Interaction between the atmospheric boundary layer and a standalone wind turbine in Gansu—Part I: Field measurement

Experiments and numerical simulations of the wake field behind a horizontal-axis wind turbine are carried out to investigate the interaction between the atmospheric boundary layer and a stand-alone wind turbine. The tested wind turbine(33 k W) has a rotor diameter of 14.8 m and hub height of 15.4 m. An anti-icing digital Sonic wind meter, an atmospheric pressure sensor, and a temperature and humidity sensor are installed in the upstream wind measurement mast. Wake velocity is measured by three US CSAT3 ultrasonic anemometers. To reflect the characteristics of the whole flow field, numerical simulations are performed through large eddy simulation(LES) and with the actuator line model. The experimental results show that the axial velocity deficit rate ranges from 32.18% to 63.22% at the three measuring points. Meanwhile, the time-frequency characteristics of the axial velocities at the left and right measuring points are different. Moreover, the average axial and lateral velocity deficit of the right measuring point is greater than that of the left measuring point. The turbulent kinetic energy(TKE) at the middle and right measuring points exhibit a periodic variation, and the vortex sheet-pass frequency is mostly similar to the rotational frequency of the rotor. However, this feature is not obvious for the left measuring point. Meanwhile, the power spectra of the vertical velocity fluctuation show the slope of-1, and those of lateral and axial velocity fluctuations show slopes of-1 and-5/3, respectively.However, the inertial subranges of axial velocity fluctuation at the left, middle, and right measuring points occur at 4, 7, and7 Hz, respectively. The above conclusion fully illustrates the asymmetry of the left and right measuring points. The experimental data and numerical simulation results collectively indicate that the wake is deflected to the right under the influence of lateral force. Therefore, wake asymmetry can be mainly attributed to the lateral force exerted by the wind turbine on the fluid.     

Simulation of the inherent turbulence and wake interaction inside an infinitely long row of wind turbines

The turbulence in the interior of a wind farm is simulated using large eddy simulation and the actuator line technique implemented in the Navier–Stokes equations. The simulations are carried out for an infinitely long row of turbines simulated by applying cyclic boundary conditions at the inlet and outlet. The simulations investigate the turbulence inherent to the wind turbines as no ambient turbulence or shear is added to this idealised case. The simulated data give insight into the performance of the wind turbines operating in the wake of others as well as details on key turbulent quantities. One of the key features of wakes behind wind turbines is the dynamic wake meandering, which is shown to be related to the wind turbine spacing and the vortex shedding from the turbine as a bluff body. The flow is analysed and reconstructed by applying proper orthogonal decomposition.     

基于LBM方法的风力发电机尾流结构仿真

针对传统CFD数值计算方法难以实现风力机动态旋转及其旋转状态下的流固耦合计算,本文结合格子玻尔兹曼(LBM)方法易于处理动态复杂边界的特点及大涡模拟(LES)方法在非稳态涡流结构捕捉上的优势,采用LBM-LES联合方法进行三维风力发电机整机气动性能及尾流结构仿真研究,同时采用尺度自适应方法对尾涡结构进行跟踪和精细化计算。针对NREL PhaseⅥ型试验机进行模拟,得到了与实验结果吻合的流动形态及尾流结构演变规律,分析了尾流区速度演变规律并对比了不同亚格子湍流模型对计算结果的影响.     

An experimental study on the effects of relative rotation direction on the wake interferences among tandem wind turbines

