Experimental study of temperature and airflow distribution inside an urban street canyon during hot summer weather conditions—Part I: Air and surface temperatures

This paper describes the measurements and analysis of an experimental campaign performed in an urban street canyon in Athens, Greece. A number of field and indoor experimental procedures were organized during summer 2002 aiming at the investigation of the impact of urban environment on the potential of natural and hybrid ventilation. The present study is focused on the experimental investigation of thermal characteristics of a typical street canyon, oriented in ESE–WNW direction, under hot weather conditions. The temporal and spatial distribution of air and surface temperatures is examined. Emphasis was given on the vertical distribution of air and surface temperatures and the air temperature profile in the centre of canyon under different weather conditions. The measured surface temperature differences across the street reached almost 30 °C and this favored the overheating of lower air levels. Buoyancy generated mainly from asphalt-street heating resulted in the development of the predominant recirculation inside the street canyon.     

An application of the thermo-radiative model SOLENE for the evaluation of street canyon energy balance

This paper presents a validation of the thermo-radiative model SOLENE and its application for analysing the street canyon energy balance. The validation data were selected from the temperature and radiation measurements obtained during the JAPEX campaign, previously described by Idczak et al. [16]: a set of four lines of steel containers buildings composing three parallel street canyons at an approximate 1:5 scale. Reference weather data and micrometeorological conditions within the canyon were measured. Numerical simulations were carried out using the meteorological measurements as model inputs. The simulated surface temperatures and radiation fluxes are compared with the measurements for a full week period, with a focus on a day with clear sky conditions. The street canyon energy balance analysis demonstrates that the most energetic surface was the street ground due to its thick surface layer of tar-coated gravels while the walls had a low heat capacity. The thermal radiation balance was negative for all canyon surfaces. The sensible heat was transferred mainly from the canyon surfaces to the ambient air, but also from the air to the ground in the morning. The effective albedo of the canyon had a diurnal value of 0.20–0.25, but dropped to 0.10 in the afternoon when the ground strongly transformed the direct and reflected solar radiation into sensible heat. This narrow street configuration enhanced solar radiation absorption and longwave radiation trapping.     

Airflow and pollutant transport in street canyons

In this work the dispersion of gaseous and particulate exhaust emissions in different street canyons were studied. For two-dimensional sections of canyon models airflow, pollutant dispersion and deposition patterns in the streets and on the surrounding buildings were analyzed. Effects of building size, street width, and wind velocity on the pollutant transport were examined. While the stress transport turbulence models were used in most of the analysis, the predictions of other turbulence models were also examined. Depending on wind speed, building height, and street width, it was found that large recirculation regions in canyons might form. Under certain conditions, also pollutants emitted from vehicle exhaust may trap inside the street canyon. Variations of transport and deposition of emitted particulate pollutants with particle size and relaxation time were also studied. It was shown that the amount of deposited particles in street canyons reduces when the wind speed increases. The simulation results were compared with the available wind tunnel experiments and favorable agreement was found.     

Experimental investigation of air flow and temperature distribution in deep urban canyons for natural ventilation purposes

Detailed experiments have been carried out in a deep canyon in Athens during the summer period. The aim of the experimental campaign was first to evaluate the potential of natural ventilation in the urban environment and second to better understand the air flow and thermal phenomena in deep urban canyons.Extensive measurements of the surface and air temperature as well as the wind speed in the middle and close to the canyon façades have been measured in a continuous basis. In parallel, the undisturbed temperature, wind speed and direction above the canyon have been measured as well. Measurements of the airflow rate in single side and cross ventilation configurations have been carried out using tracer gas techniques, in a naturally ventilated building located in the canyon.It is found that the potential of natural ventilation for both single side and cross ventilation configurations in buildings located inside urban canyons, is seriously reduced. It is estimated, that in the specific canyon, the airflow rate for single side and cross ventilation configurations, is reduced by 82 and 68%, respectively compared to an undisturbed location. The mechanism of the air flow and temperature distribution inside the canyon is extensively analysed and the specific phenomena that determine the wind speed and direction inside the canyon are described in details.     

