Canyon Aspect Ratio Research Articles

Air exchange in urban canyons with variable building width: a numerical LES approach.

The aim of this work is to gain further insight into the role played by the building aspect ratio (ARB, i.e., the ratio of the building width, WB, to the building height, H) and its influence on street canyon flow. We carried out a series of large eddy simulations with arrays of obstacles with different widths, ranging from 0.5 to 2.0, and two canyon aspect ratios (ARC, i.e., the ratio of the canyon width, W, to H) ARC = 0.5 and ARC = 1.0. Experimental data was obtained in a water channel for the corresponding configuration and used to validate numerical simulations. Results were found strongly dependent on the building aspect ratios, with two distinct behaviours identified with respect to the canyon aspect ratio. The residence time decreases with decreasing ARB, irrespective of the canyon aspect ratio, suggesting that ARB and ARC can be optimised to guarantee optimal street canyon ventilation in urban planning.

Air exchange in urban canyons with variable building width: a numerical LES approach

The aim of this work is to gain further insight into the role played by the building aspect ratio (ARB, i.e., the ratio of the building width, WB, to the building height, H) and its influence on street canyon flow. We carried out a series of large eddy simulations with arrays of obstacles with different widths, ranging from 0.5 to 2.0, and two canyon aspect ratios (ARC, i.e., the ratio of the canyon width, W, to H) ARC = 0.5 and ARC = 1.0. Experimental data was obtained in a water channel for the corresponding configuration and used to validate numerical simulations. Results were found strongly dependent on the building aspect ratios, with two distinct behaviours identified with respect to the canyon aspect ratio. The residence time decreases with decreasing ARB, irrespective of the canyon aspect ratio, suggesting that ARB and ARC can be optimised to guarantee optimal street canyon ventilation in urban planning.

Numerical simulation of pollutant dispersion in an urban street canyon - effects of elevated metro rail track

Urban air pollution is one of the unsettled issues in urban development and planning process. Urban street canyons are one of the main focused areas of investigation due to the heavy emission of the pollutants from vehicles. In the present study, the effects of elevated metro rail track (EMRT) on pollutant dispersion phenomena have been studied using computational fluid dynamics (CFD) technique. As a practical application of a CFD model, k-e turbulence model was applied for airflow modelling. Species transport model was applied for pollutant dispersion modelling. Geometrical configuration of urban street canyon of aspect ratio 'one', with elevated metro rail track, has been considered. Numerical simulations were carried out for perpendicular wind flow. Pollutant concentrations, at various locations of the street canyon, were largely affected by the introduction of elevated metro rail track. These results were compared with the street canyon without elevated metro rail track. It has been observed that the pollutant concentration increased at lower region and decreased at the upper region of the street canyon. Variations were not uniformly distributed in street canyon volume. Contours and pollutant concentrations have been compared and discussed for different sampled locations in the street canyon.

Influence of canyon aspect ratio on microclimatic conditions: case of Lodz, Poland

This study presents an analysis of the effect of height/width proportions of street canyons on microclimatic conditions in the most urbanized part of Lodz. The empirical research covered public spaces located in the so-called Greater City Zone of Lodz, which were included in the revitalization process. In this case, two street canyons were selected, which are used as pedestrian and driving canyons (the so-called woonerfs). At first, the authors determined the average weather conditions prevailing in the warm period of Typical Meteorological Year in the area of city. On their basis, microclimatic parameters prevailing in the areas of selected street canyons were evaluated with the use of computer simulations. The next stage included the determination of aspect ratio (AR) defining the ratio of building height forming the frontage of canyons to the street width. Finally, the relation between the determined coefficient and selected weather parameters was studied.

Impact of rooftop stack configuration on 2D street canyon air quality

We experimentally investigated the impact of rooftop stack position on the pollutant entrapment within 2D street canyon configurations. Analyses were performed in a water channel loop system by using the image analysis techniques: FTV (Feature Tracking Velocimetry) and LIF (Light Induced Fluorescein) respectively for measuring velocity and concentration. The set-up consists in an array of 20 identical buildings with aspect ratio equal to one (ARB = B/H, where H is the height of the eaves and B is the building width), mimicking an idealized 2D urban canopy. They were equally spaced with a unitary canyon aspect ratio (defined as ARC = W/H; where W is the distance between buildings facades), and varied in shape by using flat and gable roofs with pitch α = 45°, with different chimney positions and heights, for a total number of 9 investigated configurations. Results demonstrate that the presence of gable roof significantly varies the flow in the shear layer, which, in combination with different chimney position and height, lead to not trivial effects on pollutants dispersion.

