There is a growing concern about the threat of a malicious release of harmful substances to the air in order to cause harm to the population. In order to help decision-makers assess the consequences of such an attack, accurate predictions of the transport and dispersion of airborne contaminants in cities are needed. The complex flows produced by buildings pose difficult challenges to dispersion modelers. Among features of concern are channeling of plumes down street channels, circular transport within street canyon vortex, upwind transport, and the retention of toxic materials trapped between buildings. Intermittent spiral vortices that develop on the downwind side of tall buildings and quickly transport material from the street surface to the top of the building are also commonplace. A number of groups have developed computational fluid dynamics that have been applied to neighborhood-scale problems and have explicitly resolved hundreds of buildings in their simulations. However, CFD models are computationally intensive and for some applications turn-around time is of the essence. For example, planning and assessment studies in which hundreds of cases must be analyzed or emergency response scenarios in which plume transport must be computed quickly. For many applications, where quick turn-around is needed (e.g., emergency response)more » or where many simulations must be run (e.g., vulnerability assessments), a fast response modeling system is required. Fast running models are not only needed for emergency response and post-event applications, but for scenarios in which many cases must be run or immediate feedback is needed. We have developed the QUIC (Quick Urban & Industrial Complex) dispersion modeling system to fill that need. It can relatively quickly compute the dispersion of airborne contaminants released near buildings. It is comprised of QUIC-URB, a model that computes a 3D mass consistent wind field for flows around buildings (Pardyjak and Brown, 2001), QUICPLUME, a model that describes dispersion near buildings (Williams et al., 2003), and a graphical user interface QUIC-GUI (Boswell et al., 2004). The QUIC dispersion code is currently being used for building scale to neighborhood scale transport and diffusion problems with domains on the order of several kilometers. Figure 1 illustrates the modeled dispersion for a release in downtown Salt Lake City. This paper describes the QUIC-PLUME randomwalk dispersion model formulation, the turbulence parameterization assumptions, and shows comparisons of model-computed concentration fields with measurements from a single-building wind-tunnel experiment. It is shown that the traditional three-term random walk model with a turbulence scheme based on gradients of the mean wind performs poorly for dispersion in the cavity of the single-building, and that model-experiment comparisons are improved significantly when additional drift terms are added and a non-local mixing scheme is implemented.« less

Testing of the quic-plume model with wind-tunnel measurements for a high-rise building

CFD modeling of micro and urban climates: Problems to be solved in the new decade

The rapid urbanisation impacts on environment, climate, agriculture, water resources trigger several problems to human beings. The present study is carried out to estimate intensity and trend of Urban Heat Island (UHI) as Surface UHI (SUHI) over towns/cities of the Gangetic plain of the state of Bihar, India, in which urban areas show relatively greater Land Surface Temperature (LST) than its rural surroundings especially during night times. The LST data (2001-14) of Moderate Resolution Imaging Spectroradiometer (MODIS) is used for five major towns/cities of Bihar namely, Bhagalpur, Gaya, Patna, Purnea and Muzzaffarpur. Each city is classified into Urban, Suburban and Rural zones as per land cover of the area. During winter months (January, February, November and December), UHI is more intense over all towns/cities. Mann-Kendall Test is applied on Surface Urban Heat Island Intensity (SUHII) in which MK-Test Statistic (S) shows a significant increasing trend. This trend would alarm a risk to increase in air pollution, heat related biohazards, energy demand in the region. This study shows the need of urban greening and proper town planning over the considered areas to mitigate the changes.

Characteristics of Surface Urban Heat Island (SUHI) over the Gangetic Plain of Bihar, India

Effect of high-rise buildings on the surrounding thermal environment

Assessing local heat stress and air quality with the use of remote sensing and pedestrian perception in urban microclimate simulations

