A wind tunnel study of turbulent flow around single and multiple windbreaks, part I: Velocity fields

This paper describes wind-tunnel experiments on the flow around single and multiple porous windbreaks (height H), sheltering a model plant canopy (height H/3). The mean wind is normal to the windbreaks, which span the width of the wind tunnel. The incident turbulent flow simulates the adiabatic atmospheric surface layer. Five configurations are examined: single breaks of three solidities (low, medium, high; solidity = 1 - porosity), and medium-solidity multiple breaks of streamwise spacing 12H and 6H. The experimental emphases are on the interactions of the windbreak flow with the underlying plant canopy; the effects of solidity; the differences in shelter between single and multiple windbreaks; and the scaling properties of the flow. Principal results are: (1) the "quiet zones" behind each windbreak are smaller in multiple than single arrays, because of the higher turbulence level in the very rough-wall internal boundary layer which develops over the multiple arrays. Nevertheless, the overall shelter effectiveness is higher for multiple arrays than single windbreaks because of the "nonlocal shelter" induced by the array as a whole. (2) The flow approaching the windbreak decelerates above the canopy but accelerates within the canopy, particularly when the windbreak solidity is high. (3) A strong mixing layer forms just downwind of the top of each windbreak, showing some of the turbulence and scaling properties of the classical mixing layer formed between uniform, coflowing streams. (4) No dramatic increase in turbulence levels in the canopy is evident at the point where the deepening mixing layer contacts the canopy (around x/H = 3) but the characteristic inflection in the canopy wind profile is eliminated at this point.     

A Wind Tunnel and Numerical Investigation of Turbulent Dispersion in Coastal Atmospheric Boundary Layers

Thermal internal boundary layers in onshore air flows have a significant influence on pollutant diffusion in coastal areas. Although several models for this diffusion problem exist, measurements for model verification are scarce. In this paper, we present a set of wind tunnel observations and examine the performance of a Lagrangian stochastic model. The good agreement between the model simulation and the tunnel measurements confirms the usefulness of the Lagrangian stochastic model for practical purposes. Sensitivity tests of the model to turbulence statistics show that uncertainty in velocity skewness to the extent of observational scatter does not seem to have a significant influence on pollutant dispersion, while uncertainties in turbulence intensity (variance) significantly influence the dispersion pattern.     

Turbulent Flow Properties Around a Staggered Wind Farm

The fundamental properties of turbulent flow around a perfectly staggered wind farm are investigated in a wind tunnel. The wind farm consisted of a series of 10 rows by 2–3 columns of miniature wind turbines spaced 5 and 4 rotor diameters in the streamwise and spanwise directions respectively. It was placed in a boundary-layer flow developed over a smooth surface under thermally neutral conditions. Cross-wire anemometry was used to obtain high resolution measurements of streamwise and vertical velocity components at various locations within and above the wind farm. The results show that the staggered configuration is more efficient in terms of momentum transfer from the background flow to the turbines compared to the case of an aligned wind turbine array under similar turbine separations in the streamwise and spanwise directions. This leads to improved power output of the overall wind farm. A simplified analysis suggests that the difference in power output between the two configurations is on the order of 10%. The maximum levels of turbulence intensity in the staggered wind farm were found to be very similar to that observed in the wake of a single wind turbine, differing substantially with that observed in an aligned configuration with similar spacing. The dramatic changes in momentum and turbulence characteristics in the two configurations show the importance of turbine layout in engineering design. Lateral homogenization of the turbulence statistics above the wind farm allows for the development of simple parametrizations for the adjustment of flow properties, similar to the case of a surface roughness transition. The development of an internal boundary layer was observed at the upper edge of the wind farm within which the flow statistics are affected by the superposition of the ambient flow and the flow disturbance induced by the wind turbines. The adjustment of the flow in this layer is much slower in the staggered situation (with respect to its aligned counterpart), implying a change in the momentum/power available at turbine locations. Additionally, power spectra of the streamwise and vertical velocity components indicate that the signature of each turbine-tip vortex structure persists to locations deep within the wind farm.     