Effects of building aspect ratio and wind speed on air temperatures in urban-like street canyons

The objective of this study is to simulate the characteristic role of building aspect ratio (AR) and wind speed on air temperatures during different street canyon heating situations. A two-dimensional Renormalization Group (RNG) k–? turbulence model is employed to solve the Reynolds-averaged Navier–Stokes (RANS) and energy transport equations. A comparison of the results from the adopted model with those reported by similar experimental and numerical works demonstrated that the model is quite reliable when simulating temperature and wind profiles. The model is employed to predict air temperatures in idealized street canyons of aspect ratios (building-height-to-street-width ratio) of 0.5–8 with ambient wind speeds of 0.5–4 m/s. Three situations were identified for simulating diurnal heating of street canyon. It is noted that air temperatures are positively correlated with the bulk Richardson number (R_b) in most of the cases. The results show that the air temperature difference between high and low AR street canyon (Δθ_AR) was the highest during the nighttime (i.e., around 7.5 K between AR8 and AR0.5), but low or even negative during the daytime. It is also found that air temperatures rose as high as 1.3 K when ambient wind speed decreased from 4 m/s to 0.5 m/s. It is also revealed that the Δθ_AR during different diurnal situations and the nighttime and daytime air temperature difference between urban and rural areas (Urban Heat Island, UHI) closely resemble one another. Conclusively, the results of this study have highlighted the importance of street canyon AR and wind speed on urban heating.     

街道峡谷内汽车污染物扩散的模拟研究

对街道峡谷污染问题的研究常采用实地测量、物理风洞试验和数值模拟等方法。本文采用数值模拟对城市街道峡谷汽车污染物的扩散规律进行了研究,从描述街道峡谷内空气流动和污染物扩散的控制方程出发,采用数值模拟的方法对街道内部的空气流动和污染物的扩散进行了模拟分析,并将结果与风洞试验结果进行比较,从而获得了城市街道峡谷汽车污染物的扩散规律。研究结果可以为城市街道大气污染监测、评价以及防治提供科学依据。     

Pollution removal effectiveness of the pedestrian ventilation system

Dispersion of vehicular pollution through street canyons has been widely studied in order to find strategies for reducing concentration level. Recently, a pedestrian ventilation system (PVS), an active mitigation strategy, has been proposed to enhance pedestrian comfort indices and to induce appropriate air movement. This paper investigates the performance of PVS to control pollution dispersion within street canyons. Pollution control is achieved by exhausting/supplying air from/to the street canyon through the PVS. In the present paper, the effectiveness of these strategies was studied by varying the parameters that affect dispersion, such as aspect ratios (AR) and thermal stratifications.Computational Fluid Dynamics (CFD) has been selected as the investigation tool. Prior to simulations, the proposed model was successfully validated using two sets of experimental data. Four case-studies were also used to investigate the aspect ratio and the stratification effect. These test cases were developed based on small scale studies in a wind tunnel. Results show the ability of the PVS to change the airflow pattern through the street canyon, resulting in significant pollution removal, especially from the pedestrian level. Moreover, the air and pollution exchange rate concepts have been used for better evaluation of the PVS performance. Furthermore, a breakthrough index was proposed to evaluate the effect of the PVS airflow rate.     

Dispersion in a street canyon for a wind direction parallel to the street axis

We investigate pollutant dispersion in a street canyon for an external wind direction parallel to the street axis, a case which has been poorly documented in the literature. The study is performed numerically and analytically by means of a model based on a series of simplifying assumptions. The range of validity of these assumptions is discussed by comparing analytical and numerical results for two different street aspect ratios. Our results show that, for a critical length of the street, ground level concentration can be higher than those observed in a street canyon whose axis is perpendicular to the external wind direction. We show that this critical length depends on the street aspect ratio.     

