Plan Area Density Research Articles

Volumetric Drag Coefficients for Generic Urban Configurations: Insights from Canopy Flow Analysis

Alternative drag approaches for representing unresolved buildings were proposed in literature for computational fluid dynamics (CFD) simulation of macroscopic urban airflow. As a contribution, the present work derives the volumetric drag coefficient (Cd*) through canopy drag and velocity analysis and provides appropriate correlations for Cd*against urban morphological parameters. A total of 72 cases across various urban configurations are investigated, categorized by building typology, horizontal layout, height variability, and plan area density (λp, from 0.0625 to 0.57). Reynolds-Averaged Navier-Stokes (RANS) simulations with periodic boundary conditions are performed to model fully developed flows. Results for the normalized drag force and superficial velocity and their relations with λp are evaluated. Subsequent evaluation of the profiles for the sectional coefficients (Cd*(Z)) reveals four distinct types with variations in uniform-height cases and combinations in varying-height cases. A throughout correlations analysis, facilitated by data transformation, identifies the straightforward relations between Cd* and frontal area density (λf) and tortuosity (τ). The followed stepwise regression provides a recommended formula for Cd*, demonstrating a proper fit with the simulated values. These findings facilitate the understanding and appropriate estimation of Cd* and Cd*(Z), promoting the application of macroscopic turbulence models, for neighborhood-scale wind and air quality studies.

Numerical investigation of multiple affecting parameters on mixed convective heat transfer for arrays of buildings

This study aims to explore relationships between wind speed, wind direction, exterior surface roughness, temperature difference, plan area density and convective heat transfer coefficients (CHTCs) for external surfaces of arrays of buildings. The present investigation provides correlations for CHTCs at the windward, leeward, top, left, and right lateral surfaces. Three different plan area densities of 0.25, 0.111 and 0.027 are considered and 249 numerical simulations are conducted to investigate the influential parameters including density (λp), wind angle (θ), wind speed (U10), temperature difference (ΔT), and surface roughness (ε). Findings reveal that higher plan area density results in lower CHTC due to heightened airflow obstruction. Increase of the wind angle amplifies CHTC on leeward and right lateral facades while reduces it on the left lateral side. With increased wind speed, CHTC experiences significant increase. The results also shows that the temperature difference's impact is pronounced at lower air velocities, leading to heightened CHTC. Besides, increased surface roughness enhances heat transfer coefficients. Finally, the present study employs extensive simulations to present correlations for different facades of the buildings in the studied arrays offering insights into CHTC variations in terms of plan area density, wind angle, speed, temperature difference, and roughness height.

Simulation-based suggestions for lockdown rules in dense urban areas considering indoor-outdoor droplet transmission under natural ventilation conditions

Lockdown rules have been arbitrarily applied to curtail respiratory disease outbreaks without being certain about indoor-outdoor droplet transmission mechanisms and associated infection risks, causing enormous economic losses and arousing social unrest. This study addressed these issues by proposing lockdown rules for dense urban areas using computational fluid dynamics (CFD) simulations of indoor-outdoor droplet dispersion originating from various source locations in single-sided and cross-ventilated spaces of a six-story building in idealized urban areas with plan area densities (λp) of 0.25, 0.44, 0.59, 0.69, and 0.82. The findings suggest issuing lockdown orders based on the relative locations of the patient and vulnerable persons. For example, droplets exhaled by a patient on the upper and middle floors can enter the apartments on one or two floors below under single-sided ventilation, while the droplets are transmitted to the same level downstream spaces by cross-ventilation. Although pedestrians and building occupants pose no infection risk to each other in urban areas with λp = 0.25, they are both exposed to high droplet concentrations in dense urban areas with λp = 0.69 and 0.82. Therefore, this study recommends area lockdowns for dense urban areas to protect residents and pedestrians from virus-laden droplets carried by indoor-outdoor droplet transmission.