Pedestrian-level wind speed enhancement in urban street canyons with void decks

Low wind speeds can inhibit passive ventilation and reduce thermal comfort in tropical cities. We use both experimental and numerical approaches to investigate pedestrian-level wind speed enhancement with void decks (empty spaces at the ground floors) in urban street canyons. Water channel experiments show that void decks allow wind to pass through them, thereby enhancing pedestrian-level wind speeds up to twofold. Computational fluid dynamics models validated with the experiments were used to conduct two parametric studies. The first study varied the void deck height and showed that the wind enhancement effects increase with taller void decks, as expected. The second study varied the canyon aspect ratio and showed that variations in the canyon aspect ratio had less influence on wind enhancement compared to the variations in the void deck height. This is because a void deck induces a secondary vortex, which counter-rotates with the vortex driven by the freestream. Increasing the canyon aspect ratio stretches the vortices vertically but does not alter the (normalized) flow fields. Our findings reveal the potential of void decks to channel wind into street canyons, including narrow (high height-to-width aspect ratio) canyons, which suffer from poor ventilation. The enhanced wind speeds are beneficial for pollutant dispersion and outdoor thermal comfort in tropical cities.

Effects of the Upstream-Flow Regime and Canyon Aspect Ratio on Non-linear Interactions Between a Street-Canyon Flow and the Overlying Boundary Layer

Large-scale structures within a rough-wall boundary layer generated over a cube array have recently been linked to small-scale fluctuations close to the roughness through a dynamical mechanism similar to amplitude modulation. Demonstrating the existence of this mechanism for different roughness types is a crucial step towards the development of a generic model for wind fluctuations in the urban canopy. Here the influence of the upstream roughness geometry (two-dimensional (2D) and three-dimensional (3D)) and planform packing density ( $$ \lambda_{p} $$ ) and street-canyon aspect ratio on the non-linear interactions between large-scale momentum regions and the small scales induced by the presence of the roughness is studied within a wind tunnel using combined particle-image velocimetry and hot-wire anemometry. A multi-time delay linear stochastic estimation is used to decompose the flow into large scales that participate in modulation and the remaining small scales. Using three different upstream roughness configurations composed of either 3D cubes or 2D rectangular blocks it is shown that the upstream roughness configuration has an influence on the non-linear interactions in the rough-wall boundary layer. Analysis of the turbulence skewness decomposition shows a change in the location of the maximum of the term $$ \overline{{u_{L}^{\prime} u_{S}^{\prime 2}}} $$ , which represents the influence of the large-scale momentum regions on the small scales, whilst the temporal correlation shows a modification of the interaction located closer to the roughness with a change from 3D to 2D roughness. Furthermore, a two-point spatio–temporal correlation demonstrates that the non-linear relationship is significantly modified in the wake-interference-flow regime compared to the skimming-flow regime. Through skewness decomposition and temporal correlations the canyon aspect ratio is shown to have no influence on the non-linear interactions, indicating that the mechanism depends only on the flow developing upstream. Finally, although the upstream roughness configuration is shown to influence the non-linear interactions, the nature of the mechanism remains the same in all configurations.

