Leading edge erosion of wind turbines: Effect of solid airborne particles and rain on operational wind farms

Leading edge erosion (LEE) affects almost all wind turbines, reducing their annual energy production and lifetime profitability. This study presents results of an investigation into 18 operational wind farms to assess the validity of the current literature consensus surrounding LEE. Much of the historical research focuses on rain erosion, implying that this is the predominant causal factor. However, this study showed that the impact of excessive airborne particles from seawater aerosols or from adverse local environments such as nearby quarries greatly increases the levels of LEE. Current testing of leading edge protection coatings or tapes is based on a rain erosion resistivity test, which does little to prove its ability to withstand solid particle erosion and may drive coating design in the wrong direction. Furthermore, it was shown that there is little correlation between test results and actual field performance. A method of monitoring the expected level of erosion on an operational wind turbine due to rain erosion is also presented. Finally, the energy losses associated with LEE on an operational wind farm are examined, with the average annual energy production dropping by 1.8% due to medium levels of erosion, with the worst affected turbine experiencing losses of 4.9%.     

A simple improvement of a tip loss model for actuator disc simulations

The loading of a wind turbine decreases towards the blade tip because of the velocities induced by the tip vortex. This tip loss effect has to be taken into account when performing actuator disc simulations, where the single blades of the turbine are not modeled. A widely used method applies a factor on the axial and tangential loading of the turbine. This factor decreases when approaching the blade tip. It has been shown that the factor should be different for the axial and tangential loading of the turbine to model the rotation of the resulting force vector at the airfoil sections caused by the induced velocity. The present article contains the derivation of a simple correction for the tangential load factor that takes this rotation into account. The correction does not need any additional curve fitting but just depends on the local airfoil characteristics and angle of attack. Actuator disc computations with the modified tip loss correction show improved agreement with results from actuator line, free wake lifting line, and blade element momentum simulations.     

Estimation of torsional-flutter probability in flexible bridges considering randomness in flutter derivatives

Torsional-flutter instability is an aeroelastic phenomenon of interest to the bridge engineer, corresponding to a torsionally unstable vibration regime of the deck driven by wind excitation and appearing beyond a certain critical wind velocity. In this study a method for the derivation of the flutter probability for long-span bridges with bluff decks is proposed.In the first part of this study the deterministic problem is addressed. In contrast with the classical solution method in the frequency domain based on a numerical procedure for assessing the critical wind velocity, a single-mode “closed-form” algorithm for the derivation of the critical velocity was investigated. A polynomial representation of the aeroelastic-loading coefficients (flutter derivatives), necessary for torsional-flutter analysis, was utilized.In the second part an algorithm for estimating the torsional-flutter probability was developed, considering randomness in bridge properties, and flutter derivatives in particular due to their preeminent role in torsional-flutter velocity estimation.Experimental errors in the extraction of flutter derivatives from wind tunnel tests were analyzed. The “closed-form” algorithm, developed in the first part, allowed for a direct numerical solution of the flutter probability in a simple way.The torsional-flutter probability for three simulated bridge models with rectangular closed-box and truss-type girder deck was numerically determined. A set of experimental data, available from the literature, was employed. The simulations enabled the validation of the proposed algorithm.     

About the penetration of a horizontal axis cylindrical vortex into the nearby downwind region of a vertical porous fence

Wind tunnel experiments have been conducted on a cylindrical vortex embedded in a low turbulence stationary horizontal stream, running through a two-dimensional narrow vertical woven fence located on the wind tunnel floor.The vortex was continuously generated upwind of the fence by means of a vortex tube located well below the fence top level, with its axis aligned with the mean velocity of the external stream. The fence installed along the entire width of the tunnel had a porosity of 70%. Visualization experiments showed that approaching the fence the vortex moves away from the mean wind direction of the adjacent stream along a rising curved trajectory while the direction of the surrounding mean flow remained nearly horizontal. The results suggest that this deviation could be promoted by the vortex slanting velocity field relative to the fence, which “sees” a fence with much lower optical porosity than the fence perpendicular velocity of the nearby mean flow.The fence top shear layer flow, which dominates the downwind evolution of the mixing layer, appears to be highly sensitive to the presence of this type of vortex. The most energetic changes in the flow due to the presence of the vortex occurred in the mixing layer region. Windbreaks are usually designed in terms of mean velocity, turbulence intensity, geometric dimensions, and porosity. The results presented in this paper suggest that the sheltering ability of a porous fence depend also on the particular flow pattern of the oncoming turbulent structures embedded in the incident wind. The results show the importance for a particular wind sheltering application in knowing a priori at least some aspects of the flow pattern of the most representative turbulent structures of the local wind.     