Urban-canopy airflow dynamics: A numerical investigation of drag forces and distribution for generic neighborhoods, and their relationships with breathability

Thorough investigations of urban-canopy drag primarily stemming from pressure drag on building surfaces are necessary given the turbulent flows within complex urban areas. Moreover, a gap persists regarding the relationships between canopy drag and breathability. Therefore, this work delves into the canopy-layer airflow dynamics for generic urban neighborhoods by performing three-dimensional Reynolds-Averaged Navier-Stokes simulations. A total of 32 subcases are examined, encompassing uniform- and varying-height and diverse plan area densities (λp, categorized into groups of sparse: 0.0625/0.067, medium: 0.23/0.25, and dense: 0.53/0.56). Results for the drag distribution highlight the windward-row shelter effect for the medium and the dense, local shelter by taller buildings, and distinct shapes of sectional drag forces (F⁎Z). Local velocity and mean age of air are found strongly positively and negatively correlated to F⁎Z, respectively, with distinct slopes in relation to λp. For the uniform-height, the normalized bulk drag (F⁎bulk, referred to as drag coefficient in literature) peaks for the medium with wake-interference regime; F⁎bulk demonstrates a maximum increase of over two times with height variation; moreover, F⁎bulk for varying-height groups exhibits a marked increase from the sparse to the medium, while remaining comparable values for the dense. The frontal area averaged drag (FAf,ave) exhibits a decreasing trend against λp across all cases. Further, FAf,ave exhibits strong correlations with λp and porosity, and with bulk ventilation indices such as spatially averaged velocity, air change rate, and normalized net escape velocity. Throughout the ‘suburban-urban-suburban’ canopy, medium neighborhoods exerting larger drag cause greater streamwise outdoor pressure drops and flow reductions compared to the sparse. However, dense neighborhoods with lower drag exhibit even larger pressure losses, which should be carefully scrutinized. The findings can inform urban planners in designing more aerodynamically efficient neighborhoods and guide strategies for improving air quality within urban environments.

Influence of Various Urban Morphological Parameters on Urban Canopy Ventilation: A Parametric Numerical Study

Numerical simulation is vital for evaluating urban ventilation. However, accurate urban-scale ventilation modeling requires extensive building surface simulation for computational demand. The distributed drag force approach simplifies the urban canopy by modeling buildings as a porous volume that accounts for momentum and turbulence. This method is a practical solution for simulating urban airflow. The drag force coefficient (Cd) is a crucial aerodynamic parameter in this approach. This study examines how Cd varies with urban design parameters such as plan area density (λp), average building height (H), frontal area density (λf), floor aspect ratio (AR), and sky view factor (SVF). Employing extensive numerical simulations conducted under neutral atmospheric conditions, we explore ranges of λp = 0.04–0.07 and λf = 0.1–1.2. The numerical model has been validated against existing wind tunnel data. The results show that Cd is insensitive to the model scale and background wind speed. We discover a nonlinear relationship between Cd and the parameters λp, λf, and SVF. For urban layouts with cubic-shaped buildings, Cd peaks at different λp within the range of 0.2~0.8. When λp and H are constant, Cd has a linear relationship with AR and λf. It is recommended to use λp, SVF, and AR as predictors for Cd across various urban configurations.

Novel Geometric Parameters for Assessing Flow Over Realistic Versus Idealized Urban Arrays

AbstractUrban heterogeneity, such as the variation of street layouts, building shapes, and building heights, cannot be fully represented by density parameters commonly used in idealized urban environmental analyses. To address this shortcoming and better model flow fields over complex urban neighborhoods, we propose two novel descriptive geometric parameters, alignedness and building facet entropy, which quantify the connectivity of inter‐building spaces along the prevailing wind direction and the variation of building facet orientations, respectively. We then conducted large eddy simulations over 101 urban layouts, including realistic urban configurations with uniform building height as well as idealized building arrays with variable heights, and evaluated the resulting bulk flow properties. Urban canopy flow over realistic neighborhoods resembles staggered building arrays for low urban densities but becomes similar to aligned configurations beyond λp ∼ 0.25 where the realistic flow is less sensitive to changes in density. We further show that compared to traditional density parameters (such as plan and frontal area densities), the mean alignedness, a measure of connectivity of flow paths in street canyons, better predicts canopy‐averaged flow properties. Furthermore, for realistic urban flow, the dispersive momentum flux shows a clear increasing trend with building density, and a decreasing trend with alignedness, which is in contrast with idealized cases that exhibit no clear trend. This distinct behavior further highlights the necessity of evaluating flow over realistic urban layouts for flow parameterization. This study provides an improved method of describing urban layouts for flow characterization that can be applied in neighborhood‐scale urban canopy parameterization.

