On the dependence of an empty flanged diffuser performance on flange height: Numerical simulations and PIV visualizations

Flanged diffuser shrouding small wind turbine, is among the most tested devices for increasing wind energy. The height of the flange is between the geometric futures of the diffuser that contributes efficiently in improving diffuser performances. Results obtained from numerical simulations and PIV visualizations show that when a flange is mounted at the outlet area of the diffuser, two contra-rotating vortices were created at this location. These two vortices move away from each other in the flow direction as the flange height increases and they seem to lengthen in the streamwise direction and to extend in the two directions when the flange height becomes taller. A critical ratio (Flange height/Inlet section diffuser diameter = 0.1) has been found. Beyond this value, due to the remoteness of vortices from the flange, the flange height seems to be without significant effect on increasing wind velocity.     

水平轴风力机叶尖翼型截面流场的试验研究

利用线式互相关粒子图像测速(PIV)系统和轴编码器锁相技术,测量了不同尖速比下旋转水平轴风力机叶尖处的流场,获得了风轮叶尖处的瞬时速度场,并通过Tecplot软件处理得到了相应的时均速度场、速度云图及流线图.对瞬时图分析可知:在同一尖速比下,叶尖涡的出现使速度亏损值增加,风轮功率下降;在高尖速比下,流场中叶尖涡出现频率较高,有明显的旋涡结构出现,但随着尖速比的减小,叶尖涡出现的频率降低,只有形成的趋势,在尖速比λ-4的工况下,没有完整的涡出现.通过分析时均图表明:随着尖速比的增加,上游空气通过叶尖后的动能损失增大,叶尖尾迹区内的速度亏损范围增大.     

Characterization of aerodynamic performance of wind-lens turbine using high-fidelity CFD simulations

Wind-lens turbines (WLTs) exhibit the prospect of a higher output power and more suitability for urban areas in comparison to bare wind turbines. The wind-lens typically comprises a diffuser shroud coupled with a flange appended to the exit periphery of the shroud. Wind-lenses can boost the velocity of the incoming wind through the turbine rotor owing to the creation of a low-pressure zone downstream the flanged diffuser. In this paper, the aerodynamic performance of the wind-lens is computationally assessed using high-fidelity transient CFD simulations for shrouds with different profiles, aiming to assess the effect of change of some design parameters such as length, area ratio and flange height of the diffuser shroud on the power augmentation. The power coefficient (C_p) is calculated by solving the URANS equations with the aid of the SST k–ω model. Furthermore, comparisons with experimental data for validation are accomplished to prove that the proposed methodology could be able to precisely predict the aerodynamic behavior of the wind-lens turbine. The results affirm that wind-lens with cycloidal profile yield an augmentation of about 58% increase in power coefficient compared to bare wind turbine of the same rotor swept-area. It is also emphasized that diffusers (cycloid type) of small length could achieve a twice increase in power coefficient while maintaining large flange heights.     

Development of low-noise drag-type vertical wind turbines

In this study, the aerodynamic noise characteristics of Savonius wind turbines were investigated using hybrid computational aero-acoustics techniques, and low-noise designs were proposed based on the understanding of the noise generation mechanism. First, the flow field around the turbine was analyzed in detail by solving three-dimensional unsteady incompressible Reynolds-averaged Navier–Stokes equations using computational fluid dynamics techniques. Then, the aerodynamic noise radiating from the wind turbine was predicted using the Ffowcs Williams and Hawkings equation with the obtained flow field information. Two distinct harmonic noise components—the blade passing frequency (BPF) and harmonics with a fundamental frequency that is much higher than the BPF—were identified in the predicted noise spectrum. The origin of the higher harmonic components was found to be related to vortex shedding from the rotating turbine. Based on this finding, the proposed low-noise design for Savonius wind turbines uses S-shaped blades. S-shaped blades were found to reduce the noise levels of Savonius wind turbines by up to 2.7 dB.     

