Дисертації з теми "Airflow in street canyons"

The local wind climate within the urban environment plays a key role in the removal of heat and pollutants from pedestrian occupied areas as well as having an impact on pedestrian comfort and safety. One component of the urban landscape which is often neglected in the consideration of airflow is tree planting which can constitute a major component of the built environment. The aim of this research was therefore to gain a greater understanding into the effect of tree planting on airflow within street canyons and investigate the use of CFD modelling in predicting such effects. This aim was accomplished through the use of CFD modelling and field measurements of tree-lined and non tree-lined streets. Tree canopies were represented within the CFD model by porous subdomains containing momentum and turbulence sinks. This simple representation was found to offer favourable comparison against field measurements and would therefore provide a simple and effective method for the inclusion of trees within CFD models of the urban environment. Results of both the CFD models and field measurements found reduced wind speeds at pedestrian level as well as a significant reduction in vertical wind speeds at roof level within the tree-lined street. There was also seen to be a significant reduction in turbulence levels within the street containing trees. Based on these findings it can be concluded that trees are likely to be a useful aid in urban design helping to reduce high wind speeds and turbulence thus creating outdoor environments which are comfortable and safe for pedestrian use. However the results also indicate that the addition of trees to streets can reduce the amount of air exchange at roof top level that occurs and thus may lead to a reduction in natural ventilation and potential build-up of pollutants within pedestrian occupied areas.

Les arbres apportent de nombreux services écosystémiques en ville, ils permettent de diminuer certaines conséquences de l’urbanisation comme l’îlot de chaleur urbain et le ruissellement de l’eau. Leur effet thermo-radiatif améliore le confort thermique. Les arbres peuvent également impacter la qualité de l’air en ville via différents processus. Le dépôt de polluants gazeux et particulaires sur les feuilles des arbres peut contribuer à la diminution des concentrations. Cependant, l’effet aérodynamique des arbres modifie l’écoulement dans les rues canyons et limite la dispersion des polluants émis dans la rue. Par ailleurs, les arbres émettent des composés organiques volatils biogéniques (COVb) qui peuvent participer à la formation d’O3 et d’aérosols organiques secondaires. Les émissions de COVb varient selon l’espèce d’arbre, et sont influencées par des facteurs climatiques (température, rayonnement) mais aussi par le statut hydrique des arbres. Cette thèse a pour objectif de quantifier les impacts de ces différents processus sur la qualité de l’air en ville. Des simulations numériques sont réalisées sur la ville de Paris pendant l’été 2022 avec la chaîne de modèles CHIMERE/MUNICH afin de quantifier l’impact des arbres sur les concentrations atmosphériques de polluants à l’échelle locale et régionale. Les concentrations simulées sont comparées à des mesures. Les arbres urbains ne sont généralement pas pris en compte dans les modèles de qualité l’air, aussi bien à l’échelle régionale qu’à l’échelle de la rue. Pour intégrer les émissions de COVb dans le modèle régional CHIMERE, un inventaire est réalisé à partir de la base de données des arbres de la ville de Paris. Une méthode est développée afin d’estimer les caractéristiques des arbres qui sont utilisées en données d’entrée des différents modèles (surface de feuille, biomasse sèche, taille de la couronne, etc.). En moyenne sur les mois de juin et juillet 2022 à Paris, les émissions biogéniques locales des arbres induisent une augmentation de 1,0% d’O3, 4,6% de PM1 organiques et 0,6% de PM2.5. Les émissions biogéniques des arbres urbains augmentent très fortement les concentrations d’isoprène et de monoterpènes. Par comparaison aux mesures, les concentrations de terpènes ont tendance à être sous-estimées, compte tenu des incertitudes liées aux facteurs d’émissions et à la part de végétation manquante dans l’inventaire. Les émissions de terpène de la végétation urbaine et suburbaine influencent fortement la formation de particules organiques, il est donc important de bien les caractériser dans les modèles de qualité de l’air. Les différents effets des arbres urbains sur la qualité de l’air à l’échelle de la rue sont ensuite ajoutés dans le modèle de réseau de rue MUNICH. L’effet aérodynamique des arbres dans les rues est paramétré à partir de simulations de mécanique des fluides. Il induit une augmentation des concentrations des composés émis dans la rue. Cette augmentation peut atteindre +37% pour le NO2 dans les rues avec une surface de feuilles importante et un trafic élevé. Le dépôt sur les feuilles des arbres est calculé à partir d’une approche résistive adaptée à l’échelle de l’arbre urbain dans la rue. Cependant, son impact sur les concentrations reste limité sur les gaz et particules étudiés (< -3%).Pour finir, un couplage entre les modèles TEB (modèle de surface urbaine), SPAC (modèle de continuum sol-plante-atmosphère) et MUNICH a été mis en place. Ce couplage permet de mieux représenter les impacts des hétérogénéités du micro-climat urbain et de l’effet thermo-radiatif des arbres sur les concentrations de gaz et de particules. L’effet de ce micro-climat et du stress hydrique des arbres sur les émissions de COVb est aussi pris en compte afin d’affiner le calcul des émissions
Trees provide numerous ecosystem services in cities, helping to reduce some of the consequences of urbanization, such as the urban heat island and water run-off. Their thermo-radiative effect improves thermal comfort.Trees can also have an impact on urban air quality through various processes. The deposition of gaseous and particulate pollutants on tree leaves can help to reduce concentrations. However, the aerodynamic effect of trees modifies the airflow in street canyons and limits the dispersion of pollutants emitted in the street. Trees also emit biogenic volatile organic compounds (BVOCs), which can contribute to the formation of O3 and secondary organic aerosols. BVOC emissions vary depending on the tree species, and are influenced by climatic factors (temperature, radiation) and by the tree water status.The objective of this thesis is to quantify the impacts of these different processes on urban air quality. Numerical simulations are performed over the city of Paris during summer 2022 using the CHIMERE/MUNICH model chain in order to quantify the impact of trees on atmospheric concentrations of pollutants at the local and regional scales. The simulated concentrations are compared to measurements.Urban trees are not generally taken into account in air quality models, either at regional or street level. In order to integrate BVOC emissions into the CHIMERE regional model, an inventory is developed using the tree database of the city of Paris. A method is set up to estimate the characteristics of the trees, which are used as input data for the various models (leaf area, dry biomass, crown size, etc.). On average over the months of June and July 2022 in Paris, local biogenic emissions from trees lead to an increase of 1.0% in O3, 4.6% in organic PM1 and 0.6% in PM2.5. Biogenic emissions from urban trees strongly increase concentrations of isoprene and monoterpenes. Compared with measurements, terpene concentrations tend to be underestimated, given the uncertainties associated with emission factors and the missing part of the vegetation in the inventory. Terpene emissions from urban and suburban vegetation greatly influence the formation of organic particles, it is therefore important to characterize them properly in air quality models.The various effects of urban trees on air quality at street level are then added into the MUNICH street network model. The aerodynamic effect of street trees is parameterized using computational fluid dynamics simulations. It leads to an increase in the concentrations of compounds emitted into the street. This increase can reach +37% for NO2 in streets with a large leaf surface and high traffic. Deposition on tree leaves is computed using a resistive approach adapted to the scale of the tree in the street. However, its impact on concentrations remains limited for the gases and particles studied (< -3%).Finally, a coupling between the TEB (urban surface model), SPAC (soil-plant-atmosphere continuum model) and MUNICH models is developed. This coupling provides a better representation of the impacts of the urban micro-climate heterogeneities and of the thermo-radiative effect of trees on gas and particle concentrations. The effects of the micro-climate and of the tree water stress on BVOC emissions are also taken into account in order to refine the calculation of emissions

This thesis analyzes the characteristics of flow pattern and vehicle-emitted pollutant dispersion in roughness surface layer. In an urban environment, wind flow and transported-pollutant source interfere strongly with buildings and other roughness elements on the surface ground, which results in complex characteristics of flow pattern and pollutant dispersion in 3D circumstances. The present study intends to simplify the research domain and investigate the fundamental modeling problems that exist in the field. The current physical research topic is restricted to 2D street canyon in equilibrium conditions. The study is motivated by the fact that characteristics of flow pattern and pollutant distribution inside street canyons are important for public health. The research has applied the computational fluid dynamics (CFD) methodology. To date, insights have typically focused on idealized street canyons without strictly limited boundary conditions and turbulence models. Those approaches face challenges related to their applicability to real urban scenarios or the reliability of prediction results. The thesis examines the influence of grid density, turbulence models and turbulent Schmidt number on pollutant distribution at windward and leeward surfaces of street canyon. Since numerical results usually are validated with wind-tunnel measurement data, the results between full-size model and wind-tunnel model are compared in order to test the Reynolds number effect. The lack of measurement data means that the morphometric method is used to generate upcoming wind profile, including the mean vertical velocity and turbulence parameters. The thesis also analyzes the potential errors brought by the method (Scenario A). Based on the evaluated numerical model, the thesis continues to study the impacts of surrounding buildings and geometry of street canyon on flow and pollutant distribution inside street canyons. The effect of wind on pollutant distribution inside street canyons was also investigated (Scenario A). Furthermore, the influence of roof shape and configuration of street canyon on characteristics of flow and pollutant distribution is also systematically studied, with the results shown in scenario B. The main conclusions of the thesis are that the uncertainty of numerical results derives from different aspects. Wind profile in the inlet profile generated by morphometric method brings major error to the simulation results. Current turbulence models cannot compromise the simulation results between flow field and pollutant distribution field. Ignored small-scale obstacles also need to be handled carefully. Numerical results revealed that flow and pollutant distribution inside street canyons are mainly dominated by the geometry of the street canyon itself. Medium-spaced surrounding buildings are also better able to transport pollutant out of the street canyon. Through systematic analysis, roof shape is proven to have a significant effect on flow and pollutant distribution inside a street canyon. The major impact is altered turbulence intensity depth and strength of shear layer inside the street canyon, which is important for pollutant removal process out of the street canyon. In the future, advanced turbulence models accompanied by small-obstacle effect models need to be developed in order to reliably simulate flow and pollutant dispersion simultaneously. Based on the advanced turbulence model, simulation of flow and pollutant dispersion in a complex 3D environment is essential in the next steps for the purpose of engineering application. Accurate vertical wind profile provided for inlet profile is another interesting direction for further development. Keywords: Flow; Pollutant dispersion; CFD; Street canyon; Reliability

QC 20130215

Flows and pollutant dispersion over flat rural terrain have been investigated for decades. However, our understanding of their behaviours over urban areas is rather limited. Most cases have either focused on street level or in the roughness sub-layer (RSL) of urban boundary layer (UBL). Whereas, only a handful of studies have looked into the coupling between street-level and UBL-core dynamics, and their effects on pollutant dispersion. In this thesis, computational fluid dynamics (CFD) is employed to examine the flows and pollutant transport in and over urban roughness. Idealised two-dimensional (2D) street canyons are used as the basic units fabricating hypothetical urban surfaces. A ground-level passive and chemically inert pollutant source is applied to simulate the flows and pollutant dispersion over rough surfaces in isothermal condition. Large-eddy simulation (LES) with the one-equation subgrid-scale model is used to solve explicitly the broad range of scales in turbulent flows. Arrays of idealized street canyons of both uniform and non-uniform building height are used to formulate a unified theory for the flows and pollutant dispersion over urban areas of different morphology. The geometry of roughness elements is controlled by the building-height-to-street-width (aspect) ratio (0.083 ≤ AR ≤ 2) and/or the building height variability (BHV = 0.2, 0.4 and 0.6), in which the characteristic regimes of skimming flow, wake-interference and isolated roughness are covered. A detailed analysis on the roof-level turbulence structure reveals parcels of low-speed air masses in the streamwise flows and narrow high-speed down-drafts in the urban canopy layer, signifying the momentum entrainment into the street canyons. The decelerating streamwise flows in turn initiate up-drafts carrying pollutants away from the street canyons, illustrating the basic pollutant removal mechanism in 2D street canyons. Turbulent transport processes, in the form of ejection and sweep, are the key events governing the exchanges of air and pollutant of street canyon. Air exchange rate (ACH) along the roof level is dominated by turbulent transport, in particular over narrow street canyons. The LES results show that both the turbulence level and ACH increase with increasing aerodynamic resistance defined in term of the Fanning friction factor. At the same AR, BHV greatly increases the friction factor and the ACH in dense built areas (AR ≤ 0.25). The turbulence intensity is peaked on the windward side of street canyons that does not overlap with the maximum velocity gradient near the leeward building corners, suggesting the importance of background turbulence in street-level ventilation. Over the building roughness, pollutant plume dispersion after the ground-level area source in cross flows resumes the self-similar Gaussian shape in the vertical direction in which the vertical plume coverage is proportional to the square root of downwind distance in the streamwise direction. Moreover, the vertical dispersion coefficient is proportional to the one-fourth power of friction factor over idealised street canyons. Conclusively, friction factor can be used to parametrise ventilation and pollutant dispersion over urban areas.
published_or_final_version
Mechanical Engineering
Doctoral
Doctor of Philosophy

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 127-130).
Urban planners need a fast, simple model to assess the impact of early design phase iterations of neighborhood layout on the microclimate. Specifically, this model should be able to predict the expected urban heat island intensity and the locations in neighborhood layouts that are prone to pollutant retention. Current models are inadequate for this purpose because they use computationally intensive techniques to solve for flow through a neighborhood and often require a strong technical background for effective use of the models. In this thesis, we use analytical equations and empirical relationships to calculate the expected wind speeds in isolated, idealized street canyons. We demonstrate that flow in street canyons is driven by momentum exchange with the air above. We discuss the importance of flow separation and turbulent exchange between the urban canopy layer and the urban boundary layer for removing heat and pollutants from street canyons. Next, we introduce a method to parameterize this exchange and extend this work to more realistic street canyons and idealized neighborhoods. We evaluate this work using computational fluid dynamics and comparison to experimental results and models from the literature. We examine cases where the flow is influenced by buoyancy effects and assess the applicability of our work in these situations. Finally, we address how this work could be further developed into generalized planning guidelines and incorporated into a comprehensive model for urban planners.
by Terianne Catherine Hall.
S.M.

Recent studies on turbulent exchange processes between the urban canopy layer and the atmosphere above have focused primarily on mechanical effects and less so on thermal ones, mostly by means of laboratory and numerical investigations and rarely in the real environment. More recently, these studies have been adopted to investigate city breathability, urban comfort and citizen health, with the aim to find new mitigation or adaptation solutions to air pollution and urban heat island, to enhance the citizen wellness. To investigate the small-scale processes characterizing vegetative and non-vegetative urban canopies, two field campaigns have been carried out within the city of Bologna, Italy. New mechanical and thermal time scales, and their ratios (rates), associated with inertial and thermal flow circulations, have been derived to this scope. In the non-vegetated canopy, mechanical time scales are found to describe fast exchanges at the rooftop and slow within the canopy, while thermal ones to describe fast mixing in the whole canopy. Faster processes are found in the vegetative canopy, with rapidly mixed mechanical time scales and varying thermal ones. The exchange rates are found to identify favorable mixing conditions in the 50−75% of the investigated period, but extreme disadvantageous events can totally suppress the exchanges. The exchange rates are also found to drive the pollutant removal from vegetated and non-vegetated canopies, with an efficacy which depends on the in-canyon circulation. The impacts of real trees in a real neighborhood of the city is tackled with a simplified fluid-dynamics model, where mean flow and turbulence are studied with different vegetation cofigurations, topological and morphological characteristics. Vegetation is found to increase both blocking and channeling effects on the mean flow and to modify the production/dissipation rate of turbulence, depending on the wind direction and topology. Nevertheless, buildings maintain a predominant impact on the atmospheric flows.

Il particolato atmosferico è uno degli inquinanti atmosferici principali in quanto esercita effetti importanti su clima, tempo meteorologico, visibilità, salute umana, ambiente e beni culturali. Nonostante i numerosi studi, la conoscenza dei processi fisici che lo governano risulta ancora limitata. L'obiettivo della presente tesi è stato quello di analizzare le distribuzioni dimensionali di particelle di aerosol raccolte tramite due contatori ottici di particelle nel periodo Gennaio-Febbraio 2018 in due diverse aree urbane (street canyons) di Bologna, al fine di confrontarne l'andamento temporale e di analizzare l'importanza dei processi di rimozione dall'atmosfera. Lo studio si colloca all'interno del progetto H2020 `iSCAPE' (Improving the Smart Control of Air Pollution in Europe", GA n. 689954), avente come oggetto il controllo della qualità dell'aria tramite lo sviluppo di tecniche innovative di controllo sostenibile e passivo. Dopo aver introdotto i concetti generali, si passa a descrivere l'area di studio ed infine i dati raccolti. Questi ultimi sono stati analizzati sviluppando appositi codici MatLab per la visualizzazione grafica, nonchè per effettuare calcoli sulle distribuzioni ottenute. I risultati mostrano che entrambi i siti sono caratterizzati da fluttuazioni rapide del numero di particelle, con andamenti medi simili, soprattutto per quanto riguarda le particelle più piccole. Il confronto tra i due siti in varie condizioni meteorologiche (nebbia, cielo sereno, pioggia e neve) ha messo in evidenza i valori particolarmente elevati nelle giornate di nebbia e l'efficacia della rimozione umida tramite precipitazioni, soprattutto di quelle nevose, nella diminuzione delle concentrazioni di particelle di aerosol.

In the context of the European iSCAPE project (Improving the Smart Control of Air Pollution in Europe) an experiment was set up in order to test photocatalytic coatings effectiveness in reducing the concentration of pollutants. Further analysis on various aspects of the urban environment have been possible thanks to the experiment. The measurements collected during the experimental field campaign have been used to study the differences in the atmospheric phenomena due to the different morphology of two street canyons. Data in time periods characterized by non-synoptic conditions have been selected and then compared. The first part of the study is dedicated to the characterization of the mean flow. Subsequently, turbulent fluxes have been studied for both canyons with the Eddy Covariance method, in order to appreciate the different behaviour of the two canyons. In the end, the data collected have been elaborated with the Quadrant Analysis. This latest study has made possible to compare the results obtained by the measurements in a real urban canopy layer with the results obtained in a controlled environment and in a vegetated canopy layer. The study has examined the intensity of sweep and ejection effects in an urban canopy layer. The results show that in both canyons sweep effects prevail for the downwind case. Instead, for an upwind flow the dominant effects are those of ejection. For parallel wind directions the two canyons behave differently, due to their different morphology. Furthermore, the intensity of both effects is not as strong as in the perpendicular cases. The behaviour of the fluctuations of CO2 and H2O concentration with respect to the temperature fluctuations has been compared to the results obtained in a vegetated canopy layer. The analysis has shown that in the downwind and parallel cases the trends are in agreement with the ones obtained in the vegetated environment. For an upwind flow, however, the expected trends are not found.

In the last century, there has been a rapid growth and development in economy and modern technology around the world. This phenomenon helped improving wealth and living standard but also brought pollutions to the society and the environment. Among various kinds of pollution, air pollution takes a larger proportion. Therefore, there is increasing concern about the ventilation and pollution removal behavior in the urban environment. Among different academic studies performed, the use of computational fluid dynamics (CFD) had become more popular. Since wind tunnel experiments serve as validations for CFD results, this thesis developed the technique required for wind tunnels experiments and to investigate the pollutant removal related to urban geometry, as well as the technique for gas sampling to examine the distribution of pollutants in urban boundary layer over idealized two-dimensional (2D) street canyons. Three specific tasks are archived to accomplish the above objectives. The first task was to extend the wind tunnel in the Department of Mechanical Engineering, the University of Hong Kong. An extension duct was designed to increase the length of the test section in which the reduced-scale model could be installed. The dimensions of the test section were specified according to the required length for fully developed flow inside the test section, the environment in the laboratory and the original wind tunnel conditions. The extension duct was then constructed and mounted, with the wind profile inside the test section obtained afterwards. After construction of the extended test section for experimental purposes, the second task was to examine the pollutant transport behaviors from the ground level of idealized 2D urban street canyons to the urban atmospheric boundary layer (ABL) using both laboratory wind tunnel measurements and CFD. Movable rectangular aluminum blocks were placed in the wind tunnel in cross-flow to construct street canyons of different building-height-to-street-width (aspect) ratios. Wetted filter papers were applied on the surface of the blocks inside the street region, modeling the source of pollutant emission inside the street canyons. The wind tunnel and CFD results complemented each other to elucidate the pollutant removal mechanism that is in line with other results available in literature. From the experimental results obtained, scaling effect was observed in the mass transfer behaviors even the flows had fulfilled kinematic similarity. A new indicator, the scaled overall pollutant removal coefficient, was formulated for the comparison of pollutant removal performance. The improved agreement in the comparison with the CFD results showed that the scaled overall pollutant removal coefficient could be used to account for the scaling effects occurred in laboratory experiments at finite Reynolds number (〖10〗^(3 ) to 〖10〗^(5 ) in this study) for comparison of pollutant removal performance. The behavior of pollutants inside the street canyons was studied; however, the pollutant concentration inside a street could be affected by the pollutant source in another street, even there were several streets away from it. The pollutant escaped from the source street could act as air entrainment into other streets, affecting the air quality. The concentration profile correlated to the street geometry was thus studied. The last task of this dissertation was to study the effect of urban geometry on the concentration profile of the urban ABL by means of gas dispersion experiments. Experiments were carried out in the wind tunnels of the Department of Mechanical Engineering and Department of Civil Engineering with different sets of experimental models used. A special gas emission source was constructed in order to simulate the linear source due to busy traffic in the street regions. The required gas sampling techniques were also studied throughout the measurement. Trial experiments were carried out and preliminary results had been obtained. Furthermore, the pollutant concentration profiles downstream from a linear pollutant source in an idealized 2D street canyon were also measured. Throughout the experiments, different designs of line source were tested and factors affecting the experimental results were considered. One of the line source designs was adopted and the pollutant concentrations in street canyons of different aspect ratios were observed. The concentration decreases rapidly with increasing distance from the roof but then increases to steady value. The average pollutant concentration over the concentration profile was different at different aspect ratios. It is believed that its performance depends on the pollutant removal behavior from street regions.
published_or_final_version
Mechanical Engineering
Master
Master of Philosophy