Spatial aspects of urban air quality management: Estimating the impact of micro-scale urban form on pollution dispersion

Urban planning and design solutions affect urban ventilation conditions, thus mitigating the effects of atmospheric pollution. However, these findings are not being implemented in the planning practice to a sufficient extent, partly due to the lack of specific guidelines. Moreover, many urban air quality monitoring (AQM) sites have low represnentativeness and thus do not provide comprehensive data for effective urban air pollution control with respect to the urban spatial policy. An integrated assessment method based on modelling, simulation, and geospatial data processing tools was used to investigate the impact of micro-scale urban form on the local ventilation conditions and pollution dispersion. The proposed approach combined computational fluid dynamics (CFD) and geographic information system (GIS) tools, and particularly the newly developed Residence Time Index (RTI) - a single CFD-derived parameter quantifying the capabilities of the micro-scale built environment to retain PM10 pollution. Urban segments around monitoring sites in three Polish cities (Gdańsk, Warsaw, and Poznań), characterised by relatively low density and varied urban form typologies, were investigated. The results indicate that in these conditions the following features of the urban form have the strongest correlation with the RTI: plan area density (λP), gross floor area ratio (λGFA), and occlusivity (Oc), making them useful indicators for urban air quality management. On the other hand, PM10 data from the AQM sites are rather poorly linked with urban form indicators, which suggests that in complex urban scenarios a higher spatial resolution of air quality data is required for shaping the spatial policy. The implications from this analysis are useful for the urban planning practice. The developed approach may be also a valuable decision support tool for the assessment of the spatial representativeness of AQM sites.

Development of a Prediction Model of the Pedestrian Mean Velocity Based on LES of Random Building Arrays

Wind speed in urban areas is influenced by the interaction between wind flow and building geometry; at the pedestrian level, the interaction is more complex, particularly with high building density. This study investigated the wind velocity distribution and the mean velocity ratio at the pedestrian level using the large-eddy simulation (LES) database based on random building arrays of several plan area densities, λp. The heights of random buildings are between 0.36 h and 3.76 h where h = 0.025 m. Mean streamwise velocity profiles were obtained at the pedestrian level for all arrays and were found to decrease as λp increased. Wind flow patterns at the pedestrian level were highly influenced by adjacent buildings, especially in denser conditions, λp > 0.17. The pedestrian-level mean velocity was obtained around each building, and the relationship between the local mean velocity ratio, Vp(t) and the local frontal area density, λf(t) was analyzed. Subsequently, a prediction model was formulated based on the building’s aspect ratio, αp; the correlation for high-rise buildings with 2.64 h ≤ αp ≤ 3.76 h was high at 0.8, while a lower correlation was obtained for lower buildings due to random positioning and surrounding geometric effects. Therefore, the impact of high-rise buildings on pedestrian wind velocity can be estimated more accurately using the formulated model.

Impact of morphological parameters on urban ventilation in compact cities: The case of the Tuscolano-Don Bosco district in Rome

Air pollution and heat stress are major concerns associated with the liveability, resilience and sustainability of cities. They directly affect health and comfort and are associated with augmented morbidity and mortality and an increase in the energy demand for building ventilation, air cleaning and cooling. Nevertheless, the detrimental effects of poor air quality may partly be mitigated by increased urban ventilation. This strategy is closely related to the level of urbanization and the urban morphology. Therefore, detailed investigations on the impact of different morphologies on urban ventilation are of paramount importance. Computational Fluid Dynamics simulations have been widely used during the last decades to investigate the effects of the urban morphology on the urban ventilation. However, most of these studies focused on idealized building arrangements, while detailed investigations about the role of real urban morphologies are scarce. This study investigates the ventilation in a compact area in the city of Rome, Italy. 3D steady-state Reynolds-averaged Navier-Stokes simulations are performed to analyze the impact of Morphological Parameters (MP) on the urban ventilation. The results show a considerable worsening of urban ventilation with increasing building density with a reduction in the mean wind velocity up to 62% experienced at the pedestrian level (zp). Correlations between five MPs, e.g., plan area density, area-weighted mean building height, volume density, façade area density, and non-dimensional mean velocity at pedestrian level and at 10 m height are evaluated, and simple models are obtained using linear regression analysis. Among the selected MPs, the building façade area density shows a remarkable correlation with the non-dimensional mean velocity at zp (R2 = 0.82). Such correlations can be valuable tools for practitioners and urban designers, particularly during the first stage of planning, for highlighting areas potentially vulnerable to poor air conditions without running computationally expensive simulations.

