Search

Submicronic fluorescein aerosols were used as the tracer during four test releases in a field experiment performed at the University of Salento, Lecce (Italy) during October 2014. The role of surface building materials is investigated through the analysis of near surface thermal flow characteristics. Turbulence and thermal conditions in the flow and wall boundary layer were measured simultaneously using an array of meteorological instruments, while concentration of fluorescein deposited on the five different surface building materials were measured by spectrofluorometric techniques. Wall and boundary layer temperatures were determined. Here fluorescein concentration and deposition velocity during the first release test are shown.

The impact of urban air pollution on human health has become an important problem, and the estimation of the amount of pollutants to which people are exposed is a major challenge, due to the complex spatial distribution of pollutant concentration and the usual coarse spatial resolution data of the population distribution. This study aims to investigate the uncertainties of outdoor population exposure estimates associated to the approach used to assign concentration to population. To achieve this objective, the results of Rivas et al. (2019) are used: 1) the annual average NO2 concentration over the whole city of Pamplona (Spain)computed at high-resolution (~m) by CFD modeling and 2) distribution population from the municipal census, at a resolution of 100m x 100m. Using this detailed information, the population and concentration data were aggregated for different cell resolution, from 100 m x 100 m to 6 km x 5 km (the entire city). The total population exposure was estimated for the different cases and compared with the highest resolution case that was taken as the reference. In addition, the population exposure was estimated using the concentration at the location of different air quality monitoring stations (AQMS). Results suggest that only concentration distributions with equal or finer than 1 km x 1 km provide appropriate estimate of total population exposure. In addition, an in overall, total exposure estimates using concentration at AQMS can induce important errors.

Indoor air quality in the near-road buildings largely depends on the urban flow patterns and vehicular pollutant dispersion which are greatly affected by urban vegetation configurations and street layouts like building morphologies and aspect ratios (AR, i.e. building height/street width). Trees are usually settled to relieve the burden from urban heat island and maintain urban zoology system, however, trees also tend to reduce urban windiness and further provide adverse impacts on pollutant dilution.

This study investigates flow and turbulence within two arrays of staggered high-rise buildings. Each array is characterized by a packing density (λp) of 0.25 and a uniform building width B and height H, with H=2B in one array and H=2.67B in the other. Wind tunnel measurements and numerical simulations are performed to evaluate the wind reduction within the arrays, and explore the main mechanisms of rural wind flowing through such idealised arrays and how rural air distributes within them. Results show a quick wind reduction through the arrays. Wind strongly interacts with buildings producing downward motion near the leeward top corner of each building and upward motion near the windward top corner. These vertical flows across the street roof level mostly contribute to the air exchange and ventilation in such staggered arrays. The analysis also shows that increasing the building height does not have critical influence on flow and turbulence at pedestrian level.

The first validation exercise of the COST action 732 lead to a substantial number of simulation results for comparison with the MUST wind tunnel experiments. Validation metrics for selected simulation results of the flow field and the concentrations are presented and compared to the state of the art. In addition mean metrics and corresponding scatter limits are computed from the individual results.

The aerodynamic effects of avenue-like tree planting on air flow and traffic-originated pollutant dispersion in urban built-up areas (i.e. street canyons of width to height ratio, W/H=1) are investigated using computational fluid dynamics techniques and complemented with extensive wind tunnel experiments carried out at the University of Karlsruhe (CODASC Database, 2008).

As part of the COST 732 action more than a dozen different research groups have modelled the MUST experiment, as simulated in a wind tunnel. The model evaluation guidance developed within COST 732 recommends 'exploratory data analysis' as one of the elements in model validation. Experience has shown that such exploratory analysis is crucial to reveal shortcomings of models that might otherwise pass unnoticed. Conditions are best for detecting common patterns and anomalies if you have a situation where several models are put into a common framework – like the case at hand. The available material provides a unique opportunity to identify and explore patterns within model performance.

