Street Canyon Research Articles

Evaluation on wind and non-isothermal environment in the street canyon with and without curved arm awning

Evaluation on wind and non-isothermal environment in the street canyon with and without curved arm awning

Modeling traffic pollutants in a street canyon by CFD: Idealized line sources versus multiple realistic car sources

Idealized sources are commonly used to reproduce the traffic emission in street canyons in experimental and numerical investigations. However, it remains unclear whether idealized sources can accurately reproduce the pollutant dispersion compared to more realistic sources. The goal of this paper is to investigate the impact of idealized and realistic sources on traffic-induced pollutant concentration in a street canyon by numerical simulation with Computational Fluid Dynamics (CFD). First, the scale-adaptive simulation (SAS) results of mean velocity and concentration are compared with wind-tunnel (WT) data for idealized line sources (ILS) and a satisfactory agreement is found between the SAS and WT results. Next, SAS are performed to investigate the impact on mean velocity and mean pollutant concentration of ILS versus multiple realistic car sources (MRCS) cases, designed to mimic a configuration of traffic jam-like conditions (i.e. rows of idling cars) in a street canyon. Different levels of geometrical simplifications of sources and total emission rates are considered. Although this research study focuses on these aforementioned specific set of conditions and conclusions relate solely to these circumstances, the SAS results show that the overall-averaged concentration associated with MRCS and the simplified source-geometry cases is about 1.18–6.22 times larger than that by ILS. Thus, ILS may be inadequate to reproduce the pollutant concentration in real scenarios. This study can contribute to enhancing the pollution prediction accuracy and reduce the potential risk of human exposure.

Effects of vertical greening on the thermal environment and energy consumption in different street canyons

Vertical greening is vital for energy conservation and urban sustainability. However, previous studies have seldom considered the energy-saving effects of vertical greening within street canyons—an important representative urban model. This study employs ENVI-met and EnergyPlus to evaluate the energy savings of vertical greening in twelve typical street canyon scenarios with varying aspect ratios (H/W = 1, 2, 4) and orientations (North–South, East–West, Northeast–Southwest, Northwest–Southeast). We quantified the relative contributions of building surface temperature reduction (ΔTₛₑ) and air temperature reduction (ΔTₐ) to overall energy efficiency. Remarkably, our findings reveal that ΔTₛₑ accounts for over 97% of the total energy-saving contribution—a novel insight contrasting with previous studies that emphasized combined impacts. Additionally, the results indicate that stronger solar radiation in street canyons leads to greater reductions in building surface temperatures. To achieve maximum daily energy savings, the optimal combinations of street orientation and aspect ratio are: North–South orientation when H/W = 1, Southwest–Northeast when H/W = 2, and Northwest–Southeast when H/W = 4. This study is among the first to quantify the combined effects of different street canyon configurations and vertical greening on urban energy savings, providing effective methodologies and new insights for implementing sustainable urban vertical greening.

Large eddy simulation of droplet dispersion and deposition over street canyons

Predicting droplet transport in the atmosphere is a major issue for a large number of dispersion problems such as spray cooling systems in streets, urban lawn sprinklers [Milesi et al., “Interferometric laser imaging for respiratory droplets sizing,” Environ. Manage. 36, 426–438 (2005)], and dispersion of virus such as COVID or from a terrorist attack [Balachandar et al., “Host-to-host airborne transmission as a multiphase flow problem for science-based social guidelines,” Int. J. Multiphase Flow 132, 103439 (2020)]. Here, we investigate the propagation of droplets issued from a source near the ground between square obstacles with different spacings. The aerodynamic field is determined by large eddy simulation coupled with an immersed boundary method for the obstacles. Droplets are tracked by a Lagrangian approach. An evaporation model is used to account for varying humidity conditions. Droplet concentration, mass flux, and deposition rates are obtained for different evaporation conditions and obstacle spacings. The results show high deposition rates on the internal wall of the first obstacle due to droplet advection by the reverse flow of the primary vortex. In addition to this, droplets may escape the canyon and propagate downstream as far as eight times the obstacle distance (8H). The concentration of air-borne droplets downstream the canyon is higher for smaller street canyon openings. When evaporation is accounted for, the ground-level droplet concentration may be reduced by up to 30% in the case of small canyon openings.

