Evaluation of pedestrian winds around tall buildings by numerical approach

Summary A numerical model based on the compressible flow equations has been used to simulate the pedestrian wind fields around the programming tall buildings in Beijing city. The model results are well agreed with those of the wind tunnel experiments. Since numerical model can provide the detailed flow field data at each grid point and each time step, it has a great advantage compared with the wind tunnel experiment in accurately evaluating the wind effects on pedestrian and other environmental issues. Furthermore, advance computational technology has made the numerical modeling become fast and cheaply. It is believed that the numerical simulation will play an increasingly more important role in the coming years, especially in dealing with wind environment.     

New analytical solution of the dispersion equation

The dispersion of pollutants from a point source is analytically investigated taking into consideration the vertical variation of both wind speed and eddy diffusivity. The deposition of the diffusing particles on the ground is taken into account throughout the boundary conditions. The concentration of pollutants under different atmospheric stabilities was found assuming that the vertical diffusion is limited by an elevated inversion layer. The decay distance of a pollutant along the wind direction for different atmospheric stabilities was derived. The resulting analytical formulae have been applied on a case study namely, the emission from the research reactor at Inshas. The results are discussed and presented in illustrative figures.     

Wind Tunnel And Field Measurements Of Turbulent Flow In Forests. Part I: Uniformly Thinned Stands

Many forest management methods alterstand density uniformly. The effectsof such a change on the wind andturbulence regimes in the forest arecritical to a number of processes governingthe stability of the stand and itsmicroclimate. We measured wind speed andturbulence statistics with a Dantec tri-axialhot-film probe in model forests of variousdensities (31–333 trees m^-2), created byremoving whole trees in a regular pattern in awind tunnel, and compared them with similarmeasurements made with propeller anemometers insimilarly thinned plots (156–625 trees ha^-1)within a Sitka spruce stand in Scotland. The results agree well, in general, with measurements made inother such studies with diverse canopy types.The systematic variations with density and verticalleaf-area distribution (which differed betweenwind-tunnel and field trees) in our work can explainmuch of the variability shown in scaled profiles ofbasic turbulence statistics reported in theliterature. The wind tunnel and field results are shown to be in good agreement overalldespite the difference in vertical leaf-areadistribution. Within-canopy and isolated-treedrag coefficients in the wind tunnel showthat tree-scale shelter effects increase astree density increases. The measurements indicatethat turbulence in the canopy is dominated bylarge-scale structures with dimensions of the sameorder as the height of the canopy as found inother studies but suggest that inter-tree spacing also modulates the size of these structures. These structures are associated with the sweeps that dominatemomentum exchange in the canopy and it is thisfact that allows the tri-axial probe to operate sowell despite the relatively narrow range of anglesin which the wind vector is correctly measured. Theratio of streamwise periodicity of these structuresto vorticity thickness varies systematically withtree density in the range 2.7–5.1, which spans theexpected range of 3.5–5 found in a laboratorymixing-layer, suggesting that tree spacing imposes another relevant length scale. This test andothers show that the results are in agreement withthe idea that canopy turbulence resembles that of a mixing layer even though they disagree with, and challenge the linear relationship between, streamwise periodicity andshear length scale presented recently in theliterature. The measurements are also in goodoverall agreement with simple drag models presented recently by other researchers.     

CALPUFF在沈阳地区大气污染模拟研究中的应用

利用CALPUFF模拟了2006年1月5日沈阳地区大气污染物扩散。结果表明,CALPUFF能够直观、真实地模拟大气环境中污染物浓度的分布。污染物扩散的范围与风速相关,风速大时污染物扩散的范围大,风速小时污染物扩散的范围小。当污染物扩散的范围大时污染源附近的污染物浓度值小,污染物扩散范围小时污染源附近的污染物浓度大。其中,日平均浓度的分布受平均风场和地形的双重影响,实时浓度的分布主要受风场的影响。     

CALPUFF在沈阳地区大气污染模拟中的应用

利用CALPUFF模拟了2006年1月5日沈阳地区大气污染物扩散。结果表明,CALPUFF能够直观、真实地模拟大气环境中污染物浓度的分布。污染物扩散的范围与风速相关,风速大时污染物扩散的范围大,风速小时污染物扩散的范围小。当污染物扩散的范围大时污染源附近的污染物浓度值小,污染物扩散范围小时污染源附近的污染物浓度大。其中,日平均浓度的分布受平均风场和地形的双重影响,实时浓度的分布主要受风场的影响。     

