This study investigates the turbulent dispersion of pollutants in the wake of a two-dimensional square obstacle. Utilizing Laser Doppler Anemometry and Particle Image Velocimetry, we characterized the flow dynamics, identifying a recirculation zone downstream of the obstacle, marked by high shear and increased turbulent viscosity, and playing a crucial role in turbulent momentum exchange. We evaluated the turbulence kinetic energy budget, estimating its dissipation rate, and found traditional isotropy and Taylor hypothesis methods inadequate within the wake region. Furthermore, we explored pollutant dispersion from a linear source located downstream the obstacle. Analysis of mean concentration and variance revealed that the log-normal distribution is most effective for modelling concentrations within the recirculating region, while the Gamma distribution suits areas outside it. Testing various closure models for turbulent mass fluxes highlighted the limitations of the Simplified Gradient Diffusion Hypothesis model, favouring more complex closure models for longitudinal trends, though these still faced challenges with intensity estimation. The Simplified Gradient Diffusion Hypothesis model proved robust for vertical mass fluxes, with satisfactory results in turbulent diffusivity and turbulent Schmidt number calculations. The experimental results serve as a benchmark for validating numerical simulations and assessing the accuracy of closure models typically employed in pollutant dispersion modelling.

Inserting vegetation within the urban environment mitigates the urban heat island effect, the flooding risk, and improves air quality. However, its aerodynamic effect has remarkable impact on the pollutant transport and, consequently, on human health comfort. Indeed, the presence of vegetation within an urban canyon leads to non-trivial patterns of pollutant concentration and mass fluxes, as a consequence of complex mean and turbulent velocity fields. In addition to the vegetation density, the flow structure within canyons is influenced by their geometry and by the wind direction. The aim of the present study is to experimentally investigate the velocity field within a canyon, varying the vegetation density and the wind direction. We measured flow velocity statistics within an indefinitely long street canyon, with unit height-to-width ratio, subject to a neutrally stratified boundary layer modeled in the wind tunnel of École Centrale de Lyon. The aerodynamic impact of vegetation was reproduced by inserting plastic miniatures of trees along the two long sides of the canyon. We considered an empty canyon and a vegetated canyon, whose longitudinal axes are oriented with angles of 0°, 30°, and 60° with respect to the external wind flow. Results reveal that when the canyon is inclined with respect to the external wind direction the mean flow follows a complex helicoidal structure. The presence of trees decreases significantly the mean longitudinal velocity and weakens the transversal circulation in the inclined canyon. The dampening effect of the mean longitudinal flow is more marked increasing the inclination angle of the canyon. Turbulent fluctuations are enhanced above the tree crowns, mostly when the wind blows parallel to the canyon axis. On the contrary, turbulent fluctuations decreases at tree trunk and crown levels, in particular when the canyon is inclined of 60° with respect to the external wind direction. Spectra of the velocity signal show that the presence of trees induces an evident shift of the energy peak towards high frequencies. The collected data constitute a step forward to understand and modeling the urban microclimate.

Air pollution in cities, intensified by vehicular traffic emissions and reduced ventilation, poses a significant health risk. Obstacles, like solid or vegetation barriers, are being considered as strategies to reduce pollution exposure for pedestrians and nearby residents in street canyons. This study utilises wind tunnel experiments, to simulate a typical urban canyon with street intersections on both sides, in a 1:200 scale and an H/W=0.5, positioned perpendicular to the free stream wind flow. A passive scalar representing a vehicular pollutant is released along the length of the street canyon. Concentration measurements inside the canyon are performed to determine the effect of parked cars, boundary walls, hedges and trees on pollutant concentration exposure for pedestrians on the sidewalk. Results show that one circulating vortex is generated within the canyon, driving the pollutant to accumulate along the leeward (upwind) wall. Tightly parked cars, boundary walls and hedges, placed along the sidewalk near the leeward wall, can reduce pedestrian pollutant exposure by 15 %, 23 % and 11 % respectively along this sidewalk. Attributes such as obstacle height, surface roughness and porosity play a key role in their performance. However, trees, when placed in the same area, increase pedestrian pollutant exposure by 51 % and 17 % under dense and sparse tree arrangements, respectively. While a broader analysis that considers the variability of vegetation attributes (e.g., porosity, stand density) is desirable, this study remains crucial for validating numerical simulations and suggesting optimal urban measures to reduce pollution exposure for citizens.