Turbulence Characteristics Across a Range of Idealized Urban Canopy Geometries

Good representation of turbulence in urban canopy models is necessary for accurate prediction of momentum and scalar distribution in and above urban canopies. To develop and improve turbulence closure schemes for one-dimensional multi-layer urban canopy models, turbulence characteristics are investigated here by analyzing existing large-eddy simulation and direct numerical simulation data. A range of geometries and flow regimes are analyzed that span packing densities of 0.0625 to 0.44, different building array configurations (cubes and cuboids, aligned and staggered arrays, and variable building height), and different incident wind directions (0^circ and 45^circ with regards to the building face). Momentum mixing-length profiles share similar characteristics across the range of geometries, making a first-order momentum mixing-length turbulence closure a promising approach. In vegetation canopies turbulence is dominated by mixing-layer eddies of a scale determined by the canopy-top shear length scale. No relationship was found between the depth-averaged momentum mixing length within the canopy and the canopy-top shear length scale in the present study. By careful specification of the intrinsic averaging operator in the canopy, an often-overlooked term that accounts for changes in plan area density with height is included in a first-order momentum mixing-length turbulence closure model. For an array of variable-height buildings, its omission leads to velocity overestimation of up to 17%. Additionally, we observe that the von Kármán coefficient varies between 0.20 and 0.51 across simulations, which is the first time such a range of values has been documented. When driving flow is oblique to the building faces, the ratio of dispersive to turbulent momentum flux is larger than unity in the lower half of the canopy, and wake production becomes significant compared to shear production of turbulent momentum flux. It is probable that dispersive momentum fluxes are more significant than previously thought in real urban settings, where the wind direction is almost always oblique.

Impacts of urban morphology on improving urban wind energy potential for generic high-rise building arrays

Previous findings have indicated better performance attained by modified urban morphologies for wind energy utilization only in single and pair buildings, or medium-dense low-rise building arrays. Hence, the main purpose of this study is to address the research gaps to complete a fundamental understanding of the influences of urban morphology in compact high-rise urban areas on enhancing urban wind energy harvest for sustainable urban development. A comprehensive parametric study is conducted using the computational fluid dynamics tool to analyze the impacts of urban morphologies on the wind energy potential for a 6 × 6 array of generic high-rise buildings, including (i) urban density altered from compact to sparse urban layouts, (ii) building corner shapes of sharp and rounded corners, (iii) urban layouts of in-line and staggered patterns, and (iv) wind directions of 0° and 45°. This investigation implements the three-dimensional steady Reynolds-averaged Navier-Stokes equations with the Reynolds stress model to explore the distributions of wind speed, power density, and turbulence intensity over the building array. The results indicate that decreasing urban plan area density reduces the unacceptable turbulence areas with relatively higher wind power density on the roof. Besides, round corners can produce elevated power densities up to 201% greater than sharp corners beside the building. Even under the oblique wind direction of 45°, the rounded corner still shows better wind energy potentials than the sharp corner. The in-line urban layout demonstrates more significant areas with higher power densities and low turbulence intensities than the staggered urban layout.

The Vertical City Weather Generator (VCWG v1.3.2)

