Pollutant Dispersion In Urban Areas Research Articles

Should we care about the level of detail in trees when running urban microscale simulations?

Due to lack of information and long geometry generation times, tree geometries are usually oversimplified or even ignored in Computational Fluid Dynamic (CFD) simulations that predict wind and pollutant dispersion in urban areas. Nevertheless, trees are known to impact local wind patterns and air quality levels. Thus, in this paper we explore the effects that tree models automatically reconstructed at diverse Level of Detail (LoD) (1, 2 and 3) have in numerical wind predictions. We address this by comparing the non-dimensional velocity magnitude differences between simulations with multiple tree LoDs. To further understand these differences in changing environmental contexts we use three morphologies: an isolated tree, an idealized street, canyon, and a real urban geometry from Rotterdam, The Netherlands The numerical results show that the velocity magnitude differences between the cases with LoD1 tree models and those with LoD2 tree models can be over 1.0m/s while the differences between LoD2 and LoD3 cases are rather limited, usually lower than 0.2m/s. Consequently, through this study we highlight the importance of using tree models in LoD2 or LoD3 at least for CFD simulations of wind flows in urban areas. To further support this conclusion we also analyze the impact of changing wind directions and tree Leaf Area Density (LAD) values in the impact of tree LoDs on wind. The differences found in this work linked to the level of realism in your tree models can support future studies where researchers want to make an informed choice.

Computational fluid dynamics analysis of pollutant dispersion around a high-rise building: Impact of surrounding buildings

Understanding the detailed mechanism of pollutant dispersion in urban areas is a critical aspect of air quality control in residential areas. In this study, wind tunnel measurements and high-resolution computational fluid dynamic (CFD) based on large eddy simulation (LES) were used for the detailed analysis of the pollutant dispersion around a high-rise building for an isolated case and also a case when the central high-rise building was surrounded by several low-rise buildings. The pollutant was released on the leeward façade of the high-rise building at the pedestrian height. It was shown that the presence of the surrounding buildings significantly alters the pollutant transfer mechanism around the high-rise building. In the surrounded case, the pollutant in the wake region shifted vertically toward the roof of the high-rise building. As a result, at a location of 2/3 of the high-rise building height, the mean concentration is five times larger than the isolated case, which means the residents on higher floors experience more pollution concentration in the case of the surrounded case even though they are located much higher than the canyon rooftop and the pollutant source. In both cases, the contribution of the turbulent diffusive flux in the streamwise direction is comparable with the mean convective flux while they have opposite directions. Detailed analyses of the pollutant mass transfer rate budget around different control volumes around the high-rise buildings highlighted the effect of the surrounding buildings. It is concluded that there is a risk of underestimating pollutant concentrations around a high-rise building, especially near the upper floors if the surrounding buildings are not properly considered.

Impact of Indoor-Outdoor Temperature Difference on Building Ventilation and Pollutant Dispersion within Urban Communities

Mechanical ventilation consumes a huge amount of global energy. Natural ventilation is a crucial solution for reducing energy consumption and enhancing the capacity of atmospheric self-purification. This paper evaluates the impacts of indoor-outdoor temperature differences on building ventilation and indoor-outdoor air pollutant dispersion in urban areas. The Computational Fluid Dynamics (CFD) method is employed to simulate the flow fields in the street canyon and indoor environment. Ventilation conditions of single-side ventilation mode and cross-ventilation mode are investigated. Air change rate, normalized concentration of traffic-related air pollutant (CO), intake fraction and exposure concentration are calculated to for ventilation efficiency investigation and exposure assessment. The results show that cross ventilation increases the air change rate for residential buildings under isothermal conditions. With the indoor-outdoor temperature difference, heating could increase the air change rate of the single-side ventilation mode but restrain the capability of the cross-ventilation mode in part of the floors. Heavier polluted areas appear in the upstream areas of single-side ventilation modes, and the pollutant can diffuse to middle-upper floors in cross-ventilation modes. Cross ventilation mitigates the environmental health stress for the indoor environment when indoor-outdoor temperature difference exits and the personal intake fraction is decreased by about 66% compared to the single-side ventilation. Moreover, the existence of indoor-outdoor temperature differences can clearly decrease the risk of indoor personal exposure under both two natural ventilation modes. The study numerically investigates the building ventilation and pollutant dispersion in the urban community with natural ventilation. The method and the results are helpful references for optimizing the building ventilation plan and improving indoor air quality.