Source apportionment of particle number size distribution at the street canyon and urban background sites

Abstract. Particle size is one of the key factors influencing how aerosol particles affect their climate and health effects. Therefore, a better understanding of particle size distributions from various sources is crucial. In urban environments, aerosols are produced in a large number of varying processes and conditions. This study intended to develop the source apportionment of urban aerosols by utilising a novel approach to positive matrix factorisation (PMF). The particle source profiles were detected in particle number size distribution data measured simultaneously in a street canyon and at a nearby urban background station between February 2015 and June 2019 in Helsinki, southern Finland. The novelty of the method is combining the data from both sites and finding profiles for the unified data. Five aerosol sources were found. Four of them were detected at both of the stations: slightly aged traffic (TRA2), secondary combustion aerosol (SCA), secondary aerosol (SecA), and long-range-transported aerosol (LRT). One of the sources, fresh traffic (TRA1) was only detected at a street canyon. The factors were identified based on available auxiliary data. Additionally, the trends of the found factors were studied, and statistically significant decreasing trends were found for TRA1 and SecA. A statistically significant increasing trend was found for TRA2. This work implies that traffic-related aerosols remain important in urban environments and that aerosol sources can be detected using only particle number size distribution data as input in the PMF method.

Enhanced modeling of vehicle-induced turbulence and pollutant dispersion in urban street canyon: Large-eddy simulation via dynamic overset mesh approach

This study presented a novel application of large-eddy simulation (LES) with a dynamic overset mesh approach to simulate vehicle-induced turbulence in a two-dimensional street canyon. The simulation incorporated moving vehicle entities to emulate two-way traffic, with each vehicle equipped with a pollutant source to simulate pollutant dispersion. Comprehensive long-term statistical analyses were conducted to compare the simulated turbulence with those produced by the conventional approach (where vehicle-induced momentum was not considered) and the quasi-steady method (where vehicle motion was simplified as momentum sources). The results revealed that the presence of moving vehicle entities significantly distorted the primary circulation within the canyon, altering the transport pathways of both lateral momentum and air pollutants. The motion of vehicle entities also induced a substantial amount of turbulence, resulting in different pollutant removal mechanisms at the top of the canyon. The ensemble-average analysis revealed a downwash followed by an upwash within a cycle of vehicle movement, which largely contributed to momentum and pollutant transport. These findings underscored the need for considering the moving entities in LES approaches to enhance vehicle-induced turbulence modeling. Other factors influencing the simulation were discussed, aiming to guide more accurate and reliable turbulence modeling in urban environments.

Challenges of high-fidelity air quality modeling in urban environments – PALM sensitivity study during stable conditions

Abstract. Urban air quality is an important part of human well-being, and its detailed and precise modeling is important for efficient urban planning. In this study the potential sources of errors in large eddy simulation (LES) runs of the PALM model in stable conditions for a high-traffic residential area in Prague, Czech Republic, with a focus on street canyon ventilation, are investigated. The evaluation of the PALM model simulations against observations obtained during a dedicated campaign revealed unrealistically high concentrations of modeled air pollutants for a short period during a winter inversion episode. To identify potential reasons, the sensitivities of the model to changes in meteorological boundary conditions and adjustments of model parameters were tested. The model adaptations included adding the anthropogenic heat from cars, setting a bottom limit of the subgrid-scale turbulent kinetic energy (TKE), adjusting the profiles of parameters of the synthetic turbulence generator in PALM, and limiting the model time step. The study confirmed the crucial role of the correct meteorological boundary conditions for realistic air quality modeling during stable conditions. Besides this, the studied adjustments of the model parameters proved to have a significant impact in these stable conditions, resulting in a decrease in concentration overestimation in the range 30 %–66 % while exhibiting a negligible influence on model results during the rest of the episode. This suggested that the inclusion or improvement of these processes in PALM is desirable despite their negligible impact in most other conditions. Moreover, the time step limitation test revealed numerical inaccuracies caused by discretization errors which occurred during such extremely stable conditions.