大气污染扩散对输变电设备染污的影响

本文利用八十年代以来大气边界层的研究成果，建立起一套先进，实用的大气扩散模式系统，并对Ｂｒｉｇｇｓ的烟气穿透算法作了改进，在此基础上，结合风洞试验结果。首次实现了对输变电设备绝缘子表面积污密度和盐密的预测。结果表明，该预测精度较高。     

建筑物对称性对大气输移特性影响的模拟研究

运用修正的k-ε双方程湍流模型分别对非对称性建筑物和对称性建筑物周围的流场和污染物浓度场进行模拟,计算出地面风速为3 m/s时不同建筑物周围的流场与污染物浓度场分布。结果表明:建筑物的形状对建筑物周围风场和污染物浓度场有明显的影响,不同形状建筑物尾流区形态各不相同。数值模拟结果对研究建筑物周围的传输特性有一定的指导意义,并对污染物的扩散机理进行了探讨。     

山谷风环流控制下的大气污染物输送和扩散过程:二维数值模拟研究

本文以兰州地区的实际地形为背景,建立了一个二维小尺度数值模式,并用它对山谷之间热力差异造成的山谷风环流及其控制下山谷中高架源排放的污染物输送和扩散过程进行了模拟研究。结果表明,在山谷风环流控制下,造成山谷内高污染浓度的主要因素有两个:一是日出前和日落后山谷风的转换;二是在白天由于山谷风环流所造成的山谷上空较强的下沉气流。     

2007年冬季沈阳典型大气污染源PM10排放模拟

选取沈阳市区9个典型的大气污染源冬季的PM10。排放浓度资料,利用MM5耦合CALPUFF对2007年12月至2008年2月沈阳城市区域气象场和排放PM10。浓度分布进行月平均数值模拟。结果表明：冬季沈阳地区受高压控制,北风较强,并经过增强、减弱的过程,多呈现对大气污染物扩散不利的天气形势。高空为偏西风且风速较大时,地面和高空有明显的风向和风速的切变,切变有增强和减弱的变化。2007年冬季沈阳市区域月平均大气污染最严重的是2月,污染物分布主要集中在南部、东南部地区,南部地区大气污染最为严重。PM10。浓度分布的范围与风场、地形有直接的关系。地势平坦、风速大时,污染物扩散范围大,污染物浓度小;地势不平、风速小时,污染物扩散范围小,污染物浓度大。     

A microscale model for air pollutant dispersion simulation in urban areas: Presentation of the model and performance over a single building

A microscale air pollutant dispersion model system is developed for emergency response purposes. The model includes a diagnostic wind field model to simulate the wind field and a random-walk air pollutant dispersion model to simulate the pollutant concentration through consideration of the influence of urban buildings. Numerical experiments are designed to evaluate the model's performance, using CEDVAL(Compilation of Experimental Data for Validation of Microscale Dispersion Models) wind tunnel experiment data, including wind fields and air pollutant dispersion around a single building. The results show that the wind model can reproduce the vortexes triggered by urban buildings and the dispersion model simulates the pollutant concentration around buildings well. Typically, the simulation errors come from the determination of the key zones around a building or building cluster. This model has the potential for multiple applications; for example, the prediction of air pollutant dispersion and the evaluation of environmental impacts in emergency situations; urban planning scenarios;and the assessment of microscale air quality in urban areas.     

新都区冬季空气污染扩散与气象要素相关性研究

利用2014~2018年冬季空气质量和污染物浓度数据,结合地面观测、探空及风廓线雷达资料,对新都区冬季气象要素及其污染扩散条件进行分析。结果表明:（1）新都区不同污染物具有相同的日变化特征,在11时左右浓度最高,18时达到最低。（2）新都区污染物浓度与风速、气温、降水、相对湿度有密切关系。当风速大于(小于)平均风速时,污染物浓度减小（增加）;气温越高且相对湿度越大,污染也越强;降水较弱时,反而会加重污染。（3）新都区污染天气过程中,逆温强度与厚度的大小将影响污染物的垂直扩散,强度和厚度偏大,污染偏严重。      