Simulation of air pollution dispersion in Dhaka city street canyon

Pollutant dispersion in urban areas has been a key concern in densely populated cities because pollutants can negatively impact human health and the environment. Topography and urban obstructions, such as buildings, greatly affect the atmospheric fluid flow, leading to the dispersion process. Street canyons are usually formed in highly populous cities due to the close proximity of buildings to streets to serve commercial purposes. It can also be termed as pollutant traps that exact adverse effects on urban life. Street canyons cause changes in pollutant dispersion, particularly in the case of vehicle exhaust air pollutants, which cannot be transported by the wind owing to the presence of buildings acting as impediments regardless of the wind flow. Hence, it is imperative to completely understand the behavior of pollutants within the confined urban surroundings for further improving the urban air quality. This study explores the effects of street canyons on the busiest street of Dhaka city in Bangladesh. The Reynolds-averaged Navier–Stokes based dispersion model is used to investigate the air quality phenomenon. Building height, width, building distance, and road width have been taken into consideration in the simulation. It is expected that the findings of the current study would be helpful for urban planners and designers of Dhaka city.

A novel approach to simulate pollutant dispersion in the built environment: Transport-based recurrence CFD

The large-scale practical application of Large-Eddy Simulation (LES) for predicting long-term wind flow and pollutant dispersion in urban areas is inhibited mainly by the associated very large computational costs. To overcome this difficulty, the present study, for the first time, applies transport-based recurrence Computational Fluid Dynamics (rCFD) to simulate atmospheric pollutant dispersion around a building. A novel diffusion model is proposed to accurately predict pollutant transport with rCFD. To illustrate the feasibility and advantages of rCFD, pollutant dispersion around an isolated cubical building with a rooftop vent, immersed in neutral atmospheric boundary layer flow is used as a case study and both LES and rCFD simulations are conducted. It is shown that rCFD simulation results agree well with those from LES both in terms of mean and fluctuating concentrations while the simulation wall-clock time drops from 222 h to 16 min. The application of four evaluation metrics (FAC2, FB, NMSE and R) indicates very good agreement between LES and rCFD results. Another major advantage of rCFD is that different pollutant events can be simulated promptly once the database has been stored for a given flow configuration, as shown by the comparison of LES and rCFD results for two other cases with different release locations. This study extends the application of transport-based rCFD to pollutant dispersion in the built environment and indicates that rCFD is a promising approach to facilitate the large-scale practical application of LES for this type of applications.

Propagation of toxic substances in the urban atmosphere: A complex network perspective

The accidental or malicious release of toxic substances in the urban atmosphere is a major environmental and safety problem, especially in large cities. Computational fluid dynamics codes and simplified modelling tools have been used in the last decades to model pollutant dispersion in urban areas. These studies have shown that propagation is strongly influenced by the layout of buildings and, therefore, by the street topology of the city. This work presents a novel approach to the study of toxic propagation within the urban canopy based on the theory of complex networks. The urban canopy is modelled as a network, where the streets and the street intersections represent respectively the links and the nodes of the network. The direction and the weights of the links contain the geometrical characteristics of the street canyons and their wind conditions, so that all the key variables involved in pollutant dispersion are represented in a single mathematical structure that is a weighted and directed complex network. Thanks to this mathematical interpretation, propagation is modelled as a spreading process on a network and a depth-first search algorithm is used to rapidly delimit the zone of influence of a source node. This zone is the set of streets that are contaminated from the source. As a case study, the proposed model is applied to the urban tissue of the city of Lyon. The algorithm simulates a toxic release in all the nodes of the network and computes the number of people affected by each propagation process. In this way, the nodes with the most dangerous spreading potential are identified and vulnerability maps of the city are constructed. Moreover, various wind and concentration scenarios are easily implemented. The results highlight how the proposed method is effective in assessing the most vulnerable points in a city with a computational time that is up to three orders of magnitude lower than that of existing models. Moreover, the proposed approach paves the way to future applications of tools and metrics from the complex network theory for a deeper comprehension of the mechanisms that drive pollutant dispersion in urban areas.