The Influence of Thermally-Induced Mesoscale Circulations on Turbulence Statistics Over an Idealized Urban Area Under a Zero Background Wind

The influence of mesoscale circulations induced by urban-rural differential surface sensible heat flux and roughness on convective boundary-layer (CBL) flow statistics over an isolated urban area has been examined using large-eddy simulation (LES). Results are analyzed when the circulations influence the entire urban area under a zero background wind. For comparison, the CBL flow over an infinite urban area with identical urban surface characteristics under the same background meteorological conditions is generated as a control case (without circulations). The turbulent flow over the isolated urban area exhibits a mix of streaky structure and cellular pattern, while the cellular pattern dominates in the control case. The mixed-layer height varies significantly over the isolated urban area, and can be lower near the edge of the urban area than over the rural area. The vertical profiles of turbulence statistics over the isolated urban area vary horizontally and are dramatically different from the control case. The turbulent kinetic energy (TKE) sources include wind shear, convergence, and buoyancy productions, compared to only buoyancy production in the control case. The normalized vertical velocity variance is reduced compared to the control case except in the central urban area where it is little affected. The low-level flow convergence is mainly responsible for the enhanced horizontal velocity variance in the central urban area, while wind shear is responsible for the additional local maximum of the horizontal velocity variance near the middle of the CBL outside the central area. Parameterizations in the prognostic equation for TKE used in mesoscale models are evaluated against the LES results over the isolated urban area. We also discuss conditions under which the urban-induced circulations occur and when they may affect the entire urban area. Given that urban-induced circulations can influence the entire urban area within hours for an urban area of a realistic size, it is inappropriate to directly apply empirical relations of turbulence statistics derived under horizontally-homogenous flow conditions to an urban area.     

Laboratory and Numerical Studies of the Convective Boundary Layer Capped by a Strong Inversion

The convective boundary layer (CBL) with a wide range of stability is simulated experimentally using a thermally stratified wind tunnel, and numerically by direct numerical simulation (DNS). The turbulence structures and flow characteristics of various CBL flows, capped by a strong temperature inversion and affected by surface shear, are investigated. The various vertical profiles of turbulence statistics similar to those from the observed CBL in the field are successfully simulated in both the wind-tunnel experiment and in DNS. The comparison of the wind-tunnel data and DNS results with those of atmospheric observations and water-tank studies shows the crucial dependence of the turbulence statistics in the upper part of the layer on the strength of the inversion layer, as well as the modification of the CBL turbulence regime by the surface shear.     

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.     

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.     

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.     

A Statistical Model for the Prediction of Wind-Speed Probabilities in the Atmospheric Surface Layer

Wind fields in the atmospheric surface layer (ASL) are highly three-dimensional and characterized by strong spatial and temporal variability. For various applications such as wind-comfort assessments and structural design, an understanding of potentially hazardous wind extremes is important. Statistical models are designed to facilitate conclusions about the occurrence probability of wind speeds based on the knowledge of low-order flow statistics. Being particularly interested in the upper tail regions we show that the statistical behaviour of near-surface wind speeds is adequately represented by the Beta distribution. By using the properties of the Beta probability density function in combination with a model for estimating extreme values based on readily available turbulence statistics, it is demonstrated that this novel modelling approach reliably predicts the upper margins of encountered wind speeds. The model’s basic parameter is derived from three substantially different calibrating datasets of flow in the ASL originating from boundary-layer wind-tunnel measurements and direct numerical simulation. Evaluating the model based on independent field observations of near-surface wind speeds shows a high level of agreement between the statistically modelled horizontal wind speeds and measurements. The results show that, based on knowledge of only a few simple flow statistics (mean wind speed, wind-speed fluctuations and integral time scales), the occurrence probability of velocity magnitudes at arbitrary flow locations in the ASL can be estimated with a high degree of confidence.     

Atmospheric-stability effect on windbreak shelter and drag

Wind speed was measured near the surface, along a line normal to a single, 50% porous windbreak, to determine its wind-reducing effect. Simultaneously, undisturbed wind and temperature profiles were measured to obtain the atmospheric stability. In some of the runs, the drag on a section of the windbreak was also measured. A systematic and significant effect of the atmospheric stability was found. The relative wind speed at any distance from the windbreak could be expressed for unstable conditions as an empirical function of the Richardson number. As an extreme example, the reduction of the surface shear, at a downwind distance 7.5 times the height of the windbreak, was 86 % in adiabatic conditions, and only 62 % when the gradient Richardson number at the windbreak height was ?1.0. The minimum relative effect was found in the neighborhood of the windbreak, where the windbreak-induced turbulence was dominant. The maximum absolute effect was found at a downwind distance of about ten times the height of the windbreak. The drag coefficient of the windbreak was found to bec _d =0.77 in a neutral atmosphere, increasing slightly with instability. This trend is contrary to the trend of the surface-shear-reduction coefficient, which decreases significantly with instability.     