The presence of vegetation within urban canyons leads to non-trivial patterns of the concentration of airborne pollutants, as a result of the complex structure of the velocity field. To investigate the relationship between concentration, velocity fields and vegetation density, we have performed wind-tunnel experiments in a reduced-scale street canyon, oriented perpendicular to the external wind flow, within which we placed a steady ground-level line source of a passive tracer. The aerodynamic behavior of vegetation was reproduced by inserting plastic miniatures of trees along the two long sides of the canyon, according to three different densities. The canyon ventilation was investigated by acquiring one-point simultaneous statistics of concentration and velocity over a dense grid of points within the canyon. The results show that the presence of trees hinders the upward mean vertical velocity at the rooftop, causes a reduction of the turbulent kinetic energy inside the canyon, and reduces the energy content of the large scales. The scalar concentration is conversely characterized by an enhanced level of turbulent fluctuations, whose magnitude is not dampened increasing the tree density. Within the canyon, high tree density inhibits turbulent mass fluxes, which are instead enhanced at roof level, where the mean component of the scalar flux is however hindered. A statistical analysis of concentration time series reveals that the lognormal distribution is suitable to model concentration fluctuations and extreme events, in dispersing plumes emitted by a linear source.

The level of air pollution in a street canyon depends on the non-trivial interplay between vehicular exhaust emissions, atmospheric transport, and physico-chemical transformation of pollutants. In this study, we investigate the temporal dynamics of air pollutant concentration in a deep street canyon orthogonal to the wind direction using a two-box model. Simulations provide insights into both steady-state mean concentrations and the magnitude of concentration fluctuations as pollutant nature (inert or reactive), emission signal stochasticity, and the fundamental rates of turbulent transport and chemical transformations vary. Beyond elucidating the role of different parameters on mean pollution levels, the results reveal that extreme air pollutant concentrations are more likely to occur when the characteristic time scale of vehicular exhaust emissions significantly exceeds the ventilation time scale of the street canyon. The innovative modelling approach paves the way for various applications, particularly in the field of optimal traffic management and the study of citizens' exposure to air pollution.

We develop a linear model based on a complex network approach that predicts the effect of emission changes on air pollution exposure in urban street networks including NO–NO2–O3-chemisty. The operational air quality model SIRANE is used to create a weighted adjacency matrix A describing the relation between emissions of a passive scalar inside streets and the resulting concentrations in the street network. A case study in South Kensington (London) is used, and the adjacency matrix A0 is determined for one wind speed and eight different wind directions. The physics of the underlying problem is used to infer A for different wind speeds. Good agreement between SIRANE predictions and the model is observed for all but the lowest wind speed, despite non-linearities in SIRANE's model formulation. An indicator for exposure in the street is developed, and it is shown that the out-degree of the exposure matrix E represents the effect of a change in emissions on the exposure reduction in all streets in the network. The approach is then extended to NO–NO2–O3-chemisty, which introduces a non-linearity. It is shown that a linearised model agrees well with the fully nonlinear SIRANE predictions. The model shows that roads with large height-to-width ratios are the first in which emissions should be reduced in order to maximise exposure reduction.

To predict pollutant concentration in urban areas, it is crucial to take into account the chemical transformations of reactive pollutants in operational dispersion models. In this work, we derive and discuss two photostationary (with constant or varying transformation rates) and one non-photostationary chemical models for NO − NO2−O3 pollution in a street canyon. In the analytical derivation, we focus on the chemical and transport time scales to evaluate the applicability of the models in different urban contexts. We then assess their performance in predicting NO2, NO and O3 concentration at three locations within an urban district by comparing the model predictions with measurements acquired in a field campaign. The results are in line with analytical speculations and highlight in which street types non-photostationary models can bring substantial advantages. In courtyards with limited ventilation and without direct emissions, the performance of the photostationary model with meteorology-based transformation rates is satisfactory. On the other hand, the application of a non-photostationary model significantly improves the predictions in urban canyons with direct vehicular emissions. The applicability of the proposed models in operational tools at the city scale is finally discussed.

