Pollutant dispersion over two-dimensional idealized street canyons: a large-eddy simulation approach

Session: H14-117A series of two-dimensional (2D) street canyon models with a wide range of building-height-to-street-width (aspect) ratios are employed in this study to elucidate the pollutant transport over idealized urban areas. The large-eddy simulation (LES) is used to resolve the turbulent flows and pollutant transport in the urban boundary layer (UBL) over the street canyons. An area source of uniform pollutant concentration is applied on the ground of the first street canyon to examine the pollutant plume dispersion behaviors over the downstream building roughness elements. The LES results show that, for the street canyon with the pollutant source, the pollutant removal is governed by atmospheric turbulence in both skimming flow and wake-interference regimes. Statistical analysis reveals that the turbulent kinetic energy (TKE) is peaked near the top of the building roughness elements that contributes most to turbulent pollutant removal. The roof-level TKE distribution also demonstrates that the turbulence production is not governed by local wind shear. Instead, the descending TKE from the UBL plays a more important role. In the UBL, the vertical pollutant profiles illustrate self-similarity behaviours in the downstream region. The pollutant disperses rapidly over the buildings, exhibiting a Gaussian-plume shape. Maximum vertical pollutant dispersion coefficient is observed at aspect ratio equal to 1/10. A strong correlation between friction factor and dispersion coefficient is found, implying that the downstream air quality could be improved by increasing the roughness of urban area.postprin

On the pollutant plume dispersion in the urban canopy layer over 2D idealized street canyons: a large-eddy simulation approach

published_or_final_versionThe 2010 General Assembly of the European Geosciences Union (EGU), Vienna, Austria, 2-7 May 2010. In Geophysical Research Abstracts, 2010, v. 12, EGU2010-144

Pollutant removal, dispersion, and entrainment over two-dimensional idealized street canyons

Idealized two-dimensional (2D) street canyon models of unity building-height-to-street-width (aspect) ratio are employed to examine the pollutant transport over hypothetical urban areas. The results show that the pollutant removal is mainly governed by atmospheric turbulence when pollutant sources exist in the street canyons. Numerous decelerating, uprising air masses are located at the roof level, implying that the pollutant is removed from the street canyons to the urban boundary layer (UBL) by ejections. For the street canyons without pollutant source, the removal by ejections is limited leading to insignificant turbulent pollutant removal. The roof-level turbulent kinetic energy (TKE) distribution demonstrates that its production is not governed by local wind shear but the descending TKE from the UBL. In the UBL, the pollutant disperses rapidly over the buildings, exhibiting a Gaussian-plume shape. The vertical pollutant profiles illustrate a self-similarity behavior in the downstream region. Future studies will be focused on the characteristic plume shape over 2D idealized street canyons of different aspect ratios.postprintThe 13th International Conference on Wind Engineering (ICWE13), Amsterdam, The Netherlands, 10-15 July 2011

Effect of roughness on vertical dispersion coefficient over idealized urban street canyons under neutral stratification

Ground-level pollutants (e.g. vehicular emission) are the primary pollutant sources affecting the public health and living quality in many modern compact cities. Thus, it is necessary to estimate the pollutant concentration and distribution in urban areas in a fast and reliable manner for better urban planning. Gaussian plume dispersion model is commonly used in practice. However, one of its major parameters, dispersion coefficient, often overlooks the effect of surface roughness so its accuracy in urban application is in doubt. In the existence of large-scale roughness element, the calculation of pollutant distribution in the urban boundary layer (UBL) would be prone to error. Our previous studies, using ...published_or_final_versio

Turbulent plume dispersion over two-dimensional idealized urban street canyons

Human activities are the primary pollutant sources which degrade the living quality in the current era of dense and compact cities. A simple and reasonably accurate pollutant dispersion model is helpful to reduce pollutant concentrations in city or neighborhood scales by refining architectural design or urban planning. The conventional method to estimate the pollutant concentration from point/line sources is the Gaussian plume model using empirical dispersion coefficients. Its accuracy is pretty well for applying to rural areas. How...published_or_final_versio

On plume dispersion over two-dimensional urban-like idealized roughness elements with height variation