In view of the worsening air quality in the world, more concerns are focused on the environment. This thesis uses the technique of CFD and develops the computer model to investigate the wind and pollutant transport, as well as the chemistry of reactive pollutants in idealized two-dimensional (2D) street canyons. Three scientific questions are raised in this thesis. The first task is to find out the po- sition with the most favorable pollutant removal along the ground level over 2D idealized street canyon of different building-height-to-street-width (aspect) ratios (ARs). The di- mensionless parameter, C, represents the pollutant removal performance. In the isolated roughness regime, the two local maximum C locate at the reattachment point and the windward corner. In the wake interference regime, C is peaked on the windward side. The number of vertically aligned recirculations depends on the street depth in the skimming flow regime. The sizes of the secondary recirculation upstream and downstream deter- mine how the maximum C shifts from the street centre. After identifying the position of peaked pollutant removal rate at the ground level, the emission source should be placed with the highest constant C in order to remove the pollutants upward more quickly to safeguard the street-level air quality. After understanding the best pollutant removal in the street canyon of different ARs, the second task is to find out what AR is the most favorable for the ventilation and pollutant removal across the roof level. The three parameters, namely friction factor, air exchange rate (ACH) and pollutant exchange rate (PCH), are introduced to quantify the pressure difference to sustain the mean flow, the ventilation and pollutant removal, respectively. The turbulence contributes more than 70% to the total ACH and PCH in all the three flow regimes. By increasing the atmospheric turbulence in building geometry as well as the surface roughness, the ventilation and pollutant removal performance can be improved. The linear relation between the friction factor and ACH demonstrates the larger resistance that in turn promotes the air exchange over the roof level. The physical dispersion is studied; however atmospheric pollutants are seldom in- ert but chemically reactive instead. The last task is to include the three common air pollutants, NO, NO2 and O3, in the simple NOx ?O3 mechanism in terms of the photo- stationary state and reaction rates. The Damkohler numbers of NO and O3, DaNO and DaO3, are parameterized by the concentrations of the sources NO and O3. The normalized mean and fluctuation NO, NO2 and O3 are separately considered. The integrated pho- tostationary state (PSS) in the first canyon increases with DaO3 under the same DaNO. The integrated PSS of the second to the twelveth street canyons are compared with each case, the monotonic increase in the PSS from the second to twelveth canyon is perceived in DaNO/DaO3 1, 0.03, 0.02, 0.001 and 0.000333. Further decreases the DaNO/DaO3 to 0.000143, 0.000125, 0.000118, 0.000111 and 0.0001, the PSS is found to be non-linear and the trough appears in the fourth and fifth canyons.
published_or_final_version
Mechanical Engineering
Master
Master of Philosophy

In the summer of 2007, the number of people living in the world’s urban areas exceeded that of those living in the countryside. Such urbanisation tends to modify the climates of towns and cities as a result of a number of factors which together form the ‘urban heat island’ effect. In order to better design buildings and urban areas to cope with these effects, it is first necessary to understand the heat transfer mechanisms which are taking place. The aim of the current research has therefore been to provide convective heat transfer data appropriate for low-rise urban environments by investigating the effects of wind speed, direction and street geometry. The research has employed the naphthalene sublimation technique which has been extended in several fundamental areas including development of a novel approach to measure the rate of sublimation from wind tunnel models. This technique has permitted measurements to be made over an array of discrete locations, revealing the variation across building surfaces. The uncertainty in the convective heat transfer coefficients obtained was calculated to be approximately ±6%. Tests were conducted in the BRE wind tunnel with an atmospheric boundary layer simulation appropriate to inner city areas. Cube models were arranged so as to form long rows of flat-roofed buildings referred to as ‘street canyons’. A series of correlations have been derived from the experimental results from which the rate of convection occurring from each building surface may be obtained with respect to wind speed. The greatest rates of convective heat transfer have been shown to occur at the top of the windward wall and leading edge of the roof, the lowest rates from the leeward wall of a building. Convection was found to be reduced in narrow street canyons. In wider street canyons, the convective coefficients on the exposed windward and roof surfaces of buildings were higher, but the values on the leeward wall are lessened due to the distancing of the downstream windward vortex. The effect of wind direction was found to be relatively small and therefore it is proposed that the convective heat transfer relationships presented may be applied irrespective of wind direction.

High resolution measurements of aerosol concentrations and fluxes have been made in an urban street canyon with busy traffic in the city centre of Manchester, UK. Measurements of aerosol concentrations have been made in 150 size bins covering the range 4.6 nm < Dp < 32 μm. Vertical turbulent fluxes have been measured in the size range 0.1 J.Lm< Dp < 3 μm using the eddy covariance technique. By placing the instruments on a platform lift, some information about vertical profiles has been gained. These measurements have been supplemented by measurements of flow and turbulence using up to four ultrasonic anemometers. Measurements of turbulence were made over three week-long campaigns in February, April and May 2001, whereas turbulence and aerosol measurements were combined for a two-week campaign in October 2001.

The present work proposes methods to calculate street-level exposures to solar radiation. The methods comprise a combination of different software algorithms, online databases and real-time standard measurements of solar radiation. Firstly, the use of the free access image database “Google Street View” to reconstruct urban geometries is illustrated. Google Street View represents an enormous source of information readily available for its general use in the field of urban atmospheric studies. With the aid of existing software packages, it was possible to reconstruct urban geometries as projected fisheye images of the canyon upper-hemispheric view, and to model total-shortwave solar irradiance within an urban canyon. The models allowed the calculation of relative street-canyon irradiance as a fraction of that received under a full-sky view, depending on the visibility of the solar disc and the reduced sky view factor. The combined use of the ideal models with real-time data allows for the calculation of street-canyon irradiance under any cloud conditions. Validation of these techniques was obtained by comparing the calculations against in situ measurements of irradiance from a local street canyon. The existing software, however, does not allow the calculation of spectral irradiance, required for inferring, for example, the biological effects of solar radiation. The use of spectral radiative transfer software was explored to provide spectral irradiance, but commonly available models do not include the effects of horizon obstructions. The approach presented here followed the same general guidelines used to calculate total-shortwave irradiance. The spectral models required a spectral partitioning of global irradiance into direct and diffuse components, allowing the independent analysis of horizon obstruction effects on these components at each wavelength. To partition global irradiance, two equations were developed for the calculation of the diffuse-to-global irradiance ratio (DGR) under cloudless conditions: one based on simplified radiative transfer theory, and an empirical fit for local conditions. Afterwards, the effects of horizon obstructions were evaluated in combination with real-time measurements of unobstructed global spectral irradiance. A set of simulated obstructions were used to validate the models. Finally, it was observed that neglecting the anisotropic distribution of the diffuse component of solar radiation in these simple models could produce large uncertainties in some situations. A practical solution for including the anisotropy of diffuse radiation was proposed, requiring images from an unobstructed digital sky camera. The combination of tools described here will allow calculation of total and spectral global irradiance upon a flat horizontal surface whatever the local field of view. This is possible at any geographical location were the urban geometries can be described, either by manually obtaining digital photographs, or through the Google Street View database, and where there is a reasonably local standard measurement of radiation.