Experimental investigations using wind and water tunnels have long been a staple in fluid mechanics research. These experiments often choose a specific physical process to be investigated, whereas studies involving multiscale and multiphysics processes are rare. In the era of climate change, there is increasing interest in innovative experimental studies in which fluid (wind and water) tunnels are used in the modeling of multiscale, multiphysics phenomena of the urban climate. Fluid tunnel measurements of urban-physics-related phenomena are also required to facilitate the development and validation of advanced multiphysics numerical models. As a repository of knowledge for modeling these urban processes, we cover the fundamentals, experimental design guidelines, recent advances, and outlook of eight selected research areas, i.e., (i) absorption of solar radiation, (ii) inhomogeneous thermal buoyancy effects, (iii) influence of thermal stratification on land-atmosphere interactions, (iv) indoor and outdoor natural ventilation, (v) aerodynamic effects of vegetation, (vi) dispersion of pollutants, (vii) outdoor wind thermal comfort, and (viii) wind flows over complex urban sites. Three main challenges are discussed, i.e., (i) the modeling of multiphysics, (ii) the modeling of anthropogenic processes, and (iii) the combined use of fluid tunnels and scaled outdoor and field measurements for urban climate studies.

The exacerbation of the urban heat island due to global warming poses a serious risk to the health of citizens. Furthermore, the alteration of the urban microclimate affects air quality with an expected increase in the concentrations of harmful pollutants. Greening cities is an effective tool to mitigate these effects. However, the effect of tree planting in urban street canyons is still a debated topic. Despite their positive effect on temperature and their filtering action, trees can hinder air circulation thus limiting pollutant removal processes. In this context, it is essential to understand and model the effect of trees on the ventilation of street pollutants, heat and moisture . To this end, we present in this work the results of an experimental campaign conducted in a wind tunnel. An urban geometry with a street canyon perpendicular to the wind direction was reproduced. A linear source of passive scalar simulated the emission of pollutants from vehicular traffic. Reduced scale trees have been conceived to mimic a realistic aerodynamic behaviour. We investigated four different configurations of vegetation density: a street with no trees, two trees in the middle of the street, two rows of scattered trees and two dense rows of trees. Concentration and velocity measurements were performed in order to characterize the transfer processes of pollutants inside the street and to estimate a bulk vertical exchange rate. Results show that the presence of trees alters the concentration field in the street with a progressive shift from a nearly two-dimensional to a three-dimensional field. Despite the significant spatial variation in concentration, the presence of trees does not alter the overall efficiency of the ventilation as the vertical bulk exchange velocity remains almost constant in the different configurations. The statistical analysis of the turbulent concentration signal gives other insights in the transfer processes. The turbulent signal measured in different positions of the cavity and for different tree density follows a Gamma distribution with constant fluctuation intensity suggesting an almost universal behaviour within the canyon and providing a powerful modelling tool. Finally, combined measurements of concentration and velocity allows to measure the turbulent mass fluxes at the roof height and investigate their spectrum therefore enlightening the effect of trees on typical scales of motion.

Greening cities is a key solution to improve the urban microclimate and mitigate the impact of climate change. However, the effect of tree planting on pollutant dispersion in streets is still a debated topic. To shed light on this issue, we present a wind-tunnel experiment aimed at investigating the effect of trees on street canyon ventilation. An idealized urban district was simulated by an array of blocks, and two rows of model trees were arranged at the sides of a street canyon oriented perpendicularly with respect to the wind direction. Reduced scale trees were chosen to mimic a realistic shape and aerodynamic behaviour. Three different spacings between the trees were considered. A passive scalar was injected from a line source placed at ground level and concentration measurements were performed in the whole canyon. Results show that the presence of trees alters the concentration pattern in the street with a progressive shift from a nearly two-dimensional to a three-dimensional field depending on tree density. Despite the significant change of the concentration field induced by trees, the average level of pollution in the street, and thus the overall ventilation efficiency, does not show a specific trend with the density of trees.

