Characteristics of some organised structures in the turbulent wind above and within a spruce forest from field measurements

The present paper describes an evaluation of turbulent wind data acquired at the measuring tower installed in the Solling spruce forest by the Institute of Bioclimatology at the Georg Augustus University at Goettingen. Simultaneous wind velocity data were analysed from two measuring points: at the mixing layer above the forest canopy, and at a height of 2 m, well within the forest, by means of quadrant analysis, correlation analysis and wavelet techniques.The results suggest two types of phenomena coexisting in the upper mixing layer region of the flow: large-scale oscillations independent of the roughness type, and smaller-scale structures, of frequent penetration, related to vertical transport phenomena.The aim of the present work was to describe aspects of the mechanism involved in the generation of the known canopy size vortices, which transport momentum from the mixing layer region deep within the canopy. This work discusses aspects of the interaction between the mixing layer region above the forest and a particular low mean velocity region near the ground, submitted to intermittent shear by the nearby vortex structures and influenced by features of the canopy top mixing layer.     

Wind tunnel velocity profiles generated by differentially-spaced flat plates

Production of linear shear with low turbulence level in a wind tunnel provides a convenient environment for testing the results of computational fluid dynamics simulations and equipment calibration. Boundary layer flow over a flat plate at zero incidence provides controlled deceleration of the approach flow according to plate length, with interacting boundary layers between adjoining flat plates merging to provide fully developed duct flow. In this way, an array of differentially-spaced flat plates can be used to modify a uniform wind tunnel velocity field to a specified velocity profile. A one-step iterative scheme is offered to determine plate spacings for simulation of weakly sheared flows, constrained by zero vertical pressure gradient in the downstream flow (representative of boundary layer conditions). The scheme is tested for realisation of uniform shear flow (maximum velocity variation ±10% of centreline velocity) by wind tunnel simulation, and produces reasonable results, at least comparable with previous studies.     

Large eddy simulation of wind effects on a long-span complex roof structure

This paper presents a combined study of numerical simulations and wind tunnel tests for the determinations of wind effects on a long-span complex roof of the Shenzhen New Railway Station Building. The main objective of this study is to present an effective approach for the estimations of wind effects on a complex roof by computational fluid dynamics (CFD) techniques. A new inflow turbulence generator called the discretizing and synthesizing random flow generation (DSRFG) approach was applied to simulate inflow boundary conditions of a turbulent flow field. A new one-equation dynamic subgrid scale (SGS) model was adopted for the large eddy simulations (LES) of wind effects on the station building. The wind-induced pressures on the roof and turbulent flow fields around the station building were thus calculated based upon the DSRFG approach and the new SGS model integrated with the FLUENT software. In parallel with the numerical investigation, simultaneous pressure measurements on the entire station building were made in a boundary layer wind tunnel to determine the mean, fluctuating, and peak pressure coefficient distributions. The numerically predicted results were found to be consistent with the wind tunnel test data. The comparative study demonstrated that the recommended inflow turbulence generation technique and the new SGS model as well as the associated numerical treatments are useful tools for structural engineers to assess wind effects on long-span complex roofs and irregularly shaped buildings at the design stage.     

热力效应作用下小区风环境风洞试验研究

温度作为影响城市舒适度的主要因素之一,对流场的作用机理到目前为止尚不明确。为揭示温度场对流场的影响规律,基于大尺寸风洞,对不同热力条件下的平均风进行试验研究,得出不同理查逊数R_b作用下的人行高度风速、相关测点顺风向速度和雷诺剪切应力分布并进行详细分析。以长沙市某小区为研究背景,探究了不同热力效应下人行高度风场的分布规律,获取了热力条件下典型测点的相关性系数,并利用超越概率的方法定量评估了热力效应下的小区人行高度风环境。结果表明:在-0.38<R_b<0范围内,空风洞流场随着热力效应增大,在近地面风速增长相对较大,且随着高度的增加其增幅逐渐减缓; 实际中小区人行高度风环境受温度影响整体较小,主要影响集中在风速比0.6以下,最大均方根为0.635,风速比在0.6以上时温度对风速影响效果相对较小; 热力效应对人行高度风场的相关性与舒适度影响较小,其最大影响值分别为8%和3.87%。     