Unsteady behavior of leading-edge vortex and diffuser stall in a centrifugal compressor with vaned diffuser

The characteristics of a rotating stall of an impeller and diffuser and the evolution of a vortex generated at the diffuser leading-edge (i.e., the leading-edge vortex (LEV)) in a centrifugal compressor were investigated by experiments and numerical analysis. The results of the experiments revealed that both the impeller and diffuser rotating stalls occurred at 55 and 25 Hz during off-design flow operation. For both, stall cells existed only on the shroud side of the flow passages, which is very close to the source location of the LEV. According to the CFD results, the LEV is made up of multiple vortices. The LEV is a combination of a separated vortex near the leading- edge and a longitudinal vortex generated by the extended tip-leakage flow from the impeller. Therefore, the LEV is generated by the accumulation of vorticity caused by the velocity gradient of the impeller discharge flow. In partial-flow operation, the spanwise extent and the position of the LEV origin are temporarily transmuted. The LEV develops with a drop in the velocity in the diffuser passage and forms a significant blockage within the diffuser passage. Therefore, the LEV may be regarded as being one of the causes of a diffuser stall in a centrifugal compressor.     

Performance evaluation of a downwind diffuser on vertical axis wind turbine

Research has proven that the performance of a horizontal axis wind turbine (HAWTs) can be increased significantly by the application of a diffuser. It serves as a power augmented feature to draw higher wind flow toward the HAWT. However, research on integrating a diffuser onto vertical axis wind turbines (VAWTs) is scant, where most of the available power augmentation devices used for VAWTs are the convergent duct, deflector plate, shroud, and guide vanes which are placed in a proper configuration at the upwind. In this paper, laboratory tests and computational simulations have been carried out to study the impacts of a downwind diffuser on the performance of a VAWT. The diffuser is designed with the absence of a concentrator or flange and is placed downwind of the VAWT. Parametric computational fluid dynamics (CFD) studies were carried out for the downwind diffuser length and semi-opening angle. A five-bladed H-rotor was selected as the testing wind turbine, whereas the diffuser used was made up of flat plates. Both simulations and experiment results are consistent. From the experiments, it was found that a downwind diffuser increases the VAWT performance remarkably. The diffuser-augmented VAWT produced an increment in the maximum coefficient of power of 31.42% at the TSR 0.65 to 0.75. Moreover, the diffuser induced a better self-start ability on the VAWT. The simulation showed that the flow field at the diffuser promotes a flow expansion which created a lower-pressure region at downstream that accelerates the wind toward the VAWT, hence increasing the turbine performance significantly.     

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.     

水平轴风力机风轮子午面流场结构的实验研究

利用线式互相关PIV系统,采用轴编码器定位周期采样技术,在不同尖速比下对旋转水平轴风力机风轮不同子午面下游流场结构进行测量.分析得到不同条件下的瞬时图、时均图,重点对叶尖涡诱导效应区进行研究.实验结果表明:在风轮下游尾迹中可清晰看到叶轮近尾迹流场中的外部主流区、叶尖涡诱导效应区和中心尾迹区.其中风轮下游尾迹流管廓线是锥形螺旋体;叶尖涡核直径随轴向距离的增加而增大,随着测试方位角的增加,尾迹中各叶片产生的叶尖涡沿螺旋锥形廓线有序地向下游扩散流动;随着尖速比的增加,内部中心尾迹区轴向速度亏损值逐渐增加,并且中心尾迹区的范围逐渐扩大.     

The wake structure in a 2D grid installation of the horizontal axis micro wind turbines

In order to develop applications for micro-wind turbines, an experimental analysis of the flow field around integrated micro-wind turbines was performed. The wake flows of a single turbine and 5×5 array unit were measured by using hot-wire and ultrasonic anemometers and particle image velocimetry (PIV). The present array of turbines follows a fundamental lattice layout; however, it has the flexibility to optimize its layout according to the environmental conditions. hot-wire and ultrasonic anemometers and PIV measurements were used for stand-alone turbines and their integrated systems. Comparisons between the mean velocity field and turbulent intensity were described for stand-alone full-scale and 1/10-scale wind turbine models. Thereafter, a typical array of the 1/10-scale model was assumed and its wake flow was investigated in a wind tunnel. The velocity profile and turbulence behind the array were measured and studied at different streamwise locations. The scale effect and model similarities were discussed. The experimental results show that a zone exists with constant and linear wake deficit ratios in the downstream regions.     