Bayesian inversion of inflow direction and speed in urban dispersion simulations

Computational fluid dynamics (CFD) is a powerful tool to investigate airflow and pollutant dispersion in urban areas and can provide critical information for urban planning and public health assessment. Although CFD approaches can reproduce detailed airflow and dispersion results in complex areas, it is extremely sensitive to inflow parameters. The conventional way to set inflow conditions is to measure the mean wind direction at a single upwind detector and adopt it as the inflow direction in the CFD model. In this paper, a novel method is proposed to provide an alternative way to obtain the inflow parameters in urban dispersion simulations by using multiple near-field wind detectors within urban areas and a probabilistic inverse method, the Bayesian inference. A field release experiment in a real built-up area is adopted as the test case. Simulation results of both wind and concentration are compared between the proposed and conventional methods. The proposed method provides the probability distributions of inflow parameters, which well reproduce the near-field airflow field in the urban area. With respect to the dispersion simulation, the statistical analysis shows better performance of the proposed method in the point estimates of concentrations than the conventional method. In addition, the proposed method yields the probability distributions of predicted concentrations, which offers a rigorous quantification of uncertainties in urban dispersion simulations.

CFD Simulation of Automotive Pollutant Dispersion in High-Rise Building Urban Environment Under Deeply Stable Atmospheric Condition

The layer of atmosphere adjacent to the earth’s surface which is affected by friction, heat transfer and pollution from the surface is called the atmospheric boundary layer (ABL). At nighttime, the earth’s surface become colder than the upper atmospheric layers. The thermal stratification with heavy cold layers close to the ground and light hot ones upwards dampens mixing currents in the atmosphere; a condition named as stable ABL. The absence of mixing at night causes the pollutants released from ground sources, such as automotive transportation, to settle in the layers close to the earth which affects human health. This research is a CFD investigation of the effect of building density on pollutant dispersion in urban areas under severe atmospheric stability condition. Three plane area densities were examined; 35, 25 and 15%. Carbon dioxide was considered as the pollutant. Large eddy simulation (LES) was utilized in the simulation. The results have proven the positive effect of building structures in dispersing pollutants. However, high building densities above 25% trap high concentrations of pollutants at the pedestrian level. The research may offer recommendations for the city planners and legislators about traffic pollution and architectural planning.

Large‐eddy simulation of vortex streets and pollutant dispersion behind high‐rise buildings

Understanding turbulent flow and pollutant dispersion in urban areas is one of the important problems in urban meteorology and environmental fluid mechanics. In this study, we examine turbulent flow and pollutant dispersion in a densely built‐up area in Seoul, South Korea, using the parallelized large‐eddy simulation model (PALM). In particular, we focus on vortex streets and associated pollutant dispersion behind high‐rise buildings. The turbulence recycling method is used to produce inflow profiles. Vortices are generated near the high‐rise buildings and propagate downstream forming vortex streets behind the high‐rise buildings. To investigate characteristics of the vortex streets, spectral and correlation analyses are performed. The spectral analysis reveals that vortices have a non‐dimensional vortex shedding frequency of 0.1–0.2, and this periodicity is weakened due to the influence of other buildings. The correlation analysis shows that vortices appear frequently in regions of negative pressure perturbation. The vertical turbulent momentum fluxes induced by ejections and sweeps largely contribute to the total vertical turbulent momentum flux downstream of the high‐rise buildings. Especially, ejections in the wake region are stronger compared to other regions because ejections are induced by vortices near the top of the high‐rise buildings. It is found that pollutant dispersion is interrupted by both low‐rise and high‐rise buildings. Strong updraughts behind the high‐rise buildings transport pollutant upward and increase the mean pollutant concentration at upper levels. Vortices forming the vortex streets play a role in pollutant mixing in such a way that the vortices eject air of high pollutant concentration from the wake region behind the high‐rise buildings and entrain air of low pollutant concentration into the wake region. The mixing by vortices is verified by the correlation between vorticity and pollutant concentration.

