Scale effects in wind tunnel modeling of an urban atmospheric boundary layer

Precise urban atmospheric boundary layer (ABL) wind tunnel simulations are essential for a wide variety of atmospheric studies in built-up environments including wind loading of structures and air pollutant dispersion. One of key issues in addressing these problems is a proper choice of simulation length scale. In this study, an urban ABL was reproduced in a boundary layer wind tunnel at different scales to study possible scale effects. Two full-depth simulations and one part-depth simulation were carried out using castellated barrier wall, vortex generators, and a fetch of roughness elements. Redesigned “Counihan” vortex generators were employed in the part-depth ABL simulation. A hot-wire anemometry system was used to measure mean velocity and velocity fluctuations. Experimental results are presented as mean velocity, turbulence intensity, Reynolds stress, integral length scale of turbulence, and power spectral density of velocity fluctuations. Results suggest that variations in length-scale factor do not influence the generated ABL models when using similarity criteria applied in this study. Part-depth ABL simulation compares well with two full-depth ABL simulations indicating the truncated vortex generators developed for this study can be successfully employed in urban ABL part-depth simulations.     

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.     

Mesoscale vortex shedding from large islands: A comparison with laboratory experiments of rotating stratified flows

Summary Mesoscale vortex shedding in the wake of large islands has been analysed with respect to the influence of stratification and background rotation. Comparison with results from laboratory experiments on rotating-stratified flows past isolated obstacles based on Rossby and Froude numbers have shown that forFr<0.4 vortex shedding can be expected for moderate rotation rates. Rossby numbers for cases of atmospheric vortex streets were found to be much larger than in laboratory experiments. Depending on the Froude number different flow pattern in the wake of three-dimensional obstacles have been identified in both, laboratory experiments and atmospheric observations.With 5 Figures     

Characteristics of natural wind simulations in the TUM boundary layer wind tunnel

The boundary layer wind tunnel at the Technische Universit?t München was tested for atmospheric boundary layer (ABL) simulations. The ABLs developing above rural, suburban, and urban terrains were reproduced using the Counihan method, i.e., castellated barrier wall, vortex generators, and a fetch of surface roughness elements. A series of flow-characteristic evaluations was performed to investigate the flow development and uniformity. Experimental results presented as mean velocity, turbulence intensity, integral length scale of turbulence, Reynolds stress, and power spectral density of velocity fluctuations were compared with the ESDU data and/or theoretical models. Generated ABL wind-tunnel simulations compare well with the rural, suburban, and urban ABLs. In the test section area used for experiments on structural models, the ABL simulation is developed and uniform. Results of this study indicate the boundary layer wind tunnel at the Technische Universit?t München can be successfully employed in a broad spectrum of engineering, environmental, and micrometeorological studies, where it is required to accurately reproduce ABL characteristics.     

On atmospheric vortex streets in the wake of large islands

Summary Results from laboratory experiments on stably stratified flows over three-dimensional obstacles are related to atmospheric vortex streets formed in the lee of large islands. A quasi horizontal flow around the island can be expected if stable stratification favours the formation of a so-called dividing streamline below the islands top. The subsequent shedding of vortices with vertical axis from islands may then be due to viscous boundary layer separation, but also other possible mechanisms of vortex shedding in stably stratified flows are discussed.With 6 Figures     

涡列涡量内传的数值研究

用一个高分辨率的准地转正压模式实施了4组积分时间为24h的试验,研究台风环流区域中尺度涡列对台风强度的影响,并与单个中尺度涡旋的情况作了对比。结果表明:涡列内传过程中出现了两次涡合并现象,涡列涡量内传对台风强度的影响能力较弱,单个中尺度涡旋涡量内传可使台风增强。     

NUMERICAL EXPERIMENTS FOR THE IMPACT OF ANTARCTIC ICE COVER AND SEA SURFACE TEMPERATURE ON CLIMATE VARIABILITY*

By using a three-level atmospheric general circulation model(AGCM),we have completed several numerical experiments to study the impacts of sea surface temperature anomaly(SSTA) and antarctic ice cover anomaly(AICA) during 1981-1983 on climate variability.The results show that during the El Nino period of 1982-1983 the impact of SSTA overrides that of AICA.SSTA mainly affects equatorial zonal circulation and produces PNA wave train,and SE-NW wave train in East Asia to influence the weather of China.AICA produces west-east anomalous vortex streets in the middle latitudes of both hemispheres and affects the intensity of the polar vortex of Southern Hemisphere.     

Experiments on integral length scale control in atmospheric boundary layer wind tunnel

Accurate predictions of turbulent characteristics in the atmospheric boundary layer (ABL) depends on understanding the effects of surface roughness on the spatial distribution of velocity, turbulence intensity, and turbulence length scales. Simulation of the ABL characteristics have been performed in a short test section length wind tunnel to determine the appropriate length scale factor for modeling, which ensures correct aeroelastic behavior of structural models for non-aerodynamic applications. The ABL characteristics have been simulated by using various configurations of passive devices such as vortex generators, air barriers, and slot in the test section floor which was extended into the contraction cone. Mean velocity and velocity fluctuations have been measured using a hot-wire anemometry system. Mean velocity, turbulence intensity, turbulence scale, and power spectral density of velocity fluctuations have been obtained from the experiments for various configuration of the passive devices. It is shown that the integral length scale factor can be controlled using various combinations of the passive devices.     