Numerical simulation of wind flow near a forest edge

This paper presents a computational fluid dynamics (CFD) model to simulate wind flow near a forest edge for the purpose of wind energy applications. The model uses a porous media analogy combined with a modified k–ε turbulence model to simulate momentum losses and turbulence generation within the forest. The momentum losses are represented by a drag coefficient and a leaf area density. Two directions were investigated: wind flow entering and wind flow leaving the forest. A fully developed solution with original boundary conditions was used as the inlet boundary condition in the two-dimensional CFD model for wind flow leaving the forest. Original boundary conditions were also proposed for the ground boundary within the forest. The model was solved using FLUENT 6.2 and validated against field measurements from three different authors. A sensitivity analysis was performed on two key parameters: drag coefficient and leaf area density. The results obtained using the proposed method show good agreement with the wind velocity and turbulence intensity measured experimentally.     

台风风场作用下体育场罩棚风压分布风洞试验研究

通过刚性模型测压风洞试验研究了台风风场高湍流、强变异性等特征对大跨结构风压分布特性的影响。以某体育场罩棚为原型制作1∶300刚性模型,进行了常规B类风场和台风风场作用下的测压对比试验。基于试验数据,从测点风压和总体升力角度对两类风场作用下体育场罩棚结构的风压分布总体特性进行了分析,重点比较了典型测点在典型风向角下的风压分布规律及相互关系。结果表明：两类风场作用下平均风压的分布规律基本类似,但各风向角下台风风场中的屋盖总体升力比B类风场增大8%~25%;台风风场的高湍流特性导致基于极值负风压求得的各风向角下屋盖总体升力比B类风场大27%~46%,各测点的极值风压均明显高于常规B类风场作用下的对应值,比值约为1.13~1.70,因此对于台风多发地区的大型体育场屋盖设计,必须考虑台风风场高湍流所致的脉动风压增大效应。     

Dry deposition profile of small particles within a model spruce canopy

Data on dry deposition of 0.82 microm MMAD uranium particles to a small scale, 'model' Norway spruce (Picea abies) canopy have been determined by means of wind tunnel experiments. These are presented for both the total canopy and for five horizontal layers within the canopy. The results show a complex pattern of deposition within the canopy. The highest deposition velocity Vg (0.19 cm s(-1)) was recorded for the topmost layer within the canopy (i.e. the layer in direct contact with the boundary layer) whereas the lowest Vg (0.02 cm s(-1)) occurred at the soil surface. Vertical penetration of depositing aerosol through the canopy was influenced by variations in biomass, wind velocity and turbulence within the canopy. A total canopy Vg of 0.5 cm s(-1) was obtained and this is in line with field measurements of Vg reported in literature for both anthropogenic and radionuclide aerosols of similar size ranges. Extrapolation of wind tunnel data to 'real' forest canopies is discussed. The information presented here is of importance in predicting the likely contribution of dry deposition of aerosols to pollutant inputs to forest ecosystems, particularly in the context of radioactive aerosol releases from nuclear installations. The application of the present data may also be appropriate for other pollutant aerosols such as SO4, NO3 and NH4, which are characterised by particle sizes in the range used in this study.     

About the penetration of a horizontal axis cylindrical vortex into the nearby downwind region of a vertical porous fence

