Large‐eddy simulation of turbulent flow past wind turbines/farms: the Virtual Wind Simulator (VWiS)

A large‐eddy simulation framework, dubbed as the Virtual Wind Simulator (VWiS), for simulating turbulent flow over wind turbines and wind farms in complex terrain is developed and validated. The wind turbines are parameterized using the actuator line model. The complex terrain is represented by the curvilinear immersed boundary method. The predictive capability of the present method is evaluated by simulating two available wind tunnel experimental cases: the flow over a stand‐alone turbine and an aligned wind turbine array. Systematic grid refinement studies are carried out, for both single turbine and multi‐turbine array cases, and the accuracy of the computed results is assessed through detailed comparisons with wind tunnel experiments. The model is further applied to simulate the flow over an operational utility‐scale wind farm. The inflow velocities for this case are interpolated from a mesoscale simulation using a Weather Research and Forecasting (WRF) model with and without adding synthetic turbulence to the WRF‐computed velocity fields. Improvements on power predictions are obtained when synthetic turbulence is added at the inlet. Finally the VWiS is applied to simulate a yet undeveloped wind farm at a complex terrain site where wind resource measurements have already been obtained. Good agreement with field measurements is obtained in terms of the time‐averaged streamwise velocity profiles. To demonstrate the ability of the model to simulate the interactions of terrain‐induced turbulence with wind turbines, eight hypothetical turbines are placed in this area. The computed extracted power underscores the significant effect of site‐specific topography on turbine performance. Copyright © 2014 John Wiley & Sons, Ltd.     

Experimental validation of a computational fluid dynamics code to predict the wind speed in street canyons for passive cooling purposes

Natural ventilation is one of the most efficient passive cooling techniques for buildings. Knowledge of the wind speed in street canyons is the necessary condition for the application of such a technique in dense urban configurations. Thus, prediction techniques to evaluate the microclimate and dispersion parameters in street canyons, has become a subject of intense scientific research in recent years. In most of cases, wind flow and pollutant dispersion characteristics have been studied numerically and experimentally with a view to provide an insight in urban dispersion. The microscale model MIMO was employed in order to perform a three-dimensional modelling of the wind field within three typical deep street canyons, located in the centre of Athens. Computational results were compared to field data collected during consecutive three-day experimental campaigns that took place in the summer period. Results from the computations have shown that the wind field in urban areas is quite complex, presenting areas of very low wind speeds and convergence of vortices. The model underestimated the measured wind speed intensities, which may be partly explained by the uncertainty of specific input parameters, the necessary simplifications for the application of such models and finally the geometrical complexity of the area modelled. Finally, computations were performed for a reference velocity of 2 m/s, which is considered to be a threshold value. The wind field developed departed from the one observed in the previous cases.     

A linearized numerical model of wind‐farm flows

A fast and reasonably accurate numerical three‐dimensional wake model able to predict the flow behaviour of a wind farm over a flat terrain has been developed. The model is based on the boundary‐layer approximation of the Navier–Stokes equations, linearized around the incoming atmospheric boundary layer, with the assumption that the wind turbines provide a small perturbation to the velocity field. The linearization of the actuator‐disc theory brought additional insights that could be used to understand the behaviour, as well as the limitations, of a flow model based on linear methods: for instance, it is shown that an adjustment of the turbine's thrust coefficient is necessary in order to obtain the same wake velocity field provided by the actuator disc theory within the used linear framework. The model is here validated against two independent wind‐tunnel campaigns with a small and a large wind farm aimed at the characterization of the flow above and upstream of the farms, respectively. The developed model is, in contrary to current engineering wake models, able to account for effects occurring in the upstream flow region, thereby including more physical mechanisms than other simplified approaches. The conducted simulations (in agreement with the measurement results) show that the presence of a wind farm affects the approaching flow far more upstream than generally expected and definitely beyond the current industrial standards. Despite the model assumptions, several velocity statistics above wind farms have been properly estimated providing an insight into the transfer of momentum inside the turbine rows. Copyright © 2016 John Wiley & Sons, Ltd.     