Sky-GVIO: Enhanced GNSS/INS/Vision Navigation with FCN-Based Sky Segmentation in Urban Canyon

Accurate, continuous, and reliable positioning is critical to achieving autonomous driving. However, in complex urban canyon environments, the vulnerability of stand-alone sensors and non-line-of-sight (NLOS) caused by high buildings, trees, and elevated structures seriously affect positioning results. To address these challenges, a sky-view image segmentation algorithm based on a fully convolutional network (FCN) is proposed for NLOS detection in global navigation satellite systems (GNSSs). Building upon this, a novel NLOS detection and mitigation algorithm (named S−NDM) uses a tightly coupled GNSS, inertial measurement units (IMUs), and a visual feature system called Sky−GVIO with the aim of achieving continuous and accurate positioning in urban canyon environments. Furthermore, the system combines single-point positioning (SPP) with real-time kinematic (RTK) methodologies to bolster its operational versatility and resilience. In urban canyon environments, the positioning performance of the S−NDM algorithm proposed in this paper is evaluated under different tightly coupled SPP−related and RTK−related models. The results exhibit that the Sky−GVIO system achieves meter-level accuracy under the SPP mode and sub-decimeter precision with RTK positioning, surpassing the performance of GNSS/INS/Vision frameworks devoid of S−NDM. Additionally, the sky-view image dataset, inclusive of training and evaluation subsets, has been made publicly accessible for scholarly exploration.

Mechanism of Wind and Buoyancy Driving on Ventilation and Pollutant Transport in an Idealized Urban Street Canyon

The mechanisms underlying the effects of wind and buoyancy on ventilation in urban street canyons are unclear. This study investigated the effects of facade heating on ventilation and pollutant transport in an idealized street canyon with a 1.67 aspect ratio through computational fluid dynamics simulations. The dispersion pattern of discharged hot pollutants was also studied. A primary recirculation was observed when facade heating was not applied; this recirculation was promoted in leeward-wall and ground heating cases. However, the recirculation was bifurcated into two recirculations in windward-wall heating cases, restricting ventilation. Enhanced recirculation increased the ventilation and decreased the pollution level; by contrast, air pollution increased considerably when the recirculation was bifurcated and ventilation was restricted. In the hot-pollutant case, similar results to those in the ground-heating case were observed. The hot discharged pollutant enhanced ventilation, reducing pollution. The pollutant transport mechanism was determined through an analysis of pollutant fluxes. For the one-recirculation pattern, air convection transported the pollutant from the ground level to the top boundary, and turbulent diffusion then caused pollutant removal. For the two-recirculation pattern, turbulent diffusion contributed substantially to pollutant transport both in the junction between the recirculations and through the top boundary of the street canyon.

Sound propagation modelling in a street canyon by combining an image source model and impulse response measurements

This research introduces an innovative approach to achieve a computationally efficient model for calculating impulse responses in an urban environment, more precisely a street canyon, for auralization purposes. The image source method has shown its merits before, but when used for auralization, it typically lacks realism. In this study the image source method is used to create a basic impulse response of a street canyon, enriched by a stochastic response based on an envelope function. While the image source model accounts for the most prominent reflections, the stochastic part is due to all scattered sound energy in the street originating mainly from facade irregularities and street furniture. To obtain realistic frequency dependent envelope functions for various street canyon geometries, the function is based on measured impulse responses in nearly 100 inner-city street canyons in the city of Ghent, which were conducted by Thomas et al. (2013). By the proposed approach, we are able to generate impulse responses for street canyons suitable for sound perception research in an efficient way. More studies are needed to verify the applicability of the model for streets outside the Ghent dataset.