An experimental study was conducted to investigate the effects of relative rotation direction on the wake interferences among two tandemwind turbines models.While the oncoming flow conditions were kept in constant during the experiments,turbine power outputs,wind loads acting on the turbines,and wake characteristics behind the turbines were compared quantitatively with turbine models in either co-rotating or counter-rotating configuration.The measurement results reveal that the turbines in counter-rotating would harvest more wind energy from the same oncoming wind,compared with the co-rotating case.While the recovery of the streamwise velocity deficits in the wake flows was found to be almost identical with the turbines operated in either co-rotating or counter-rotating,the significant azimuthal velocity generated in the wake flow behind the upstream turbine is believed to be the reason why the counter-rotating turbines would have a better power production performance.Since the azimuthal flow velocity in the wake flow was found to decrease monotonically with the increasing downstream distance,the benefits of the counter-rotating configuration were found to decrease gradually as the spacing between the tandem turbines increases.While the counter-rotating downstream turbine was found to produce up to 20%more power compared with that of co-rotating configuration with the turbine spacing being about 0.7D,the advantage was found to become almost negligible when the turbine spacing becomes greater than 6.5D.It suggests that the counter-rotating configuration design would be more beneficial to turbines in onshore wind farms due to the smaller turbine spacing(i.e.,~3 rotor diameters for onshore wind farms vs.~7 rotor diameters for offshore wind farms in the prevailing wind direction),especially for those turbines sited over complex terrains with the turbine spacing only about 1–2 rotor diameters.     

Darrieus风力透平气动设计参数优化

针对Darrieus垂直轴风力透平,本文首先引入多流管方法,并在验证基础上,开展了样机设计和参数化优化研究。结果表明,多流管模型具有较高的准确度。利用此方法获得了Darrieus垂直轴风力透平参数优化的规律性认识。     

LES studies of the flow in a swirl gas combustor

Environmental and other practical concerns have led to the development of compact gas turbine combustors burning lean mixtures leading to potentially low CO and NO_x emissions. The compact design requires efficient atomization and mixing together with a compact premixed flame. Associated with these requirements are higher temperatures, increased heat transfer, and thermal load, thus increasing the danger of combustion instabilities (causing performance deterioration and excessive mechanical loads), and possible off-design operation. Numerical simulations of reacting flows are well suited to address these issues. To this end, large eddy simulation (LES) is particularly promising. The philosophy behind LES is to explicitly simulate the large scales of the flow and the thermochemistry, affected by boundary conditions whilst modeling only the small scales, including the interaction between the flow and the combustion processes. Here, we examine the flow and the flame in a model gas turbine combustor (General Electric’s lean premixed dry low NO_x LM6000) to evaluate the potential of LES for design studies of engineering applications and to study the effects of the combustor confinement geometry on the flow and on the flame dynamics. Two LES models, a Monotone Integrated LES model with 1 and 2 step Ahrrenius chemistry, and a fractal flame-wrinkling LES model coupled to a conventional one-equation eddy-viscosity subgrid model, are used. Reasonable agreement is found when comparing predictions with experimental data and with other LES computations of the same case. Furthermore, the combustor confinement geometry is found to strongly affect the vortical flow, and hence also the flame and its dynamics.     

Investigation of a wake decay behind a circular disk in a hydro channel at high Reynolds numbers

LDA and PIV techniques were used to study the decay of an axisymmetrical turbulent wake originated downstream an immobile disk in a water flume for Reynolds numbers Re = 1.5?2.4?10⁵. Data were compared with experiments performed with a set of thermo-anemometers behind a disk in a wind tunnel at lower Reynolds numbers (Re = 1.3?2.6?10⁴). Observations for a new range of Reynolds number confirmed that the velocity distribution in the disk wake keeps self-similarity. The decay of a wake by the power law ?2/3 in the presented experiments remains until the maximum deficit of velocity becomes comparable with the turbulent pulsation level in the free-stream (less than 2 %).     

The fluctuation property of blown sand particles and the wind-sand flow evolution studied by numerical method

Sand particles blown by wind cause serious environmental problems and many researchers are trying to understand the dynamic properties of blown sand better. But the existing numerical approaches have not been able to simulate many important characteristics of wind-sand flow. In this paper, the evolution and fluctuation properties of blown sand at a dynamic steady state are investigated by using a more effective method. Using the LES (large eddy simulation) method for air phase movement and the DEM (discrete element method) for solid phase movement along with the existing particle-bed splashing function, we have characterized the whole movement property of the wind-sand system. The results indicate that the saturation time decreases with the inlet friction velocity, and it gradually reaches the shortest saturation time of about 1s; the saturation length, which is about 14 m at the usual wind velocity, first increases with wind velocity and then reaches a plateau; within the saturation length, the sand transport rate at different positions varies with time; the sand transport rate of the stable wind-sand flow is non-uniform with distance downwind and time, and has a notable correlation with the inflow friction velocity.     

Wake flow in a wind farm during a diurnal cycle

The atmospheric boundary layer (ABL) undergoes substantial changes in its structure and dynamics in the course of a day due to the transient nature of forcing factors such as the surface fluxes of heat and momentum. The non-stationary nature of the mean wind and turbulence in the ABL, associated with the diurnal cycle, can in turn affect the structure of wind turbine wakes and their effects on power losses within wind farms. In this research, large-eddy simulation is used to study the evolution of the turbine wakes and their effects on power losses inside an idealised finite-size wind farm in the course of a full diurnal cycle. The simulation results show a strong effect of atmospheric stability on the wind farm wakes and associated power losses. During the day, the positive buoyancy flux and associated turbulence production lead to a relatively high turbulence level in the background ABL flow, which enhances turbulent mixing and wake recovery. In contrast, during the night, the relatively low turbulence intensity of the ambient ABL flow results in a relatively slow rate of entrainment of momentum into the wake and, consequently, a slow wake recovery. As a result, the averaged power deficit in the wind farm is found to increase with increasing thermal stability. In particular, the averaged power deficit increased from 28% under the most convective condition to about 57% under the most stable condition. It is also found that the low-level jet formed before dawn is weakened owing to the energy extraction by the turbines and deflected upward over the farm. In addition, strong vertical wind veer at that time, associated with the Coriolis force and shallow boundary layer, causes important lateral wake stretching effects, which in turn significantly impacts the performance of the waked turbines.     

Vortex identification in the wake of a model wind turbine array

Vortex identification techniques are used to analyse the wake of a 4 × 3 array of model wind turbines. The Q-criterion, Δ-criterion, and λ₂-criterion are applied to particle image velocimetry data gathered fore and aft of the last row centerline turbine. The Q-criterion and λ₂-criterion provide a clear indication of regions where vortical activity exists, while the Δ-criterion is not successful. Galilean decomposition, Reynolds decomposition, vorticity, and swirling strength are used to further understand the location and behaviour of the vortices. The techniques identify and display the high-magnitude vortices in high-shear zones resulting from the blade tips. Using Galilean and Reynolds decomposition, swirling motions are shown encapsuling vortex regions in agreement with the identification criteria. The Galilean decompositions selected are 20% and 50% of a convective velocity of 7 m/s. As the vortices convect downstream, the strength of the vortices decreases in magnitude, particularly in the far wake of the array, to approximately 25% of those present in the near wake. A high level of vortex activity is visualised as a result of the top tip of the wind turbine blade -- the location where the highest vertical entrainment is present. Analysing the full frame set, the Q-criterion, λ₂-criterion, and swirling strength prove comparable, while the Δ-criterion under-performs in regions of high turbulence activity, namely in the back of the turbine. Entraining flow into the turbine canopy interacting with the turbine generates high-magnitude vortices concentrated at the blade tips. The count of vortices decreases when moving from the top tip down to the wall, as well as their strength for each Galilean technique when a non-zero threshold is applied. Vortex sizes in the near wake are found comparable to turbine blade, hub, and mast dimensions. In the far wake, the resulting size of the vortices is approximately 30% of those in the near wake. These vortices increase in velocity as they convect downstream, following the mean velocity behaviour. The lowest magnitude vortices reside at the hub height in the near-wake region, where they convect at nearly half the speed of those at the blade tips.     

大型风力机叶片非线性气弹模型

大型柔性叶片在实际运行过程中存在明显的几何非线性变形问题.本文基于自由涡尾迹方法,与FAST进行耦合建立了完善的风力机叶片气弹模型.并通过NREL Phase Ⅵ风力机对气动部分进行了验证,最后用耦合后的气弹模型对两种极端工况进行了数值仿真.结果表明,自由涡尾迹方法能够更好的模拟风力机叶片的气动变化过程.耦合后的气弹模...     

