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

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

postprin

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

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

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

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

A comparative study of overlap and staggered fermions in QCD

We perform a comparative study of the infrared properties of overlap and staggered fermions in QCD. We observe that the infrared spectrum of the APE/HYP improved staggered Dirac operator develops a four-fold near-degeneracy and is in quantitative agreement with the infrared spectrum of the overlap operator. The near-degeneracy allows us to identify the zero modes of the staggered operator and we find that the number of zero modes is in line with the topological index of the overlap operator.Comment: Talk presented at Lattice2004(chiral), Fermilab, June 21-26, 2004, 3 pages, 2 figure

Chronic hepatitis B infection and liver cancer

Hepatitis B virus (HBV) is one of the most well recognised human carcinogens. Since its discovery about 40 years ago, HBV has been studied extensively. This article summarises the evidence derived from various studies including epidemiological, animal model, histopathology studies and molecular genetics studies leading to the establishment of HBV as the main aetiological agent for hepatocellular carcinoma (HCC). The reduction in the incidence of childhood HCC due to mass hepatitis B vaccination in Taiwan is a dramatic demonstration of the critical aetiological role of hepatitis B in HCC. Thus it is essential for interventionalists to understand the epidemiological and pathogenesis of HCC to ensure optimal patient care

Modeling saturated and unsaturated ferroelectric hysteresis loops : an analytical approach

Author name used in this publication: C. H. TsangAuthor name used in this publication: C. K. WongAuthor name used in this publication: F. G. Shin2005-2006 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe