Street Canyon Research Articles

Ventilation and Pollutant Dispersion Analysis in the Uneven Building Arrays along with the Green Roof System

AbstractIn this study, uneven urban streets are explored with different standard deviations (σH = 0–0.6) through numerical simulations. The ventilation and pollutant diffusion conditions are investigated under the combined action between the thermal buoyancy force and mechanical wind force. When the thermal buoyancy is uniformly distributed, the upstream vortex gradually occupies the most area of the flow unit with the increase in σH. Three configurations of green roof systems (roof, leeward, and windward cooling schemes) are assumed to represent different thermal buoyancy distributions in the street canyon. The average air exchange rate and pollutant retention time (τ) are proposed to evaluate the ventilation efficiency and pollutant dispersion condition. The “free airflow” phenomenon occurs and gradually expands with the increase in the standard deviation of each model. The flow structure and pollutant distribution inside the urban canopy are similar in the uniformly heated and leeward cooling scheme. The windward cooling scheme presents the least local retention time in most working conditions. The roof cooling scheme aggravates the pollutant accumulation conditions inside the street canyon. This study can provide some constructive guidelines in the urban planning process and benefit the design of urban greening with different building morphologies.

Outdoor Thermal Comfort Optimization in a Cold Climate to Mitigate the Level of Urban Heat Island in an Urban Area

Climatic and micro-climatic phenomena such as summer heat waves and Urban Heat Island (UHI) are increasingly endangering the city’s livability and safety. The importance of urban features on the UHI effect encourages us to consider the configuration of urban elements to improve cities’ sustainability and livability. Most solutions are viable when a city redevelops and new areas are built to focus on aspects such as optimum design and the orientation of building masses and streets, which affect thermal comfort. This research looks beyond outdoor thermal comfort studies using UHI data and geoprocessing techniques in Tallinn, Estonia. This study supposes that designing urban canyons with proper orientation helps to mitigate the UHI effect by maximizing outdoor thermal comfort at the pedestrian level during hot summer days. In addition, optimizing the orientation of buildings makes it possible to create shaded and cooler areas for pedestrians, reducing surface temperature, which may create more comfortable and sustainable urban environments with lower energy demands and reduced heat-related health risks. This research aims to generate valuable insights into how urban environments can be designed and configured to improve sustainability, livability, and outdoor thermal comfort for pedestrians. According to the study results, researchers can identify the most effective interventions to achieve these objectives by leveraging UHI data and geoprocessing techniques and using CFD simulations. This evaluation is beneficial in guiding urban planners and architects in proposing mitigation solutions to enhance thermal comfort in cities and creating suitable conditions for approved thermal comfort levels. Results of the study show that in the location used for the survey, Tallinn, Estonia, the orientation of West-East offers the optimum level of comfort regarding thermal comfort and surface temperature in the urban environment.

Large-eddy simulations on pollutant reduction effects of road-center hedge and solid barriers in an idealized street canyon

This study conducted large-eddy simulations (LES) on the pollutant reduction effects of hedge and solid barriers in a three-dimensional idealized street canyon with an aspect ratio of 0.5. The wind direction was perpendicular and oblique (45°) to the street. The results were validated with data from wind tunnel experiments. LES accurately predicted the concentration distribution in the barrier-free case and reproduced well the barrier-induced concentration reduction. In the barrier-free case, a large recirculation vortex was observed. However, the central barriers forced the recirculated airflow in the middle of the canyon and newly formed vortices near the leeward walls. The two counter-direction vortices in the hedge and solid barrier cases transported pollutants toward the center of the canyon and enhanced the vertical pollutant removal at the top of the street canyon. The hedge barrier (solid barrier) reduced spatially-averaged concentration by about 59% (45%) near the leeward wall, 64% (20%) near the windward wall, and 45% (17%) in the whole street canyon compared to the barrier-free case. The effects of leaf area density (LAD) and barrier width were further investigated under the perpendicular wind direction. Increasing the LAD or the width of the hedge barrier decreased concentration near the leeward walls but increased canyon-averaged concentration. Increasing the width of the solid barrier decreased the concentration near the leeward walls and the canyon-averaged concentration. In an oblique wind direction, the hedge and solid barriers reduced by about 30% and 60% the spatially-averaged concentration near the building walls compared to the barrier-free case.

A DL-Based High-Precision Positioning Method in Challenging Urban Scenarios for B5G CCUAVs

Unmanned aerial vehicles (UAVs) facilitate services in civilian and industrial fields but suffer from a limited direct link operating range and unreliable satellite positioning in urban canyons. Fortunately, cellular-connected UAVs (CCUAVs) overcome these shortcomings, benefitting from the beyond 5th generation (B5G) network’s <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">city-level coverage</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">high-precision positioning capabilities</i> , and are considered a paradigm of 5G-advanced and beyond. However, in a challenging airspace (e.g., urban canyon), the CCUAV localization accuracy deteriorates due to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">low signal-to-interference-plus-noise (SINR) air-ground channels</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">strong multipath effects</i> . To solve these problems, we first construct channel amplitude-phase response (CAPR) images to characterize the cellular channel in a challenging airspace for CCUAV positioning. In particular, the effect of down-tilted antennas and high-dimensional channel features are embedded into CAPR images, to meet the relevant cellular communication criteria. Subsequently, a deep learning (DL) model, the scale-shared quarter network (SSQ-Net), is devised for CAPR image-based positioning, along with a robustness enhancement method. With this method, the multipath effects and interference in challenging environments are exploited to improve positioning accuracy and robustness, instead of being treated as detriments. Finally, the experimental results in a typical urban canyon show that our method outperforms state-of-the-art methods in terms of accuracy and robustness.

The Ventilation Capacity of Earthen Vernacular Buildings With Timber Projections (Sachnisi) in Dense Urban Canyons – Findings From a Field Study in the Mediterranean

Vernacular architecture employs numerous elements for enhancing natural ventilation, especially in the Mediterranean climate. However, the cooling effectiveness of natural ventilation also relies on occupant behaviour and is highly determined by urban scale attributes. Still, airflow in real, inhomogeneous historic urban settings is rarely integrated in the thermal comfort studies of historic buildings. In this study, a multi-scale approach is adopted in order to assess the role of ventilation in establishing thermal comfort in earthen buildings, in real field conditions. This is achieved by addressing field monitoring of air velocity, temperature and relative humidity in: a) a dense (packing density λp=0.60) and inhomogeneous historic neighbourhood in Nicosia, Cyprus (measurement at a reference height); b) the street-canyon (measured at the mid-height of a narrow canyon with aspect ratio 2); and c) the interior of a typical adobe building with semi-open, pass-through spaces (portico) and closed timber projections with multiple openings, called sachnisi. Building ventilation was addressed as a function of in-street and rooftop airflow. Indoor thermal comfort was assessed in the case of specific ventilation patterns, comparing three adaptive comfort models. The results highlight the complexity of airflow in real inhomogeneous urban canopies and demonstrate that air velocity level in the street canyon is approximately one third of the reference free-stream wind velocity, while indoor air velocity remains at minimum levels. The study also reveals the contribution of night-time ventilation in cooling. Finally, spatial differentials of indoor environmental conditions in the portico were recorded for the first time, demonstrating the role of the sachnisi as a wind-capture element.

TOM-Odometry: A Generalized Localization Framework Based on Topological Map and Odometry

The localization of autonomous vehicles mainly depends on GPS-based methods in large-scale outdoor scenarios. Unfortunately, GPS is vulnerable and not reliable in some regions, such as urban canyons. Odometry is taken as a complementary method for positioning in GPS-limited areas, but it suffers from accumulated errors, which limits its application. Topological map is a concise and low-cost representation of real world environment, which can be deemed as one of the information sources for localization. To realize accurate localization in the GPS-limited and large-scale scenes for autonomous vehicles, a generalized localization framework based on the topological map and odometry is proposed in this paper. The odometry trajectory can be automatically aligned with the correct paths of the map through matching without the initial global position. Under this framework, odometry is deemed as the kinematic model while topological map is regarded as a global sensor that is utilized to correct the drift errors of odometry. As an example, the integration of visual odometry and topological map is used to validate the effectiveness of the proposed method on KITTI benchmark. Experimental results demonstrate that the proposed method can effectively constrain the drift errors of odometry, and it achieves state-of-the-art localization performance compared with existing map-aided methods.

Reduced order model for simulation of air pollution model and application in 2D urban street canyons via the meshfree gradient smoothing method

Reduced order model for simulation of air pollution model and application in 2D urban street canyons via the meshfree gradient smoothing method

Effect of morphology and an upstream tall building on the mean turbulence statistics of a street canyon flow

The effects on turbulent street canyon flow of the morphological model and the existence of an upstream tall building are investigated in a wind tunnel, using Stereoscopic Particle Image Velocimetry (S-PIV) measurements on two orthogonal vertical planes. The cathedral in the “Rue de Strasbourg, (Nantes, France)” morphological model is regarded as a local tall building and is tested and then replaced by generic, square planform buildings of 4 different heights. The cathedral induces an asymmetrical wake while no obvious different influence on the street canyon flow can be identified, in comparison with a generic building of the same height. A strong along-canyon flow occurs which may be due to the intersections, while the along-canyon ventilation decreases with increasing building height. The higher the building, the stronger its sheltering effect, leading to a reduction of the vertical ventilation and mass transport across the canyon opening, at least in the canyon portion directly downstream of the building.

Performance Analysis of the GNSS Instantaneous Ambiguity Resolution Method Using Three Collinear Antennas

Attitude determination technology based on the global navigation satellite system (GNSS) has been widely studied due to its low cost and no accumulation of errors. The essential task of attitude determination using GNSS is to estimate the pointing vector of the baseline. However, when the user is located in the complex urban canyon environment, the success rate of ambiguity will be significantly reduced, resulting in inaccurate attitude determination results. This paper presents improvements to existing methods for ambiguity resolution using collinear antennas. The collinear receiver system is built with low-cost commercial antennas to verify the method’s performance. Results indicate a success rate of over 68% with just four satellites, demonstrating significant improvement over traditional LAMBDA method. This method reduces operation time while maintaining a high success rate, with a potential reduction of up to three orders of magnitude compared to the baseline constrained LAMBDA method. The method also exhibits potential anti-multipath capability, reducing RMSE of the pointing vector coordinate component by 60.3%, 87.5%, and 94.2% in the urban canyon environment with significant multipath effects. These results demonstrate the practical effectiveness and potential of the method for ambiguity resolution in challenging environments.

Detection and quantification of seasonal human heat and cold stress frequencies in representative existing and future urban canyons: the case of Ankara

Detection and quantification of seasonal human heat and cold stress frequencies in representative existing and future urban canyons: the case of Ankara

SMARTPHONE LEVEL INDOOR/OUTDOOR UBIQUITOUS PEDESTRIAN POSITIONING 3DMA GNSS/VINS INTEGRATION USING FGO

Abstract. This paper discusses ubiquitous smartphone pedestrian positioning challenges in urban canyons and GNSS-denied areas such as indoor spaces. Existing sensor-based techniques, including GNSS, INS, and VIO, have limitations that affect positioning accuracy and reliability. A machine learning-based approach is suggested to employ Support Vector Machine (SVM) to classify indoor/outdoor (IO) detection using GNSS measurement data. The proposed system integrates local estimates on VIO and 3D mapping aided (3DMA) GNSS measurements using Factor Graph Optimization (FGO) with an IO detection switch to estimate precise pose and eliminate global drift. The effectiveness of the system is evaluated through real-world experiments that produce notable outcomes.

PCDA™ SLAM-BASED TECHNOLOGY FOR POINT CLOUD AND TRAJECTORY OPTIMIZATION FOR AIRBORNE, LAND, AND INDOOR APPLICATIONS IN GNSS-DENIED ENVIRONMENTS

Abstract. This paper presents the results of assessing the performance of Trimble Applanix PCDA™ SLAM-based technology to simultaneously optimize any mobile mapping system trajectory and LiDAR point cloud data in a GNSS-denied environment. The simultaneous use of inertially-aided GNSS data along with LiDAR point clouds to optimally correct shifts and/or drifts in the trajectory in GNSS-denied environments is addressed in detail in this paper. A number of Trimble MX50 Mobile Mapping System data sets were acquired in Germany particularly to assess the performance of PCDA™. The land mobile mapping data sets were acquired in deep urban canyons which were purposely acquired that way to reach the most challenging land mobile mapping data sets in a GNSS-denied environment. The PCDA™ technology assessment results are presented in detail. In summary, the results show how LiDAR data can successfully be used to correct the trajectory shifts and drifts due to GNSS outages by simultaneously optimizing both point cloud and trajectory data.

Air Quality Improvement in Urban Street Canyons: An Assessment of the Effects of Selected Traffic Management Strategies Using OSPM Model