Evaluation of a micro-scale wind model’s performance over realistic building clusters using wind tunnel experiments

The simulation performance over complex building clusters of a wind simulation model(Wind Information Field Fast Analysis model, WIFFA) in a micro-scale air pollutant dispersion model system(Urban Microscale Air Pollution dispersion Simulation model, UMAPS) is evaluated using various wind tunnel experimental data including the CEDVAL(Compilation of Experimental Data for Validation of Micro-Scale Dispersion Models) wind tunnel experiment data and the NJU-FZ experiment data(Nanjing University-Fang Zhuang neighborhood wind tunnel experiment data). The results show that the wind model can reproduce the vortexes triggered by urban buildings well, and the flow patterns in urban street canyons and building clusters can also be represented. Due to the complex shapes of buildings and their distributions, the simulation deviations/discrepancies from the measurements are usually caused by the simplification of the building shapes and the determination of the key zone sizes. The computational efficiencies of different cases are also discussed in this paper. The model has a high computational efficiency compared to traditional numerical models that solve the Navier–Stokes equations, and can produce very high-resolution(1–5 m) wind fields of a complex neighborhood scale urban building canopy(～ 1 km ×1km) in less than 3 min when run on a personal computer.     

深凹露天矿内复环流和高湍能区的三维细网格非静力模拟

建立了一个准定常的三维非静力能量闭合的PBL模式模拟了深凹露天矿内复环流和高湍能区。针对露天矿具有范围小、地形复杂的特点,放弃了静力近似的假定,且采用了在复杂地形下应用较好的能量闭合方案,采用细网格系统对一个水平只有2 km×2 km的实际矿区进行了模拟,并将结果与同样条件下的风洞试验结果进行了比较。结果表明,矿坑的开口宽度与深度比和来流的性质对坑内气流和湍流情况有了较大影响。     

风电场流场特性及风机布局数值模拟研究

针对风电场流场特性研究对风力机工作性能提高的重要意义,采用计算流体力学（CFD）方法在单机风力机模拟验证的基础上,对某风电场单风力机和三种布局条件下的风电场流场特性进行了数值模拟研究。考察了不同布局条件下风电场速度、叶轮表面压力以及湍流涡的分布特性。结果表明：叶轮后方尾流效应明显,速度损失随着相对距离的增加而逐渐减小,风力机处中心尾流速度比率最低降为0.4;当相对距离超过4 d后,风速可恢复为初始速度的90%以上;尾流速度与涡流粘性呈负相关性;三风力机平行布置时,各风力机尾流速度比率基本一致;三风机组在错落布置时,尾流效应对下游风力机工作性能影响较小,错落角度越大,尾流损失越小。     

Numerical study of flow and gas diffusion in the near-wake behind an isolated building

To assist validation of the experimental data of urban pollution dispersion, the effect of an isolated building on the flow and gaseous diffusion in the wake region have been investigated numerically in the neutrally stratified rough-walled turbulent boundary layer. Numerical studies were carried out using Computational Fluid Dynamics (CFD) models. The CFD models used for the simulation were based on the steady-state Reynolds-Average Navier-Stoke equations (RANS) with κ-ε turbulence models; standard κ-ε and RNG κ-ε models. Inlet conditions and boundary conditions were specified numerically to the best information available for each fluid modeling simulation. A gas pollutant was emitted from a point source within the recirculation cavity behind the building model. The accuracy of these simulations was examined by comparing the predicted results with wind tunnel experimental data. It was confirmed that simulation using the model accurately reproduces the velocity and concentration diffusion fields with a fine-mish resolution in the near wake region. Results indicated that there is a good agreement between the numerical simulation and the wind tunnel experiment for both wind flow and concentration diffusion. The results of this work can help to improve the understanding of mechanisms of and simulation of pollutant transport in an urban environment.     

