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Street canyons are the unit elements of the urban morphology. Fluid dynamics within these elements has been widely studied especially in the context of air pollution associated to traffic emissions. Gases and particulate matter resulting from the combustion processes are the main components of these emissions, but also pneumatic abrasion, brake discs consumption and road dust resuspension contribute to air quality deterioration. In general terms, the presence of dense built up areas limits the efficacy of atmospheric winds to disperse pollutants, enhancing near ground concentrations. It is also known that the presence of trees in urban street canyons affects pollutants dispersion and exchange of momentum and scalars with the free atmosphere. Depending on wind direction, the establishment of canyon vortices, corner vortices at street entrance and intersections, intermittent flows within the canyon are drivers for near surface pollutant removal. The presence of trees alter the flow structure and may lead to a reduction of ventilation in streets. In real world applications, the actual quantification of tree influence depends on different and mutual interacting factors: synoptic meteorological conditions, wind stress at the top of the canyon, geometry of the street canyon and vegetation aerodynamic characteristics. Within the recently EU-funded project iSCAPE, the role of vegetation in street canyons has being analyzed in real street canyons in the city of Bologna (Italy). Specifically, during summer 2017, turbulent fluxes are being measured at different heights in two typical street canyons of similar aspect ratio and orientation with respect to the prevailing wind but with different vegetative elements i.e. one is almost free of vegetation and the other has trees at the edge of each traffic lane. Data interpretation is assisted by Computational Fluid Dynamics (CFD) modelling using large eddy simulations (LES). Validated simulations are used to formulate ventilation indices typical for Italian cities. Results are sought for extension to other cities in Europe and elsewhere that are characterized by low synoptic conditions and similar morphological structures.

Motivated by the increasing interest on passive control solutions to lower pollutant concentrations in cities, this paper introduces a novel methodology to demonstrate the potential of photocatalytic coatings in abating air pollution in real-world urban environments. The methodology introduced in this paper is based on an original application of Computational Fluid Dynamic (CFD) modelling to simulate the effect of photocatalytic coatings in real yet simplified urban setting. The numerical approach is validated against observations gathered during an ad-hoc designed intensive experimental campaign performed in a real urban area in the city of Bologna, Italy (44.5075 N, 11.3514E), under semi-controlled conditions. Comparison of the model output with observations show a concentration reduction in the range 10–20%. After validation and choice of the proper model set-up, numerical simulations are analyzed by focusing on the mechanisms enhancing the flow circulation within the canyon, an effect that may increase the effect of coatings within street canyons. Results show that application of photocatalytic coatings can give pollutant reductions up to 50% in a confined region close to the walls. A parametrization for the pollutant reduction within the street canyon is suggested to summarize these results, providing a characterization of the photocatalytic coatings performances as a function of the geometric characteristic of the street canyon.

This paper is devoted to the study of pollutant concentration distribution within urban-like geometries. By applying efficiency concepts originally developed for indoor environments, the term ventilation is used as a measure of city " breathability" It can be applied to analyse pollutant removal within a city in operational contexts. This implies the evaluation of the bulk flow balance over the city and of the mean age of air. The influence of building packing density on flow and pollutant removal is, therefore, evaluated using those quantities. Idealized cities of regular cubical buildings were created with packing density ranging from 6.25% to 69% to represent configurations from urban sprawl to compact cities. The relative simplicity of these arrangements allowed us to apply the Computational Fluid Dynamics (CFD) flow and dispersion simulations using the standard k- e{open} turbulence model. Results show that city breathability within the urban canopy layer is strongly dependent from the building packing density. At the lower packing densities, the city responds to the wind as an agglomeration of obstacles, at larger densities (from about 44%) the city itself responds as a single obstacle. With the exception of the lowest packing density, airflow enters the array through lateral sides and leaves throughout the street top and flow out downstream. The air entering through lateral sides increases with increasing packing density.At the street top of the windward side of compact building configurations, a large upward flow is observed. This vertical transport reduces over short distance to turn into a downward flow further downstream of the building array. These findings suggest a practical way of identifying city breathability. Even though the application of these results to real scenarios require further analyses the paper illustrates a practical framework to be adopted in the assessment of the optimum neighbourhood building layout to minimize pollution levels.

The present paper is aimed at the analysis of flow and pollutant dispersion in a portion of the Canal Grande (Grand Canal) in Venice (Italy) by means of both Computational Fluid Dynamics (CFD) FLUENT simulations and wind tunnel experiments performed at the University of Gävle (Sweden). For this application, Canal Grande can be viewed as a sort of street canyon where the bottom surface is water and bus boat emissions are the major source of pollution. Numerical investigations were made to assess the effect of the water surface on air flow and pollutant concentrations in the atmosphere. One of the challenges has been to deal with the interface between two immiscible fluids which requires ad-hoc treatment of the wall in terms of the numerical scheme adopted and the grid definition which needs to be much finer than in typical numerical airflow simulations in urban street canyons. Preliminary results have shown that the presence of water at the bottom of the street canyon modifies airflow and turbulence structure with direct consequences on concentration distribution within the domain.

