Riccardo Buccolieri

Aerodynamic effects of trees on pollutant concentration in street canyons

This paper deals with aerodynamic effects of avenue-like tree planting on flow and traffic-originated pollutant dispersion in urban street canyons by means of wind tunnel experiments and numerical simulations. Several parameters affecting pedestrian level concentration are investigated, namely plant morphology, positioning and arrangement. We extend our previous work in this novel aspect of research to new configurations which comprise tree planting of different crown porosity and stand density, planted in two rows within a canyon of street width to building height ratio W/H=2 with perpendicular approaching wind. Sulfur hexafluoride was used as tracer gas to model the traffic emissions. Complementary to wind tunnel experiments, 3D numerical simulations were performed with the Computational Fluid Dynamics (CFD) code FLUENT using a Reynolds Stress turbulence closure for flow and the advection-diffusion method for concentration calculations. In the presence of trees, both measurements and simulations showed considerable larger pollutant concentrations near the leeward wall and slightly lower concentrations near the windward wall in comparison with the tree-less case. Tree stand density and crown porosity were found to be of minor importance in affecting pollutant concentration. On the other hand, the analysis indicated that W/H is a more crucial parameter. The larger the value of W/H the smaller is the effect of trees on pedestrian level concentration regardless of tree morphology and arrangement. A preliminary analysis of approaching flow velocities showed that at low wind speed the effect of trees on concentrations is worst than at higher speed. The investigations carried out in this work allowed us to set up an appropriate CFD modelling methodology for the study of the aerodynamic effects of tree planting in street canyons. The results obtained can be used by city planners for the design of tree planting in the urban environment with regard to air quality issues.

Recent advancements in numerical modelling of flow and dispersion in urban areas: a short review

This paper reviews recent findings in the field of flow and pollutant dispersion modelling around buildings and within complex urban geometries. Complexity is not only associated to the packing density of buildings, but originates also from building-height variability, buoyancy effects close to the building walls, traffic-produced turbulence and from the presence of vegetation. Recent results are discussed in light of progress made in operational urban dispersion models as a way forward for the application of those models in real scenarios.

COST 732 in practice: the MUST model evaluation exercise

The aim of this paper is to describe the use of a general methodology tailored to the evaluation of micro-scale meteorological models applied to flow and dispersion simulations in urban areas. This methodology, developed within COST 732, has been tested through a large modelling exercise involving many groups across Europe. The major test case used is the Mock Urban Setting Test (MUST) experiment representing an idealised urban area. It is emphasised that a full model evaluation is problem-dependent and requires several activities including a statistical validation that requires a careful choice of the metrics for the comparison with measurements.

On the contribution of mean flow and turbulence to city breathability : the case of long streets with tall buildings

This paper analyses the contribution of mean flow and turbulence to city breathability within urban canopy layers under the hypothesis that winds from rural/marine areas are sources of clean air (inhale effect) and main contributors to local-scale pollutant dilution (exhale effect). Using Computational Fluid Dynamics (CFD) simulations, several idealized long streets flanked by tall buildings are investigated for wind flow parallel to the street axis. Aspect ratios (building height/street width) ranging from 2 to 4 and street lengths ranging from neighborhood scales (~1km in full scale) to city scales (~10km in full scale) are analyzed. To assess the inhale effect, the age of air concept is applied to quantify the time taken by a parcel of rural/marine air to reach a reference location within the urban canopy layer. To simulate the exhale effect, removal of pollutants released from a ground level source is considered. Numerical results agree with wind tunnel observations showing that a bulk portion of rural/marine air enters the streets through windward entries, a smaller part of it leaves through street roofs and the remaining fraction blows through the street aiding pollutant dilution. Substantial differences between neighborhood-scale and city-scale configurations are found. For neighborhood-scale models, pollutant removal by rural/marine air is mainly associated to mean flow along the streets. Breathability improves in streets flanked by taller buildings since in this case more rural/marine air is captured inside canyons leading to stronger wind along the street. For city-scale models, pollutant removal due to turbulent fluctuations across street roofs competes with that due to mean flows along the street. Breathability improves in streets flanked by lower buildings in which less rural/marine air is driven out and pollutant removal by turbulent fluctuations is more effective. Based on these findings, suggestions for ventilation strategies for urban areas with tall buildings are provided.