This paper deals with the development of a simple urban model for flow and dispersion in the urban canopy layer (UCL). The flow module of the model calculates spatially-averaged wind profiles adopting a technique recently proposed in the literature, which is based on a balance equation between the obstacle drag force and the local shear stress. Spatially-averaged wind profiles are used as input for a newly proposed dispersion model which solves the advection-diffusion equation at neighbourhood scale. In the model, the effects of the buildings within the UCL are taken into account by means of morphological parameters λf and λp (the ratios of plan area and frontal area of buildings to the lot area). Spatially-averaged mean concentrations output by the developed model are compared with numerical results obtained from the computational fluid dynamics (CFD) model FLUENT. In particular, two configurations of constant height UCL have been considered, which refer to as λ p = λf = 0.16 and λ p = λf = 0.44. The originality of the study is that the dispersion model itself integrates the equations without explicitly resolving the flow around individual buildings but still accounts for their effects. The computational costs are much reduced which makes it suitable for the predictions of concentrations over the neighbourbood scale in an operational context.

The impact of urban air pollution on human health has become an important problem, and the estimation of the amount of pollutants to which people are exposed is a major chanllenge. Usually, population exposure is only related to outdoor pollutant concentration, however people spend the most time inside buildings. In this context, the objective of this contribution is to relate the outdoor pollutant concentration at pedestrian level with the concentration inside a standard building. Computational Fluid Dynamics (CFD) simulations of pollutant dispersion are performed in an array of buildings where the interior of the central building (target building) is also simulated. Traffic-related pollutant are considered and several configurations of open/closed windows under different wind directions are modelled. The relationship between indoor concentration in each room and the spatially-averaged pollutant concentration in the street at pedestrian level is investigated. The results show a large variability of indoor concentrations depending on the configuration of open/closed windows, the floor of the room and the room location and the incidence angle of wind. In general, indoor concentration decreases with floor and indoor concentration is lower than average concentration in the street. However, for some conditions in the first-floor rooms indoor concentration is higher than the spatially-averaged concentration in the street at pedestrian level. Taking the average over all cases the indoor-outdoor concentration ratio is 0.6±0.2.

This study analyses the effects of trees on local meteorology in a medium-size Mediterranean city (Lecce, IT) using field measurements and Computational Fluid Dynamics (CFD) simulations. Measurements were taken in two parallel street canyons with and without trees. Building façades and ground temperatures were estimated from infrared (IR) images, while flow and turbulence were measured by three ultrasonic anemometers. CFD simulations were performed by employing the Reynolds Stress Model (RSM). Overall the analysis shows that trees reduce the wind speed and alter the typical diurnal cycle of surface and air temperature within the canyon. In particular, the flow interaction with trees induces wind direction fluctuations below tree crowns and lower velocities which are expected to lead to larger pollutant and heat trapping in the lower part of the street canyon, especially in nocturnal hours.

This paper presents the impact of different scenarios of future emissions from ships taking into account the implementation of the Directive 2012/33/EU as far as the sulphur content in fuels is concerned in the port of Brindisi. This is one of the most important of the Adriatic Sea, located in the south-east coast of Italy. Ship emissions are estimated through the MEET methodology using appropriate emission factors for manoeuvring and hoteling phases. Numerical simulations of NOx, SO2 and primary PM emissions and dispersion are performed by means of the mesoscale model BOLCHEM coupled with ADMS-Urban dispersion model. This study extends our previous work presented at the 15th Harmo Conference in Madrid, by considering different sulphur content in the fuel oil to analyse the impact of future emission scenarios on the foreseen reduction of pollutant concentrations and assess the potential improvement of air quality in the port area.