To predict pollutant concentration in urban areas, it is crucial to take into account the chemical transformations of reactive pollutants in operational dispersion models. In this work, we derive and discuss different NO-NO2-O3 chemical street canyon models with increasing complexity and we analytically evaluate their applicability in different urban contexts. We then evaluate the performance of the models in predicting NO2 concentration at different locations within an urban district by comparing their predictions with measurements acquired in a field campaign. The results are in line with analytical speculations and give indications as to which model to use according to the conditions of the urban street canyon. In courtyards with limited ventilation and without direct emissions, the performance of the photostationary model is satisfactory. On the other hand, the application of a non-photostationary model significantly improves the predictions in urban canyons with direct vehicular emissions. The applicability of the proposed models in operational tools at the city scale is finally discussed.

Modélisation de la dispersion des polluants à l'échelle de la ville et de la rue : expériences en soufflerie et théorie des réseaux complexes La pollution atmosphérique dans les zones urbaines constitue une menace sérieuse pour la santé et la sécurité des résidents. Centres d'activité économique, politique et sociale, les villes sont densément peuplées et exposées à un grand nombre de sources de pollution. Ces facteurs, ainsi que le risque d'actes malveillants à des fins terroristes, les rendent extrêmement vulnérables aux rejets gazeux. Afin de prévoir et gérer la pollution atmosphérique, il est très important de comprendre et de modéliser les phénomènes de dispersion dans l'atmosphère urbaine. Contrairement au transport loin des parois, l'écoulement et la dispersion dans la canopée urbaine sont principalement régis par la géométrie complexe de la ville. L'objectif de cette thèse est d'étudier et de modéliser l'effet de la forme urbaine sur la dispersion des polluants à deux échelles spatiales différentes: à l'échelle du quartier et à l'échelle de la rue. Le tracé des rues est la géométrie dominante à l'échelle du quartier. Partant de ce constat, la première partie de cette thèse propose une nouvelle perspective basée sur la théorie des réseaux complexes pour analyser et modéliser le transport de polluants le long des rues d'une ville. La canopée urbaine est modélisée comme un réseau. Les rues-canyons et leurs intersections façonnent la structure spatiale du réseau. La direction et la capacité de transport de l’écoulement dans les rues déterminent les directions et le poids des liens du réseau. Avec cette interprétation mathématique, la propagation est modélisée comme un processus de diffusion sur un réseau et les zones les plus dangereuses d'une ville sont identifiées comme les nœuds avec le potentiel de diffusion le plus élevé. Dans ce but, nous introduisons une nouvelle métrique de centralité adaptée au transport de masse dans les réseaux de flot. L'approche proposée fournit un outil opérationnel pour identifier les lieux d'une ville avec le potentiel de dispersion le plus élevé. En outre, le modèle est utile pour identifier les propriétés structurelles qui rendent les villes vulnérables à la pollution atmosphérique. La comparaison entre quatre villes emblématiques, aux plans urbains différents, montre que la vulnérabilité dépend des propriétés topologiques du tissu urbain: la connectivité des rues et la variabilité de l'orientation des rues par rapport à la direction du vent extérieur. À l'échelle de la rue, les dynamiques d'écoulement et de dispersion sont régies par la géométrie du canyon. Le transport des polluants peut être décomposée en transport longitudinal le long de la rue et en transfert vertical entre le canyon et l'écoulement extérieur. La vitesse moyenne de l'écoulement régit la dispersion le long de la rue. En revanche, les mécanismes d'échange de masse dans le sens vertical ne sont pas encore parfaitement compris. La deuxième partie de cette thèse analyse ces processus à l'aide d'expériences en soufflerie qui simulent la dispersion de polluants dans un canyon orienté perpendiculairement à la direction du vent. Dans cette configuration, l'échange vertical est le seul mécanisme de ventilation du canyon. Au cours des expériences, les conditions du canyon sont modifiées en chauffant alternativement les parois latérales, en modifiant le rapport hauteur/largeur du canyon et en introduisant des éléments de rugosité sur les parois. Deux rangées de maquettes d'arbres sont disposées le long du canyon pour simuler la végétation urbaine. Des mesures de vitesse et de concentration sont effectuées dans le canyon et une vitesse d'échange caractéristique entre la rue-canyon et l'atmosphère sus-jacente est estimée pour quantifier la ventilation globale du canyon dans les différentes configurations. Les résultats montrent que l'efficacité de l'échange vertical entre le canyon et l'écoulement externe est principalement déterminée par l'énergie cinétique turbulent à l'intérieur du canyon. [...]