Variable‐sized wind turbines are a possibility for wind farm optimization

The potential benefits associated with harnessing available momentum and reducing turbulence levels in a wind farm composed of wind turbines of alternating size are investigated through wind tunnel experiments. A variable size turbine array composed of 3 by 8 model wind turbines is placed in a boundary layer flow developed over both a smooth and rough surfaces under neutrally stratified thermal conditions. Cross‐wire anemometry is used to capture high resolution and simultaneous measurements of the streamwise and vertical velocity components at various locations along the central plane of the wind farm. A laser tachometer is employed to obtain the instantaneous angular velocity of various turbines. The results suggest that wind turbine size heterogeneity in a wind farm introduces distinctive flow interactions not possible in its homogeneous counterpart. In particular, reduced levels of turbulence around the wind turbine rotors may have positive effects on turbulent loading. The turbines also appear to perform quite uniformly along the entire wind farm, whereas surface roughness impacts the velocity recovery and the spectral content of the turbulent flow within the wind farm. Copyright © 2013 John Wiley & Sons, Ltd.     

垂直轴风力机三维效应的数值模拟研究

文章针对二维和三维垂直轴风力机的数值模拟的差异,提出了风力机的三维效应是造成模拟差异的主要原因。运用计算流体力学方法对某直线翼垂直轴风力机模型进行了二维和三维的数值模研究。通过比对实验得到的风力机功率系数,发现三维模拟结果与实验值吻合。观察尖速比为1.5时二维和三维垂直轴风力机的速度型分布曲线、流向速度云图和涡量云图,研究了阻塞效应、叶梢涡、支撑结构和塔架对数值模拟结果的影响。研究发现:在二维的数值模拟中,风力机没有受阻塞效应影响,功率系数被严重高估;三维的数值模拟能够模拟出全部的流畅细节,受叶梢涡和支撑结构的影响,风力机的功率系数明显降低。     

Real‐time wind field reconstruction from LiDAR measurements using a dynamic wind model and state estimation

The use of light detection and ranging (LiDAR) instruments offer many potential benefits to the wind energy industry. Although much effort has been invested in developing such instruments, the fact remains that they provide limited spatio‐temporal velocity measurements of the wind field. Moreover, LiDAR measurements only provide the radial (line‐of‐sight) velocity component of the wind, making it difficult to precisely determine wind magnitude and direction, owing to the so‐called ‘cyclops’ dilemma. Motivated by a desire to extract more information from typical LiDAR data, this paper aims to show that it is possible to accurately estimate, in a real‐time fashion, the radial and tangential velocity components of the wind field. We show how such reconstructions can be generated through the synthesis of an unscented Kalman filter that employs a low‐order dynamic model of the wind to estimate the unmeasured velocities within the wind field, using repeated measurement updates from typical nacelle‐mounted LiDAR instruments. This approach is validated upon synthetic data generated from large eddy simulations of the atmospheric boundary layer. The accuracy of the wind field estimates are validated across a variety of beam configurations, look directions, atmospheric stabilities and imperfect measurement conditions. The main outcome of this paper is a technique that offers the potential to accurately reconstruct wind fields from LiDAR data, overcoming the cyclops dilemma in the process. The ultimate aim of this research is to provide reliable gust detection warning systems to offshore construction workers, in addition to accurate wind field estimates for use in preview turbine pitch control systems. © 2014 The Authors. Wind Energy published by John Wiley & Sons Ltd.     

A telescopic local grid refinement technique for wind flow simulation over complex terrain

A telescopic local grid refinement technique is developed in order to enhance the accuracy level of wind field predictions in a subregion of a complex topography. A 3D simulation of the wind flow field over complex terrains has been carried out. The governing Navier–Stokes conservation equations of the flow field are solved numerically in a three‐dimensional generalized curvilinear non‐orthogonal grid, using Cartesian velocity components, following the finite volume approximation and a pressure correction method. Turbulence is simulated by a two‐equation transport model. The reliability of the general flow solver is first tested by simulating the flow past a cube. The second test case simulated is the flow over the Askervein hill, with a detailed comparison of predicted and measured velocities. The third case presented concerns flow field simulation over a complete island. Comparison with measurements reveals the significance of the accurate discretization of the topography and the use of telescopic meshes on the results. Copyright © 2001 John Wiley & Sons, Ltd.     