A novel, flexible method to derive urban morphometric parameters is presented. Through selected examples, it demonstrates its employability in a wide range of applications. This method builds upon an extension of an image-based technique for the treatment of building data to discuss objective criteria for model grid choice and related consequences. Starting from an estimation of aerodynamic parameters, and their validation by computational fluid dynamics using an existing simulation of downtown Oklahoma City in the US, the method is used to evaluate improvements in the performance of an operational dispersion model. Results are applied to flow over a neighbourhood for the determination of ventilation parameters. It is suggested that the grid used for calculation of morphometric parameters provides the best agreement with data from laboratory experiments when the selection of grid size is made upon the spatial profile of building height standard deviation and maximum building height. The implication is that when a mesoscale numerical model is employed, morphometric parameters should be calculated by positioning the computational grid based on physical boundaries, while for finer resolution (namely, smaller scale) numerical models, morphometric parameters should be calculated using the street grid as external boundary, and the maximum building height criterion performs well.

The impact of ship emissions on the surface concentration of nitrogen oxides (NOx) and ozone (O-3) in the Mediterranean area of the harbour of Brindisi (IT) has been investigated. Numerical simulations have been performed for a summer period of the year 2012, at different spatial scale, using the meso-scale BOLCHEM and the local-scale ADMS-Urban models. Results show that while average ground concentration of primary pollutant NOx increases in the area surrounding the port, a decrease in O-3 concentration is observed.

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 (TM) 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.

Focusing on the effect of street morphology on the ventilation efficiency, this paper presents 3D computational fluid dynamics (CFD) simulations of airflow and pollutant dispersion within urban-like three-way intersections, four-way intersections and roundabouts. The steady-state Reynolds-averaged Navier-Stokes (RANS) k-ε turbulence model is adopted and eight directions of the approaching wind are considered. The ventilation efficiency is evaluated using the ventilation indices purging flow rate (PFR) and the net escape velocity (NEV). Results show the sensitivity of the ventilation efficiency to the type of intersection, to the wind direction and to the number of branches. Specifically, the ventilation efficiency of the investigated three-way intersections is found to be better than that of the other intersections, especially when the angle between the streets is large, while that of roundabouts is also considerable, even with a similar average wind velocity, among the cases evaluated in this paper. Further, the influence of the wind direction for the three-way and four-way intersections is greater than that on roundabouts. Studies on the ventilation efficiency at urban intersections are not common in the literature and this work may help urban planners to better design such hub nodes of urban traffic, where traffic-related pollutants are not easily dispersed, thus avoiding harm to the health of pedestrians and surrounding residents.

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

Air pollution in cities, especially in existing urban canyons, poses a global significant long-term health risk to the urban population, and urban planners are faced with the challenge of increasing the transport of pollutants out of cities. In recent years, many research and review articles have explored passive methods of altering the urban canyon geometry using ‘obstacles’ that alter the dispersion of air flow in these canyons, thereby enhancing pollutant dispersion. These obstacles have been categorised (in previous review articles) into porous and non-porous barriers. Porous barriers include vegetated shrubs and trees, while non-porous barriers include parked cars, low boundary walls, roadside barriers, wind catchers and solar chimneys. This paper expands on previous reviews to highlight newer studies, their findings, and challenges in implementing these measures for existing urban canyons. In addition, it attempts to bring us one step closer to developing urban design guidelines that improve air quality in the urban environment.

Current European legislation aims to reduce the air pollutants emitted by European countries in the coming years. In this context, this article studies the effects on air quality of the measures considered for 2030 in the Spanish National Air Pollution Control Programme (NAPCP). Three different emission scenarios are investigated: a scenario with the emissions in 2016 and two other scenarios, one with existing measures in the current legislation (WEM2030) and another one considering the additional measures of NAPCP (WAM2030). Previous studies have addressed this issue at a national level, but this study assesses the impact at the street scale in three neighborhoods in Madrid, Spain. NO2 concentrations are modelled at high spatial resolution by means of a methodology based on computational fluid dynamic (CFD) simulations driven by mesoscale meteorological and air quality modelling. Spatial averages of annual mean NO2 concentrations are only estimated to be below 40 µg/m3 in all three neighborhoods for the WAM2030 emission scenarios. However, for two of the three neighborhoods, there are still zones (4–12% of the study areas) where the annual concentration is higher than 40 µg/m3. This highlights the importance of considering microscale simulations to assess the impacts of emission reduction measures on urban air quality.