Characterising the near-wake of a cross-flow turbine

The performance and detailed near-wake characteristics of a vertical axis, cross-flow turbine (CFT) of aspect ratio 1 were measured in a large cross-section towing tank. The near-wake at one turbine diameter downstream was examined using acoustic Doppler velocimetry, where essential features regarding momentum, energy, and vorticity are highlighted. Dominant scales and their relative importance were investigated and compared at various locations in the measurement plane. Estimates for the terms in the mean streamwise momentum and mean kinetic energy equation were computed, showing that the unique mean vertical velocity field of this wake, characterised by counter-rotating swirling motion, contributes significantly more to recovery than the turbulent transport. This result sheds light on previous CFT studies showing relatively fast downstream wake recovery compared to axial-flow turbines. Finally, predictions from a Reynolds-averaged Navier–Stokes simulation with the commonly used actuator disk model were compared with the experimental results, evaluating its use as an engineering tool for studying flow in CFT arrays. Unsurprisingly, the model was not able to predict the near-wake structure accurately. This comparison highlights the need for improved parameterised engineering models to accurately predict the near-wake physics of CFTs.     

尺度自适应模拟和大涡模拟的关联性分析

采用理论分析和数值模拟相结合的方法,系统研究了尺度自适应模拟（scale-adaptive simulation,SAS）和大涡模拟（large-eddy simulation,LES）的关联性问题.在理论分析方面,对比分析了系综平均和滤波的定义、Spalart-Allmaras（SA）湍流模型和动态亚格子（subgrid-scale,SGS）模型关于湍流黏性系数的求解方式.理论分析结果表明,系综平均等价于盒式直接滤波,SAS和LES的控制方程在数学形式上具有一致性;SAS存在过多的湍流耗散,主要来自于SA输运方程中的扩散项.在数值模拟方面,选取来流Mach数0.55,Reynolds数2×105的圆柱可压缩绕流为分析算例.计算结果表明,SAS和LES预测的大尺度平均流场信息几乎一致,SAS预测的湍流脉动信息略低于LES.SAS在圆柱近尾迹区的湍流耗散过大,而在稍远的尾迹区几乎能够完全等效于LES.      

Flow diagnostics downstream of a tribladed rotor model

This paper presents results of a study of vortex wake structures and measurements of instantaneous 3D velocity fields downstream of a triblade turbine model. Two operation modes of flow around the rotor with different tip speed ratios were tested. Initially the wake structures were visualized and subsequently quantitative data were recorded through velocity field restoration from particle tracks using a stereo PIV system. The study supplied flow diagnostics and recovered the instantaneous 3D velocity fields in the longitudinal cross section behind a tribladed rotor at different values of tip speed ratio. This set of data provided a basis for testing and validating assumptions and hypothesis regarding classical theories of rotors.     

Large eddy simulations and parameterisation of roughness element orientation and flow direction effects in rough wall boundary layers

We conduct a series of large eddy simulations (LES) of turbulent boundary layers over arrays of cuboidal roughness elements at arbitrary orientation angles (non-frontal orientations with the incident flow). Flow response to changing roughness orientation is systematically studied at two ground coverage densities, λ_p = 0.06 and 0.11. As expected, the effective roughness heights z_o measured from LES are higher for λ_p = 0.11 than for λ_p = 0.06, although appreciable changes both in z_o and wall shear stress (friction velocity) are observed at both ground coverage densities as the roughness orientation angle changes. This suggests the necessity of accounting for detailed rough wall topology (including more information than just λ_p, λ_f) when relating rough wall morphology to its aerodynamic properties. To this end, a recently developed analytical rough wall parameterisation is used to predict the aerodynamic properties of the simulated rough surfaces. In this rough wall model, wake interactions among roughness elements are explicitly modelled using the concept of sheltering height and exponential attenuation coefficient. As a result, the parameterisation is responsive to detailed ground roughness arrangements and flow conditions, including roughness height variations, element orientation, incident flow direction, transverse displacements, etc. Model-predicted effective roughness heights, wall stress, mean velocity at the height of the roughness, and in some cases displacement height, are compared against the LES measurements from this study as well as numerical/experiment measurements from other authors. The predictions from the model are found to agree well with the measurements both in trends and in absolute values, thus extending the applicability of the analytical rough wall model to more general surfaces than those previously tested.     