An improved k‐ ϵ model applied to a wind turbine wake in atmospheric turbulence

An improved k‐ ? turbulence model is developed and applied to a single wind turbine wake in a neutral atmospheric boundary layer using a Reynolds averaged Navier–Stokes solver. The proposed model includes a flow‐dependent C_μ that is sensitive to high velocity gradients, e.g., at the edge of a wind turbine wake. The modified k‐ ? model is compared with the original k‐ ? eddy viscosity model, Large‐Eddy Simulations and field measurements using eight test cases. The comparison shows that the velocity wake deficits, predicted by the proposed model are much closer to the ones calculated by the Large‐Eddy Simulation and those observed in the measurements, than predicted by the original k‐ ? model. Copyright © 2014 John Wiley & Sons, Ltd.     

Numerical simulations of fatigue loads on wind turbines operating in wakes

Wake effects increase the fatigue loads on wind turbines in operation. However, the wake flow is considerably different from the traditional boundary layer flow, and poses many challenges in determining the fatigue loads on wind turbines operating in a wake. Therefore, in the present study, the actuator‐line model was adopted to numerically simulate the wake flow and an in‐house code named AOWT, which is based on a generalized coordinate method, was developed for analyzing the dynamics of wind turbines under an arbitrary distribution of the turbulent flow field varying in time and space. Using the numerically modeled instantaneous wake flow fields and AOWT, the dynamic response of a wind turbine, located at specified positions in both tandem and staggered arrangements in a wake, was examined, and the fatigue loads were determined. Furthermore, to determine the major contributions to the fatigue loads, the loads induced by the spatial variation of the mean flow fields were predicted. To the best of the authors' knowledge, no such analysis has been conducted thus far. Importantly, it was found that in the near‐wake region, the mean flow field had a significant influence on the fatigue loads, especially in the staggered layout. However, there is no analytical wake model available in the literature capable of predicting the near‐wake mean flow fields. Therefore, in this study, a near‐wake model was proposed, which yielded satisfactory predictions of the mean velocities in the near‐wake region.     

Effect of Inlet Window Deflectors on the Performance of a Natural-Draft Cooling Tower Subjected to Crosswinds

The effect of inlet window deflectors on the performance of a natural draft cooling tower subjected to a crosswind was studied numerically. Three patterns of the inlet window deflectors with different setting angles (62, 72, and 82 degrees) were tested under various crosswind velocities. The assumption of heat and mass transfer equivalence was adopted for simulation. A numerical model was developed and verified by comparison with experimental data. The simulations show that the 72-degree inlet window deflectors are efficient in a wide range of crosswind velocities. The effect of the number of deflectors was also investigated and the link between the velocity profile and the critical wind velocity was clarified. It was shown that the use of deflectors in the inlet window increases the thermal efficiency of a cooling tower up to 8.6% at wind velocities greater than 2 m/s and does not exhibit any effect at the wind velocities close to critical wind velocity. The investigation may be helpful in optimal design of natural draft and “hybrid” cooling towers.     

动态风工况下复杂地形风电场流场多尺度仿真

首先在秒级风速数据的基础上构建动态风速函数模拟真实风速工况,同时基于高程数据构建某真实复杂地形的三维结构图。基于格子玻尔兹曼方法并结合自适应格子排布,对复杂地形风电场非定常流场进行数值计算,得到该风电场的风资源分布。之后在典型位置布置2台2 MW风力发电机,考虑真实风力机叶片的动态旋转计算风力机及真实复杂地形在动态风工况下的流场。研究实际复杂地形和动态风速下风电场的风速分布及尾流结构演变规律。结果表明：该方法可实现对复杂地形在动态来流风速作用下的风资源分布预测,并考虑风力机小尺度尾流结构实现对真实风电场流场的多尺度仿真。     

Investigation of Electrohydrodynamically-Enhanced Convective Heat and Mass Transfer from Water Surface

Enhancement of forced flow evaporation rate by applying electric field (corona wind) has been experimentally evaluated in this study. Corona wind produced by a fine wire electrode which was charged with positive high DC voltage impinges to water surface and leads to evaporation enhancement by disturbing the saturated air layer over the water surface. The study was focused on the effects of corona wind velocity, electrode spacing and air flow velocity on the level of evaporation enhancement. Two sets of experiments, i.e., with and without electric field, have been conducted. Data obtained from the first experiment were used as reference for evaluation of evaporation enhancement at the presence of electric field. Applied voltages ranged from corona threshold voltage to spark over voltage at 1 kV increments. The results showed that corona wind has great enhancement effect on the water evaporation rate, but its effectiveness gradually diminishes by increasing air flow velocity. Maximum enhancement ratios were 7.3 and 3.6 for air velocities of 0.125 and 1.75 m/s, respectively. Finally two empirical correlations were obtained for prediction of electrohydrodynamic evaporation enhancement and its coefficient of thermal performance.     