A comprehensive study into the spatial and temporal variations in concentrations of a traffic-related pollutant was undertaken in two urban street canyons located in York, U. K. During the field experiment, investigations were carried out in order to determine the influence of meteorology, urban topography and traffic on the measured concentrations of a traffic-related pollutant. The pollutant measured in this study was carbon monoxide (CO). Results are presented from simultaneous and continuous measurements of the background (or reference) wind speed and direction, the in-canyon wind and turbulence fields, traffic characteristics and CO concentrations collected over a period of one month. The background wind was found to influence the development of in-canyon wind flow features, which became mechanisms for pollutant dispersion. Under certain background winds, evidence of across-canyon recirculating flows with horizontally- and vertically-aligned axes is presented. During these conditions, turbulence and traffic-related pollutants are likely to have been transported in the across-canyon re-circulating flows. During background wind orientated perpendicular to the street axis, the 15-minute mean concentrations of CO were a factor of 2 or 3 higher on the leeward (or upwind) side of the street canyons compared to the windward side. This was caused by the development of an across-canyon recirculating flow with a horizontally-aligned axis. Down draughts were measured on the windward side of the canyon during these conditions, while updraughts were measured on the leeward side. The wind direction at street level opposed the direction of the above-roof flow, causing the transport of CO towards the leeward side of the street canyon where concentrations increased. Evidence Results are presented from simultaneous and continuous measurements of the background (or reference) wind speed and direction, the in-canyon wind and turbulence fields, traffic characteristics and CO concentrations collected over a period of one month. The background wind was found to influence the development of incanyon wind flow features, which became mechanisms for pollutant dispersion. Under certain background winds, evidence of across-canyon re circulating flows with horizontally- and vertically-aligned axes is presented. During these conditions, turbulence and traffic-related pollutants are likely to have been transported in the across-canyon re circulating flows. During background wind orientated perpendicular to the street axis, the 15-minute mean concentrations of CO were a factor of 2 or 3 higher on the leeward (or upwind) side of the street canyons compared to the windward side. This was caused by the development of an across-canyon recirculatingf low with a horizontally-aligned axis. Downdraughts were measured on the windward side of the canyon during these conditions, while updraughts were measured on the leeward side. The wind direction at street level opposed the direction of the above-roof flow, causing the transport of CO towards the leeward side of the street canyon where concentrations in creased. Evidence of counter-rotating comer vortices with vertically-aligned axes are also presented and are thought to have been caused by along-canyon converging flows during certain background winds. However, across-canyon flow during these conditions also accounted for the higher concentrations of CO measured on the leeward side of the canyon, compared to the windward side. Background winds orientated oblique to the street axis were found to produce across-canyon rearticulating flows. On the other hand, parallel background winds produced channel flows, which had a 'flushing' effect, causing some of the lowest concentrations of CO to be measured during these conditions. Traffic-produced turbulence was investigated in the street canyon during the field experiment. Comparisons were made between calculated and measured turbulence parameters. The results indicate that the parameterisation performs reasonably well, particularly during weak perpendicular background winds when traffic-produced turbulence effects are likely to have been most dominant. The traffic was also investigated due to the implications of traffic characteristics on the variability in concentrations of CO. The influence of free-, unstable- and congested-traffic flows on measured CO concentrations was determined. The highest mean concentrations were measured during congested traffic conditions, when the emission levels are likely to have been elevated due to stop-start driving events. The combined influence of the background wind and traffic characteristics was investigated. Results are presented which show that the highest mean CO concentrations were measured during perpendicular background winds and during congested traffic conditions. The implications of using variable emission rates in a computational fluid dynamics (CFD) dispersion model were investigated in a sensitivity study. Results are also presented from the flow model study, which was conducted to help in the interpretation of the field experiment data.

When simulating air flows in an urban environment, for e.g. pollutant dispersion investigations, today's main tool is advanced computational fluid dynamics simulations. These simulations take a lot of time and resources to perform, even for small geometries. In some situations, one would like to be able to run approximate simulations, possibly with large geometries, without such a significant investment. The model described in this thesis is a graph network model which have streets and intersections of an urban environment modeled as connections and nodes in a graph. The model uses a pressured pipe model, based on the Darcy-Weisbach equation, to simulate air flow in the network. Such a model requires only rough measurements of the urban geometry and an estimated Darcy's friction factor, to be able to solve the system. Furthermore, using the same rough geometrical parameters, together with shear velocity, the model solves atmospheric exchange rates of the streets. Intersections play a major role when investigating urban dispersion. The way this model deals with dispersion in any complex intersections, represented by single nodes, is by using wind direction variance together with a distribution parameter based on computational fluid dynamics intersection simulations made in Comsol Multiphysics - also present in this paper. Using the simple model described above, I have simulated urban air flows in a complex urban geometry of a part of Paris. This specific geometry has already been investigated by computational fluid dynamics simulations as well as wind tunnel experiments. By comparing the computational fluid dynamics simulation with my model, I have validated its accuracy. 40% and 45% of all streets reach a relative and absolute error below 25% respectively. Directions of the street velocities have been simulated with approximately 90% accuracy - with distinct error indications. Atmospheric exchange rates of the streets are within an order of magnitude accurate, however, showing a systematic error by overestimating the vast majority of the exchange rates. The model could become even better by covering error sources discussed in the discussion section. Excess theory for simulating each of the above-described flows is presented, which might change the results. For example, slightly altering the modeling of the atmospheric exchange rate might fix the overestimation offset we have seen. Potential error sources could be the varying building heights and the streets angle relative the overlaying wind direction. The pressured pipe simulated flows have shown tendencies to be bad at picking up the effects of high/low buildings following low/high buildings, as well as accurately capture the behavior of streets close to perpendicular to the wind direction. Main streets with plenty of exits have been modeled with intersections at each exit, which results in strong flow variation along a street that should have a flow close to constant. Solving main streets like this separately could improve this behavior drastically.

