Turbulence Characteristics Over Complex Terrain In West China

Meteorological data of velocity components and temperature have been measured on a mast of height 4.9 m at one site in the Heihe River Basin Field Experiment (HEIFE) conducted in west China. Mean and individual turbulence parameters, power spectra/cospectra, phase angles and their changes withfetch downwind of a change in surface roughness were analyzed. The turbulence characteristics depend strongly on the prevailing wind direction, which in turn is associated with changes in the upwind surface roughness pattern. The results show that values of horizontal velocity standard deviations sigma;_u,v scaled with local friction velocity u under different stratifications are larger than those over flat terrain, while the values of _w/u have the same values as over flat terrain. The differences between variance values of the horizontal velocity components, u and v, over inhomogeneous terrain were found to be significantly smaller than those over flat terrain. Since energy densities of the w spectra, uw and wT cospectra at low frequencies are relatively lower than those of longitudinal velocity spectra, total energies of w spectra, uw and wT cospectra tend to be in equilibrium with the local terrain. The values of phase angles at the low frequency end of the frequency showed obvious differences associated with changes of roughness.     

广州市近地层大气的湍流微结构和谱特征

本文利用UVW脉动风速仪资料分析了广州市区近地层大气的湍流强度、相关系数、尺度和速度谱,并获得了不同稳定度条件下的速度谱模式.结果表明,城市近地层大气湍流在惯性副区接近局地各向同性、速度谱符合Kolmogorov相似理论;气流方向上下垫面粗糙度的增加,使沿海城市近地层大气湍流能量(特别是铅直方向)比平坦、均匀下垫面上的增加.     

稳定条件下盆地上空大气边界层湍谱特征

本文利用100m~3系留气艇携带超声风温仪在复杂的盆地地形上空对1000m以下大气进行观测所获得的资料,研究小风稳定条件下大气边界层湍流结构特征。结果表明,在双对数坐标中,纵向速度u谱,垂直速度w谱,温度T谱在惯性区均遵循Kolmogorov的-2/3次律;横向速度V谱有其特殊的情形;协谱uw,wT及近地层的uT协谱在惯性区服从-4/3次律。和平坦、均一、开阔下垫面不同的是谱的峰值频率向高频移动,且没有发现近地层具有的谱峰随高度的明显变化关系。     

基于CFD动力降尺度的复杂地形风电场风电功率短期预测方法研究

复杂地形导致近地层风场时空变化大,是影响风电场短期风电功率预测准确率的重要因素。为此,基于中尺度数值预报模式和微尺度计算流体力学模式,建立了风电场短期风电功率动力降尺度预测系统。该系统由中尺度数值预报模式、微尺度风场基础数据库、风电功率预测集成系统组成,能够预测复杂地形风电场中每台风电机组未来72 h逐15 min的发电量。提高了复杂地形风场发电功率预测准确率,同时还可以在上报电网的风电功率预测结果中考虑运行维护计划和限电等因素对实际并网功率的影响。2014年7月-2015年1月的业务预测试验表明,风电场短期风电功率动力降尺度预测系统的月预测相对误差均小于0.2,满足中国国家电网对风电功率预测误差和时效性的业务要求。动力降尺度技术不受具体项目地形复杂程度和历史观测数据样本量的限制,可以在新建风电场中推广应用,具备实际的可操作性。      

Turbulence spectra in the near-neutral surface layer over the Loess Plateau in China

We present the power spectra of wind velocity and the cospectra of momentum and heat fluxes observed for different wind directions over flat terrain and a large valley on the Loess Plateau. The power spectra of longitudinal (u) and lateral (v) wind speeds satisfy the −5/3 power law in the inertial subrange, but do not vary as observed in previous studies within the low frequency range. The u spectrum measured at 32 m height for flow from the valley shows a power deficit at intermediate frequencies, while the v spectrum at 32 m downwind of the valley reaches another peak in the low frequency range at the same frequency as the u spectrum. The corresponding peak wavelength is consistent with the observed length scale of the convective outer layer at the site. The v spectrum for flat terrain shows a spectral gap at mid frequencies while obeying inner layer scaling in its inertial subrange, suggesting two sources of turbulence in the surface layer. All the spectra and cospectra from the valley direction show a height dependency over the three levels.     