A wind-tunnel experiment concerning atmospheric vortex streets

Cloud patterns, in the upper part of the atmospheric boundary layer, with eddies similar to Karman vortex streets, have been observed in the lee of several isolated islands. An experiment was conducted to see if major features of these atmospheric vortex streets and associated atmospheric conditions could be simulated in a wind tunnel. A visible vortex street was observed in a wind tunnel while six atmospheric simulation criteria were at least qualitatively satisfied. The physical modeling, the atmospheric data used, the simplifying assumptions and approximations required, as well as the wind tunnel methods and results are described.     

2015年热带气旋“彩虹”的高分辨率数值模拟与风场诊断分析

利用WRF模式,对热带气旋“彩虹”（1522）登陆过程开展了高分辨率数值模拟,模拟验证表明:模式较成功地模拟出了“彩虹”的移动路径、强度变化和降水分布。利用高分辨率模拟资料,分析了“彩虹”登陆期间的大风分布以及热、动力结构特征,结合切向风动量方程开展了模拟诊断,结果表明:切向风动量方程各项在近地面层和边界层顶代表高度上的平面结构诊断分析显示,近地面（0.4 km）层上,VVOR（角动量径向平流项）和VPGF（切向气压梯度项）是切向风变化的最大贡献项;边界层顶（1.3 km）附近,VPGF和VVA（切向动量垂直平流项）对切向风起到正加速作用,而VVOR则根据入流（出流）特征,起到正（负）加速作用。切向风动量方程各项的轴对称平均结构诊断分析显示,“彩虹”登陆前,VVA和VVOR是决定切向风变化的主要贡献项。     

Operating ranges of meteorological wind tunnels for the simulation of convective boundary layer (CBL) phenomena

The operating ranges of meteorological wind tunnels for convective boundary-layer (CBL) simulation are defined in this paper based on a review of the theoretical and practical limitations of the flow phenomena and the facilities available. Wind-tunnel operating ranges are limited by the dimensions of the simulated circulations and of the tunnel itself, the tunnel flow speed and turbulence processes, and the characteristics of the measurement instrumentation. When it is desired to simulate both the CBL and the behavior of other flows imbedded within the boundary layer, such as power-plant plume rise and dispersion, then additional constraints exist on the fluid modeling process. The capabilities of meteorological wind tunnels can also be extended through the judicious use of boundary and side wall flow controls.     

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.     

Flow around a windbreak in oblique wind

Wind speed was measured near the surface, along a line normal to a single, 50 % porous windbreak. The results for near-neutral atmospheric conditions were strongly dependent on the incidence angle of the wind, a, and slightly dependent on the roughness of the surface. A measure of the shelter effect - the protected distance - was found to decrease faster than cos . The drag coefficient of the windbreak decreased proportionally with cos . The effect of increased roughness was to increase the wind speed near the surface in the lee of the windbreak.     

Numerical simulation of pollutant dispersion in a small valley under conditions with supercritical Richardson numbers

The numerical drainage wind model of Wonget al. (1987) is used together with a Lagrangian particle model in the simulation of carbon monoxide (CO) dispersion within a small urban valley in Edmonton, Alberta, Canada. The conditions studied are those of strong static stability when vertical mixing is suppressed. These are conditions with the Richardson number exceeding its critical value (hereafter referred to as supercritical conditions). Observations showed that under such conditions, vertical turbulence is suppressed but horizontal turbulence still exists. The effects of turbulence in the dispersion and transport of pollutants under such conditions are small. However, in the present simulation, a simple turbulence parameterization based on observations is used for supercritical conditions. Some field experiments were performed and the observations are compared with model results. For a location downwind of the CO source, two peaks can be observed during the course of the drainage flow regime. The model results suggest that these represent an initial flux from the drainage flow and a second flux later from drainage wind recirculation. Another main feature of the model-predicted concentration field is zones of maximum concentration at and above the valley floor. There is a drainage wind cell on each side of the valley slope and the cells are effectively decoupled from the prevailing wind above. The present modelling results show that when the prevailing wind exists before the development of the drainage wind, it can be instrumental in transporting CO from one drainage wind cell to the other. Otherwise, the CO released within one drainage wind cell is well contained.