Investigation of wet acidic deposition episodes capable of damaging red spruce in the Mt. Mitchell State Park

Cloud events whose wet deposition is comparable to that shown to trigger negative health symptoms in trees constitute environmentally significant episodes of wet deposition. They accounted for approximately one fourth of all cloud events that traversed the Mt. Mitchell State Park, NC, during the summer field seasons of 1986–1988 and 1993. The environmentally significant episodes of wet deposition most frequently occurred during the month of June. The June occurrence is equivalent to the frequency of acid mist treatments shown to initiate health related decline in Red Spruce. The slightly more than three seasons of data were analyzed for a possible relationship between cloudwater acidity, liquid water content, wind speed and wind direction. The data analysis shows: (i) A scatter plot of the liquid water flux versus the H⁺ ion flux differentiates between environmentally significant and non-environmentally significant cloud events of wet deposition. (ii) The synoptic and resulting local wind speed and direction are important in determining whether a cloud event will be environmentally significant or not, since they are related to the origin and the approach of the airflow into the Mt. Mitchell site. (iii) A characteristic 850 mb height field associated with the most frequently occurring environmentally significant cloud only event of wet deposition. Further investigation of wet acidic deposition episodes capable of damaging Red Spruce in the Mt. Mitchell State Park     

The characteristics of a laboratory produced turbulent Ekman layer

The characteristics of the atmospheric turbulent Ekman boundary layer have been qualitatively simulated in an annular rotating wind tunnel. Observed velocity spirals found to exist within the wind tunnel resembled qualitatively those found in the atmosphere in that a two-layer structure was evident, consisting of a log-linear portion topped by an outer spiral layer. The magnitude of the friction velocity u _* obtained from the log-linear profile agreed with that measured directly, i.e., that obtained from the relation: u _* = (u′w′)^1/2. Also, the effects of surface roughness on the characteristics of the boundary layer agreed with expected results. In cases where the parametric behaviour predicted by theory departed from the observed behaviour, the probable cause was the inherent size limitations of the wind tunnel. The ability to maintain dynamic similarity is constrained by the limited radius of curvature of the wind tunnel. The vertical distribution of turbulent intensity in the wind tunnel was found to agree qualitatively with an observed atmospheric distribution. Also, a vertical distribution of eddy diffusivity was calculated from tunnel data and found to give qualitatively what one might expect in the atmosphere.     

Advection, Diffusion, and Deposition From Distributed Sources

An efficient method for solving the advection-diffusionequation for a distributed source is proposed, using the Fast Fourier Transform. The governingequations include deposition, gravitational settling, in situ decay andanisotropic diffusion. The model uses the Fickian formulation, neglecting the verticalvariation in wind and turbulence. Various local balances and far field decay laws arediscussed. The model is extended to secondary pollutants.     

街谷大气扩散试验研究

为研究机动车辆排出的污染物在大气中的扩散规律，在北京做了小风条件下的街谷示踪试验。当楼顶风速u接近或大于1米/秒时，街谷内可形成一稳定的原生涡；u＜0.6米/秒时原生涡将消失。对于楼之间空间较小的街谷，背风面和迎风面的示踪剂浓度平均比值可达8。浓度值沿楼层高度无明显变化；由于快车路旁松墙的阻挡和抬升作用，可能造成沿高度方向楼层中段的浓度偏高。在街谷外，除下风方路面上有一按下风距离的负幂指数衰减的浓度分布外，上风方路面上也有一按较大负幂指数衰减的分布。根据上述试验，给出了用以预测街谷中机动车辆排出的惰性气体污染物的扩散模式；模式中，对原生涡和小尺度湍流，做了分别处理。     

Measurements of the upstream turbulent flow during wind tunnel simulations of agricultural field burning

Mean streamwise and vertical velocities as well as streamwise and vertical turbulence intensities were measured in a combustion wind tunnel used to collect pollutant emission data for agricultural field burning. Objectives were to compare the flow field upstream of a fire to that without a fire present and to compare the wind tunnel flow upstream of a fire to field conditions. Vertical centerline traverses with an anemometer were conducted for 32 separate wind tunnel operating configurations (wind speed, position in the tunnel, with or without fire, ceiling position, and floor condition) with one replication for each configuration (total of 64 traverses). Certain configurational changes in the wind tunnel had substantial effects on the flow field. Turbulence intensities and velocity profiles (as modeled by the log law-of-the-wall to determinez ₀ andu _* values) in the wind tunnel were comparable to those in the field as reported in the literature. Velocities and turbulence intensities were generally higher, however, with a fire present in the tunnel and all other conditions constant.     