Quantifying the impact of vertical greenery systems (VGS) on Mediterranean urban microclimate during heat wave events

This research investigates the potential of vertical greenery systems (VGS) to modulate the outdoor microclimate in a real-scale Mediterranean climate street canyon with respect to a non-vegetated control during four heat waves and summer conditions. The cooling effect of VGS on air temperature was assessed using an identical set of sensors in both canyons. The effect on thermal comfort was evaluated using radiation fluxes, Mr.T tool, and the ENVI-met model that was validated against measured data.The VGS cooling effect during the heat waves was up to 0.36–2.04°C, and its duration was significantly extended. The correlation (R2 = 0.75) between the cooling effect, relative humidity, and wind direction, highlights that warmer and drier conditions increase the VGS cooling effect during heat waves.VGS improved the mean radiant temperature (Tmrt) above the canyon (3.9–4.1°C) and at the pedestrian level (2.21–2.8°C), while the physiological equivalent temperature (PET) and universal thermal climate index (UTCI) decreased by an average of 0.67–1.07 °C and 0.63–0.8 °C, respectively. Shaded walls improved the average Tmrt (11.7°C), PET (3.2°C), and UTCI (2.7°C), reflecting the importance of casting shadow in the urban environment.

A numerical study on the issue of Reynolds independence of flow and dispersion within isolated street canyons

Achieving Reynolds number independence is a prerequisite for conducting scale model experiments. This paper presents a numerical model of the two-dimensional isolated street canyon, utilizing the validated reliable k-epsilon turbulence model based on wind tunnel experimental data, to investigate the influence of approaching wind profiles (α) and windward building width (WB) on achieving Reynolds number independence in the isolated street canyon and compare these characteristics with those observed in urban street canyon. quantitative criteria, including revised relative change ratios (RRCs) ≤ 20% and dimensionless concentration relative difference (K_RD) ≤ 5%, were employed in conjunction with velocity, concentration contours, and streamlines to determine the corresponding critical Reynolds numbers. The results suggest that variations in approaching wind profiles primarily influence the flow field structure within the isolated street canyon after achieving Reynolds number independence, particularly impacting the scale of the counterclockwise vortex. However, these variations do not significantly alter the critical Reynolds number (Recrit) required for achieving Re-independence, which is estimated to be Recrit = 2.2×10⁴ for the isolated street canyon. Varying WB affects the formation of counterclockwise vortex within the isolated street canyon. A reduction in WB facilitates its formation and accelerates reaching Re-independence. The Recrit is found to be linearly proportional to WB variation. The flow field structure in urban street canyons remains relatively stable, with a recommended Recrit = 3.1×10⁴. When WB is 18 cm, the Recrit for the isolated street canyon aligns with that of the urban street canyon.

Coupling MATSim and the PALM Model System—Large Scale Traffic and Emission Modeling with High-Resolution Computational Fluid Dynamics Dispersion Modeling

To effectively mitigate anthropogenic air pollution, it is imperative to implement strategies aimed at reducing emissions from traffic-related sources. Achieving this objective can be facilitated by employing modeling techniques to elucidate the interplay between environmental impacts and traffic activities. This paper highlights the importance of combining traffic emission models with high-resolution turbulence and dispersion models in urban areas at street canyon level and presents the development and implementation of an interface between the mesoscopic traffic and emission model MATSim and PALM-4U, which is a set of urban climate application modules within the PALM model system. The proposed coupling mechanism converts MATSim output emissions into input emission flows for the PALM-4U chemistry module, which requires translating between the differing data models of both modeling systems. In an idealized case study, focusing on Berlin, the model successfully identified “hot spots” of pollutant concentrations near high-traffic roads and during rush hours. Results show good agreement between modeled and measured NOx concentrations, demonstrating the model’s capacity to accurately capture urban pollutant dispersion. Additionally, the presented coupling enables detailed assessments of traffic emissions but also offers potential for evaluating the effectiveness of traffic management policies and their impact on air quality in urban areas.

