华南一些城市近地层大气扩散与输送特征

本文利用1980—1983年广州、湛江、汕头等地的实测资料，讨论了华南一些城市近地层大气污染物扩散与输送特征。发现由于垂直扩散较强，最大浓度常于近源处发生，且向下风方浓度衰减较快。受近地层局地小扰动的影响，大气污染浓度分布常呈现不规则起伏。季风对华南一些城市污染物输送规律有较大影响，从而使平均污染浓度高值区具有明显的季节性变化。沿海城市大气污染还在一定程度上受到海陆风的作用，而使浓度分布有明显的日变化。最后讨论了雨水的冲洗作用。     

化学转化作用下大气污染物的浓度分布

本文针对稳定和不稳定层结二种情况,求出了大气边界层中在输送、扩散和化学转换、衰减的共同作用下的次生污染物浓度分布的解析解,同时也完善了初生污染物的浓度分布公式。由于化学转换和衰减的作用,初生污染物随着下风距离而较快地减少,特别是在10公里以后的中距离上更加明显;相应地,一个较高的次生污染物浓度将稳定地维持在一个较长的下风距离上。     

街谷环流生热力结构的数值模拟

文章介绍一种用于模拟街谷流场和温度场的动力学模式和热力学模式。应用动力模式模拟了方桩体塔楼和圆柱体塔楼形成的流场，应用动力和热力模式模拟了街谷中流场和温度场的日变化过程。计算实例表明，上述模式可用于城市街谷和建筑群风环境和热力环境研究以及街谷中空气污染物传输和扩散的计算。     

城市湍流边界层内汽车尾气扩散规律数值模拟研究

以纳维斯托克斯方程组、大气平流扩散方程、湍流动能及湍流动能耗散率方程组为基础．采用伪不定常方法,建立了一个数值模式．利用该模式列城市湍流边界层内流场结构及汽车排放污染物扩散规律进行了研究。结果表明：街谷内会形成一个涡旋型流场．汽车排放污染物浓度在地面及建筑物背风面产生堆积,且其沿高度方向的梯度变化在背风面大．迎风而小。随着街谷两侧建筑物屋顶风速的增大,峡谷内形成的涡旋流场的强度增大,污染物扩散速率增大：当屋顶来流与街道之间的夹角逐渐增大时．涡旋中心位置由街道中心偏向于背风面及更高层且污染物扩散速度加快。     

山区城市大气污染物传输扩散的数值模拟

利用Monte-Carlo模式预测了山区城市（贵阳市）的SO2地面浓度分布。为实现城市面源的数值模拟，对Monte-Carlo模式的浓度计量和水平扩散等方面做了改进。作为模式的输入参数，风场得自于对三维观测数据的客观分析，湍流的参数化公式按现场超声风速仪实测数据进行了调整和校核。其风廓线不完全遵循一般规律，在距地面50—500m之间，常出现一极值风层；该地区具有典型的山区小风扩散特征，稳定时u、v湍谱峰值明显地移向低频，水平扩散明显偏大。对模式做了理论和实验校核。在均匀湍流场条件下，和解析结果一致；冬季日平均浓度的预测值和监测值的相关系数为0.96。数值模拟结果表明:贵阳市冬季大气污染物的传输扩散主要依赖于强不稳定的天气条件和极值风层的扩散和通风作用。     

北京交通环境污染诊断模拟初步研究

建立了能够用于评估大型城市交通环境污染与气象条件关系的数值模拟技术,即三维边界层光化学数值模式.其中,高分辨率的精细城市边界层气象场、细致的城市地理与环境参数、空气污染物输送扩散与大气化学模拟以及交通废气污染源的分布等诸多因子是实施模拟研究的几个重要环节.从模拟结果看出,各污染物在地面附近浓度相对高值区位于排放源附近和下风方.在边界层顶附近,污染物浓度分布较均匀,分布趋势和气流一致.虽然仅仅考虑了城区污染物本底浓度和主要交通干道机动车的排放,但交通排放对城区污染物浓度的贡献是较大的.     

用K理论研究南京大厂地区污染物扩散

本文用三维K理论扩散方程对南京大厂地区污染物浓度分布特征进行了讨论。在K理论扩散方程中,输入的风速廓线、扩散系数廓线采用Businger通量廓线关系式。用塔层风资料求取了南京地区近地面特征量。用具有四阶精度的Chapeau函数方法对三维K理论扩散方程进行数值求解,计算了大厂地区SO_2年平均浓度分布,和实测值基本上吻合。本文还研究了忽略水平扩散系数、不考虑风速随高度变化,对污染物地面浓度分布的影响。结果表明:在点源的下风方1—5千米范围内,不管是较强的还是较弱的风,忽略水平扩散系数都会使计算的地面浓度估计过高;忽略风速随高度的变化,会给计算带来一定的误差。     