Parabolic RANS solver for low‐computational‐cost simulations of wind turbine wakes

A numerical framework for simulations of wake interactions associated with a wind turbine column is presented. A Reynolds‐averaged Navier‐Stokes (RANS) solver is developed for axisymmetric wake flows using parabolic and boundary‐layer approximations to reduce computational cost while capturing the essential wake physics. Turbulence effects on downstream evolution of the time‐averaged wake velocity field are taken into account through Boussinesq hypothesis and a mixing length model, which is only a function of the streamwise location. The calibration of the turbulence closure model is performed through wake turbulence statistics obtained from large‐eddy simulations of wind turbine wakes. This strategy ensures capturing the proper wake mixing level for a given incoming turbulence and turbine operating condition and, thus, accurately estimating the wake velocity field. The power capture from turbines is mimicked as a forcing in the RANS equations through the actuator disk model with rotation. The RANS simulations of the wake velocity field associated with an isolated 5‐MW NREL wind turbine operating with different tip speed ratios and turbulence intensity of the incoming wind agree well with the analogous velocity data obtained through high‐fidelity large‐eddy simulations. Furthermore, different cases of columns of wind turbines operating with different tip speed ratios and downstream spacing are also simulated with great accuracy. Therefore, the proposed RANS solver is a powerful tool for simulations of wind turbine wakes tailored for optimization problems, where a good trade‐off between accuracy and low‐computational cost is desirable.     

Large‐eddy simulations of wind‐farm wake characteristics associated with a low‐level jet

In this study, we performed a suite of flow simulations for a 12‐wind‐turbine array with varying inflow conditions and lateral spacings, and compared the impacts of the flow on velocity deficit and wake recovery. We imposed both laminar inflow and turbulent inflows, which contain turbulence for the Ekman layer and a low‐level jet (LLJ) in the stable boundary layer. To solve the flow through the wind turbines and their wakes, we used a large‐eddy simulation technique with an actuator‐line method. We compared the time series for the velocity deficit at the first and rear columns to observe the temporal change in velocity deficit for the entire wind farm. The velocity deficit at the first column for LLJ inflow was similar to that for laminar inflow. However, the magnitude of velocity deficit at the rear columns for the case with LLJ inflow was 11.9% greater because of strong wake recovery, which was enhanced by the vertical flux of kinetic energy associated with the LLJ. To observe the spatial transition and characteristics of wake recovery, we performed statistical analyses of the velocity at different locations for both the laminar and LLJ inflows. These studies indicated that strong wake recovery was present, and a kurtosis analysis showed that the probability density function for the streamwise velocity followed a Gaussian distribution. In a quadrant analysis of the Reynolds stress, we found that the ejection and sweep motions for the LLJ inflow case were greater than those for the laminar inflow case.     

PIV measurements of flows around the wind turbines with a flanged-diffuser shroud

The wind turbines with a flanged-diffuser shroud -so called ＂wind lens turbine＂- are developed as one of high performance wind turbines by Ohya et al. In order to investigate the flow characteristics and flow acceleration, the paper presents the flow velocity measurements of a long-type and a compact-type wind turbines with a flanged-diffuser shroud by particle image velocimetry. In the case of the long type wind turbine, the velocity vec- tors of the inner flow field of the diffuser for turbine blades rotating and no blades rotating are presented at Rey- nolds number, 0.9x105. Fur~thermore the flow fields between with and without rotating are compared. Through the PIV measurement results, one can realize that the turbine blades rotating affects as suppress the disturbance and the flow separation near the inner wall of the diffuser. The time average velocity vectors are made on the av- erage of the instantaneous velocity data. There are two large vortices in downstream region of the diffuser. One vortex behind the flange acts as suck in wind to the diffuser and raise the inlet flow velocity. Another large vortex appears in downstream. It might be act as blockage vortex of main flow. The large blockage vortex is not clear in the instantaneous velocity vectors, however it exists clearly in the time average flow field. The flow field around the wind turbine with a compact-type flanged-diffuser shroud is also investigated. The flow pattern behind the flange of the compact-type turbine is the same as the long-type one. It means that the effect of flow acceleration is caused by the unsteady vortices behind the flange. The comparison with CFD and PIV results of meridional time-average streamlines after the compact-type diffuser is also presented.     