Abstract. The Vertical City Weather Generator (VCWG) is a computationally efficient urban microclimate model developed to predict temporal and vertical variation of potential temperature, wind speed, specific humidity, and turbulent kinetic energy. It is composed of various sub-models: a rural model, an urban vertical diffusion model, a radiation model, and a building energy model. Forced with weather data from a nearby rural site, the rural model is used to solve for the vertical profiles of potential temperature, specific humidity, and friction velocity at 10 m a.g.l. The rural model also calculates a horizontal pressure gradient. The rural model outputs are applied to a vertical diffusion urban microclimate model that solves vertical transport equations for potential temperature, momentum, specific humidity, and turbulent kinetic energy. The urban vertical diffusion model is also coupled to the radiation and building energy models using two-way interaction. The aerodynamic and thermal effects of urban elements, surface vegetation, and trees are considered. The predictions of the VCWG model are compared to observations of the Basel UrBan Boundary Layer Experiment (BUBBLE) microclimate field campaign for 8 months from December 2001 to July 2002. The model evaluation indicates that the VCWG predicts vertical profiles of meteorological variables in reasonable agreement with the field measurements. The average bias, root mean square error (RMSE), and R2 for potential temperature are 0.25 K, 1.41 K, and 0.82, respectively. The average bias, RMSE, and R2 for wind speed are 0.67 m s−1, 1.06 m s−1, and 0.41, respectively. The average bias, RMSE, and R2 for specific humidity are 0.00057 kg kg−1, 0.0010 kg kg−1, and 0.85, respectively. In addition, the average bias, RMSE, and R2 for the urban heat island (UHI) are 0.36 K, 1.2 K, and 0.35, respectively. Based on the evaluation, the model performance is comparable to the performance of similar models. The performance of the model is further explored to investigate the effects of urban configurations such as plan and frontal area densities, varying levels of vegetation, building energy configuration, radiation configuration, seasonal variations, and different climate zones on the model predictions. The results obtained from the explorations are reasonably consistent with previous studies in the literature, justifying the reliability and computational efficiency of VCWG for operational urban development projects.

Numerical study of stable stratification effects on the wind over simplified tall building models using large-eddy simulations

Atmospheric stratification is a critical factor affecting wind around buildings. However, very few studies have dealt with the atmosphere under stable stratification. In addition, the building model is typically simplified as a cubic block in previous studies concerning stable stratifications; however, the building aspect ratio (height/width) in the downtown area is often larger than 1. Furthermore, the spatial correlation of the velocities in tall buildings, which is important in determining the length scale of the flow and in evaluating pollutant dispersion, has seldom been studied. Therefore, in the present study, the flow around a simplified tall building and the flow around two groups of buildings with the plan area densities of 12.5% and 25.0%, were investigated under stable stratifications with stratified numbers K equaling to 0, 1, 2, and 3, using large eddy simulations (LES). The aspect ratio of the building models is 2. It was found that, for a single building model, the average wind acceleration above the building is directly proportional to K. Regarding the fluctuating wind velocity, the stratification effect has a weak impact. For building groups, the impact region of stratified flow usually appears when z/H < 1.2. A higher building density reduces the streamwise fluctuating wind velocity and the streamwise velocity spatial correlation.

A framework for pedestrian-level wind conditions improvement in urban areas: CFD simulation and optimization

Given the complexity of urban wind flow, acceptable wind conditions can hardly be achieved in all regions within an urban area. In some regions, high wind speeds can lead to wind discomfort and danger, while some regions suffer from poor outdoor urban ventilation. In order to evaluate and improve urban wind conditions, the combined impacts of wind flow and morphological and aerodynamic characteristics of the urban area should be taken into account. In this perspective, performing extensive and time-consuming parametric analyses is required – which is very difficult – even impossible. This paper, therefore, presents an evolutionary-based framework to optimize building heights and plan area densities to improve wind conditions in complex real urban areas. The framework consists of four steps: (i) urban area identification, (ii) optimization algorithms, (iii) CFD simulations, and (iv) evaluation. The optimization consists of Genetic Algorithm and Particle Swarm Optimization. The framework is applied to a real urban area in the city of Tehran, Iran. The focus is on minimizing low-wind-speed regions. In total, 920 possible combinations of building height variations and plan area densities are assessed. 3D steady Reynolds-averaged Navier-Stokes (RANS) simulations are performed based on a detailed validation study and grid-sensitivity analysis. The results show that the use of this framework can effectively reduce computational time and significantly improve the wind conditions in the entire urban area. Urban planners, architects, and building engineers can benefit from this framework in the early stages of design for better wind conditions in complex real urban areas.