A wind-tunnel facility for simulating mountain and heated-island gravity waves

A wind tunnel was constructed to study mountain lee-wave phenomena and heated-island effects. The present wind tunnel appears to have several advantages over a towing tank facility for modeling atmospheric gravity-wave phenomena, i.e., non-uniform approach flows, stationary measuring conditions, variation of boundary temperature condition in the longitudinal direction, and no slip at the wall. Simple design criteria and characteristics of the present wind tunnel are discussed. A numerical program was utilized to explain peculiar standing-wave disturbances in the wind-tunnel test section. The above disturbances seemed unavoidable under the chosen mechanical constants. Nevertheless, a practical solution was found by extending the tunnel length.Postdoctoral Fellow, Princeton University, New Jersey, U.S.A.Associate Professor, Fluid Mechanics Program, Colorado State University, Fort Collins, U.S.A.     

Particle Image Velocimetry Measurements of Turbulent Flow Within Outdoor and Indoor Urban Scale Models and Flushing Motions in Urban Canopy Layers

We investigate the spatial characteristics of urban-like canopy flow by applying particle image velocimetry (PIV) to atmospheric turbulence. The study site was a Comprehensive Outdoor Scale MOdel (COSMO) experiment for urban climate in Japan. The PIV system captured the two-dimensional flow field within the canopy layer continuously for an hour with a sampling frequency of 30 Hz, thereby providing reliable outdoor turbulence statistics. PIV measurements in a wind-tunnel facility using similar roughness geometry, but with a lower sampling frequency of 4 Hz, were also done for comparison. The turbulent momentum flux from COSMO, and the wind tunnel showed similar values and distributions when scaled using friction velocity. Some different characteristics between outdoor and indoor flow fields were mainly caused by the larger fluctuations in wind direction for the atmospheric turbulence. The focus of the analysis is on a variety of instantaneous turbulent flow structures. One remarkable flow structure is termed ‘flushing’, that is, a large-scale upward motion prevailing across the whole vertical cross-section of a building gap. This is observed intermittently, whereby tracer particles are flushed vertically out from the canopy layer. Flushing phenomena are also observed in the wind tunnel where there is neither thermal stratification nor outer-layer turbulence. It is suggested that flushing phenomena are correlated with the passing of large-scale low-momentum regions above the canopy.     

An Actively Controlled Wind Tunnel And Its Application To The Reproduction Of The Atmospheric Boundary Layer

An actively controlled wind tunnel equipped with multiple fansand airfoils has been developed, mainly for the purpose of reproducing the atmospheric boundary layer (ABL) for wind engineering applications. Various fluctuating flows can be achieved in this wind tunnel by altering the input data of the fans and airfoils through computer control. In this study, the ABL is physically simulated in this wind tunnel, and particular attention ispaid to the simulation of the profile of Reynolds stress. The method of generating the fluctuating flow and the experimental results of reproducing the ABL are presented. As the results show, the spatial distribution of Reynolds stress is satisfactorily simulated, and the profiles of other statisticalturbulent parameters, such as mean velocity, turbulent intensity, integral scale and power spectrum are successfully reproduced simultaneously.     

中国1月份降水异常与司期里海低涡强弱的关系

利用中国气象局国家气象信息中心提供的1951年-2008年中国160站月平均降水、气温资料,美国提供的NCEP／NCAR再分析月平均资料,运用EOF分解的方法定义了一个里海低涡指数。应用相关系数检验的方法统计分析了1月份指数与160站降水量相关系数分布,发现1月份里海低涡指数与全国大部分地区的降水量成正相关。本文还运用合成分析的方法简单的分析了这种相关关系的原因,发现里海低涡活跃的年份,同期的大气环流、风场和垂直运动都有利于中国的降水。     

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。     

正压大气中尺度半平衡和准平衡动力学模式

应用描写正压大气运动的基本方程组，分析了中尺度大气运动的物理特征，指出非平衡强迫运动是引起中尺度重要天气演变的根本原因。中尺度动力学方程组是中尺度动力学理论研究的基础，因此，结合中尺度大气运动的基本特征，依据严格的尺度分析理论和摄动理论，简化基于流体力学和热力学的大气动力学方程组使之能够恰当地描述出中尺度运动的基本特征，对于中尺度动力学的发展是极为必要的。基于非线性平衡方程所得到的半平衡和准平衡动力学模式分别与半地转和准地转模式极为相似，它们可以较精确地描述中尺度大气运动的基本特征，因而，可作为中尺度动力学研究的理论基础。将准平衡动力学模式应用于中尺度涡旋系统的研究，结论表明中尺度平衡涡旋系统主要是受梯度风控制，其流场和气压场的发展演变则由一个演化方程来描写，获得了较为理想的结果。     