四川省会理县风能资源分析研究

本研究利用四川省会理县测风塔短期2012年2月1日~2013年1月31日期间的70m高度实测逐时数据,经会理和会东气象观测站1981年1月~2010年12月共计30年的风速逐时数据采用长年代法订正推算后,对会理县该研究区域风能资源参数进行计算。结果表明:该区域风能资源应用于并网风力发电分别为“很好”和“好”的等级,具有较好开发潜力;风能和风向较为集中,有利于风机布设;测风塔地处高海拔地形较为平坦的山脊地带,不占用耕地,临近公路,气候较为温和,利于风电场修建。      

Turbulence Closures In Neutral Boundary Layer Over Complex Terrain

In this work, three turbulence closure models, Mellor andYamada level 2.5, E - l and E - implemented in a circulation model, are compared in neutral condition over complex terrain. They are firstly applied to a one-dimensional case on flat terrain and then to a schematic two-dimensional valley. The simulation results, in terms of wind field and turbulent kinetic energy, are tested against measurements from a wind-tunnel experiment. The empirical constants defining the characteristic length scales of the closures are modified based on turbulence parameters estimated in the experiment. The formulation of the diffusion coefficients is analysed to explain the differences among the various closures in the simulation results. Regarding the mean flow, both on flat and complex terrain, all the closures yield satisfactory results. Concerning the turbulent kinetic energy, the best results are obtained by E - l and E - closures.     

A Wind-Tunnel Investigation of Wind-Turbine Wakes: Boundary-Layer Turbulence Effects

Wind-tunnel experiments were performed to study turbulence in the wake of a model wind turbine placed in a boundary layer developed over rough and smooth surfaces. Hot-wire anemometry was used to characterize the cross-sectional distribution of mean velocity, turbulence intensity and kinematic shear stress at different locations downwind of the turbine for both surface roughness cases. Special emphasis was placed on the spatial distribution of the velocity deficit and the turbulence intensity, which are important factors affecting turbine power generation and fatigue loads in wind energy parks. Non-axisymmetric behaviour of the wake is observed over both roughness types in response to the non-uniform incoming boundary-layer flow and the effect of the surface. Nonetheless, the velocity deficit with respect to the incoming velocity profile is nearly axisymmetric, except near the ground in the far wake where the wake interacts with the surface. It is found that the wind turbine induces a large enhancement of turbulence levels (positive added turbulence intensity) in the upper part of the wake. This is due to the effect of relatively large velocity fluctuations associated with helicoidal tip vortices near the wake edge, where the mean shear is strong. In the lower part of the wake, the mean shear and turbulence intensity are reduced with respect to the incoming flow. The non-axisymmetry of the turbulence intensity distribution of the wake is found to be stronger over the rough surface, where the incoming flow is less uniform at the turbine level. In the far wake the added turbulent intensity, its positive and negative contributions and its local maximum decay as a power law of downwind distance (with an exponent ranging from −0.3 to −0.5 for the rough surface, and with a wider variation for the smooth surface). Nevertheless, the effect of the turbine on the velocity defect and added turbulence intensity is not negligible even in the very far wake, at a distance of fifteen times the rotor diameter.     

HEIFE戈壁地区近地层大气的湍流结构和输送特征

根据黑河地区地气相互作用实验研究(HEIFE)1988年POP和1990年PIOP实验的观测资料,分析了戈壁地区近地层大气的湍流结构,主要是风速分量和温度方差的相似关系,以及各有关量的功率谱和湍流通量的协谱。1988年POP期间发现的戈壁近地层大气中白天经常出现的水汽由上而下输送等特殊现象,在1990年PIOP中得到进一步的验证。文中分析了有关物理机制,认为这一现象和众所周知的“绿洲效应”是戈壁-绿洲这一地区性中尺度环流影响的两个侧面。     

Turbulence measurements over complex terrain

Horizontal turbulence measurements obtained from 22 wind sensors located on 9 towers in a mountainous coastal area are described and categorized by stability and terrain. Vector wind time series are high-pass filtered, and lateral and longitudinal wind speed variance is calculated for averaging times ranging from 15 s to 2 h. Parameterizations of the functional dependence of variance on averaging time are discussed, and a modification of Panofsky's (1988) uniform terrain technique applicable to complex terrain is presented. The parameterization is applied to the data and shown to be more realistic than a less complicated power law technique. The parameter values are shown to be different than the flat terrain cases of Kaimal et al. (1972), and are primarily a function of sensor location within the complex terrain. The parameters are also examined in terms of their dependence upon season, stability, marine boundary-layer height, and measurement height.     