The incidental or malicious release of toxic gases in the atmosphere is one of the most critical scenarios for cities. The impact of these releases varies with the ventilation potential of the urban environment. To disentangle this crucial aspect, vulnerability to airborne releases is here traced back to essential properties of the urban fabric. To this aim, pollutant dispersion is disassembled in its fundamental bricks and the main drivers of the process are captured. The analysis is based on four cities with emblematic architectures: Paris, Firenze, Lyon and New York. Results show that vulnerability is driven by the topology of the city and by its interaction with the approaching wind. In this sense, fragility to toxic releases is written in the layout of the urban fabric and results from its historical evolution. This study paves the way to the assessment of air pollution-related issues from a morphological point of view.

Due to the overall growth of the world population and to the progressive shift from rural to urban centres, 70% of the world population is expected to live in urban areas in 2050. This trend is alarming when related to the constant decline of urban air quality at the global level. To cope with this rapid urbanization, solutions for sustainable cities are extensively sought. In this framework, the mitigation of air pollution in street canyons plays a crucial role. The street canyon (a street flanked by high buildings on both sides) is the fundamental unit of the urban tissue, as well as a vital public and residential space. Street canyons are particularly vulnerable to air pollution due to traffic emissions, low ventilation conditions, and the number of citizens exposed. Tree planting in street canyons is often used as a pollution mitigation strategy, due to the filtering effect of vegetation on airborne pollutants. However, from the aerodynamic point of view, trees can obstruct the wind flow thus reducing canyon ventilation and leading to higher pollutant concentrations. In this framework, we present the results of an experimental study aimed at evaluating how tree planting influences the flow and concentration fields within a street canyon. The study was carried out in a recirculating wind tunnel. An idealised urban district was simulated by an array of square blocks, whose orientation with respect to the incident wind was varied. Within this urban geometry, two rows of model trees were arranged at the sides of a street canyon. Three configurations with different spacing between the trees were considered. A passive scalar was injected from a line source placed at ground level to simulate traffic emissions. Concentration and flow field measurements were performed in several cross-sections of the street canyon. Results showed the effect of trees on the spatial distribution of pollutants. Moreover, a characteristic exchange velocity between the street canyon and the overlying atmosphere was estimated to quantify the overall canyon ventilation under several wind directions and different planting densities. These preliminary results provide city planners with first recommendations for the sustainable design of urban environments. Moreover, the experimental dataset is valuable in validating numerical simulations of air pollution in cities accounting for urban vegetation.

Understanding the dynamics of mass exchange between a street canyon and the overlying atmosphere is crucial to predict air quality in urban areas. Despite the large number of studies on this topic, there are many aspects that still need to be clarified. Among these, one is certainly the role of thermal stratification in street canyon ventilation. In order to fill this gap, this study evaluates how the combined effect of street canyon geometry, wall roughness and differential heating of the building facades influences pollutant dispersion within the canyon and out of it. The study was carried out in a wind tunnel, adopting an idealized urban geometry made up of square bars placed normal to the wind direction. The boundary conditions inside the canyon were modified by heating its windward and leeward walls, by changing its aspect ratio and by introducing roughness elements at the walls. A passive scalar was injected from a line source at ground level. The flow and concentration fields were measured in a cross‐section of the canyon. Characteristic exchange velocities within the canyon and towards the external flow were estimated comparing the experimental data with an analytical model for the cavity wash‐out. Results show that the transition from one recirculating cell to two counter‐rotating cells inhibits canyon ventilation, with a consequent increase in pollutant concentration at the pedestrian level. This transition occurs as the cavity aspect ratio increases and is facilitated by adding roughness elements at the windward wall. Heating the leeward wall has negligible effects on canyon ventilation. Heating the windward wall accelerates pollutant removals in square cavities, while it contributes to a worsening of air quality in narrow cavities. Finally, the wash‐out times of the cavity are discussed in terms of a relative contribution of the mean advective motion and its turbulent counterpart.