Self-similarity of far wake behind tandem of two disks

In this work we used digital particle image visualization (PIV) to experimentally establish the self-similarity of far wake behind a tandem of two disks of a diameter D (300 mm) with a common axis along the incident flow. The research was performed in a water flume (Re ≈ 2 · 10⁵) with variation of L, the longitudinal dimension of the tandem. The self-similarity of the velocity profile in the wake behind the tandem has been established; the level of turbulent fluctuations of the profile has been measured. Due to the influence of the second disk, the velocity deficit in the wake behind the tandem exceeded the corresponding value for a single disk, being independent of the distance between the disks (L = 4–8D). The velocity fluctuations behind the tandem did not differ much from the level of fluctuations in the case of a single disk up to a distance of forty calibers downstream, where the wake ceased to differ from the background of natural turbulent fluctuations of the incident flow. It has been found that the position of the second disk in the tandem affects the energy loss in the wake due to its expansion but does not influence the decay. The revealed patterns in the wake development behind tandems of bodies will enable optimization of construction of systems of repetitive elements and their movement in different flows.     

Impact of subgrid fluid turbulence on inertial particles subject to gravity

Two-phase turbulent flows with the dispersed phase in the form of small, spherical particles are increasingly often computed with the large-eddy simulation (LES) of the carrier fluid phase, coupled to the Lagrangian tracking of particles. To enable further model development for LES with inertial particles subject to gravity, we consider direct numerical simulations of homogeneous isotropic turbulence with a large-scale forcing. Simulation results, both without filtering and in the a priori LES setting, are reported and discussed. A full (i.e. a posteriori) LES is also performed with the spectral eddy viscosity. Effects of gravity on the dispersed phase include changes in the average settling velocity due to preferential sweeping, impact on the radial distribution function and radial relative velocity, as well as direction-dependent modification of the particle velocity variance. The filtering of the fluid velocity, performed in spectral space, is shown to have a non-trivial impact on these quantities.     

Experimental investigation of wake evolution behind a couple of flat discs in a hydrochannel

The decay of a far wake and its turbulent fluctuations behind two thin discs of the same diameter D, oriented normal to the incident flow, have been studied using the Particle Image Velocimetry (PIV). The experimental study was carried out in a water flume (Re ≈ 2·10⁵) with varying distances between the discs (L _х = 4–8D) and their axes shift relative to each other (0, 0.5D and 1D). It is found that the velocity deficit behind two discs depends weakly on L _x , and at L _х > 40D, it becomes indistinguishable from the level of turbulent fluctuations of the incident flow. It is found that the decay of the average velocity deficit and its turbulent fluctuations in a wake of a tandem of discs can be described by the same analytical dependence with exponent–2/3 as for the wake decay of a single disc. However, at the same distance downstream, the value of deficit behind two discs is substantially higher than the corresponding value behind a single disc. Velocity fluctuations in a far wake behind a pair of discs depend weakly on longitudinal dimension L _x , but at the same time, in contrast to the velocity deficit, their level does not differ significantly from the level of fluctuations behind a single disc.     

On the effect of an anisotropy-resolving subgrid-scale model on large eddy simulation predictions of turbulent open channel flow with wall roughness

A large eddy simulation (LES) was conducted of turbulent flow in a channel with a rough wall on one side and a free surface on the other by adopting an anisotropy-resolving subgrid-scale (SGS) model. A shear Reynolds number of Re_τ = 395 was used based on the mean friction velocity and channel height. To investigate the grid dependency of the LES results caused by the SGS model, three grid resolutions were tested under the same definition of a roughness shape by using the immersed boundary method. The results obtained were compared with direct numerical simulation data with and without the wall roughness and those without the extra anisotropic term. The primary focus was on how the present anisotropic SGS model with coarser grid resolutions can properly provide the effects of roughness on the mean velocity and turbulent stresses, leading to a considerable reduction of the computational cost of LES.     

风电场地形绕流的CFD结果确认研究

掌握复杂地形区域的风能分布对风电场微观选址至关重要.为把CFD技术应用于风场中实际地形的风流动模拟,本文选择三种具有实验数据的典型地形进行数值计算,并对数值结果进行确认研究.文中简述了不同网格和不同湍流模型对旋涡位置和速度分布的影响,并探讨了为增加风能利用率,如何确定风力机的最佳安装位置.