Wind tunnel experiments have been conducted on a cylindrical vortex embedded in a low turbulence stationary horizontal stream, running through a two-dimensional narrow vertical woven fence located on the wind tunnel floor.The vortex was continuously generated upwind of the fence by means of a vortex tube located well below the fence top level, with its axis aligned with the mean velocity of the external stream. The fence installed along the entire width of the tunnel had a porosity of 70%. Visualization experiments showed that approaching the fence the vortex moves away from the mean wind direction of the adjacent stream along a rising curved trajectory while the direction of the surrounding mean flow remained nearly horizontal. The results suggest that this deviation could be promoted by the vortex slanting velocity field relative to the fence, which “sees” a fence with much lower optical porosity than the fence perpendicular velocity of the nearby mean flow.The fence top shear layer flow, which dominates the downwind evolution of the mixing layer, appears to be highly sensitive to the presence of this type of vortex. The most energetic changes in the flow due to the presence of the vortex occurred in the mixing layer region. Windbreaks are usually designed in terms of mean velocity, turbulence intensity, geometric dimensions, and porosity. The results presented in this paper suggest that the sheltering ability of a porous fence depend also on the particular flow pattern of the oncoming turbulent structures embedded in the incident wind. The results show the importance for a particular wind sheltering application in knowing a priori at least some aspects of the flow pattern of the most representative turbulent structures of the local wind.     

罩棚式低矮房屋屋面风荷载特性及气动抗风措施研究

采用模型的风洞试验详细研究了矩形和圆形罩棚屋面结构的平均风压和峰值风压分布特征,分析了屋面风致破坏的主要原因,在此基础上实施了7种不同的屋面局部修改方案的对比试验,从中筛选出可以有效消减屋面风荷载的抗风措施。两种平顶矩形和圆形罩棚屋面结构均以负压为主,试验测得两结构屋面的最高平均负压系数分别为-1.83和-0.97,相应最高极值负压系数为-5.41和-3.11,结果远高于GB 50009-2001《建筑荷载规范》推荐的平均风压乘以阵风系数的方法,这显示规范中的阵风系数方法并不适合于计算该类屋面结构的风压值。根据分析结果给出了平顶矩形和圆形罩棚屋面结构风压体型系数取值的建议值,采用斜切角形式的屋檐或在屋面板和侧面围板交界处开贯通透风槽方式可以使屋面风敏感区域的极值负压削减25%~35%。     

基于风洞试验的风沙两相流耦合流场特性

风沙两相流结构的理论与研究方法已趋于成熟,主要集中在风沙物理运动本身和防风固沙工程方面,然而,将风沙运动现象及其对建筑结构物的作用效应相结合的研究还比较鲜见,继续开展风沙地区工程结构的抗风沙研究具有重要的现实与工程意义。通过风沙风洞试验,模拟了实际沙漠地貌下的风场特征,重点通过风洞顶部落沙研究了类似沙尘暴环境下的沙浓度、风沙流速度廓线以及湍流强度随高度的变化情况。通过控制相同风速、变化不同输沙率进行落沙,以此形成多种不同类型的风沙两相流耦合流场,并与净风工况相比较。试验结果表明:沙浓度梯度分布与落沙孔数量、控制风速以及高度均相关;风沙流场中沙颗粒的运动对风速剖面有一定的削弱作用,对湍流强度却有增强作用;风场中沙质量浓度沿高度方向的分布特征直接影响了各高度处风速和湍流强度的大小,沙浓度越大的高度处对风速的削弱程度越显著,且对湍流强度的增强程度越大。     

Boundary layer wind structure from observations on a 325 m tower

This paper is concerned with vertical distributions of mean wind speed and atmospheric turbulence characteristics over a typical urban area. The wind data were measured from more than 30 anemometers installed at 15 different height levels on the 325 m high Beijing Meteorological Tower during numerous windstorms. Profiles of mean wind speed are presented based on the field measurements and are compared with empirical models’ predictions. Aerodynamic parameters of atmospheric boundary layer are determined from the measured wind speed profiles. Furthermore, wind velocity data in longitudinal, lateral and vertical directions, which were recorded by ultrasonic anemometers at three height levels during windstorms, are analyzed and discussed. Atmospheric turbulence information such as turbulence intensity, gust factor, turbulence integral length scale and power spectral densities of the three-dimensional fluctuating wind velocity are presented and used to evaluate the adequacy of the existing theoretical and empirical models. A comparative study between the measured profiles of turbulence parameters and those estimated by AIJ-RLB-1996 and ASCE (1999) is performed. The objective of this study aims to provide useful information on boundary layer wind characteristics for the wind-resistant design of tall buildings and high-rise structures in urban areas.     

