S Di Sabatino

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.

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.

Modelling the Urban Boundary-Layer Over a Typical Mediterranean City Using WRF: Assessment of UHI and Thermal Comfort

The aim of this work is to simulate the Urban Heat Island (UHI) in a medium size Mediterranean city (Lecce, IT) and to analyze its consequences for thermal comfort. We use the Weather Research and Forecasting (WRF) model (version 3.2), that accounts for the urban structure with a multilayer urban parameterization (BEP+BEM i.e. the Building Effect Parameterization (BEP) combined with the Building Energy Model (BEM)). Three hot and cloudless summer days have been simulated and results have been compared with field data collected during an experimental campaign performed over the whole summer in the city of Lecce, Italy. In the model, the structure and shape of the city are reproduced using detailed data related to different urban classes, urban fraction and building morphometry. For the residential urban classes, different thermal parameters that are representative of building materials in the oldest and the newer part of the city, are used. Results show that UHI reaches, on average, its maximum intensity (4–5 °C) just before sunrise, and its minimum (2 °C) occurs during the day. Model validation inferred through statistical analysis shows overall a better model performance for the historical city centre than for the suburban area. This suggests that further refinement of the building representation in the outskirts might still be required. Consequences of the increased urban temperature are evaluated in terms of thermal comfort. The maximum thermal stress occurs during the central hours of the day, while, the minimum thermal stress occurs during the twilight hours.Copyright

Chapter 1.6 Modelling urban heat island in the context of a Mediterranean city

Abstract Urban heat island (UHI) is one of the most well known forms of localised anthropogenic climate modification. It causes a local alteration of atmospheric stability. According to a top-down methodology, mesoscale meteorological modelling is a commonly used approach to study the impact of UHI on atmospheric stability at the urban scale. Our work is an effort to investigate UHI using a bottom-up approach by looking at the UHI through a computational fluid dynamics (CFD) model applied to the street canyons of a neighbourhood area. The CFD code is set up to model the thermal response (structure surface temperature and ambient air temperature) of an urban system to the outside climate. The determination of the air temperature in an urban unit allows the calculation of the Δ T u−r factor representing the difference between the air temperature in the urban system (u) and the air temperature recorded at the closest meteorological station (r), generally situated in the countryside. This factor, introduced by Oke in Boundary Layer Climate, (1987), enables the analysis of the heat island generated by an urban system. The simulation results obtained from the CFD model allows the estimation of the Δ T u−r factor in relation to physical aspects and geometrical configurations. We apply this technique to study UHI of a Mediterranean city of which some urban temperature measurements and morphometry from a digital elevation model (DEM) are available.

A Method of Analysis for Turbulent Flows Using the Streamline Coordinate System

A procedure is described for analysing the transport equations for Reynolds stresses written in a streamline coordinate system, starting from the fields of first- and second-order moments of wind velocity measured in a terrain-following system over topography. In the analysis, the equations are split into two parts: the first contains the terms that can be calculated directly from measurements; the second contains third-order moments that are parameterized using suitable models. To evaluate the error associated with both parts, a Monte-Carlo technique that takes into account the experimental errors is proposed. An example of the application of this method for the Reynolds shear stress equation, using wind-tunnel data for non-separating flow over a two-dimensional valley, is reported. The comparison between the measured and modelled parts is fair near the surface, while at higher levels, the modelled part can be shown to miss a correct treatment of the third-order moments. In the frame of this analysis, the use of the correct derivative transformation has been found to be significant even for moderately sloping topography.