Monte-Carlo 法模拟复杂地形对扩散的影响

不考虑边界层中层结作用，引入Kao得到的复杂地形中的平均流场分布和近十年来PBL实验和理论研究导出的新的湍流统计量参数化关系，本文用Monte-Carlo模式模拟了复杂地形对扩散的影响，结果表明:地形的影响主要是迎风坡抬升和背风坡下沉；陡峭地形和平缓地形的影响不完全相同；在陡峭地形和大的平缓地形的背风坡能够形成空腔区，空腔区内出现闭合的浓度中心，且地形越陡峭，闭合浓度中心的范围越大；大地形对扩散的影响可以掩盖其下风方小地形的影响。     

山谷地形流场和扩散的数值研究

运用三维非表力E－ε闭合模式，模拟了山体和山谷地形下的流场、湍流场和不同位置低矮点源扩散的污染物浓度分布。发现在不稳定时湍能的热力产生率并非在任何位置都是主要的。当风速较大时，在山谷底部机械产生率也可能超过热力产生率。对污染扩散的分析表明：在近地面源条件下，稳定层结时如果出现小风，无论源在何处，都人造成一定范围内的严重污染，当污染源位于山前或山顶时，谷底的污染不剧烈。当污染源位于谷底时，无论何种层结、何种来流风速，都会造成山谷地区的严重污染。污染源位于山体沿来流方向的中心线上的谷底时，如果风速大，污染物沿回流输送；如果风速小，则沿来流方向输送。     

2003年6月24～25日江南特大暴雨数值模拟和诊断分析

采用NCAR、NCEP和FSL／NOAA等共同研制的新一代细网格WRF中尺度数值模式，对2003年6月24～25日江南地区出现的一次特大暴雨过程进行了数值模拟，并利用模式输出的高分辨率动力协调资料进行了初步诊断分析。结果表明：WRF模式比较成功地再现了高低空环流形势的演变及暴雨带的分布特征；低层切变线上分布不均匀的辐合区，中低层正涡度区、高层负涡度区的配置有利于造成较强烈的中尺度上升运动；同时有很强的湿度锋区，大气处于不稳定状态。分析发现，沿850hPa切变线的局地风暴相对螺旋度与暴雨有很好的对应关系。     

城市街道峡谷对称性对内部气流场的影响研究

应用雷诺应力湍流模型,模拟了不同高度比的城市街道峡谷的气流场。结果表明:峡谷的对称性对其内部气流场有显著影响。前高后低型峡谷下部为逆时针旋涡,上部为顺时针旋涡,峡谷越深,流场发展的越充分;峡谷内部墙面存在明显的驻点。前低后高型峡谷只存在一个大的顺时针旋涡,随着峡谷的加深,内部气流速率有减小的趋势;峡谷达到一定深度后出现驻点。对称型峡谷内部形成了顺时针旋涡,强度不大;随着峡谷的加深,内部流场转为一顺一反2个旋涡的二元结构;仅当峡谷很深时才出现明显驻点。前低后高型峡谷的气流场形式更有利于污染物的迁移、扩散,在城市规划中应尽量结合主导风向设计这类建筑布局。     

Numerical Study on the Impact of Ground Heating and Ambient Wind Speed on Flow Fields in Street Canyons

The impact of ground heating on flow fields in street canyons under different ambient wind speed conditions was studied based on numerical methods.A series of numerical tests were performed,and three factors including height-to-width(H/W) ratio,ambient wind speed and ground heating intensity were taken into account.Three types of street canyon with H/W ratios of 0.5,1.0 and 2.0,respectively,were used in the simulation and seven speed values ranging from 0.0 to 3.0 m s 1 were set for the ambient wind speed.The ground heating intensity,which was defined as the difference between the ground temperature and air temperature,ranged from 10 to 40 K with an increase of 10 K in the tests.The results showed that under calm conditions,ground heating could induce circulation with a wind speed of around 1.0 m s 1,which is enough to disperse pollutants in a street canyon.It was also found that an ambient wind speed threshold may exist for street canyons with a fixed H/W ratio.When ambient wind speed was lower than the threshold identified in this study,the impact of the thermal effect on the flow field was obvious,and there existed a multi-vortex flow pattern in the street canyon.When the ambient wind speed was higher than the threshold,the circulation pattern was basically determined by dynamic effects.The tests on the impact of heating intensity showed that a higher ground heating intensity could strengthen the vortical flow within the street canyon,which would help improve pollutant diffusion capability in street canyons.     

A Laboratory Model for the Flow in Urban Street Canyons Induced by Bottom Heating

Water tank experiments are carried out to investigate the convection flow induced by bottom heating and the effects of the ambient wind on the flow in non-symmetrical urban street canyons based on the PIV (Particle Image Visualization) technique. Fluid experiments show that with calm ambient wind,the flows in the street canyon are completely driven by thermal force, and the convection can reach the upper atmosphere of the street canyon. Horizontal and vertical motions also appear above the roofs of the buildings. These are the conditions which favor the exchange of momentum and air mass between the street canyon and its environment. More than two vortices are induced by the convection, and the complex circulation pattern will vary with time in a wider street canyon. However， in a narrow street canyon， just one vortex appears. With a light ambient wind, the bottom heating and the associated convection result in just one main vortex. As the ambient wind speed increases, the vortex becomes more organized and its center shifts closer to the leeward building.     