局地废气排放污染影响的实验模拟

本文给出了一项在南京大学NJU环境风洞中实施的实验模拟研究。试验模拟一座位于城市街区的排放塔的废气排放，通过流场测量，放烟显示和示踪气体扩散试验，分析气流分布与污染物浓度分布，得出对局地环境影响的一些结论。文中还以模拟扩散试验资料为实验基础，建立修正的扩散模式，探讨污染物浓度预测的可靠途径。研究结果表明，风洞流体模拟手段是有成效且实用的。     

Wind tunnel modeling of small-scale meteorological processes

After introductory remarks on similarity laws to be satisfied in wind tunnel experiments simulating small-scale meteorological processes, mean and turbulence characteristics of wind tunnel boundary layers are presented and compared with the characteristics of the atmospheric boundary layer. The results are used to evaluate the possibilities and limitations of physical modeling of pollutant dispersion in general. In the second part of the paper, the potential of wind tunnels to solve micro-meteorological problems of real practical interest will be demonstrated. The example involves the investigation of the effects of building downwash on ground-level concentrations for flue gases discharged from natural draft wet cooling towers.     

Secular Variability of the Coupled Tropospheric and Stratospheric Circulation in the GCM ECHAM 3/LSG

Summary  Secular or multi-decadal variability is a widely observed phenomenon, apparent in instrumental and paleo climatic records. These long time oscillations are found in many variables of the climate system. The ocean especially experiences low frequency variations. But also atmospheric variables such as temperature, wind velocity and sea level pressure can show secular variability. The low frequency variability here is examined in the coupled atmosphere-ocean model ECHAM3/LSG T21. A coupled stratospheric and tropospheric mode is detected oscillating with a period of approximately 100 years. The atmospheric pressure system mainly involved in this oscillation is the northern hemispheric winter stratospheric polar vortex. The near surface temperature experiences variations of the same magnitude as the observed temperature trends of the last decades. Multi decadal variability is also shown in the North Atlantic Oscillation Index. A shift of the length of the oscillation period between longer and shorter time scales indicates that chaotic processes might be responsible for the variability. Received February 22, 1999/Revised August 5, 1999     

Operating ranges of mesoscale numerical models and meteorological wind tunnels for the simulation of sea and land breezes

The operating ranges of mesoscale numerical models and meteorological wind tunnels for sea- and land-breeze simulations are defined in this paper based on a review of the theoretical and practical limitations of these two approaches. Numerical-model operating ranges are limited by the choice of governing equations, the numerical methods used to solve the governing equations, the scales of the surface or atmospheric forcing and the atmospheric response, the specified grid resolution and domain size, and the available computer resources. Wind-tunnel operating ranges are limited by the dimensions of the simulated circulations and of the tunnel itself, the tunnel flow speed and turbulence characteristics, the temperature gradients within the tunnel, the lack of Coriolis force and moist processes, and the characteristics of the measurement instrumentation. The operating ranges of these two simulation methods are shown to overlap. In this common range, results of simulations from both approaches can be compared so as to strengthen the validity of the results and to help in the development and improvement of parameterizations of physical processes in numerical models. In addition, the coupling of meteorological wind tunnels and mesoscale numerical models offers a larger range of operating conditions than can be achieved by either approach alone. Together, they can be used in a hybrid form to predict atmospheric conditions at the scale of a few meters for complex terrain (e.g., buildings, hills, etc.) within larger mesoscale atmospheric flow regimes. In the case of sea and land breezes, the sea-land transition zone and coastal internal boundary layer can be studied using both approaches.     

东北冷涡降水集中期的客观识别研究

基于区域关键天气过程客观化识别和监测的需求及东北雨季应包括冷涡降水的事实,采用东北三省及内蒙古东四盟共147站逐日降水量资料,通过对东北区域多年平均5点平滑处理的逐日降水量序列的综合分析及对历年逐日滑动平均雨量的对比试验,确定了判别东北冷涡降水集中期开始日期的阈值及持续时间,进而研制了东北冷涡降水集中期开始日期的客观识别方法。基于该方法的客观识别,得到1981-2010年气候平均态的东北冷涡降水集中期的开始日期为每年的5月26日。同时,定义盛夏降水集中期开始日的前一日为冷涡降水集中期的结束日期,发现冷涡降水集中期的结束日期为6月25日。在此基础上,采用NCEP/NCAR逐日再分析的风场、位势高度场资料,通过对东北冷涡降水集中期前、中和后期各层大气环流场及各系统的逐日变化特征的对比分析,验证了该客观识别方法的合理性。     

A note on vertical coherence of streamwise turbulence inside and above a model plant canopy

Measurements of longitudinal turbulent velocity were made at pairs of levels inside and above a model plant canopy in the wind tunnel. It was found that above approximately the zero-plane displacement level, the coherence and phase results were similar in many respects to atmospheric data, but that deviations from this behaviour appeared deeper in the model canopy.