Impact of shortwave multiple reflections in an urban street canyon on building thermal energy demands

The urban fabric plays a fundamental role in convective and radiative heat exchanges between buildings. The main parameters which influence these heat exchange mechanisms are due to climate conditions such as air temperatures/humidity, wind speed/direction and solar irradiance. These weather data are related on where our cities are located rather than on how they are built. In this work, a building energy simulation tool is exploited to study the impact of multiple shortwave inter-reflections exchanges in an urban environment with the aim of evaluating their influence on the thermal energy demand of a building. These multiple radiative exchanges modify the buildings envelope energy budget influencing space cooling and heating demand. A street canyon model validated in a previous work was used to investigate the effects of the urban radiative trapping. Due to multiple shortwave reflections, the effective solar radiation absorbed by the buildings envelope surfaces is higher than in a street canyon building where only shadowing phenomena due to canyon geometry are considered. A comparison has been performed between these two configurations as a function of several driving parameters such as street canyon aspect ratio, orientation, transparent/opaque surfaces ratio and solar absorption. The goal is to characterize how these parameters influences the inter reflections inside an urban canyon and thus the buildings energy demands. Increases in cooling demand up to 35% and decreases in heating demand up to 7.5% are found.

Flow and temperature dynamics in an urban canyon under a comprehensive set of wind directions, wind speeds, and thermal stability conditions

Atmospheric flow and temperature dynamics in the urban roughness sublayer exhibit numerous complexities that cannot all be investigated using models or scaled-down experiments, thus these complexities necessitate careful field observations. Such dynamics were studied under a comprehensive set of wind directions, wind speeds, and thermal stability conditions in a field campaign held in Guelph, Ontario, Canada, from 13th to 25th of August 2017. The urban site was a quasi-two-dimensional canyon with unit canyon aspect ratio. Beside characterizing thermal stability, inertial effects, urban heat island intensity, and mean properties of the atmosphere, the turbulence statistics were studied carefully as functions of roof-level wind angle, diurnal time, the building Reynolds number, or the bulk Richardson number. Turbulence statistics in the vertical direction were observed to be influenced by local conditions, such as flow properties and nearby surface temperatures. These statistics also indicated presence of small integral lengthscales and short integral timescales. On the other hand, turbulence statistics in the horizontal directions were influenced by non-local conditions, such as horizontal heterogeneity in heating or urban morphology. These statistics indicated presence of large integral lengthscales and long integral timescales. In addition, a rigorous scaling analysis was performed to seek significant correlations between turbulence statistics and other known flow variables both locally or as measured at a nearby non-urban rural station. Variances scaled more successfully to mean quantities than covariances (Reynolds stresses and turbulence kinematic heat fluxes). In addition, the statistics in the vertical direction scaled more successfully compared to horizontal directions. Statistics in the horizontal directions, particularly in the along-canyon axis direction, were poorly scaled, suggesting presence of unorganized and irregular turbulence structures influenced by non-local conditions. Temperature difference between the atmosphere and nearby surfaces as well as measured velocity scales showed to scale vertical heat fluxes successfully.

Air pollutant sinks on noise barriers: Where do they perform the best?

While laboratory experiments, numerical simulations as well as field tests have underlined the influence of noise barriers in dispersing vehicular emissions and reducing downwind peak concentrations, these pollutants still remain in the atmosphere. Artificial pollutant sinks (for example, particle capturing or toxic gas treating devices) installed on top of noise barriers can further alleviate this problem by eliminating the pollutants passing through it. However, it is not known how the installation of a semi-permeable pollutant sink affects the aerodynamics of the pollutants' flow. By finding an optimal position and orientation for these sinks, the mass of the pollutants reaching the sink inlet can be maximized. Scaled down water tunnel experiments have been used to investigate the effectiveness of installing such a pollutant sink, of fixed dimensions, on top of a noise barrier adjacent to a highway. It is found that installing a sink is more beneficial on top of shorter barriers and that vertically elevating the sink, only slightly, can enhance its pollutant capturing performance. Using a sink in a ‘highway canyon’ (two noise barriers placed symmetrically with respect to the highway) must be done cautiously as there are several flow regimes observed, which are sensitive not only to the canyon aspect ratio (ratio between canyon width and height), but also to the presence/absence of the sink. The results here not only demonstrate the effectiveness of installing pollutant sinks on noise barriers, but also provide ballpark estimates on the optimal placement, orientation and performance of these devices, prior to field tests or even large-scale installation.