Constantly changing vehicle stock, modification of road infrastructure, and other conditions result in a need to update the knowledge on the effectiveness of individual traffic management strategies, which could form the basis for actions taken by local authorities to improve air quality in crowded city centers, especially in street canyons. The article presents research results that evaluate the theoretical effects of introducing select traffic reorganization scenarios in the example of four street canyons located in Krakow (Poland) that are different in terms of vehicle traffic volume and canyon geometry. These scenarios were based on a reduction in the average traffic speed, road capacity or the admission of cars meeting certain exhaust emission standards. The authors estimated changes in emissions of nitrogen oxides (NO, NO2 and total NOx) and particulate matter (PM10 and PM2.5) as well as investigated the effect of these changes on air quality in the canyons using the Operational Street Pollution Model (OSPM). Significant effects in terms of improving air quality were identified only in scenarios based on a significant reduction in traffic volume and the elimination of passenger cars and light commercial vehicles with internal combustion engines that did not meet the requirements of the Euro 4, Euro 5 or Euro 6 emission standards. For these scenarios, depending on the variant and canyon analyzed, the emission reduction was achieved at a level of approximately 36–66% for NO, 28–77% for NO2, 35–67% for NOx and 44–78% for both PM10 and PM2.5. The expected effect of improving air quality in individual street canyons for these substances was 15–44%, 5–14%, 11–36% and 3–14%, respectively. The differences obtained in the percentage reduction of emissions and pollutant concentrations in the air were the result of a relatively high background of pollutants that suppress the achieved effect of improving air quality to a large extent.

Estimating individual vehicle emission factors from near-road measurements in India

Over the years, vehicular emissions have been the main contributor to the deterioration of urban air quality. However, quantification of real-world vehicular emissions is quite limited in low- and middle-income countries like India. Developing real-world vehicle emission factors (EFs) using reference-grade instruments requires a significant amount of resources. This study aims to develop the individual and fleet vehicle EFs and the fraction of high-emitting vehicles using high-time resolution, low-cost sensors from near-road measurements – a first-of-its-kind study in India. Traffic and air pollutant measurements were conducted at the kerbside of a street canyon in Mumbai. The individual vehicle fuel-based EFCO and EFNOx were estimated using the plume-identification technique coupled with the information obtained from the vehicle registration number plates. The fleet mean (±SD) EFCO, EFNO, and EFNO2 were 6.30 (±3.16), 1.38 (±1.17), and 0.43 (±0.32) g/kg, respectively, while for EFPM1, EFPM2.5, EFPM2.5-10, and EFPM10 were 0.70 (±0.34), 1.19 (±0.57), 0.90 (±0.65), and 2.09 (±1.05) g/kg, respectively. The developed individual vehicle EFCO and EFNOx were greatly varied within each vehicle type due to differences in emission control technology, engine size, and the prevalence of “super-emitters”. There was no substantial difference in EFCO and EFNOx among the different BS emission standards across almost all vehicle types. The reconstructed fleet EFCO and EFNOx from the developed single-vehicle EFs were 1.4 and 1.9 times higher than the recorded fleet EFs. Approximately 14% of vehicles in the fleet were identified as super-emitters, responsible for 37–54% of total emissions, primarily from private passenger vehicles such as cars and two-wheelers. The EFCO and EFNOx from these high-emitters were 3–30 times greater than the laboratory-reported emissions. Our study suggests that improving emission standards alone is not enough to decrease tailpipe emissions from vehicles. Proper vehicle inspection and maintenance programs are crucial in controlling these emissions.

Impact of shading on evapotranspiration and water stress of urban trees

AbstractEvapotranspiration of urban street trees is essential in mitigating urban heat islands due to its cooling effect. However, current shifts in rainfall and temperature regimes towards drier and hotter periods in Central Europe have caused substantial water stress for street trees. Quantifying and subsequently managing these changing dynamics as well as estimating evapotranspiration and water availability is necessary but at the same time extremely challenging in urban environments. Both dynamics are influenced by soil sealing and complex shading patterns of the surrounding street canyon, which vary on a small spatial scale as a function of the canyon layout and orientation. In the present study, the diurnal patterns of six typical urban shading types for street trees were derived by considering a large set of street orientations, widths and tree positions within the street canyon. A shading model was integrated into a hydrological urban tree model to assess the impact of those shading types on diurnal patterns of radiation and evapotranspiration rates calculated using the Penman–Monteith approach and the resulting soil moisture conditions for several vegetation seasons and water‐supply scenarios. The modelling results showed that the six shading patterns significantly influenced the simulated hourly, daily and seasonal potential and actual evapotranspiration rates and water availability. Shaded trees have a substantially reduced, simulated water stress period, regardless of initial water supply, and are able to provide a longer‐lasting cooling function during dry periods due to higher evapotranspiration rates later in the summer season.