The complex interaction between the Urban Heat Island (UHI), local circulation, and air quality requires new methods of analysis. To this end, this study investigates the multiple scale nature of the UHI and its relationship with flow and pollutant dispersion in urban street canyons with and without the presence of vegetation. Two field experimental campaigns, one in summer and one in winter, were carefully designed in two parallel urban street canyons in the city of Bologna (44°29′ N, 11°20′ E; Italy) characterized by a similar orientation with respect to the impinging background flow but with a different aspect ratio and a different presence of vegetation. In addition to standard meteorological variables, the dataset collected included high-resolution flow data at three levels and concentration data of several pollutants. The UHI has been evaluated by combining surface temperature of building facades and ground surfaces acquired during two intensive thermographic campaigns with air temperature from several stations in order to verify the presence of intra-city neighborhood scale UHIs additional to the more classical urban–rural temperature differences. The presence of trees together with the different morphologies was shown to mitigate the UHI intensity of around 40% by comparing its value in the center of the city free of vegetation and the residential area. To capture the multiple-scale nature of UHI development, a simple relationship for the UHI convergence velocity, used as a surrogate for UHI strength, is proposed and used to establish the relationship with pollutant concentrations. The reliability of the proposed relationship has been verified using a Computational Fluid Dynamics (CFD) approach. The existence of a robust relationship between UHI strength and pollutant concentration may indicate that the positive effect of mitigation solutions in improving urban thermal comfort likely will also positively impact on air pollution. These results may be useful for a quick assessment of the pollutant accumulation potential in urban street canyons.

The Multiple-Scale Nature of Urban Heat Island and Its Footprint on Air Quality in Real Urban Environment

An experimental field campaign is designed to unveil mechanisms responsible for turbulent exchange processes when mechanical and thermal effects are entwined. The focus is an urban street canyon with a mean aspect ratio H/W of 1.65 in the business centre of a mid-size Italian city (H is the mean building height and W is the mean canyon width). The exchange processes can be characterized by time scales and time-scale ratios specific to either mechanical or thermal process. Time scales describe the mixing caused by momentum and heat exchange within different canyon layers, while their rates are surrogates of their efficacy. Given that homogeneous mixing does not always occur within the canyon, several time scales are estimated at different levels, showing that mechanical and thermal processes may both contribute to enhance mixing. By computing mechanical time scales, it is found that the fastest mixing occurs at the canyon rooftop level for perpendicular or oblique wind directions, while slow mixing occurs for parallel directions. Thermal processes are faster than the mechanical ones and are particularly efficient for perpendicular wind directions. By calculating the time-scale ratios, exchange processes are found to facilitate mixing for most wind directions and to regulate the pollutant-concentration variability in the canyon. This variability can be associated with the local-circulation regime, demarcated as thermally driven or inertially driven using a buoyancy parameter, i.e., the ratio between thermal and inertial forcings. Using this approach, a generalization of the results is proposed, enabling the extension of the current investigation to different street-canyon aspect ratios.

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Characteristic Scales for Turbulent Exchange Processes in a Real Urban Canopy

A numerical study of the impact of vegetation on mean and turbulence fields in a European-city neighbourhood

Numerical simulation of air pollution mitigation by means of photocatalytic coatings in real-world street canyons

Anabatic flows are common phenomena in the presence of sloping terrains, which significantly affect the dynamics and the exchange of mass and momentum in the low-atmosphere. Despite this, very few studies in the literature have tackled this topic. The present contribution addresses this gap by utilising high-resolved large-eddy simulations for investigating an anabatic flow in a simplified configuration, commonly used in laboratory experiments. The purpose is to analyse the complex thermo-fluid dynamics and the turbulent structures arising from the anabatic flow near the slope. In such a flow, three main dynamic layers are identified and reported: the conductive layer close to the surface, the convective layer where the most energetic motion develops, and the outer region, which is almost unperturbed. The analysis of instantaneous fields reveals the presence of thermal plumes, which are stable turbulent structures enhancing vertical transport and mixing of momentum and temperature. Such structures are generated by thermal instabilities in the conductive layer that trigger the rise of the plumes above them. Their evolution along the slope is described, identifying three regions responsible for the plumes generation, stabilisation, and merging. To the best of the authors’ knowledge, this is the first numerical experiment describing the along-slope behaviour of the thermal plumes in the convective layer.

Francesco Barbano

Papers

The Multiple-Scale Nature of Urban Heat Island and Its Footprint on Air Quality in Real Urban Environment

Characteristic Scales for Turbulent Exchange Processes in a Real Urban Canopy

A numerical study of the impact of vegetation on mean and turbulence fields in a European-city neighbourhood

Numerical simulation of air pollution mitigation by means of photocatalytic coatings in real-world street canyons

Anabatic Flow along a Uniformly Heated Slope Studied through Large-Eddy Simulation