侧向风对自然通风直接空冷塔性能影响的数值分析

采用多孔介质简化模型分析了在不同风速情况下300 MW自然通风直接空冷系统空冷塔内的流场和各凝汽器换热量.结果表明:在没有侧向风时,流场是均匀对称的,此时塔内空气流量最大,而各凝汽器换热量均匀;当有侧向风时,由于空冷塔底部气流存在漩涡,使空冷塔内空气回流,空气流量减小;在风速大于4 m/s时,各凝汽器换热量差异较大,随着风速增加,这种现象加剧.     

Testing basic performance of a very small wind turbine designed for multi-purposes

A very small wind turbine system for multi-purposes was developed and its performance was reported in this paper. The rotor diameter of the turbine is 500 mm. The tests of the energy output, turbine speed, power coefficient, and torque of turbine were carried out for a wide rage of free stream velocity. The flow around the wind turbine and the influence of the turbulence were investigated with a particle image velocimetry. Experimentally obtained power coefficient was 0.4 in maximum and 0.36 in the rated running condition, respectively. The tip speed ratio corresponding to the optimum driving condition was 2.7. Comparing with the other commercial turbines, the performance was excellent at a slow turbine speed. By the flow visualization and PIV measurement around the wind turbine, the approaching flow velocity and the accelerated flow field passing the blade tip was obtained. It was confirmed that the actual flow passed through the blades was about 20% slower than the ideal flow. Tip vortex shed from the blade tip was also visualized clearly.     

The spectral signature of wind turbine wake meandering: A wind tunnel and field‐scale study

Field‐scale and wind tunnel experiments were conducted in the 2D to 6D turbine wake region to investigate the effect of geometric and Reynolds number scaling on wake meandering. Five field deployments took place: 4 in the wake of a single 2.5‐MW wind turbine and 1 at a wind farm with numerous 2‐MW turbines. The experiments occurred under near‐neutral thermal conditions. Ground‐based lidar was used to measure wake velocities, and a vertical array of met‐mounted sonic anemometers were used to characterize inflow conditions. Laboratory tests were conducted in an atmospheric boundary layer wind tunnel for comparison with the field results. Treatment of the low‐resolution lidar measurements is discussed, including an empirical correction to velocity spectra using colocated lidar and sonic anemometer. Spectral analysis on the laboratory‐ and utility‐scale measurements confirms a meandering frequency that scales with the Strouhal number St = fD/U based on the turbine rotor diameter D. The scaling indicates the importance of the rotor‐scaled annular shear layer to the dynamics of meandering at the field scale, which is consistent with findings of previous wind tunnel and computational studies. The field and tunnel spectra also reveal a deficit in large‐scale turbulent energy, signaling a sheltering effect of the turbine, which blocks or deflects the largest flow scales of the incoming flow. Two different mechanisms for wake meandering—large scales of the incoming flow and shear instabilities at relatively smaller scales—are discussed and inferred to be related to the turbulent kinetic energy excess and deficit observed in the wake velocity spectra.     

Effects of operating parameters on transport phenomena and cell performance of PEM fuel cells with conventional and contracted flow field designs

A contracted parallel flow field design was developed to improve fuel cell performance compared with the conventional parallel flow field design. A three-dimensional model was used to compare the cell performance for both designs. The effects of the cathode reactant inlet velocity and cathode reactant inlet relative humidity on the cell performance for both designs were also investigated. For operating voltages greater than 0.7 V because the electrochemical reaction rates are lower with less oxygen consumption and less liquid water production, the cell performance is independent of the flow field designs and operating parameters. However, for lower operating voltages where the electrochemical reaction rates gradually increase, the oxygen transport and the liquid water removal efficiency differ for the various flow field designs and operating parameters; therefore, the cell performance is strongly dependent on both the design and operating parameters. For lower operating voltages, the cell performance for the contracted design is better than for the conventional design because the reactant flow velocities in the contracted region significantly increase, which enhances liquid water removal and reduces the oxygen transport resistance. For lower operating voltages, as the cathode reactant inlet velocity increases and the cathode reactant inlet relative humidity decreases, the cell performance for both designs improves.