Wind-induced pressure at a tunnel portal

In order to properly size the mechanical ventilation system of a tunnel, it is essential to estimate the wind-driven pressure difference that might rise between its two portals. In this respect, we explore here the pressure distribution over a tunnel portal under the influence of an incident atmospheric boundary layer and, in particular, its dependency on wind direction and on tunnel geometry. Reduced scale models of generic configurations of a tunnel portal are studied in an atmospheric wind tunnel. Pressure distributions over the front section of different open cavities are measured with surface taps, which allows us to infer the influence of the tunnel aspect ratio and wind direction on a pressure coefficient $$C_{P}$$ , defined as a spatially and time averaged non-dimensional pressure. Experiments reveal that the magnitude of the coefficient $$C_{P}$$ , as a function of the wind direction, is significantly influenced by the portal height-to-width ratio and almost insensitive to its length. The experimental data set is completed by hot-wire anemometry measurements providing vertical distribution of velocity statistics. The same configurations are simulated by numerically solving the Reynolds-averaged Navier–Stokes equations, adopting the standard $$k - \varepsilon$$ turbulence model. Despite some discrepancies between numerical and experimental estimates of some flow parameters (namely the turbulent kinetic energy field), the numerical estimates of the pressure coefficients $$C_{P}$$ show very good agreement with experimental data. The latter is also compared to the predictions of an analytical model, based on the estimate of a spatially averaged velocity within an infinitely long street canyon. The results of the model, which takes into account varying canyon aspect ratios, are in reasonable agreement with experimental data for all cases studied. Notably, its predictions are significantly better than those provided by the simple analytical relations usually adopted as a reference in tunnel ventilation studies.

Pollutant fluxes in two-dimensional street canyons

The turbulent dispersion of a passive tracer emitted by a line source simulating vehicular traffic in an idealized urban canyon in neutral conditions is studied in the water channel by means of simultaneous measurements of velocity and concentration fields. The experiments are conducted for two geometrical arrangements of two-dimensional obstacles reproducing the skimming flow (AR = 1) and the wake-interference regime (AR = 2), where AR is the canyon aspect ratio. The results show a strict connection between the dynamics of the shear layer developing at the top of the canyon and the vorticity field. For AR = 1, it is found that the shear layer flaps upwards and downwards according to two different frequencies. The greatest of them matches the vortex shedding frequency as measured at the canyon top, while the lower is comparable to H/u∗ (H is the building height and u∗ a reference friction velocity). The shear layer flapping modulates in time the pollutant exchange rate between the canyon and the outer layer. The different characteristics of the shear layer found for the two flow regimes also explain the larger pollutant re-entrainment observed for AR = 1, which turns out to be greater than the emission rate at the source. Sweep and ejection modes are identified via quadrant analysis and used to quantify the weights of the factors involved in the turbulent exchanges of tracer and momentum between the canyon and the outer layer. It is found that the venting of polluted fluid at the canyon top increases substantially passing from AR = 1 to 2, while an opposite trend is observed for the entrainment of polluted fluid. The vertical flux of pollutant at the canyon top for AR = 2 is largely of turbulent nature, with the contribution of the mean flux being practically negligible. On the other hand, the latter becomes comparable or even exceeds the magnitude of the turbulent flux when AR = 1. The maps of the first three statistical moments of the pollutant concentration for the two geometrical arrangements are also reported and discussed.

“Optimisation of canyon orientation and aspect ratio in warm-humid climate: Case of Rajarhat Newtown, India”

This study investigates the impact of different orientations and canyon aspect ratios to improve street-level microclimate in a warm humid climate. It also contributes to the development of a methodology to develop design guidelines while planning urban neighborhoods. The study was carried out using ENVI_met version 4.0. Simulations were run for typical residential neighborhoods in the Rajarhat Newtown, Kolkata, India, characterized by a warm humid climate as per the National Building Code of India, for a typical summer day with a high level of discomfort. Different orientations and different canyon aspect ratios are simulated and compared to find the most suitable design alternative for the study area. The floor area ratio has been kept constant in order to keep the design alternative more realistic. Performance analysis of the design alternatives was carried out by comparing air temperature, wind speed, MRT and PET. It was found that an orientation angle between 30° and 60° with the wind direction and a Canyon Aspect Ratio of 2.5 can reduce the PET value by 5 to 9°C throughout most of the study area during a mid-afternoon summer day. Improvement in the thermophysical assessment classes cannot be achieved for the adopted simulation date and time (May 31st, 14:00h).