A series of large-eddy simulation (LES) models over two-dimensional (2D) urban-like idealized roughness elements with height variation were performed. Results show that building-height variability (BHV) could enhance the aerodynamic resistance of the urban surfaces. Both the air exchange rate (ACH) and the vertical dispersion coefficient ðz increase with increasing the friction factor, implying that the air quality in both street canyons and urban boundary layer (UBL) could be improved by increasing the surface roughness via BHV. In addition, the parameters used in the estimates of dispersion coefficient are modified substantially by the friction factor, suggesting that friction factor could be used to parameterize dispersion coefficient of urban Gaussian plume model.postprin

A theory on the ventilation over hypothetical urban areas

Urban roughness is one of the major factors affecting the flows and turbulence structures in the bottom of the atmospheric boundary layer (ABL). Whereas, our understanding of their relation is limited. In this paper, we attempt to examine the interaction among aerodynamic resistance (friction factor f), ventilation (air exchange rate ACH), and pollutant removal (pollutant removal rate PCH). Using the method of characteristic, analytical solution shows that the turbulent ventilation of a hypothetical urban area is directly proportional to the square root of friction factor (ACH? f1-2) regardless of the building geometry. Next, a series of computational fluid dynamics (CFD) sensitivity tests are performed to verify the theory. In addition to the commonly employed rectangular building mod...published_or_final_versio

Pollutant removal, dispersion and entrainment over two-dimensional idealized street canyons: an LES approach

postprintThe 2011 General Assembly of the European Geosciences Union (EGU), Vienna, Austria, 3-8 April 2011. In Geophysical Research Abstracts, 2011, v. 13, EGU2011-201

Plume dispersion over idealized urban-like roughness with height variation: an LES approach

Human activities (e.g. vehicular emission) are the primary pollutant sources affecting the health and living quality of stakeholders in modern compact cities. Gaussian plume dispersion model is commonly used for pollutant distribution estimate that works well over rural areas with flat terrain. However, its major parameters, dispersion coefficients, exclude the effect of surface roughness that unavoidably prone to error handling the pollutant transport in the urban boundary layer (UBL) over building roughness. Our recent large-eddy simulation (LES) has shown that urban surfaces affect significantly the pollutant dispersion over idealized, identical two-dimensional (2D) street canyons of uniform height. As an extension to our on-going effort, this study is con...published_or_final_versio

Asymptomatic Atrial Fibrillation among Hospitalized Patients:clinical correlates and in-hospital outcomes in Improving Care for Cardiovascular Disease in China-Atrial Fibrillation

AIMS: The clinical correlates and outcomes of asymptomatic atrial fibrillation (AF) in hospitalized patients are largely unknown. We aimed to investigate the clinical correlates and in-hospital outcomes of asymptomatic AF in hospitalized Chinese patients.METHODS AND RESULTS: We conducted a cross-sectional registry study of inpatients with AF enrolled in the Improving Care for Cardiovascular Disease in China-Atrial Fibrillation Project between February 2015 and December 2019. We investigated the clinical characteristics of asymptomatic AF and the association between the clinical correlates and the in-hospital outcomes of asymptomatic AF. Asymptomatic and symptomatic AF were defined according to the European Heart Rhythm Association score. Asymptomatic patients were more commonly males (56.3%) and had more comorbidities such as hypertension (57.4%), diabetes mellitus (18.6%), peripheral artery disease (PAD; 2.3%), coronary artery disease (55.5%), previous history of stroke/transient ischaemic attack (TIA; 17.9%), and myocardial infarction (MI; 5.4%); however, they had less prevalent heart failure (9.6%) or left ventricular ejection fractions ≤40% (7.3%). Asymptomatic patients were more often hospitalized with a non-AF diagnosis as the main diagnosis and were more commonly first diagnosed with AF (23.9%) and long-standing persistent/permanent AF (17.0%). The independent determinants of asymptomatic presentation were male sex, long-standing persistent AF/permanent AF, previous history of stroke/TIA, MI, PAD, and previous treatment with anti-platelet drugs. The incidence of in-hospital clinical events such as all-cause death, ischaemic stroke/TIA, and acute coronary syndrome (ACS) was higher in asymptomatic patients than in symptomatic patients, and asymptomatic clinical status was an independent risk factor for in-hospital all-cause death, ischaemic stroke/TIA, and ACS.CONCLUSION: Asymptomatic AF is common among hospitalized patients with AF. Asymptomatic clinical status is associated with male sex, comorbidities, and a higher risk of in-hospital outcomes. The adoption of effective management strategies for patients with AF should not be solely based on clinical symptoms.</p