城市街谷几何结构对街谷内流场及污染物浓度场影响的数值模拟

利用数值模拟方法研究了微尺度街道峡谷范围内街谷几何结构及街道两侧建筑物高度对称性对街谷内流场及机动车排放污染物扩散规律的影响。结果表明:当街道峡谷高宽比>2.1时,街谷内的流场结构由一个完整的垂直涡旋变为上下两个反向运动的强弱不同的垂直涡旋。各类型街谷内污染物扩散水平从强到弱依次为迎风面建筑物高度大于背风面建筑物高度的街道峡谷,迎风面建筑物高度小于背风面建筑物高度的街道峡谷;平行型街道峡谷。     

二维街谷地面加热引起的流场特征的水槽实验研究

利用拖曳式水槽,采用激光粒子成像速度场测量系统(PIV),模拟了街谷存在地面加热时流场特征;讨论了环境风场对其的影响。我们发现在静风条件下,街谷中环流完全由热力驱动,对流活动可伸展至街谷上方;在建筑物层顶以上,也可发现水平和垂直方向的运动。这些对流活动有助于基本风场为零时,街谷内外动量和物质的交换。当街谷较宽时,对流形成的涡旋可能为两个以上,形态较为复杂并随时间变化,当街谷变窄时,涡旋蜕化成只有一个。当有弱环境风场存在时,街谷中的对流呈现为一个主涡旋,随着风速增加,涡旋形状更加规则,其中心并向下风向移动。     

Micrometeorological Measurements in a Street Canyon during the Joint ATREUS-PICADA Experiment

In order to investigate the microclimatic conditions in a street canyon, a physical model was used to conduct the Joint ATREUS-PICADA Experiment (JAPEX) in situ experimental campaign. Four lines of buildings simulated by steel containers were installed to form three parallel street canyons at 1:5 scale, with width/height aspect ratio approximately 0.40. The reference wind and atmospheric conditions were measured, as well as the flow velocity and direction in the street. Preliminary results concern street canyon ventilation and thermal effects on in-canyon airflow, and show that vortical motions appear for reference wind directions perpendicular to the street axis. The presence of adjacent rows of buildings did not appear to significantly influence the flow character within the canyon for the case of a low aspect ratio corresponding to a skimming flow regime. The flow structure was not significantly affected by the thermal effects although some slight interference occurred in the lower part of the canyon. An analysis of horizontal temperature gradients indicated that a thin boundary layer develops near the heated facade. These facts imply that the thermal effects are considerable only very close to the wall.     

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.     

An Improved Three-Dimensional Simulation of the Diurnally Varying Street-Canyon Flow

The impact of diurnal variations of the heat fluxes from building and ground surfaces on the fluid flow and air temperature distribution in street canyons is numerically investigated using the PArallelized Large-eddy Simulation Model (PALM). Simulations are performed for a 3 by 5 array of buildings with canyon aspect ratio of one for two clear summer days that differ in atmospheric instability. A detailed building energy model with a three-dimensional raster-type geometry—Temperature of Urban Facets Indoor-Outdoor Building Energy Simulator (TUF-IOBES)—provides urban surface heat fluxes as thermal boundary conditions for PALM. In vertical cross-sections at the centre of the spanwise canyon the mechanical forcing and the horizontal streamwise thermal forcing at roof level outweigh the thermal forces from the heated surfaces inside the canyon in defining the general flow pattern throughout the day. This results in a dominant canyon vortex with a persistent speed, centered at a constant height. Compared to neutral simulations, non-uniform heating of the urban canyon surfaces significantly modifies the pressure field and turbulence statistics in street canyons. Strong horizontal pressure gradients were detected in streamwise and spanwise canyons throughout the day, and which motivate larger turbulent velocity fluctuations in the horizontal directions rather than in the vertical direction. Canyon-averaged turbulent kinetic energy in all non-neutral simulations exhibits a diurnal cycle following the insolation on the ground in both spanwise and streamwise canyons, and it is larger when the canopy bottom surface is paved with darker materials and the ground surface temperature is higher as a result. Compared to uniformly distributed thermal forcing on urban surfaces, the present analysis shows that realistic non-uniform thermal forcing can result in complex local airflow patterns, as evident, for example, from the location of the vortices in horizontal planes in the spanwise canyon. This study shows the importance of three-dimensional simulations with detailed thermal boundary conditions to explore the heat and mass transport in an urban area.     

Model of air flow and air pollution concentration in urban canyons

A model for the calculation of the turbulence flow field and air pollutant concentrations in urban canyons is developed. A two-dimensional set of hydrodynamical equations and a threedimensional diffusion equation are solved numerically with a personal computer. Different boundary conditions were investigated. Three flow regimes were found: without vortex, with one vortex, and with two vortexes, within an urban canyon. The influence of building density and wind speed components along the street was also investigated.