碩士
淡江大學
水資源及環境工程所
83
The transport of air pollutants in a street canyon is simul- ated for different atmospherical stabilities, traffic conditions and street caynon geometry. The simulation domain includes the atmospheric boundary layer which cover both sub- urban and urban areas. The simulation tool is a finite element method based CFD package:FIDAP. The results of the numerical calculations shows that a sing- le vortex is maintained within a canyons.A second vortex appears when aspect ratio is greater than certain critical value under various meteorological conditions. This phenomenon then results in local high concentration on leeward、 windward side or near ground within a canyon rather than that of the one vortex case. In addition, the pollutant concentration will increases as the aspect ratio increases, and decreases exponentially in the vertical direction.

碩士
淡江大學
水資源及環境工程學系
86
This research uses Finite Analytic Method to simulate "Street Canyons" formed by city buildings and streets in atmospheric boundary layer. Because Street Canyons are complex structures on the Earth's surface, they may produce different kinds of nonuniform flow field.This thesis first compares the flow field variations in both laminar flow and turbulent flow under different height-width ratios. The laminar flow fields were solved from Navie-Stokes motion equation and continuity equation. For turbulent flow we used k-ε-E(ddy) turbulent model to simulate the turbulent flow field.The height-width ratios of "Street Canyons" are divided into three categories. The first category of the Street Canyon is low on the left and high on the right. The height-width ratios of it are 0.75, 1.0, 1.25, 1.5 and 1.75. The second category of the Street Canyons has equal heights. The height-width ratios of it are 0.5,1.0,1.5 and 2.0. The third category is one which is higher on the left hand side and lower in the right hand side. And the height-width ratios are 0.75, 1.0, 1.25, 1.5, 1.75. Then, we compare the size, location, and numbers of vortexes when laminar flow and turbulent flow are of the same height-width ratios. Finally, we compare the differences between turbulent flow using Wall- Function and turbulent flow not using Wall-Function The Results Of Simulation Are: Laminar flow:Only one vortex is produced in the first category of Street Canyons. The center of vortex in the second category increases with the rising of height-width ratios and the second vortex shows up in this category, too. When the second vortex appears in the third category and the difference between the heights of two structures is high, the center of the first vortex will move to down stream.Turbulent flow: There are two vortexes in the first category. When the second vortex shows up in the second category and the height- width ratio is 1.0, the center of vortex is about half high of the structure. When the second vortex appears in the third category and the difference of heights between two structures is high, the first vortex center will move to down stream. Differences between laminar and turbulent flows: The second vortex of turbulent flow always shows up earlier than that of laminar flow. The vortex center of turbulent flow is lower than that of laminar flow when the height-width ratio is 1.0. Thus, we can know the momentum transmission ability of turbulent flow is stronger than that of laminar flow. Wall-Function: The main structure is basically the same except there are some differences in the boundary, when comparing the circumstances between using and not using the Wall-Function.

碩士
淡江大學
水資源及環境工程學系
84
Carbon monoxide distribution in street canyons under various geometry''s,atmospheric stability, temperature gradient induced buoyant effect and trafficload were simulated. We intend to use the simulation results to evaluatepedestrian pollutant exposure and to provide external conditions forevaluating indoor air quality of the buildings beside urban street. In thisstudy, we consider several factors those may affect the pollutant distributionin street canyons. The considered factors include geometry of the streetcanyons, atmospheric stability, traffic load and turbulent buoyant effect. Atwo-dimensional domain that includes suburban roughness and urban streetcanyons was considered as the simulation domain. The considered influentialfactors were imposed into the simulation through the associated boundaryconditions. The simulated results show that serious pollutant accumulationoccurred at where double vortices encountered and the situation only occurredat specific aspect ratios of the street canyons. Usually, the stagnation zoneformed at where the encounter of two vortices that reduce the transport ofpollutant and the pollutant tent to accumulate at the region. Buoyant effectto the vortices formation was found that the double vortices occurred atsmaller aspect ratio of the canyons with buoyancy than those without buoyancy. The buoyant effect also change the pollutant transport pattern in the canyons.The pedestrian exposure to the street air pollutant under various trafficloads and atmospheric stability were evaluated. This study concluded that thepatterns of pollutant distribution in different street canyons are mostlyaffected by their aspect ratios where the affect of buoyancy and atmosphericstability played less role. The pedestrian exposure to the street airpollution is considered to be serious under specific conditions when pollutantconcentrations at 3 m above ground level are very high. Those specificconditions were also found in this study.

碩士
淡江大學
水資源及環境工程學系
92
Air pollution in big city areas resulting from exhaust emissions is a major urban problem. Often traffic pollution excess controls air pollution management decisions. The pollutants emitted from motor vehicles have direct impact on human health, especially on the drivers, bicyclists, motorcyclists, pedestrians, people working nearby and vehicle passengers in urban street canyon with high-rise buildings in most large cities. This study presents ventilation behavior in different height of street canyon configurations. To evaluate dispersion in a model urban street canyon, a series of tests with various street canyon aspect ratios (B/H) in upwind and downwind of street canyon are presented. Physical modeling in wind tunnels and numerical modeling can be used for dispersion simulation when investing air quality. The flow and dispersion of gases emitted by line source located between two buildings. A two-dimension numerical model based on Reyonlds-averaged Navier-Stokes equations coupled with a serious of standard, Renormalization Group(RNG) and realizable κ-εturbulence models was developed to simulate the fluid-flow development and pollutant dispersion within different height street canyon using the FLUENT code.Calculations are compared against fluid modeling in an Environmental Wind Tunnel of Wind Engineering Center at Tamkang University. These buildings were arranged in various symmetric configurations with different height ratios in upwind and downwind of street canyon. The fluid-flow development has demonstrated that the vortices generated within the urban street canyon can transport the pollutants form a line source to wall surfaces of the building. Different height arrangement of street canyons were found that pollutant concentration in leeward side is not bigger than windward side . The Pollutant concentration in wind tunnel test and simulation by FLUENT code had corresponding resluts.