Google-Earth Based Visualizations for Environmental Flows and Pollutant Dispersion in Urban Areas.

In the present study, we address the development and application of an efficient tool for conversion of results obtained by an integrated computational fluid dynamics (CFD) and computational reaction dynamics (CRD) approach and their visualization in the Google Earth. We focus on results typical for environmental fluid mechanics studies at a city scale that include characteristic wind flow patterns and dispersion of reactive scalars. This is achieved by developing a code based on the Java language, which converts the typical four-dimensional structure (spatial and temporal dependency) of data results in the Keyhole Markup Language (KML) format. The visualization techniques most often used are revisited and implemented into the conversion tool. The potential of the tool is demonstrated in a case study of smog formation due to an intense traffic emission in Rotterdam (The Netherlands). It is shown that the Google Earth can provide a computationally efficient and user-friendly means of data representation. This feature can be very useful for visualization of pollution at street levels, which is of great importance for the city residents. Various meteorological and traffic emissions can be easily visualized and analyzed, providing a powerful, user-friendly tool for traffic regulations and urban climate adaptations.

Modelling Pollutant Dispersion in a Street Network

This study constitutes a further step in the analysis of the performances of a street network model to simulate atmospheric pollutant dispersion in urban areas. The model, named SIRANE, is based on the decomposition of the urban atmosphere into two sub-domains: the urban boundary layer, whose dynamics is assumed to be well established, and the urban canopy, represented as a series of interconnected boxes. Parametric laws govern the mass exchanges between the boxes under the assumption that the pollutant dispersion within the canopy can be fully simulated by modelling three main bulk transfer phenomena: channelling along street axes, transfers at street intersections, and vertical exchange between street canyons and the overlying atmosphere. Here, we aim to evaluate the reliability of the parametrizations adopted to simulate these phenomena, by focusing on their possible dependence on the external wind direction. To this end, we test the model against concentration measurements within an idealized urban district whose geometrical layout closely matches the street network represented in SIRANE. The analysis is performed for an urban array with a fixed geometry and a varying wind incidence angle. The results show that the model provides generally good results with the reference parametrizations adopted in SIRANE and that its performances are quite robust for a wide range of the model parameters. This proves the reliability of the street network approach in simulating pollutant dispersion in densely built city districts. The results also show that the model performances may be improved by considering a dependence of the wind fluctuations at street intersections and of the vertical exchange velocity on the direction of the incident wind. This opens the way for further investigations to clarify the dependence of these parameters on wind direction and street aspect ratios.

Investigation of Re -independence of turbulent flow and pollutant dispersion in urban street canyon using numerical wind tunnel (NWT) models

Abstract This paper numerically and experimentally studied the Reynolds number independence (Re-independence) of turbulent flow and pollutant dispersion in urban areas. The concept of numerical wind tunnel (NWT) is proposed and validated by prototype wind-tunnel experimental measurements. A new physical quantity: the ratio of relative change, RRC, is proposed to investigate the Re-independence quantitatively. For the given street canyon, numerically predicted variations of RRC vs. building Reynolds number (ReH) from three k–ɛ turbulence models agree well with each other, and the variation trend shows that there exist two flow regimes in the range of ReH studied: flow in low ReH region is strongly affected by ReH, while that in higher ReH region is weakly affected by ReH, giving a strong support to the concept of Re-independence of turbulent flow. A criterion of RRC less than 5% is suggested to determine the value of the critical Reynolds number. For the street canyon studied, such determined critical building Reynolds number (ReH,crit) is 3.3 × 104. Examinations of dimensionless velocity contours, local velocity vectors, and concentration contours demonstrate the feasibility of the suggested critical Reynolds number.