Numerical simulation of the wind field around different building arrangements

A numerical model with the RNG κ−ε turbulence closure model and a pressure correction algorithm of SIMPLEC is used to examine three different building configuration effects on wind flow. Comparisons of computational results with experimental data have been carried out for the vertical velocity profiles at some measurement points. For the experimental study, the building arrangements were presented by 1:150 scale models and tested in a low-speed wind tunnel. It was found that the wind environment for two improved arrangements with lower interval-to-height ratio is better than that for the reference layout with higher aspect ratio in terms of the natural ventilation. The interference effect is more obvious for two improved arrangements than the reference one. The numerical results also show that changing wind direction from perpendicular to the building facades to a 45°-incidence angle has significant effect on the flow field for different configurations.     

Flow around a cube in a turbulent boundary layer: LES and experiment

We present a numerical simulation of flow around a surface mounted cube placed in a turbulent boundary layer which, although representing a typical wind environment, has been specifically tailored to match a series of wind tunnel observations. The simulations were carried out at a Reynolds number, based on the velocity U at the cube height h, of 20,000—large enough that many aspects of the flow are effectively Reynolds number independent. The turbulence intensity was about 18% at the cube height, and the integral length scale was about 0.8 times the cube height h. The Jenson number Je=h/z₀, based on the approach flow roughness length z₀, was 600, to match the wind tunnel situation. The computational mesh was uniform with a spacing of h/32, aiding rapid convergence of the multigrid solver, and the governing equations were discretised using second-order finite differences within a parallel multiblock environment. The results presented include detailed comparison between measurements and LES computations of both the inflow boundary layer and the flow field around the cube including mean and fluctuating surface pressures. It is concluded that provided properly formulated inflow and surface boundary conditions are used, LES is now a viable tool for use in wind engineering problems concerning flow over isolated bodies. In particular, both mean and fluctuating surface pressures can be obtained with a similar degree of uncertainty as usually associated with wind tunnel modelling.     

Single-sided ventilation — Part I. The flow between a cavity and external air stream

This paper considers in detail particular flows of the kind which give rise to single-sided ventilation. It presents an experimental study of turbulent flow in a square cavity, with particular reference to the shear layer between cavity and external flows. Hot-wire measurements were carried out in a cubical cavity placed beneath the floor of the test section of the boundary-layer tunnel. Results are presented for zero, positive and negative external pressure gradients, and include time-mean velocity, as well as turbulence intensities, kinetic energy, and shear stress. Flow visualization conducted in a smoke tunnel reveals the behaviour in the shear layer and within the cavity. Predictions of two-dimensional flow fields have been undertaken using a finite-difference scheme and a two-equation model of turbulence. Comparisons with measurements show good agreement in regions where the comparison is valid, and reference to the flow-visualization studies reveals that the essential features of the entire flow field are reproduced by the analytical model.     

Wind-tunnel and field investigation of the effect of local wind direction on speed-up over hills

Measurements of flow past simulated sinusoidal hills were taken in an atmospheric boundary layer wind tunnel (ABLWT) that modeled typical full-scale complex terrain for many wind turbine locations in the Altamont Pass, California, USA. Velocity profiles and speed-up factors for several model hills were determined. All hills modeled had the same height and sinusoidal cross-section, and length-to-width aspect ratios of infinity, four and one. Each of the three models was tested with approach wind directions from 0° to 90°, in 15° increments. It was observed that speed-up can vary significantly depending on the approaching wind direction. The effect of wind direction on speed-up was also investigated using field data from a site in the Altamont Pass. Average speed-up factor was found to vary significantly at the site in time, and as a function of atmospheric stability.     