Effects of Topographical Slope Angle and Atmospheric Stratification on Surface-Layer Turbulence

The effect of topographical slope angle and atmospheric stratification on turbulence intensities in the unstably stratified surface layer have been parameterized using observations obtained from a three-dimensional sonic anemometer installed at 8 m height above the ground at the Seoul National University (SNU) campus site in Korea for the years 1999–2001. Winds obtained from the sonic anemometer are analyzed according to the mean wind direction, since the topographical slope angle changes significantly along the azimuthal direction. The effects of the topographical slope angle and atmospheric stratification on surface-layer turbulence intensity are examined with these data. It is found that both the friction velocity and the variance for each component of wind normalized by the mean wind speed decrease with increase of the topographical slope angle, having a maximum decreasing rate at very unstable stratification. The decreasing rate of the normalized friction velocity (u _* /U) is found to be much larger than that of the turbulence intensity of each wind component due to the reduction of wind shear with increase in slope angle under unstable stratification. The decreasing rate of the w component of turbulence intensity (σ _w /U) is the smallest over the downslope surface whereas that of the u component (σ _u /U) has a minimum over the upslope surface. Consequently, σ _w /u _* has a maximum increasing rate with increase in slope angle for the downslope wind, whereas σ _u /u _* has its maximum for the upslope wind. The sloping terrain is found to reduce both the friction velocity and turbulence intensity compared with those on a flat surface. However, the reduction of the friction velocity over the sloping terrain is larger than that of the turbulence intensity, thereby enhancing the turbulence intensity normalized by the friction velocity over sloping terrain compared with that over a flat surface.     

Surface-layer flow in complex terrain: Comparison of models and full-scale observations

Four models of surface boundary-layer flow in complex terrain are compared with observations made at Blashaval Hill, North Uist, Scotland. The field experiment is described by Mason and King (1985). Three of the models are derived from the two-dimensional theory of Jackson and Hunt (1975) and are described in Mason and King (1985), Walmsley et al. (1986) and Troen and Petersen (1989). The fourth is a mass-consistent code based on Traci et al. (1979). The model results are in good agreement with each other and are generally within the observed range of variation ( ~ ± 16%) in normalized wind speed. For most wind direcions (7 of 11), model results of normalized wind speed at the summit were within 7% of the observed mean values. For some wind directions, calculations using the Guidelines of Walmsley et al. (1989) suggested that variations in surface roughness were important. This led us to apply one of our models incorporating nonuniform surface roughness. The lack of significant improvement for cases when water lay upstream of Blashaval Hill is attributed to compensating changes at summit and reference sites and to very local effects on the wind data. Sensitivity to topography lying to the west and northwest of Blashaval was also investigated. Results suggested an influence from those distant topographic features for some wind directions. When those features were incorporated, maximum errors in normalized wind speed at the summit were reduced from 18 to 13%.     

The Effects of Building Representation and Clustering in Large-Eddy Simulations of Flows in Urban Canopies

We perform large-eddy simulations of neutral atmospheric boundary-layer flow over a cluster of buildings surrounded by relatively flat terrain. The first investigated question is the effect of the level of building detail that can be included in the numerical model, a topic not yet addressed by any previous study. The simplest representation is found to give similar results to more refined representations for the mean flow, but not for turbulence. The wind direction on the other hand is found to be important for both mean and turbulent parameters. As many suburban areas are characterised by the clustering of buildings and homes into small areas separated by surfaces of lower roughness, we look at the adjustment of the atmospheric surface layer as it flows from the smoother terrain to the built-up area. This transition has unexpected impacts on the flow; mainly, a zone of global backscatter (energy transfer from the turbulent eddies to the mean flow) is found at the upstream edge of the built-up area.     