Validation of temperature-perturbation and CFD-based modelling for the prediction of the thermal urban environment: the Lecce (IT) case study

This paper discusses the performance of the temperature perturbation-type ADMS-Temperature and Humidity Model (ADMS-TH) and the Computational Fluid Dynamics (CFD)-based model ENVI-met for the prediction of urban air temperature using measurements collected in the city of Lecce (IT) in summer 2012. The goal is to identify the most important factors influencing numerical predictions. Direct comparisons with measured data and statistical indices show that modelled results are within the range of acceptance. Daily trends are well captured although an underestimation of maximum temperature is observed. In ADMS-TH this is due to an underestimation of sensible heat fluxes during daytime, while in ENVI-met it can be attributed to an underestimation of turbulent momentum and thermal diffusivity. Overall, ADMS-TH did predict the temperature cycle with higher accuracy than ENVI-met and its performance was particularly good during the night. ENVI-met required an ad-hoc tuning of surface boundary conditions to predict nocturnal cooling, satisfactorily.

The effects of trees on micrometeorology in a real street canyon: consequences for local air quality

This study analyses the effects of trees on local meteorology of a Mediterranean City (Lecce, IT) using field measurements and computational fluid dynamics simulations. Measurements were collected for 51 days in a street canyon with trees to cover different meteorological and foliage conditions. Building facades and ground temperatures were estimated from infrared images, flow and turbulence measured by ultrasonic anemometers. In the case of approaching wind parallel to the street axis, trees induce large wind direction fluctuations below tree crowns and velocities up to about 80% lower than those at roof top. This, combined with the obstruction by tree crown, lead to lower ventilation in the bottom part of the street, especially during nocturnal hours, and to in-canyon volume-averaged pollutant concentration about 20% larger than in the tree-free case. Ignoring trapping effects of trees, as typically done in many air quality models, may lead to underestimation of ground level concentrations.

MUST experiment simulations using CFD and integral models

This paper looks at the application of Computational Fluid Dynamics (CFD) and integral approaches to the study of effects of obstacles on pollutant dispersion from a point source placed within an idealised urban area (MUST). This study is part of a modelling exercise within the COST Action 732. Numerical results are compared with wind tunnel data. We use the CFD code FLUENT and the dispersion model ADMS-Urban. The CFD model predicts concentrations more accurately than the integral model. However, both models results satisfy accepted statistical criteria, showing that those criteria should not be the only way of evaluating a model.

An application of ventilation efficiency concepts to the analysis of building density effects on urban flow and pollutant concentration

This paper is devoted to the study of flow and pollutant dispersion within different urban configurations by means of wind tunnel experiments and Computational Fluid Dynamics (CFD) simulations. The influence of the building packing density was evaluated in terms of ventilation efficiency. We found that air entering the array through the lateral sides and that leaving through the street roofs increased and the lateral spread of the pollutant released from a ground level line source decreased with increasing packing density. Ventilation efficiency concepts developed for indoor environments appear a promising tool for evaluating the urban air quality as well.

A fast model for pollutant dispersion at the neighbourhood scale

This paper is devoted to the development and evaluation of a fast three-dimensional Eulerian model for dispersion inside and above the urban canopy layer. Spatially averaged wind and diffusivity coefficient profiles obtained from the commercial Computational Fluid Dynamics (CFD) code FLUENT are used as input in the developed model. This model is numerically solved by means of a finite volume method and mean concentration outputs are compared with the corresponding results from FLUENT. We considered several canopies made of arrays of cubes laid in staggered position. Results from the comparison suggest the potential of this type of simple modelling approach. As the spatially averaged wind and diffusivity profiles are strongly dependent from the mean morphometry properties of the urban canopy, these results, though preliminary, highlight the necessity of using specific turbulence closure models where building effects at the neighbourhood scale are taken into account.

Chapter 1.1 Application and validation of FLUENT flow and dispersion modelling within complex geometries

Abstract Flow patterns around buildings have a strong influence on pollutant dispersion derived from sources placed within the urban area. Computational fluid dynamics (CFD) codes are used to provide solutions to the fundamental fluid dynamics equations at spatial scales smaller than the typical urban ones. In this work, dispersion of pollutant from sources near buildings forming various street canyons is studied by means of the general purpose CFD code FLUENT to investigate the influence of small geometric features on pollutant concentration distributions. Firstly, we study the effects of a complex geometry on the flow near the ground by considering a finite array of rectangular and square-shaped rings of buildings with different aspect ratios. Secondly, we study transport and diffusion of pollutant within a finite array of rectangular buildings. FLUENT concentration results are validated against wind tunnel data (CEDVAL, 2002). Numerical simulations are performed using the Reynolds Averaged Nervier–Stokes (RANS) k–e turbulence model and the advection–diffusion model. The paper documents the potential of a general purpose CFD model for the simulation of pollutant dispersion close to emission sources and within complex building arrangements in an operational context.