Computational Fluid Dynamics Analysis for Evaluating the Urban Heat Island Effects

The quality of the thermal environment within the built environment is dependent by local climate and urban design features. Therefore, the scientific knowledge on urban design and microclimate are fundamental for obtaining a tolerable thermal environment at the neighborhood scale. Such problematic interested the huge part of the world population that lives in urban area, which is currently about 50% and it is expected will increase to 66 % by 2050.The specificity of the urban climate is frequently associated with the urban heat islands phenomenon, which refers to the elevated temperatures within the city areas compared to the rural surroundings. In this context a typical urban geometry is represented by the so called “urban canyon “that denotes an ideal infinite urban street confined by buildings on both sidesIn this study an urban geometry, constituted by three urban street canyons with a canyon aspect ratio H/W of 1.0, has been examined. CFD simulations were performed to evaluate the fields of temperature and velocity of the air within the urban canyons and their surroundings. Several scenarios were examined considering alternatively the leeward or the windward walls hitted by the sunrays, while the opposite façade was shaded, as well as varying the reflective properties of the surfaces and the wind velocity. The results of simulations evidence that the adoption of materials of the building envelope with high albedo coefficient guarantees a decrease of the temperatures at least of 1.5°C. Therefore, an increase of the knowledge of urban climate may provide valuable contribution to promote energy efficiency in the built environment.

Large-eddy simulation of the impact of urban trees on momentum and heat fluxes

Trees in urban environment have a profound impact on the microclimate and environmental sustainability. Realistically representing them in urban models is an ongoing area of research in urban environmental study. In this paper, we develop a novel large-eddy simulation (LES) model (LES-UrbanTree) that resolves the buildings and parameterizes urban trees by accounting for their aerodynamic impact. The shading effect of trees is explicitly taken into account in LES-UrbanTree by a subsurface conduction model coupled to LES. Two-dimensional street canyons with trees in the middle of the street are used as a prototype for case studies. It is found that under moderate canyon aspect ratio (i.e. height/width being 0.5 and 1), trees taller than the mean building height leads to the strongest modification of the flow and temperature fields. Tall trees strongly impact the downward transport of high momentum (i.e. sweeping events) and therefore alter the momentum and heat fluxes most significantly through direct interaction with the strong shear layer near the roof top. Simulations of street canyons of different aspect ratios also produce physically consistent results, thus demonstrating the application potential of LES-UrbanTree. The study overall highlights the importance of representing both the aerodynamic and thermodynamic changes due to trees in urban models.

Numerical Investigation on the Effect of Avenue Trees on PM2.5 Dispersion in Urban Street Canyons

The Reynolds-averaged Navier-Stokes (RANS) model and revised generalized drift flux model were used to investigate the characteristics of airflow fields and PM2.5 dispersion in street canyons with a variety setting on tree crown morphologies (i.e., conical, spherical, and cylindrical), leaf area densities (LADs = 0.5, 1.5, and 2.5 m2/m3), and street canyon aspect ratios (H/W = 0.5, 1.0, and 2.0). Results were as follows: (1) airflow fields were reversed in the presence of trees and enhanced with higher LAD; (2) air velocity decreased negligibly when LAD increased from 1.5 to 2.5, but significantly when LAD increased from 0.5 to 1.5; (3) tree crown morphologies, building aspect ratios, and LADs were interrelated. The comparison of PM2.5 showed that the most critical situations in H/W = 0.5, 1.0, and 2.0 corresponded to LAD = 0.5 with a conical canopy; (4) the H/W = 1.0 and LAD = 1.5 scenario was identified as the most efficient combination for PM2.5 capture; (5) the maximum PM2.5 reduction ratio was ordered from low to high in the sequence of conical, spherical, and cylindrical canopies. At predestinated LADs and aspect ratio, Populus tomentosa with cylindrical crown morphology exhibited the best efficiency on PM2.5 capture with a reduction ratio of 75% to 85% at pedestrian height.

