and much less sensitive to the width and length of the upstream building. It is also shown that dilution value has an asymptotic behavior which defines the particular point where dilution becomes independent of the upstream building configuration. Some discrepancies between CFD and wind tunnel data were found, specifically for extreme configurations e.g. significantly taller upstream building. These differences are mainly due to the inherent unsteady fluctuations in the wake of buildings which are not detectable by RANS.

A wind tunnel study of the effect of downstream buildings on near-field pollutant dispersion

The effect of near-field pollutant dispersion characteristics for the case of downstream buildings in the urban environment has been presented in this paper. Wind tunnel data were obtained for nine different building configurations, three exhaust momentum ratios (M) and three stack heights (hs), for wind azimuth of 0°. Tracer gas concentrations were measured on the roof, windward and leeward walls of each building. When a tall downstream building was located within the recirculation length of the emitting building, higher rooftop concentration was measured on the emitting building than for the isolated building case. Results also show that the height and across wind dimension of the downstream building, as well as the spacing between buildings are critical parameters in assessing plume dilution. ASHRAE 2007 and ASHRAE 2011, which apply Gaussian-based models for the evaluation of dilution, are unable to model the effect of adjacent buildings; the former yielded lower dilution for all cases examined whilst the latter was found to be suitable only for specific limited cases. Design guidelines for the placement of stack and intakes to avoid or minimize plume re-ingestion are proposed.

The effect of upstream buildings on near-field pollutant dispersion in the built environment

This paper examines the effects of near-field pollutant dispersion characteristics of upstream buildings in the built environment and compares them to the ASHRAE 2007 model. Wind tunnel simulations were performed for nine different building configurations for three exhaust momentum ratios (M) and three stack heights (hs). The effect of spacing (S) between the buildings and stack location from the upwind edge of the emitting building (X) were also investigated. Measurements of gas concentrations were performed on the roof and leeward wall of the emitting and upstream buildings. Data show that within the recirculation zone a change in along wind dimension of the upstream building has a negligible effect on the dilution of emissions from the downwind building. However, spacing between buildings and the height of the upstream building were found to be critical parameters in assessing plume dilution. The plume geometry is largely governed by the upwind dimensions of the upstream building. ASHRAE (2007) predicts lower dilution for all cases examined, leading to conservative or very conservative design. However, the ASHRAE 2007 cannot model the effect of upstream buildings, thus further investigation of its formulations is required. Guidelines for placement of intake and stack on the roof of the building to avoid problems of re-ingestion are discussed.

Near-field pollutant dispersion in the built environment by CFD and wind tunnel simulations

Buildings are always found to be in the vicinity of other buildings, especially in urban areas. This causes effluents released from stacks located on one of the buildings to re-enter the same or an adjacent building, generating potential health problems to the occupants of the building. Earlier, Computational Fluid Dynamics (CFD) has been used in simulating pollutant transport for isolated buildings, with only few studies examining the effects of adjacent buildings. In this paper three cases that include an isolated low-rise building (source), a taller building placed upwind of the source and a case with taller buildings placed upwind and downwind of the source were considered. CFD simulations using the Realizable k– ε model for different turbulent Schmidt numbers ( Sc t ) and wind tunnel experiments were performed for these cases. ASHRAE, 2007 was also used to assess plume dispersion for the isolated building. It was found that a strong dependence of Sc t on CFD simulations of pollutant transport exists for the isolated building configuration. However, variations of Sc t have less impact on assessing pollutant dispersion in the presence of adjacent buildings. The ASHRAE, 2007 model predicted very low dilutions for the isolated building, making it necessary to re-visit its formulations.

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

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.