Outdoor Thermal Comfort Integrated with Energy Consumption for Urban Block Design Optimization: A Study of the Hot-Summer Mediterranean City of Irbid, Jordan

With an increasing awareness of urban health and well-being, this study highlights the growing importance of considering environmental quality in urban design beyond mere energy performance. This study integrates outdoor and indoor quality by investigating the effect of design parameters at an urban block scale (building form restricted to width and length as rectangular and square, building orientation, block orientation, building combination, building height, facade length, built-up percentage, setbacks, and canyon aspect ratio) on outdoor thermal comfort and energy use intensity. In addition, it explains the different correlations between outdoor thermal comfort and energy use intensity in different urban block designs in a hot-summer Mediterranean climate in Jordan. The study adopts a performance-driven approach using simulation tools of Ladybug, Honeybee, Dragonfly, and Eddy3d plugins across the grasshopper interface and evaluates 59 different urban block designs with nine different orientations (0°, 1°, 45°, 85°, 87°, 90°, 355°, 358°, and 359°). The results show that there is a positive correlation between the canyon aspect ratio and the environmental performance of the urban block designs. North–south street canyons are more effective at enhancing microclimates. Negatively increasing the street aspect ratio by more than four affected outdoor thermal comfort by increasing longwave radiation. Further results suggest a positive correlation between the compactness of urban blocks and their environmental performance, with north–south street canyons found to be more effective in enhancing microclimates. The study emphasizes the need to understand the distribution of open spaces formed by buildings and to strike a balance between day and night, as well as summer and winter conditions in outdoor spaces.

Impact of urban surfaces’ solar reflectance on air temperature and radiation flux

This study evaluates the effect of urban surfaces’ solar reflectance on the microclimate of a Brazilian city, Balneário Camboriú. A case study of two different geometrical areas of the city's central region was conducted. The influence of solar reflectance was evaluated through computer simulations on the ENVI-met model, validated for a winter and a spring day, according to field measurements. The simulations were performed for twelve combinations of solar reflectance on the roofs, façades, and pavements. The results confirmed that surfaces of high solar reflectance generally contribute to an air-temperature decrease. However, the degree to which each surface's solar reflectance influences the air temperature depends on the solar access (urban geometry, time, and season). In denser areas, the solar reflectance of facades has a greater impact on temperature variation, whereas in open spaces, roofing and paving surfaces are more critical. Impacts on urban microclimate extend beyond air temperatures. The radiation fluxes on two urban canyons revealed that the solar radiation incidence on the paving of a high solar reflectance canyon could be up to three times greater than one with low reflectance.

Evaluation on the performance of tree view factor in a high-density subtropical city: A case study in Hong Kong

Outdoor thermal comfort is a primary concern when designing urban environments suitable for pedestrian activities. In previous research, urban greenery had been considered as an effective strategy to mitigate the deterioration of the street-level thermal environment. Tree view factor (TVF), a parameter that can estimate the amount of tree cover visible in the overlying hemisphere, was applied in recent thermal environment assessment studies. However, a detailed understanding of TVF's suitability and application in a subtropical high-rise compact metropolis with diversified land uses is still lacking. To fill the knowledge gap, this study used Hong Kong as a study case and conducted ENVI-met simulations to analyse the human thermal comfort at two levels of TVF sites and two weather conditions. Results showed that high TVF (Htvf) sites had a pronounced reduction in mean radiant temperature (TMRT) of at least 15 °C after 12:00 under both typical and extreme weather scenarios. A lower mean physiological equivalent temperature (PET) of below 30 °C and 35 °C remained in the daytime of typical and extreme weather scenarios, respectively. It was also found that low TVF sites inside parks received more solar radiation, while Htvf sites inside parks exhibited a more substantial cooling potential than that inside street canyons. Htvf sites could maintain better pedestrian thermal comfort due to the shading effect. This study also proved that TVF is more suitable than green coverage ratio (GCR) in urban thermal environment assessments and urban green infrastructure design to create a thermally comfortable high-rise and high-density metropolis.

Applying vertical greening systems to reduce traffic noise in outdoor environments: Overview of key design parameters and research methods

Vertical greening can be used to absorb and scatter sound, which may reduce noise levels in street canyons. In this paper, a literature review is presented, which combines results and methods from over 40 individual studies. The article describes the guiding principles behind the acoustic effects of vertical greening and provides an overview of the prevalent research methods. The article shows that vertical greenery is effective for the reduction of mid and high frequency noise, unless air cavities or resonators are introduced inside, or behind, the systems. The review also reflects on studies with an emphasis on the application of vertical greenery in streets and urban blocks. The aim of the article is to set out the key design parameters for noise reduction that can be achieved by vertical greening, focusing on designers and engineers.

Numerical Simulation of the Distribution of Vehicle Emissions in a Street Canyon

Numerical Simulation of the Distribution of Vehicle Emissions in a Street Canyon