我国不同下垫面的近地层风切变指数研究

利用国家气候中心2009-2011年在内蒙古锡林浩特市、河北黄骅市、江西湖口县和星子县开展的100 m铁塔大气湍流观测试验数据,分析不同下垫面、不同稳定度条件下地面到100 m的风切变指数变化情况,得到了一些有意义的结论:(1)内陆简单地形整体风切变指数α0.3的情况出现在稳定和非常稳定层结状态下,分层风切变指数α0.4的频率为2.95%;(2)沿海平坦地形大气层结越稳定,整体风切变指数α的高值出现频率越高,分层风切变指数α0.4的频率略高,为4.2%;(3)对于内陆湖复杂地形,星子地形更为复杂,大于0.3的整体风切变指数出现频率高,分层风切变指数α0.4的频率高达10%,而湖口大于0.3的整体风切变指数值出现频率略低,分层风切变指数α0.4的频率为3.37%。(4)按照风力发电机组设计要求中假设风切变指数α为0.2,一般会过高的估计轮毂高度的风速。     

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.     

山谷城市的近地层大气湍流谱特征

山谷城市地区的大气湍流结构研究至今报道很少.本文根据兰州市一次观测实验,对各风速分量谱、温度谱以及动量和感热通量的协谱进行了分析.结果表明,尽管仪器的安装高度较低(6.45m),湍谱特征仍与平坦均匀下垫面得到的一些典型结果基本一致,从而认为,在类似地区进行湍流通量等的直接观测时,对观测高度和地点的选择,可以不必象一些文献要求的那样高.     

连云港沿海地区及近海风能资源评估

通过对连云港市30多年气象整编资料及沿海高密度自动气象站、6座测风塔、4座海岛气象站等加密观测资料研究,在充分考虑当地特殊地形特点的基础上,探讨了该地区沿海及近海风能资源的时空分布规律及特点,并重点对风湍流强度的特征及强湍流对风电的影响进行了剖析;研究表明,近海地区风能资源储量相对丰富,风速变化稳定,且风垂直切变较小,具有更为广阔的开发利用前景.     

Prediction of wind properties in urban environments using artificial neural network

Artificial neural network (ANN) modeling has been performed to predict turbulent boundary layer characteristics for rough terrain based on experimental tests conducted in a boundary-layer wind tunnel to simulate atmospheric boundary layer using passive roughness devices such as spires, barriers, roughness elements on the floor, and slots in the extended test section. Different configurations of passive devices assisted to simulate urban terrains. A part of the wind tunnel test results are used as training sets for the ANN, and the other part of the test results are used to compare the prediction results of the ANN. Two ANN models have been developed in this study. The first one has been used to predict mean velocity, turbulence intensity, and model length scale factor. Results show that ANN is an efficient, accurate, and robust modeling procedure to predict turbulent characteristics of wind. In particular, it was found that the ANN-predicted wind mean velocities are within 4.7%, turbulence intensities are within 6.2%, and model length scale factors are within 3.8% of the actual measured values. In addition, another ANN model has been developed to predict instantaneous velocities that enables calculating the power spectral density of longitudinal velocity fluctuations. Results show that the predicted power spectra are in a good agreement with the power spectra obtained from measured instantaneous velocities.     

étude expérimentale de la couche limite de montagne

The characteristics of the boundary layer over complex terrain (Lannemezan - lat.: 43.7° N and, long.: 0.7 ° E) are analyzed for various scales, using measurements obtained during the COCAGNE Experiment. In this first part, the dynamic characteristics of the flow are studied with respect to atmospheric stability and the relief at small (~20 km) and medium scales (~100 km). These relief scales depend on the topographical profile of the Lannemezan Plateau along the dominant axis of the wind (E-W) and the Pyrénées Mountains located at the south of the experimental site. The terrain heterogeneities have a standard deviation of ~48 m and a wavelength of ~2 km.The averaged vertical profiles of wind speed and direction over the heterogeneous terrain are analyzed. The decrease of wind speed within the boundary layer is greater than over flat terrain (WANGARA Experiment). However, a comparison between ETTEX (complex terrain) and COCAGNE vertical wind speed profiles shows good agreement during unstable conditions. In contrast, during neutral conditions a more rapid increase with normalized height is found with COCAGNE than with ETTEX and WANGARA data. The vertical profiles of wind direction reveal an influence of the Pyrénées Mountains on the wind flow. The wind rotation in the BL is determined by the geostrophic wind direction-Pyrénées axis angle (negative deviation) as the geostrophic wind is connected with the Mountain axis.When the geostrophic wind does not interact with the Pyrénées axis, the mean and turbulent wind flow characteristics (drag coefficient C _D, friction velocity u _*) depend on the topography of the plateau. When the wind speed is strong (>6 m s ^-1), an internal boundary layer is generated from the leading edge of the Plateau.