Large-eddy simulation of turbulent transports in urban street canyons in different thermal stabilities

Five sets of large-eddy simulations (LES) were performed to examine the characteristics of flows and pollutant dispersion in two-dimensional (2D) urban street canyons of unity building-height-to-street-width ratio in neutral, unstable, and stable thermal stratifications. The characteristic flows fall into the skimming flow regime for all the cases tested. The mean wind speed is increased and decreased, respectively, in unstable and stable conditions. Turbulence is enhanced in unstable conditions. Whereas, in stable conditions, the low-level temperature inversion weakens the recirculating flows forming another layer of stagnant air in the vicinity of the ground level. Unexpectedly, an increase in turbulence is found in the street canyon core in the slightly stable condition (Richardson number Rb=0.18). The turbulence promotion could be caused by the unique geometry of 2D street canyon in which the stable stratification slows down the primary recirculation. The rather stagnant flows in turn sharpen the roof-level vertical velocity gradient and deter the entrainment penetrating down to the ground level, leading to a substantial pollutant accumulation. While the pollutant tends to be well mixed in the street canyons in neutral and unstable conditions, a mildly improved pollutant removal in unstable conditions is observed because of the enhanced roof-level buoyancy-driven turbulence.     

Numerical simulation of the thermal environment of urban street canyon and a design strategy

The thermal environment of urban street canyons is closely related to design factors. By establishing a dynamic model of street canyons numerical simulations of temperature fields for typical street canyons are conducted. Through numerical analysis the influence of the most common variables related to environment in China’s urban residential quarters is compared and contrasted. Environmental factors include the height-width ratio of the street canyon, ground paving materials, canyon directions, and facades of buildings on canyon sides. Simulation results reveal how these factors influence the street canyon thermal environment. Results also give the recommended optimal height-width ratios for urban street canyons and question and revise some design ideas prevalent in China today.     

Effect of uneven building layout on air flow and pollutant dispersion in non-uniform street canyons

Uneven building layouts and non-uniform street canyons are common in actual urban morphology. To study the effects of building layouts on air flow in non-uniform street canyons, various building arrangements are designed in this study. Simulations are carried out under four cases (i.e., a uniform street canyon as Case 1 and three non-uniform canyons as Cases 2–4) with parameter change of the occupying ratio of high buildings (ORHB) in the computational domain and their bilateral allocation as well as the combinations of stepup and/or stepdown notches. In the three non-uniform canyons, stepup and stepdown notches are separating (with ORHB of 25% for Case 2 and 75% for Case 4) or adjoining (with ORHB of 50% for Case 3). The air flow and pollutant dispersion in these street canyons are investigated using Large-eddy Simulation (LES). The air flow structures in the non-uniform street canyons are more complicated than in the uniform street canyon. Inside the non-uniform street canyons, the tilting, horizontal divergence and convergence of wind streamlines are found. Large-scale air exchanges of air mass inside and above the street canyons are found as well. At the pedestrian level, the concentrations of simulated pollutants (e.g., the mean and maximum concentrations) in the non-uniform street canyons are lower than those in the uniform one, suggesting that uneven building layouts are capable of improving the dispersion of pollutants in urban area. Further studies on Case 2–4 show that the separation of stepup and stepdown notches along the street increases the wind velocities in the vicinity of high buildings, while the adjoining of stepup and stepdown notches decreases the wind velocities. Low concentrations of pollutant at the pedestrian level are found in Case 2 compared to Cases 3 and 4. Thus, the separation of stepup and stepdown notches in non-uniform street canyons might be a good choice for uneven building layout arrangements from the point of view of pollutant dispersion and human health.     

CFD modeling of pollution dispersion in a street canyon: Comparison between LES and RANS

CFD modeling using RANS and LES of pollutant dispersion in a three-dimensional street canyon is investigated by comparison with measurements. The purpose of this study is to confirm the accuracy of LES in modeling plume dispersion in a simple street canyon model and to clarify the mechanism of the discrepancy in relation to RANS computation. Simple LES modeling is shown by comparison with wind tunnel experiments to give better results than conventional RANS computation (RNG) modeling of the distribution of mean concentration. The horizontal diffusion of concentration is well reproduced by LES, mainly due to the reproduction of unsteady concentration fluctuations in the street canyon.     