Cross Comparisons of CFD Results of Wind and Dispersion Fields for MUST Experiment: Evaluation Exercises by AIJ

In order to apply Computational Fluid Dynamics (CFD) techniques to estimate pollutant dispersion in urban areas, it is important to assess the performance of numerical models used to estimate these phenomena. Recently, the MUST (Mock Urban Setting Test) has often been adopted as a test case for evaluating numerical models for micro-scale urban meteorology. This paper presents the results of model evaluation exercises carried out using MUST based on the Architectural Institute of Japan (AIJ) experience. The results of model evaluation exercises conducted by AIJ are broadly consistent with the results obtained by the COST group. However, the variety of results by each computation can be minimized by setting standard computational conditions. All computations including both RANS and LES show good agreement with wind experiment data. In general, LES cases show comparable accuracy to RANS in predicting U and W. However, LES shows better agreement than RANS in TKE and concentration.

Evaluation of a neighbourhood scale, street network dispersion model through comparison with wind tunnel data

This study compared dispersion calculations using a street network model (SIRANE) with results from wind tunnel experiments in order to examine model performance in simulating short-range pollutant dispersion in urban areas. The comparison was performed using a range of methodologies, from simple graphical comparisons (e.g. scatter plots) to more advanced statistical analyses. A preliminary analysis focussed on the sensitivity of the model to source position, receptor location, wind direction, plume spread parameterisation and site aerodynamic parameters. Sensitivity to wind direction was shown to be by far the most significant. A more systematic approach was then adopted, analysing the behaviour of the model in response to three elements: wind direction, source position and small changes in geometry. These are three very critical aspects of fine scale urban dispersion modelling. The overall model performance, measured using the Chang and Hanna (2004) criteria can be considered as ‘good’. Detailed analysis of the results showed that ground level sources were better represented by the model than roof level sources. Performance was generally ‘good’ for wind directions that were very approximately diagonal to the street axes, while cases with wind directions almost parallel (within 20°) to street axes gave results with larger uncertainties (failing to meet the quality targets). The methodology used in this evaluation exercise, relying on systematic wind tunnel studies on a scaled model of a real neighbourhood, proved very useful for assessing strengths and weaknesses of the SIRANE model, complementing previous validation studies performed with either on-site measurements or wind tunnel measurements over idealised urban geometries.

Micrometeorological Modeling of Radiative and Convective Effects with a Building-Resolving Code

AbstractIn many micrometeorological studies with computational fluid dynamics, building-resolving models usually assume a neutral atmosphere. Nevertheless, urban radiative transfers play an important role because of their influence on the energy budget. To take into account atmospheric radiation and the thermal effects of the buildings in simulations of atmospheric flow and pollutant dispersion in urban areas, a three-dimensional (3D) atmospheric radiative scheme has been developed in the atmospheric module of the Code_Saturne 3D computational fluid dynamic model. On the basis of the discrete ordinate method, the radiative model solves the radiative transfer equation in a semitransparent medium for complex geometries. The spatial mesh discretization is the same as the one used for the dynamics. This paper describes ongoing work with the development of this model. The radiative scheme was previously validated with idealized cases. Here, results of the full coupling of the radiative and thermal schemes with the 3D dynamical model are presented and are compared with measurements from the Mock Urban Setting Test (MUST) and with simpler modeling approaches found in the literature. The model is able to globally reproduce the differences in diurnal evolution of the surface temperatures of the different walls and roof. The inhomogeneous wall temperature is only seen when using the 3D dynamical model for the convective scheme.