格尼襟翼垂直轴风力机风场研究

为研究垂直轴风力机风场中机组气动性能受格尼襟翼的影响,采用TSST湍流模型对直线翼垂直轴风力机进行数值模拟研究.结果表明:风场上游风力机组尖速比越大,机组间流体加速效果越显著,使风力机组气动性能高于单风力机;在中低尖速比时,格尼襟翼可有效提升单个风力机气动效率,在尖速比较高时,提升效果并不明显;在风力机组中安装格尼襟翼...     

New vortex‐lift and tangential‐force models for HAWT aerodynamic load prediction

Horizontal axis wind turbines (HAWTs) experience three‐dimensional rotational and unsteady aerodynamic phenomena at the rotor blades sections. These highly unsteady three‐dimensional effects have a dramatic impact on the aerodynamic load distributions on the blades, in particular, when they occur at high angles of attack due to stall delay and dynamic stall. Unfortunately, there is no complete understanding of the flow physics yet at these unsteady 3D flow conditions, and hence, the existing published theoretical models are often incapable of modelling the impact on the turbine response realistically. The purpose of this paper is to provide an insight on the combined influence of the stall delay and dynamic stall on the blade load history of wind turbines in controlled and uncontrolled conditions. New dynamic stall vortex and nonlinear tangential force coefficient modules, which integrally take into account the three dimensional rotational effect, are also proposed in this paper. This module along with the unsteady influence of turbulent wind speed and tower shadow is implemented in a blade element momentum (BEM) model to estimate the aerodynamic loads on a rotating blade more accurately. This work presents an important step to help modelling the combined influence of the stall delay and dynamic stall on the load history of the rotating wind turbine blades which is vital to have lighter turbine blades and improved wind turbine design systems.     

Three‐dimensional viscous‐inviscid coupling method for wind turbine computations

In this paper, a computational model for predicting the aerodynamic behavior of wind turbine wakes and blades subjected to unsteady motions and viscous effects is presented. The model is based on a three‐dimensional panel method using a surface distribution of quadrilateral sources and doublets, which is coupled to a viscous boundary layer solver. Unlike Navier‐Stokes codes that need to solve the entire flow domain, the panel method solves the flow around a complex geometry by distributing singularity elements on the body surface, obtaining a faster solution and making this type of codes suitable for the design of wind turbines. A free‐wake model has been employed to simulate the wake behind a wind turbine by using vortex filaments that carry the vorticity shed by the trailing edge of the blades. Viscous and rotational effects inside the boundary layer are taken into account via the transpiration velocity concept, applied using strip theory with the cross sectional angle of attack as coupling parameter. The transpiration velocity is obtained from the solution of the integral boundary layer equations with extension for rotational effects. It is found that viscosity plays a very important role in the predictions of blade aerodynamics and wake dynamics, especially at high angles of attack just before and after boundary layer separation takes place. The present code is validated in detail against the well‐known MEXICO experiment and a set of non‐rotating cases. Copyright © 2014 John Wiley & Sons, Ltd.     

吹气控制策略对垂直轴风力机气动性能的影响

为改善垂直轴风力机周围流场结构并提升气动性能,在风力机叶片中采用外吹式流动控制,并提出5种吹气控制策略,通过数值模拟的方法研究不同吹气控制策略对垂直轴风力机气动性能的影响,进一步分析在最佳吹气流动控制策略时的涡量场与载荷波动。研究表明:采用上开口抛物线控制策略时的风力机气动性能最佳,当最大吹气动量系数为0. 025时,风能利用系数及平均力矩系数提升26%,并可抑制大涡的形成及发展,同时改善翼型表面压力分布。     

A vortex model for Darrieus turbine using finite element techniques

Since 1970 several aerodynamic prediction models have been formulated for the Darrieus turbine. We can identify two families of models: stream-tube and vortex. The former needs much less computation time but the latter is more accurate. The purpose of this paper is to show a new option for modelling the aerodynamic behaviour of Darrieus turbines. The idea is to combine a classic free vortex model with a finite element analysis of the flow in the surroundings of the blades. This avoids some of the remaining deficiencies in classic vortex models. The agreement between analysis and experiment when predicting instantaneous blade forces and near wake flow behind the rotor is better than the one obtained in previous models.     