Transport characteristics of PM<Subscript>2.5</Subscript> inside urban street canyons: The effects of trees and vehicles

Particles emitted from vehicles can threaten human health. Therefore the investigations about the particle dispersion characteristics inside the street canyon are of significance. For the current work, the airflow fields and the particle movement inside an isolated street canyon with three aspect ratios under perpendicular wind direction and two typical wind speeds are studied numerically, for which the particle distributions are obtained by solving the drift-flux model. The conclusions demonstrate that the trees will result in the particles gathering. However, the particle deposition fluxes on the trees are so small which cannot effectively remove the particles from the ambient air. Besides, the vehicles tend to cluster particles around the street axis and slow down the dispersion to the curbside buildings. Therefore, the vehicles should not be ignored in the numerical models, especially for the cases in the wind direction of 90°. Moreover the cases with the aspect ratio of 0.5 are more sensitive to the effects of trees and vehicles. The shop vendors expose to the highest particle concentrations in most of the scenarios.     

Concentration and flow distributions in urban street canyons: wind tunnel and computational data

The goal of this paper is to present bluff body flow and transport from steady point sources of pollutants, or chemical and biological agents in an idealized urban environment This paper includes ventilation behavior in different street canyon configurations. To evaluate dispersion in a model urban street canyon, a series of tests with various street canyon aspect ratios (B/H) are presented. Both open-country roughness and urban roughness cases are considered. The flow and dispersion of gases emitted by a point source located between two buildings inside an urban street canyon were determined by the prognostic model FLUENT using four different RANS turbulent closure approximations and in the model fire dynamics simulator using a large eddy simulation methodology. Calculations are compared against fluid modeling in the Industrial Meteorological Wind Tunnel at Colorado State University. A basic building shape, the Wind Engineering Research Field Laboratory building (WERFL) at Texas Tech University, was used for this study. The urban street canyon was represented by a 1:50 scale WERFL model surrounded by models of similar dimensions. These buildings were arranged in various symmetric configurations with different separation distances and different numbers of up- or downwind buildings. Measurements and calculations reveal the dispersion of gases within the urban environment are essentially unsteady, and they are not always well predicted by the use of steady-state prediction methodologies.     

Impact of non-uniform urban surface temperature on pollution dispersion in urban areas

Airflow pattern through street canyons has been widely studied to understand the nature of pollution dispersion in order to develop guidelines for urban planners. One of the major contributing parameters in pollution dispersion is thermal-induced flow caused by surface and air temperature difference. However, most of the previous studies assumed isothermal condition for street canyons. Those addressed the thermal-induced flow, have assumed a uniform wall surface temperature distribution. The external building wall surface temperature distribution is not uniform, and is influenced by many factors including the wall surface characteristics, and shading. The non-uniform temperature distribution significantly impacts on 3-dimensional airflow within street canyons. Moreover, effect of intersection is barely considered in the literature where L/H<3 (L and H are respectively length and height of street canyon). This Paper reports the development of a 3-dimensional model to study the effect of non-uniform wall surface temperature distribution on the pollution dispersion and flow pattern within the short street canyons (L/H<3). For this purpose, a computational fluid dynamics (CFD) model is developed to investigate these effects on pollution dispersion in various prevailing wind velocities and directions. Moreover, active and passive techniques to reduce the level of concentration are examined. The study clearly shows that thermal-induced flow dominates during fair-weather condition.     

Study on the influence of meteorological conditions and the street side buildings on the pollutant dispersion in the street canyon

As cities grow, automobile exhaust pollution is worsening, which has become a major problem of air pollution, even it is a serious threat to the physical and mental health of residents. Thus, to study its diffusion law and influential factors occupies a count for much position. The paper analyzes the factors that affect the dispersion of urban vehicle emissions in street canyon to evaluate the research method of pollutant dispersion. In addition, the influences of different wind speeds and wind directions, the roof shape of buildings on both street sides, and the relative height of the two sides of buildings on the street canyon, on airflow field and pollutant dispersion are simulated. It is shown that the wind speed, the wind direction and the buildings on both sides have a great impact on the airflow and contaminant dispersion in the street canyon. The results provide scientific basis for controlling, monitoring and evaluating the urban motor vehicle emissions, besides the reasonable layout and the programme of urban streets.     