When-to-Loop: Enhanced Loop Closure for LiDAR SLAM in Urban Environments Based on SCAN CONTEXT

Global Navigation Satellite Systems (GNSSs) frequently encounter challenges in providing reliable navigation and positioning within urban canyons due to signal obstruction. Micro-Electro-Mechanical System (MEMS) Inertial Measurement Units (IMUs) offers an alternative for autonomous navigation, but they are susceptible to accumulating errors. To mitigate these influences, a LiDAR-based Simultaneous Localization and Mapping (SLAM) system is often employed. However, these systems face challenges in drift and error accumulation over time. This paper presents a novel approach to loop closure detection within LiDAR-based SLAM, focusing on the identification of previously visited locations to correct time-accumulated errors. Specifically, the proposed method leverages the vehicular drivable area and IMU trajectory to identify significant environmental changes in keyframe selection. This approach differs from conventional methods that only rely on distance or time intervals. Furthermore, the proposed method extends the SCAN CONTEXT algorithm. This technique incorporates the overall distribution of point clouds within a region rather than solely relying on maximum height to establish more robust loop closure constraints. Finally, the effectiveness of the proposed method is validated through experiments conducted on the KITTI dataset with an enhanced accuracy of 6%, and the local scenarios exhibit a remarkable improvement in accuracy of 17%, demonstrating improved robustness in loop closure detection for LiDAR-based SLAM.

GNSS positioning enhancement based on NLOS signal detection using spatio-temporal learning in urban canyons

GNSS positioning enhancement based on NLOS signal detection using spatio-temporal learning in urban canyons

An Investigation of Globe Temperature in Street Canyons: A Scaled Model Study Implementing Cool Materials

The measurement of globe temperature (GT) is essential for investigating pedestrian thermal comfort in street canyons. The globe thermometer is the most common instrument used to measure GT; however, its application in scale models has not been thoroughly investigated to date. Therefore, this study explicitly investigates globe thermometer measurements in scale models and analyzes the need for customization of the globe thermometer for more reliable measurements. Scaling down with respect to the size of the globe thermometer and the effect of solar orientation/envelope materials are investigated in this study. The initial experiments were carried out in an outdoor setting using a typical street canyon model (scale 1:100) with an east-west street orientation. The results of the experiment are presented to compare a low solar reflectance street canyon (albedo of 0.4) and a high solar reflectance canyon (albedo of 0.6) in terms of surface temperatures, heat flux, and globe temperature. It is observed that although the wall and road surface temperatures are lower for the high solar reflectance canyon compared to those for the low solar reflectance canyon, the GT (measured at pedestrian height) is higher in a high reflectance canyon during the daytime, which could be due to the combined effect of direct radiation and short-wave reflection. However, for the hours after sunset, a reverse effect is observed, i.e., the GT becomes lower (up to 0.8 °C) in the case of a high reflectance canyon compared to that for the low reflectance canyon. Furthermore, the study emphasizes the impact of solar reflectance of canyon surfaces on GT values, due to the view factors that the globe thermometer on those surfaces.

Well-planned greenery improves air urban quality - Modelling the effect of altered airflow and pollutant deposition

Urban air quality is influenced by vegetation through alterations in airflow and pollutant deposition processes. We investigated these interactions by integrating the Vegetation Impact Dynamic Assessment model (VIDA) with the Large-Eddy Simulation model PALM. Our analysis focus on nitrogen dioxide (NO₂) and particulate matter (PM) concentrations at the local scale, considering three tree genera. Our findings reveal the necessity of accounting for both gaseous pollutants and particles separately due to their differing mechanisms of deposition onto leaves. The coupled PALM-VIDA model demonstrates a significant reduction in PM levels across the modelling domain and within street canyons when deposition to vegetation is incorporated. Reduction in NO₂ through deposition to vegetation is lower but human NO2 exposure can still be decreased if tree species selection and placement leads to desirable effects on air flow. Sparse tree arrangements or species with sparse crowns facilitate ventilation and are often better at reducing NO₂ concentrations in street canyons compared to denser vegetation with higher deposition but negative effects on ventilation. Our study informs urban planning and green infrastructure design, underscoring the multifaceted role of urban greenery in air pollution mitigation strategies. Its main conclusion is that both deposition processes and the influence of air mixing and ventilation need to be considered to accurately assess the effects of urban trees on local air quality. Ill-considered placement and species selection may cause a net increase in pollutants underneath the trees. However, careful planning can address this risk and instead improve overall air quality.

Dynamic patterns of particulate matter concentration and size distribution in urban street canyons: insights into diurnal and short-term seasonal variations.