A one-dimensional model of turbulent flow through “urban” canopies (MLUCM v2.0): updates based on large-eddy simulation

Abstract. In mesoscale climate models, urban canopy flow is typically parameterized in terms of the horizontally averaged (1-D) flow and scalar transport, and these parameterizations can be informed by computational fluid dynamics (CFD) simulations of the urban climate at the microscale. Reynolds averaged Navier–Stokes simulation (RANS) models have previously been employed to derive vertical profiles of turbulent length scale and drag coefficient for such parameterization. However, there is substantial evidence that RANS models fall short in accurately representing turbulent flow fields in the urban roughness sublayer. When compared with more accurate flow modeling such as large-eddy simulations (LES), we observed that vertical profiles of turbulent kinetic energy and associated turbulent length scales obtained from RANS models are substantially smaller specifically in the urban canopy. Accordingly, using LES results, we revisited the urban canopy parameterizations employed in the one-dimensional model of turbulent flow through urban areas and updated the parameterization of turbulent length scale and drag coefficient. Additionally, we included the parameterization of the dispersive stress, previously neglected in the 1-D column model. For this objective, the PArallelized Large-Eddy Simulation Model (PALM) is used and a series of simulations in an idealized urban configuration with aligned and staggered arrays are considered. The plan area density (λp) is varied from 0.0625 to 0.44 to span a wide range of urban density (from sparsely developed to compact midrise neighborhoods, respectively). In order to ensure the accuracy of the simulation results, we rigorously evaluated the PALM results by comparing the vertical profiles of turbulent kinetic energy and Reynolds stresses with wind tunnel measurements, as well as other available LES and direct numerical simulation (DNS) studies. After implementing the updated drag coefficients and turbulent length scales in the 1-D model of urban canopy flow, we evaluated the results by (a) testing the 1-D model against the original LES results and demonstrating the differences in predictions between new (derived from LES) and old (derived from RANS) versions of the 1-D model, and (b) testing the 1-D model against LES results for a test case with realistic geometries. Results suggest a more accurate prediction of vertical turbulent exchange in urban canopies, which can consequently lead to an improved prediction of urban heat and pollutant dispersion at the mesoscale.

Roof-level large- and small-scale coherent structures in a street canyon flow

The characteristics of large- and small-scale turbulent motions at roof-level in a street canyon flow were experimentally investigated along with their spatio-temporal organization and their mutual interaction in this region. Quadrant analysis was conducted to identify sweep and ejection events, whilst a spanwise spatial filter was used to identify the large-scale motions in the flow. The present analysis was conducted for six configurations; three upstream roughness arrays and two canyon width (W) to height (h) aspect ratios (AR = W/h = 1 and 3). The upstream roughness arrays consisted of three-dimensional cubes (plan area density, λp = 25%), 1h spaced two-dimensional bars (λp = 50%, corresponding to the skimming-flow regime) and 3h spaced two-dimensional bars (λp = 25%, corresponding to the wake-interference flow regime). It was found that the roughness configuration has a strong effect on the size of the quadrant events in the street canyon, with the wake-interference flow regimes producing significantly larger sweep and ejection events than the skimming-flow regimes. Upstream wake-interference flow regimes were found to have a significantly greater temporal correlation than in the skimming-flow regimes. Large-scale streamwise and spanwise velocity components were found to have large spatio-temporal correlation integral scales, in agreement with the large-scale structures existing in the overlying boundary layer. It was also confirmed that there is a coupling between large- and small-scales at roof-level of the street canyon. While sweep and ejection events were found to be due to the dynamical contribution of the small-scales, their occurrence is shown to be coupled with that of large-scale low- and high-momentum regions, respectively.

Quantifying impacts of wind speed and urban neighborhood layout on the infiltration rate of residential buildings

Urban neighborhood layouts affect wind flow patterns and infiltration rates in the built environment and ultimately, local thermal comfort and energy consumption of buildings. This study uses Computational Fluid Dynamics (CFD) to quantify infiltration rates in low-rise residential buildings. The validated simulation results using the experimental data propose k-ω SST turbulence model to simulate outdoor airflow. Three urban plan area density with four common types of urban morphologies including (i), (ii) Cases A and B: regular and staggered cubical buildings, respectively and (iii), (iv) Cases C and D: street canyon with parallel and vertical wind flow, respectively, are examined. The simulation results indicate that wind speed does not have a decisive role in determining the infiltration or exfiltration phenomenon, and it only is able to increase or decrease the rates of them. The absolute infiltration rate for wind speeds of 1, 4, and 8 m/s is of the considerable scale of 1:2:12 in the all investigated building rows and layouts. The urban layouts findings demonstrate that decreasing the compactness of the buildings reduces the negative pressure gradients. Besides, if the leakage area is parallel to the dominant wind flow, the exfiltration rate is barely a function of the wind speed.