Large-Eddy Simulation of Windbreak Flow

A large-eddy simulation has been performed of turbulent flow around multiple windbreaks set within a wheat canopy under neutral stability conditions. The simulation is validated against a wind tunnel data set taken under similar conditions. Velocity profiles and second-order statistics are presented and compared to those found in the wind tunnel. From the numerical simulation, we discuss spatial distributions of instantaneous velocity fields and pressure statistics, which are important and telling features of the flow that are difficult to measure experimentally. We present a discussion of the momentum balance at various locations with respect to the windbreak, and similarly, we introduce the budget of a passive scalar. These discussions show the importance of the terms in each budget equation as they vary upstream and downstream of the windbreak.     

采暖期间区域气象条件与天津大气污染概率关系

利用2002～2005年11、12、1月冬季采暖期NCEP气象分析资料和同期日空气污染物监测资料,在开设的关键区、识别区内,分析区域污染气象特征,探讨区域气象条件对天津大气污染的影响.采暖期大气污染高发是与冬季的污染气候特征和气象条件密切相关的,区域污染环流特征、大气动力特征、热力条件对污染物的扩散、积累、输送有着重要的影响.当区域大气环流由北到西北气流向西到西南气流转换时,大气稳定度逐渐增加,区域大气扩散能力随之减弱,天津大气污染概率增大.另外,区域风速与天津污染的关系,在一定程度上反映了华北区域污染对天津污染的贡献.     

A wind tunnel study of air flow in waving wheat: Single-point velocity statistics

We analyse single-point velocity statistics obtained in a wind tunnel within and above a model of a waving wheat crop, consisting of nylon stalks 47 mm high and 0.25 mm wide in a square array with frontal area index 0.47. The variability of turbulence measurements in the wind tunnel is illustrated by using a set of 71 vertical traverses made in different locations, all in the horizontally-homogeneous (above-canopy) part of the boundary layer. Ensemble-averaged profiles of the statistical moments up to the fourth order and profiles of Eulerian length scales are presented and discussed. They are consistent with other similar experiments and reveal the existence of large-scale turbulent coherent structures in the flow. The drag coefficient in this canopy as well as in other reported experiments is shown to exhibit a characteristic height-dependency, for which we propose an interpretation. The velocity spectra are analysed in detail; within and just above the canopy, a scaling based on fixed length and velocity scales (canopy height and mean horizontal wind speed at canopy top) is proposed. Examination of the turbulent kinetic energy and shear stress budgets confirms the role of turbulent transport in the region around the canopy top, and indicates that pressure transport may be significant in both cases. The results obtained here show that near the top of the canopy, the turbulence properties are more reminiscent of a plane mixing layer than a wall boundary layer.     

Pollutant Concentrations in Street Canyons of Different Aspect Ratio with Avenues of Trees for Various Wind Directions

This study summarizes the effects of avenues of trees in urban street canyons on traffic pollutant dispersion. We describe various wind-tunnel experiments with different tree-avenue models in combination with variations in street-canyon aspect ratio W/H (with W the street-canyon width and H the building height) and approaching wind direction. Compared to tree-free street canyons, in general, higher pollutant concentrations are found. Avenues of trees do not suppress canyon vortices, although the air ventilation in canyons is hindered significantly. For a perpendicular wind direction, increases in wall-average and wall-maximum concentrations at the leeward canyon wall and decreases in wall-average concentrations at the windward wall are found. For oblique and perpendicular wind directions, increases at both canyon walls are obtained. The strongest effects of avenues of trees on traffic pollutant dispersion are observed for oblique wind directions for which also the largest concentrations at the canyon walls are found. Thus, the prevailing assumption that attributes the most harmful dispersion conditions to a perpendicular wind direction does not hold for street canyons with avenues of trees. Furthermore, following dimensional analysis, an estimate of the normalized wall-maximum traffic pollutant concentration in street canyons with avenues of trees is derived.