基于气动弹性理论的风力机叶片耦合分析

针对风力机叶片,建立其结构动力学方程,推导分析了叶片旋转所产生的振动速度及其对来流的影响。基于BEM(Blade Element Momentum)理论,在风力机空气动力学基础上,建立了风力机的气动耦合分析模型。应用该模型,对某2MW风力机进行了计算分析,得到了叶片在额定工作风速下的振动变形、速度、加速度以及叶片沿展向的变形和载荷分布。充分考虑叶片的结构振动特性与来流风速的耦合效应,使得风力机空气动力学特性模型更加准确,对于风力机的设计和分析具有重要意义。     

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。     

不同材质风力机叶片绕流流场特征变化PIV实验研究

考虑风力机叶片变形对绕流流场的影响,通过粒子图像测速（PIV）方法,采集两副翼型相同、材质分别玻璃聚酯（叶片Ⅰ）和玻璃聚醋内部填充泡沫（叶片Ⅱ）的水平轴风力机叶片绕流流场信息,对不同测试截面处流场数据进行分析。研究结果表明：相比于叶片Ⅰ,相同工况下,叶片Ⅱ的总体涡量、回流区的面积和时均速度更大,轴向雷诺应力更小;随着相对轴向距离的增加,时均速度在相对高度方向波动减小,材质对轴向雷诺应力影响逐渐减弱。实验工况下,叶片材质对绕流流场特征的影响大于叶尖速比对其的影响,并沿径向方向有增大的趋势。     

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.     

Comparison of the near‐wake between actuator‐line simulations and a simplified vortex model of a horizontal‐axis wind turbine

The flow around an isolated horizontal‐axis wind turbine is estimated by means of a new vortex code based on the Biot–Savart law with constant circulation along the blades. The results have been compared with numerical simulations where the wind turbine blades are replaced with actuator lines. Two different wind turbines have been simulated: one with constant circulation along the blades, to replicate the vortex method approximations, and the other with a realistic circulation distribution, to compare the outcomes of the vortex model with real operative wind‐turbine conditions (Tjæreborg wind turbine). The vortex model matched the numerical simulation of the turbine with constant blade circulation in terms of the near‐wake structure and local forces along the blade. The results from the Tjæreborg turbine case showed some discrepancies between the two approaches, but overall, the agreement is qualitatively good, validating the analytical method for more general conditions. The present results show that a simple vortex code is able to provide an estimation of the flow around the wind turbine similar to the actuator‐line approach but with a negligible computational effort. Copyright © 2015 John Wiley & Sons, Ltd.     

Experimental investigation of the root flow in a horizontal axis wind turbine

This research investigates the flow behavior and its features in the blade's root region of a horizontal axis wind turbine by using stereoscopic particle image velocimetry (PIV) technique. Wind tunnel tests are conducted to measure the velocity field, phase‐locked with the blade motion, at different azimuth angles and at different spanwise positions. The pressure distribution is obtained from PIV velocity field by solving the Navier–Stokes momentum equations. In this paper, we aim to answer two questions: (i) How is the flow behavior in the root region? (ii) How is the evolution of the root vortex? The analysis of the velocity fields shows an outboard radial flow motion in the root region and a vorticity driven inboard motion at the blade?s maximum chord position. As a result of this vorticity driven flow, an increase in the axial velocity close to nacelle is measured. Wake sheets are observed and further discussed in the measured velocity and vorticity distributions. The formation and evolution of the root vortices conveyed downstream by the axial velocity are analyzed through vorticity and pressure distributions. Although the azimuthal vorticity in 3D representation is showing the trailing vorticity, the tilting of the root vortex tube is observed in the axial vorticity distribution. Moreover, the radial vorticity and azimuthal velocity from chordwise measurements show separation on the suction surface of the blade. This research concluded that the flow in the blade wake is driven by the root vortex; hence, the local effects of the root vortex cannot be ignored. Copyright © 2013 John Wiley & Sons, Ltd.