碩士
國立臺北科技大學
能源與冷凍空調工程系碩士班
104
Arcade is a partly open comfortable space for people to take shelter from the wind, rain and sunlight in a highly urbanized environment. Accompanying with the urban renewal incentives proposed by domestic country and municipal governments, those buildings having arcade spaces were increasingly replaced by the design concept of "setback of architectures", leading to gradual disappearance of unique Taiwan street culture. Above all, there are no studies conducted to probe the influences of arcade buildings on the microclimates. This research aims to conduct the computational fluid dynamics (CFD) simulations and field measurements for resolving the microclimate environments around the subject building complex with arcade spaces in highly urbanized areas. It is expected to implement the results obtained to the urban design guidelines for improving the pedestrian comfort level and microclimate state.

碩士
國立臺灣大學
生物環境系統工程學研究所
105
Urban heat islands rapidly increase energy demand for air conditioning. To reduce the energy demand for cooling the environment, some possible solutions have been studied and applied. Among these methods, the water spray system is considered most effective and flexible with its dynamic controls. To simulate the cooling effect of water spray system, numerical simulation with Computational Fluid Dynamics (CFD) is used. This simulation was validated with water channel and wind tunnel experiments. The goal of this study is to simulate the cooling effect in the street canyon with different aspect ratio in high relative humidity (70% and 80%) environment, which is often the case in Taipei city. The results showed that if relative humidity is larger than 70%, the air cooled by small water droplets was easily saturated. Large water droplets almost saturated the air just under the nozzles. If the nozzle height was increased from 2.5 m to 3.5 m, the air under the nozzles was completely saturated, and reached wet bulb temperature, which is the lowest bound of temperature. The coolest region is just below the nozzles because the wind in street canyon is too weak to blow the cold air away. However, in a narrow street, people may feel the cooling effect in the middle of the street because the accumulation of the cold air.

碩士
國立臺北科技大學
能源與冷凍空調工程系碩士班
106
The development of high-density urban street canyons toward dense high-rise buildings causes the decline of ventilation rate and pollutant accumulation resulting from the breezeless state in the downtown area. Lately, the corridor concept has been extensively proposed during the layout, design and construction processes of city buildings. Corridors are arranged to align with the prevailing wind for bringing the wind into cities, and therefore to disperse the outdoor air pollution and promote the indoor air circulation. This study implements the environmental CFD based analysis procedures to construct a three-dimensional computational model for replicating the high-density urban buildings with corridors. By varying the effects of configurations of corridor, shape of buildings, and the distance between two neighboring streets, the iii fluid fields and air pollution dispersal were thoroughly investigated to evaluate the microclimate environments around the subject building with corridors. Besides, field measurements were conducted to provide the database for model validation. We then apply the verified CFD tool to predict the detailed airflow characteristics of urban environment as well as perform multivariable regression analysis for determining the correlation of the corridor design parameters with the air exchange rate per hour (ACH) for ventilation and the lessening of pollutant levels in environs.

碩士
國立臺灣海洋大學
河海工程學系
101
In the present thesis, wind tunnel experiments were conducted to study the dispersion characteristics of the airborne pollution on the triangle terrain with different slope angles and various distances between the terrain and street canyons. In the experiments, the airborne pollution source was located at the toe of the downwind slope of terrain. The triangle terrain with equal upwind and downwind slope which were 15∘and 30∘. Building heights in the street were the same with terrain model height. The height denoted by H. The vortex generators and roughness elements were appropriately arranged in the test section of wind tunnel to simulate the neutral atmospheric boundary layer flows which used as the approaching flow for the experiments. The distance between the triangle terrain and front row building of street canyon were 2H and 4H. Effects of the downwind slope angle of terrain and the distance between the triangle terrain and front row building of street canyon on the pollution dispersion characteristics (such as pollution plume averaged height and dispersion parameters) were investigated in this study.

碩士
國立臺北科技大學
能源與冷凍空調工程系
107
Pedestrian bridges are designed to separate pedestrians from vehicles on the road to improve traffic and pedestrian safety. However, the presence of pedestrian bridges may deteriorate the microclimates in street canyons. The development of high-density urban street canyons toward dense high-rise buildings causes the decline of ventilation rate, heat island effect and pollutant accumulation resulting from the breezeless state in the downtown area. Based on the local microclimate and urban conditions, this study applies the computational fluid dynamics (CFD) simulation technology to explore the influences of pedestrian bridge design between buildings on the air ventilation, thermal comfort and pollution dispersion of the indoor and outdoor spaces of the bridge and street canyons. The results show that the impact of pedestrian bridge structures on the local microclimate environment is insignificant for low and medium-rise buildings. Change the width (3m, 6m, 9m), interior height (3m, 4m, no cap) and barrier’s height (1.25m, 2m, 2.75m) of pedestrian bridge to analyze the velocity, temperature and pollution distributed over the internal passage space of pedestrian bridge. In addition, this paper explores the air ventilation, thermal comfort results and pollution removal rate in terms of the air exchange rate per hour (ACH), physiological equivalent temperature (PET) and purging flow rate (PFR) performance to maximize the comfort of pedestrians. The results show that the reduction of bridge’s width reduces the path of airflow, and flow resistance is lower, the velocity is relatively higher. Therefore, the bridge with narrow width (Case1 of pedestrian bridge’s width is 3m) shows the best ACH result, but the bridge’s width variation has no obvious influences on the temperature inside the bridge with effect of solar radiation and convection of heat transfer. No top cap (Case5) and higher barrier’s height (Case7 of barrier’s height is 2.75m) cause PET thermal sensitivity to be hot (38°C~42°C), except for them, the other Case’s PET results are warm (34°C~38 °C). Change the configuration of the pedestrian bridge on blocking the pollution is not significant, the wider pedestrian bridge (Case3 of pedestrian bridge’s width is 9m) has the best PFR value.