CFD simulations of gas dispersion around high-rise building in non-isothermal boundary layer

Urban heat island phenomena and air pollution become serious problems in weak wind regions such as behind buildings and within street canyons, where buoyancy effect cannot be neglected. In order to apply CFD techniques for estimation of ventilation and thermal and pollutant dispersion in urban areas, it is important to assess the performance of turbulence models adopted to simulate these phenomena. As the first step of this study, we carried out wind tunnel experiments and CFD simulations of gas and thermal dispersion behind a high-rise building in an unstable non-isothermal turbulent flow. The standard k– ε model and a two-equation heat-transfer model as RANS models, and LES, were used for the CFD simulation. One of the important purposes of this study was to clarify the effect of inflow turbulence (both velocity and temperature) on flow field and gas/thermal dispersion for the LES calculation. Thus, LES calculations with/without inflow turbulence were conducted. The inflow turbulence was generated through a separate precursor simulation. The calculated results showed that both RANS models overestimated the size of the recirculation region behind the building and underestimated the lateral dispersion of the gas. Turbulent flow structures of LES with and without inflow turbulence were completely different. The LES result with inflow turbulence achieved better agreement with the experiment.

Tracer Flux Balance at an Urban Canyon Intersection

Despite their importance for pollutant dispersion in urban areas, the special features of dispersion at street intersections are rarely taken into account by operational air quality models. Several previous studies have demonstrated the complex flow patterns that occur at street intersections, even with simple geometry. This study presents results from wind-tunnel experiments on a reduced scale model of a complex but realistic urban intersection, located in central London. Tracer concentration measurements were used to derive three-dimensional maps of the concentration field within the intersection. In combination with a previous study (Carpentieri et al., Boundary-Layer Meteorol 133:277-296, 2009) where the velocity field was measured in the same model, a methodology for the calculation of the mean tracer flux balance at the intersection was developed and applied. The calculation highlighted several limitations of current state-of-the-art canyon dispersion models, arising mainly from the complex geometry of the intersection. Despite its limitations, the proposed methodology could be further developed in order to derive, assess and implement street intersection dispersion models for complex urban areas.

Street canyon ventilation and atmospheric turbulence

Operational models for pollutant dispersion in urban areas require an estimate of the turbulent transfer between the street canyons and the overlying atmospheric flow. To date, the mechanisms that govern this process remain poorly understood. We have studied the mass exchange between a street canyon and the atmospheric flow above it by means of wind tunnel experiments. Fluid velocities were measured with a Particle Image Velocimetry system and passive scalar concentrations were measured using a Flame Ionisation Detector. The mass-transfer velocity between the canyon and the external flow has been estimated by measuring the cavity wash-out time. A two-box model, used to estimate the transfer velocity for varying dynamical conditions of the external flow, has been used to interpret the experimental data. This study sheds new light on the mechanisms which drive the ventilation of a street canyon and illustrates the influence of the external turbulence on the transfer process.

Modelling of vehicle-induced turbulence in air pollution studies for streets

Vehicle-induced turbulence can be an important factor of pollutant dispersion in urban areas, especially under conditions of low wind speeds which are typical for street canyons. An experimental concept (Plate, 1982) for modelling the effects of vehicle-induced turbulence was applied in the present study. The movement of vehicles was simulated in a boundary-layer wind tunnel by small metal plates mounted on two belts moving along a modelled street canyon. The scaling factor was based on the ratio of turbulence production by cars to that by wind flow. The traffic was represented by the velocity, density, frontal area and drag coefficients of the vehicles. The velocity and traffic density were varied, and the influence of the vehicle-induced turbulence on concentration patterns at the canyon walls was studied. It was found that concentration decreases with an increasing ratio of vehicle to wind velocity and with an increase of traffic density. A dimensionless combination of vehicle to wind-velocity ratio and density factor was proved to be a universal parameter describing the dependence of the concentration on vehicle-induced turbulence. The wind tunnel measurements were compared with predictions by the numerical Operational Street Pollution Model (OSPM; Hertel and Berkowicz, 1989a). Differences between the wind tunnel and numerical results regarding effects of vehicle-induced turbulence are discussed. The comparison revealed general agreement between wind tunnel and numerical data. Turbulence and concentration measurements in a street canyon in Copenhagen have been additionally employed for analysis of the model results.