Understanding the dynamics of mass and heat exchange between a street canyon and the overlying atmosphere is crucial to predict air quality and microclimatic conditions within dense urban areas. Previous studies have demonstrated that the bulk transfer between the street and the overlying flow is entirely governed by the intensity of turbulent fluctuations within the street. The aim of this experimental study is to evaluate how the geometry of the street canyon and the solar radiation on building façades influence the turbulent velocity field within a two-dimensional street canyon and thus the global street canyon ventilation. The study was carried in a wind tunnel. The boundary conditions inside the canyon were modified by heating its windward and leeward walls and by changing the cavity aspect-ratio. The flow field in a cross-section of the street canyon was measured with particle image velocimetry. Temperatures were measured by means of thermocouples. The velocity and vorticity fields are analysed and discussed.

The dispersion of airborne pollutants in the urban atmosphere is a complex, canopy-driven process. The intricate structure of the city, the high number of potential sources, and the large spatial domain make it difficult to predict dispersion patterns, to simulate a great number of scenarios, and to identify the high-impact emission areas. Here we show that these complex transport dynamics can be efficiently characterized by adopting a complex network approach. The urban canopy layer is represented as a complex network. Street canyons and their intersections shape the spatial structure of the network. The direction and the transport capacity of the flow in the streets define the direction and the weight of the links. Within this perspective, pollutant contamination from a source is modeled as a spreading process on a network, and the most dangerous areas in a city are identified as the best spreading nodes. To this aim, we derive a centrality metric tailored to mass transport in flow networks. By means of the proposed approach, vulnerability maps of cities are rapidly depicted, revealing the nontrivial relation between urban topology, transport capacity of the street canyons, and forcing of the external wind. The network formalism provides promising insight in the comprehensive analysis of the fragility of cities to air pollution.

This work presents an algorithm for real-time fault detection in the SCADA system of a modern water supply system (WSS) in an Italian alpine valley. By means of both hardware and analytical redundancy, the proposed algorithm compares data and isolates faults on sensors through analysis of residuals. Moreover, the algorithm performs a real-time selection of the most reliable measurements for the automated control of the WSS operations. A coupled model of the hydraulic and remote-control system is developed to test the performance of the WSS when the proposed algorithm is applied or not. Simulations show that the occurrence of errors in the sensors causes significant worsening in the economic, energy and mechanical performance of the infrastructure. In many cases, the operations of the WSS are seriously compromised. The error detection and measurement assessment performed by the proposed algorithm proves to be crucial for the safe control of the WSS.

The stability of the tank-level controller of hydraulic infrastructures is a complex issue. Past studies have focused on run-of-river hydropower plants with a short penstock, and where the upstream tank level is regulated by a variable-speed turbine. The short penstock entails that the flow regulation performed by the turbine can be considered instantaneously effective in the upstream tank. This is not true in water supply systems in mountain areas, where the length of the water main easily exceeds 10 km. This work is devoted to assessing the stability of the water-level controller of hydraulic systems made up of very long conduits. To do this, we numerically studied the stability of a large number of ideal case studies representative of real hydraulic infrastructures. The key results are (1) the understanding of the causes of the water-level controller instability; and (2) a set of charts that predicts whether the control system is unstable. These charts should be used during the design stage of this type of systems to select values of the tank area that avoid tank-level-controller instabilities.

This work presents the technical characteristics and the regulation system of a complex water supply system (WSS) in an Italian Alpine valley. The WSS faced multiple challenges: water supply over a large area, hydropower generation, and coordination among multiple local sources and networks. The development of an optimal feedback-control algorithm for the supervisory control system was key to guaranteeing the operation of this modern WSS. This regulation scheme and the rationale for its development are described in this paper. A customized numerical model of the WSS was developed in order to test the operating rules through suitable numerical simulations. Results show that the proposed algorithm satisfies the objectives of the WSS and respects its tight constraints. The analysis of the case study evidences the advantages of the coordination among the municipal water networks, quantifies the hydropower generation potential in the WSS, and highlights the key roles of automation and remote control in modern water systems. Finally, the case study presented here provides an efficient technical solution for the hydraulic regulation of a high-pressure water main connecting a cascade of small tanks in mountainous regions. (c) 2017 American Society of Civil Engineers.