Wind tunnel simulation studies on dispersion at urban street canyons and intersections—a review

Increased traffic emissions and reduced natural ventilation cause build up of high pollution levels in urban street canyons/intersections. Natural ventilation in urban streets canyons/intersections is restricted because the bulk of flow does not enter inside and pollutants are trapped in the lower region. Wind vortices, low-pressure zones and channeling effects may cause build up of pollutants under adverse meteorological conditions within urban street canyons. The review provides a comprehensive literature on wind tunnel simulation studies in urban street canyons/intersections including the effects of building configurations, canyon geometries, traffic induced turbulence and variable approaching wind directions on flow fields and exhaust dispersion.     

Impact of wedge-shaped roofs on airflow and pollutant dispersion inside urban street canyons

The objective of this study is to investigate numerically the effect of wedge-shaped roofs on wind flow and pollutant dispersion in a street canyon within an urban environment. A two-dimensional computational fluid dynamics (CFD) model for evaluating airflow and pollutant dispersion within an urban street canyon is firstly developed using the FLUENT code, and then validated against the wind tunnel experiment. It was found that the model performance is satisfactory. Having established this, the wind flow and pollutant dispersion in urban street canyons of sixteen different wedge-shaped roof combinations are simulated. The computed velocity fields and concentration contours indicate that the in-canyon vortex dynamics and pollutant distriburtion are strongly dependent on the wedge-shaped roof configurations: (1) the height of a wedge-shaped roof peak is a crucial parameter determining the in-canyon vortex structure when an upward wedge-shaped roof is placed on the upwind building of a canyon; (2) both the heights of upstream and downstream corners of the upwind building have a significant impact on the in-canyon vortical flow when a downward wedge-shaped roof is placed on the upwind building of a canyon, due to flow separation as wind passes through the roof peak; (3) the height of upstream corner of the downwind building is an important factor deciding the in-canyon flow pattern when a wedge-shaped roof is placed on the downwind building of a canyon; (4) the characteristics of pollutant dispersion vary for different wedge-shaped roof configurations, and pollution levels are much higher in the “step-down” canyons relative to the “even” and “step-up” ones.     

The effect of surroundings with different separation distances on surface pressures on low-rise buildings

Very large roof suctions on low-rise buildings occur for isolated buildings during both full-scale experiments and wind tunnel tests performed by many investigators. This paper investigates the sensitivity of these high suctions to the presence of multiple surrounding building configurations. This study uses the Wind Engineering Research Field Laboratory (WERFL) building studied during the CSU/TTU Cooperative Program in Wind Engineering as a basic building shape. A model of the WERFL structure was constructed to a 1:50 scale and instrumented with multiple pressure ports. Pressure taps on the 1:50 scale building model were connected to two 48-channel PSI transducer units. A large number of “dummy” models of similar dimensions were constructed to represent surrounding buildings. These model buildings were arranged in various symmetric configurations with different separation distances, and placed in the Industrial Wind Tunnel of the Wind Engineering and Fluids Laboratory, Colorado State University. Measurements include mean, RMS and peak pressures, street canyon velocity profiles and laser-sheet flow visualizations. Shelter effects produced by the surrounding buildings on the central instrumented building were found to be significant, such that flow patterns are displaced and mean and peak induced loads are significantly different from the isolated building base case. The surface pressures of master WERFL model inside the urban street canyons were determined by the prognostic model FLUENT using the four differences closure approximation and FDS, large eddy simulations methodology. Calculations are compared against fluid modeling from wind-tunnel test.     

城市街道峡谷空间形态及其污染物扩散研究——以杭州市中山路为例

概述了城市街谷污染物的扩散机制及相关研究方法。选择CFD数值模拟法,对杭州市中山路街谷进行试验分析。根据试验结果,从水平断面、纵断面、横断面等对街谷形态及其内部污染物的扩散、分布进行了三维的分析研究,进而总结了街谷形态及其污染物扩散的基本规律,并提出基于促进空气流通和污染物扩散、稀释的相关规划设计策略。