This paper is devoted to the study of flow and traffic exhaust dispersion in urban street canyons with avenue-like tree planting. The influence of tree planting with different crown porosity was investigated. Wind tunnel experiments for perpendicular approaching flow showed that avenue-like tree planting cause increases in exhaust concentrations at the leeward wall as tree crowns reduce the vortex found in the outer regions of the tree-free street canyon and the vertically entering volume flow rate at the canyon-roof top interface. This results in less ventilation and consequently larger concentrations in proximity of the leeward wall. At the windward wall, decreases in concentration are due to the upward moving stream in front of the leeward wall which extends farther into the skimming above roof flow and is better mixed. The clean air entrained in front of the windward wall mixes with air inside the street canyon leading to smaller concentrations. Experiments performed in the wind tunnel with different tree crown porosities did not indicate substantial changes in the flow and concentration fields. The porous model crowns investigated behaved almost like impermeable objects when arranged in a sheltered position and wind speeds are relatively small as in the street canyon. The above described experiments have been also investigated by means of numerical simulations with the CFD code FLUENT (TM), rarely applied to this type of problems. The standard k-epsilon turbulence model and the Reynolds Stress Model were used for flow while the Eulerian advection diffusion scheme has been used for dispersion. Both models reproduced qualitatively the main aspects found in wind tunnel experiments, even though they underestimated flow velocities. Improvement of CFD dispersion performance was obtained by increasing the diffusivity through the turbulent Schmidt number Sc(t). Overall we found that the k-epsilon model failed to capture the complex structure of dispersion process in the presence of tree planting as it would require unphysical low Sc(t) values. On the other hand the RSM turbulence model agreed fairly well with experiments by slightly reducing the standard Sc(t). The results obtained in this work by combining wind tunnel experiments and CFD based simulations to investigate this novel aspect of research suggest ways to obtain quantitative information for assessment, planning and implementation of exposure mitigation using trees in urban street canyons

Until recently, urban air quality modelling has been based on operational models of an integral nature. The use of computational fluid dynamics (CFD) models to address the same problems is increasing rapidly. Operational models e.g. OSPM, AERMOD, ADMS-Urban have undergone many comprehensive formal evaluations as to their “fitness for purpose” while CFD models do not have such an evaluation record in the urban air quality context. This paper looks at the application of both approaches to common problems. In particular, pollutant dispersion from point and line sources in the simplest neutral atmospheric boundary layer and line sources placed within different regular building geometries is studied with the CFD code FLUENT and the atmospheric dispersion model ADMS-Urban. Both the effect of street canyons of different aspect ratios and various obstacle array configurations consisting of cubical buildings are investigated. The standard k – e turbulence model and the advection–diffusion (AD) method (in contrast to the Lagrangian particle tracking method) are used for the CFD simulations. Results from the two approaches are compared. Overall CFD simulations with the appropriate choice of coefficients produce similar concentration fields to those predicted by the integral approach. However, some quantitative differences are observed. These differences can be explained by investigating the role of the Schmidt number in the CFD simulations. A further interpretation of the differences between the two approaches is given by quantifying the exchange velocities linked to the mass fluxes between the in-canopy and above-canopy layers.

Flow and traffic-originated pollutant dispersion in an urban street canyon with avenue-like tree planting have been studied by means of wind tunnel and CFD investigations. The study comprises tree planting of different crown porosity, planted in two rows within a canyon of street width to building height ratio W/H = 2 and street length to building height ratio L/H= 10 exposed to a perpendicular approaching boundary layer flow. Numerical simulations have been performed with the commercial CFD code FLUENT(tm) by employing the RSM turbulence model. In the presence of tree planting, both measurements and simulations show considerable larger pollutant concentrations in proximity of the leeward wall and slightly lower concentrations in proximity of the windward wall in comparison to the tree-less street canyon. In particular, FLUENT slightly underestimated pollutant concentrations in proximity of the leeward wall in all cases studied, while near the windward wall there is no general tendency towards underestimation or overestimation. Overall, numerical computations compare qualitatively well with experimental data. Results from commonly used statistical tests also suggest the CFD predictions to he satisfactory. Results obtained in this work by combining wind tunnel experiments and CFD based simulations in a novel aspect of research suggest ways to obtain quantitative information for planning and implementation of exposure mitigation using trees in urban street canyons.