On the wind-tunnel simulation of the atmospheric surface layer for the study of wind loads on low-rise buildings

Mean velocity and turbulence measurements are made in a shear layer generated in a short test-section wind tunnel. This shear layer is developed by the spire-roughness technique and simulates as closely as possible the lower part of the neutral atmospheric surface layer for the study of pressures on 1:70 scale models of low-rise buildings. In Ref. [1] and in a companion paper [2], to be published at a later date, the statistical quantities describing the fluctuating pressures acting on a low-rise building in model and in full scale are compared.Comparison of the generated shear flow with measurements obtained in the atmospheric surface layer indicates that simultaneous simulation of mean-velocity power index, surface-roughness length, turbulence intensity and turbulence integral scale is extremely difficult to achieve. On the basis of results obtained to date, it is tentatively concluded that close simulation of the turbulence intensity and development of a turbulence integral scale at least as large as the largest model dimension are required for proper simulation of the fluctuating wind pressures. In order to satisfy these requirements, the scaling of the upstream roughness elements must be exaggerated.     

A new boundary-layer wind tunnel at the Danish Maritime Institute

The paper describes a new boundary-layer wind tunnel at the Danish Maritime Institute in Lyngby. The wind tunnel is of the open-circuit type and is integrated into a building raised for the purpose. The design was constrained by the need to provide a simulation of the atmospheric boundary layer (without thermal stratification) and alternatively a uniform flow without excessive turbulence.The wind tunnel has a working section of principal dimensions: length 20.8 m, width 2.6 m and height 1.8 m (adjustable from 1.8 to 2.3 m). The maximum wind speed in the working section when empty is about 26 m s^?1.Data acquisition, analysis and instrumentation in relation to the new wind tunnel are discussed and, finally, an example of a simulated natural wind is presented.     

Some characteristics of the wind flow in the lower Urban Boundary Layer

The characteristics of the wind flow at low levels in the urban environment (Roughness Sublayer (RS)) are quite different from, and to some extent independent of the characteristics of the flow in the upper part of the Urban Boundary Layer. In the RS, in fact, the flow is influenced more by the local geometry, than by a homogeneous energy transfer between horizontal layers. In this paper, the results of wind tunnel flow measurements in and above a regular urban geometry pattern, with street canyons parallel and orthogonal to the oncoming wind, are reported. The statistics and the spectral characteristics of the flow in the RS are discussed. The influence of the oncoming turbulence on the RS flow is analysed, and the differences between the flow in transversal and longitudinal street canyons are evidenced.     

Development of a shrouded wind turbine with a flanged diffuser

We have developed a wind turbine system that consists of a diffuser shroud with a broad-ring flange at the exit periphery and a wind turbine inside it. The flanged-diffuser shroud plays a role of a device for collecting and accelerating the approaching wind. Emphasis is placed on positioning the flange at the exit of a diffuser shroud. Namely, the flange generates a low-pressure region in the exit neighborhood of the diffuser by vortex formation and draws more mass flow to the wind turbine inside the diffuser shroud. To obtain a higher power output of the shrouded wind turbine, we have examined the optimal form of the flanged diffuser, such as the diffuser open angle, flange height, hub ratio, centerbody length, inlet shroud shape and so on. As a result, a shrouded wind turbine equipped with a flanged diffuser has been developed, and demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about 4-5 compared to a standard (bare) wind turbine. In a field experiment using a prototype wind turbine with a flanged diffuser shroud, the output performance was as expected and equalled that of the wind tunnel experiment.     

Simulation of honeycomb-screen combinations for turbulence management in a subsonic wind tunnel

Honeycomb and screens, mainly used for turbulence reduction, are the key elements of a subsonic wind tunnel. In this paper, design aspects of these elements are addressed for an open-circuit wind tunnel, installed at Indian Institute of Technology Guwahati (IITG), India. The effectiveness of honeycomb and honeycomb-screen combinations, in reducing swirl and turbulence level in the test section, is studied by simulating the flowfield using commercial computational fluid dynamics (CFD) package ANSYS-CFX. RNG k−ε turbulence model, with scalable wall functions, is used for modeling turbulence. Results of these simulations for turbulence management, using honeycombs of different lengths, cell shapes and screens of different open area ratios, are found to be in good agreement with experimental and theoretical results available in open literature. These simulations have confirmed the methodology to be adopted for design of wind tunnel subsections. Thus validated design parameters have been used for fabricating the honeycomb and screens for the IITG wind tunnel.