Structure of high Reynolds number boundary layers over cube canopies

The influence of a cube-based canopy on coherent structures of the flow was investigated in a high Reynolds number boundary layer (thickness $\unicode[STIX]{x1D6FF}\sim 30\,000$ wall units). Wind tunnel experiments were conducted considering wall configurations that represent three idealised urban terrains. Stereoscopic particle image velocimetry was employed using a large field of view in a streamwise–spanwise plane ($0.55\unicode[STIX]{x1D6FF}\times 0.5\unicode[STIX]{x1D6FF}$) combined to two-point hot-wire measurements. The analysis of the flow within the inertial layer highlights the independence of its characteristics from the wall configuration. The population of coherent structures is in agreement with that of smooth-wall boundary layers, i.e. consisting of large- and very-large-scale motions, sweeps and ejections, as well as smaller-scale vortical structures. The characteristics of vortices appear to be independent of the roughness configuration while their spatial distribution is closely linked to large meandering motions of the boundary layer. The canopy geometry only significantly impacts the wall-normal exchanges within the roughness sublayer. Bi-dimensional spectral analysis demonstrates that wall-normal velocity fluctuations are constrained by the presence of the canopy for the densest investigated configurations. This threshold in plan area density above which large scales from the overlying boundary layer can penetrate the roughness sublayer is consistent with the change of the flow regime reported in the literature and constitutes a major difference with flows over vegetation canopies.

The Spanwise Variation of Roof-Level Turbulence in a Street-Canyon Flow

The effect of upstream roughness and canyon width on turbulent street-canyon flow is investigated, using wind-tunnel measurements made in a horizontal plane at near roof level of a street canyon and stereoscopic particle image velocimetry. Three upstream roughness arrays and two canyon width (W) to height (h) aspect ratios (AR = W/h = 1 and 3) are used; the arrays consist of three-dimensional cubes (plan area density, λp = 25%), 1h-spaced two-dimensional bars (skimming flow, λp = 50%) and 3h-spaced two-dimensional bars (wake-interference flow, λp = 25%). Understanding the spanwise structure of the flow and how it interacts with large-scale structures is necessary to reliably predict the mean pollutant transport in the lateral direction along the canyon and to further investigate the three-dimensional behaviour of turbulent street-canyon flows. The mean turbulent statistics are presented, whilst two-point correlations and integral length scales are computed for the different configurations. The results show a significant effect of upstream roughness on these quantities. The total turbulent kinetic energy and shear stress are found to be highest for the wake-interference flow regimes and lowest for the skimming-flow regimes. It is found that the three-dimensional upstream roughness configurations result in a significantly weaker correlation in the spanwise direction at canyon roof level, with a similar trend observed in the spanwise integral length scales. The shear-layer thickness is found to be related to the magnitude of the correlations near roof level of the street canyon.

Effect of High-rise Apartment Building Shape and Densities on Outdoor Thermal Environment by Floor in Summer Season

The object of this study is to analyze the outdoor thermal environment of pedestrian and residents of high-rise apartment buildings complexes considering the vertical temperature distribution in summer by evaluating the trend of vertical temperature distribution according to the shape and densities of high-rise apartment buildings. For this purpose, the outdoor vertical temperature of the buildings was analyzed for each apartment complex using the ENVI-met simulation program. Also, the outdoor thermal environment analysis technique in the high-rise apartment complex was validated through the outdoor vertical temperature measurement in the apartment building complex. As a result of the analysis of the outdoor thermal environment, ground surface temperature was analyzed to be strongly influenced by the surface heating by solar radiation. In addition outdoor temperature at the pedestrian level increases with increasing the plan area density (λp) and the frontal area density (λF). However, in the case of high-rise apartment buildings, the trend of increasing the outdoor temperature is relatively large, which suggests that the increase in the density of the buildings affects the outdoor thermal environment. As a result of the analysis of the outdoor thermal environment, when planning a high-rise apartment complex, it is suggested that the density and the shape should be actively considered to improve the outdoor thermal environment.