Street canyon design and improvement potential for urban open spaces; the influence of canyon aspect ratio and orientation on microclimate and outdoor comfort

Urban morphology and material properties are critical parameters in the formation of outdoor microclimates and their effects on the thermal comfort of pedestrians. Despite considerable amounts of previous and ongoing research the scientific research results have not yet been fully adopted on urban projects. The present paper aims to contribute empirical and analytical results that highlight potential improvements that can be achieved through urban design. The paper draws upon environmental measurements, taken in both winter and summer periods, in 18 street canyons located in a dense central area of the city of Thessaloniki in northern Greece. The measurements are complemented by microclimate simulation studies involving modelling of the 18 street canyons as well as a number of generic cases. The results of these studies suggest the most favourable canyon geometries in terms of pedestrian thermal comfort and support the incorporation of basic climatic parameters into urban design.

Effect of Urban Street Canyon Aspect Ratio on Thermal Performance of Road Pavement Solar Collectors (RPSC)

Studies on RPSC (road pavement solar collectors) have shown the potential of reducing the urban heat island effect by dissipating the heat from the pavement for energy harness. In our previous work, performance analysis of RPSC system was carried out to compare the RPSC embedment in two scenarios; within an urban street canyon and within suburban or rural area. The current study expands the analysis of the RPSC system in urban areas by assessing the impact of varying canyon aspect ratios on the performance of RPSC. De-coupled Computational Fluid Dynamic (CFD) approach was proposed to investigate the integration of RPSC system in an urban canyon. The CFD tool ANSYS Fluent 15.0 was used to simulate the fluid flow and heat transfer on the pavement/road surface by enabling three models: (i) energy model, (ii) standard k-epsilon model, and (iii) coupled DO-solar load radiation model. The results showed that a significant pavement surface temperature increase was found when the aspect ratio (AR) was increased from 1 to 2 while minimal increase was observed for the canyon with AR above 2. At the particular simulated time (13:00) and location, it was found that the overall performance of the RPSC system significantly increased by up to 13.0% when AR was increased from 1 to 2, but the performance of RSPC in shadow area (due to the shading effect of building) had significantly dropped (up to 30.0%) from AR 3 to 4. Findings of this study showed that the canyon aspect ratio had a significant impact on the temperature distribution of the ground surface and should be taken into consideration when assessing the performance of RPSC in urban areas.

On the effect of gable roof on natural ventilation in two-dimensional urban canyons

Flow regimes occurring in urban canyons are strongly influenced by the geometrical shape of the buildings; however, fluid dynamic investigations are typically carried out using parallelepiped obstacles. The present study is focused on assessing the effect of gable roofs on the flow regimes characterizing urban canyons (skimming flow, wake interference, isolated roughness) and the implications in terms of integral parameters (air exchange rate and friction factor), which are useful in practical applications. Numerical simulations are performed by means of RANS modeling of idealized two dimensional urban canyons between series of identical gable roof buildings with pitch ranging from 0° up to 40°, and wind direction perpendicular to the canyon axis. Simulations performed for different canyon aspect ratios show the key role played by the roof pitch in enhancing turbulence and in increasing ventilation, in particular for narrow canyons. Furthermore, turbulence-driven ventilation is observed to be related to the square root of the friction coefficient by a single linear relation, despite of the roof pitch. These results may have an impact on design and planning strategies aimed at enhancing natural ventilation and promoting efficient pollutant and heat dispersion in urban areas.

The Effectiveness of Light Shelf in Tropical Urban Context

Abstract Light shelf was developed to create uniform indoor illuminance. However, in hot climates the unshaded clerestory above the shelf transmits high solar heat gain. In dense urban context, these advantages and disadvantages might vary regarding the context and position of the fenestration. This study employed an integrated energy simulation software to investigate the effectiveness of light shelf application in a tropical urban context in terms of building energy consumption. Radiance and EnergyPlus based simulations performed the effects of urban canyon aspect ratio and external surface albedo on the daylighting performances, space cooling load, as well as the lighting energy consumption of the building equipped with lightshelves in 2 humid tropical cities. Comparison of the energy performances of 3 fenestration systems, i.e. fenestration without any shading device, with overhangs, and with light shelves, yielded some recommendations concerning the best application of light shelf on the certain floor levels and aspect ratio of the urban context.