Time-varying characteristics of particulate matter (PM) pollution play a crucial role in shaping atmospheric dynamics, which impact the health and welfare of urban commuters. Previously published studies on the diurnal patterns of PMs are not consistent, especially in the context of field experiments in central China, and most field studies have only focused on particles with a single particle size. This study conducted regional-scale studies across 72 street canyon sets in Wuhan, China, investigated diurnal and seasonal PM concentration variations while also evaluating various PM size and the key driving factors. During summer (July, August, and September), evergreen tree-lined street canyons maintained a stable linear trend for smaller dp particulates (i.e., PM1, PM2.5, and PM4), while deciduous street canyons exhibited a bimodal distribution. In winter (January and February), fine particulates (i.e., PM1 and PM2.5) remained a linear trend in evergreen street canyons, while deciduous street canyons show a slightly wavy fluctuating pattern. Meanwhile, it exhibited quadrimodal-peak and triple-trough patterns in both PM7, PM10, and TSP concentrations. The lowest PM concentrations were observed between 14:00 and 16:00 for all particle sizes, with decreased summer pollution (7.81% lower in PM2.5, 53.47% lower in PM10, and 50.3% lower in TSP) noted in our seasonal analysis. Among the various meteorological factors, relative humidity (RH) was identified as the dominant influencing PM factor in both summer and winter. Results from this study will help us better understand field-based air pollutant dispersion processes within pedestrian spaces while laying the groundwork for future research into street PM experiments.

ASSESSING URBAN HEAT ISLAND IMPACT AND IDENTIFYING VULNERABILITY ZONES THROUGH GEOSPATIAL AND GEO-STATISTICAL TECHNIQUES

An urban heat island emerged due to micro urban temperature variations are also referred to as urban heat islands or urban hot spots. The high-rise buildings along the roads form "Urban Canyons" that inhibit reflected radiation from the built-up surface. Urban heat island develops over the cities due to man-made activity and the landscape. An understanding of the urban heat island and its formation is not only helpful in understanding urban thermal characteristics but also helps in understanding human comfort. A geospatial technique has the ability to acquire updated and cost-effective data over large regions. For urban climatology studies, remote sensing and geographic information systems are an important source of information and an effective methodology. Since 1971, the city of Krishnanagar and its vicinity have been witnessing rapid urban growth. Due to its dense population, urban climate and rapid urban expansion, they cause environmental degradation. Appraisal and Impact of urbanization on micro-climate in the Krishnanagar city complex based on satellite derived parameters. For the years 1995, 2007 and 2018, several satellite image analysis approaches such as NDVI, NDWI and NDVI were computed. Significant differences in land surface temperature were observed between 1995 and 2007, as compared to 2007 and 2018.

Pedestrian Dynamic Thermal Comfort Analysis to Optimize Using Trees in Various Urban Morphologies: A Case Study of Cairo City

Considering the impacts of climate change on the goal of obtaining sustainable and healthier cities, this research aimed to analyze and assess the impact of different urban forms with different trees densities on the dynamic physiological equivalent temperature (DPET) for pedestrians while walking further than the average walking distance (750 m) using ENVI-met. This study included five different areas within Greater Cairo, which is suffering from extreme heat stress. The selected study areas had lots of urban variety in terms of the canyons’ aspect ratios, orientations, urban form, green areas, mixed uses, and tree densities. Two tree scenarios were analyzed: the current tree density situation and a scenario where the tree density of each study area was increased to its capacity. The results proved that the DPET had different values than the steady physiological equivalent temperature (SPET) at each point within the walking routes. However, the DPET was closely related to changes in the SPET. Keeping the SPET lower or higher for a long time reduced or increased the DPET, and frequent changes (up and down) in the SPET kept the DPET stable. Changes between DPET values were driven more by the microclimate conditions of a space or canyon than the conditions of the overall area, and controlling the microclimate conditions of a whole urban canyon controlled the DPET. Changes in the DPET could reach as high as 10 °C between different walking routes, and increasing the tree density could help lower the DPET by as much as 6 °C in some cases.