非线性力矩作用下气动偏心弹丸强迫圆锥运动稳定性条件

为了指导无伞末敏弹等气动非对称弹丸的结构设计和气动设计,建立了气动偏心弹丸在三次方非线性静力矩和二次方非线性赤道阻尼力矩作用下的攻角方程,运用平均法求解了方程的近似解析解及其线性变分方程。在此基础上,根据Hurwitz判别准则,得到了气动偏心弹丸做强迫圆锥运动的渐近稳定条件,分析了该条件的物理意义,并应用数值计算算例对该条件进行了验证。结果表明,当自转角速度和气动偏心角满足一定的约束条件时,三次方非线性静力矩和二次方非线性赤道阻尼力矩作用下的弹丸可以实现固定攻角的稳定强迫圆锥运动。     

汽车侧风稳定性动态耦合方法研究

针对静态耦合方法对汽车侧风稳定性预测不足等问题,通过对流体仿真分析软件FLUENT中动网格及UDF技术的研究,结合双向耦合数据通信技术,提出了一种动态耦合数值仿真计算方法,实现了空气动力学与汽车动力学之间的动态耦合,并将该方法与静态耦合数值计算方法进行了对比分析。利用动态耦合数值计算方法分析了不同侧风风速条件对汽车侧风稳定性问题的影响,结果表明:动态耦合下汽车侧风稳定性较静态耦合下汽车侧风稳定性差;随侧风风速的增大,汽车侧风稳定性降低。     

定向反射膨胀装置作用特性研究

为实现冲击波尽量小的高效后坐,提出了定向反射膨胀原理,设计了相应的装置。运用一维准定常气体动力学理论,建立了气体动力学模型,根据实际情况选定模型参数进行数值分析,得到了装置减后坐效率及弹丸炮口初速等理论分析结果; 根据理论分析结果确定了实验装置的基本参数,设计实验并在某30 mm口径的火炮上进行定向反射膨胀装置的实验验证。实验测试结果表明,理论分析结果与实验测试结果基本一致,验证了定向反射膨胀装置作用特性研究的正确性,为定向反射膨胀原理及装置的继续研究打下了基础。     

Prediction of the wind turbine performance by using BEM with airfoil data extracted from CFD

Blade element momentum (BEM) theory with airfoil data is a widely used technique for prediction of wind turbine aerodynamic performance, but the reliability of the airfoil data is an important factor for the prediction accuracy of aerodynamic loads and power. The airfoil characteristics used in BEM codes are mostly based on 2D wind tunnel measurements of airfoils with constant span. Due to 3D effects, a BEM code using airfoil data obtained directly from 2D wind tunnel measurements will not yield the correct loading and power. As a consequence, 2D airfoil characteristics have to be corrected before they can be used in a BEM code. In this article, we consider the MEXICO (Model EXperiments In Controlled cOnditions) rotor where airfoil data are extracted from CFD (Computational Fluid Dynamics) results. The azimuthally averaged velocity is used as the sectional velocity to define the angle of attack and the coefficient of lift and drag is determined by the forces on the blade. The extracted airfoil data are put into a BEM code without further corrections, and the calculated axial and tangential forces are compared to both computations using BEM with Shen's tip loss correction model and experimental data. The comparisons show that the recalculated forces by using airfoil data extracted from CFD have good agreements with the experiment.     

A CFD study into the influence of unsteady aerodynamic interference on wind turbine surge motion

To improve knowledge of the unsteady aerodynamic characteristics and interference effects of a floating offshore wind turbine (FOWT), this article focuses on the platform surge motion of a full configuration wind turbine with the rotating blades, hub, nacelle, and tower shapes. Unsteady aerodynamic analyses considering the moving motion of an entire configuration wind turbine have been conducted using an advanced computational fluid dynamics (CFD) and a conventional blade element momentum (BEM) analyses. The present CFD simulation is based on an advanced overset moving grid method to accurately consider the local and global motion of a three-dimensional wind turbine. The effects of various oscillation frequencies and amplitudes of the platform surge motion have been widely investigated herein. Three-dimensional unsteady flow fields around the moving wind turbine with rotating blades are graphically presented in detail. Complex flow interactions among blade tip vortices, tower shedding vortices, and turbulent wakes are physically observed. Comparisons of different aerodynamic analyses under the periodic surge motions are summarized to show the potential distinction among applied numerical methods. The present result indicates that the unsteady aerodynamic thrust and power tend to vary considerably depending on the oscillation frequency and amplitude of the surge motion.     

On the generation of vorticity by force fields in rotor- and actuator flows

In most rotor design methods, the blade load is found by a blade element analysis in an iterative procedure with flow solvers like actuator disc and -line analyses as well as momentum balances. For the flow solvers the force field is the input. In most other aerodynamic analyses the force field is the output result instead of input. This is done by applying boundary conditions at the lifting surface with which the flow is solved and the pressure at the surface, so the load, is determined (only inviscid flows are considered here). Both approaches are consistent, but appear to differ with respect to the generation of vorticity. In the lifting surface approach, usually Helmoltz's laws are used to show that bound and free vorticity is conserved instead of being generated, while in the force field approach vorticity is generated instead of conserved. It is shown that both methods are consistent since sometimes Helmholtz's laws are incorrectly referred to. These laws have been derived in absence of non-conservative forces, while the surface pressure distribution is shown to be such a force field. Besides this, the question is discussed how a force field creates vorticity in an inviscid flow, since some papers consider viscosity to be necessary to generate vorticity. A literature study contradicts this, showing that in inviscid flows vorticity is generated by tangential pressure gradients or, equivalently, a non-uniform force field. This makes the Euler equation including the force field term well suited to express the generation of vorticity in characteristics of the force field. A comparison of the convection of vorticity in the wake of a disc, rotor blade and wing shows several differences. The azimuthal vorticity in the disc wake does not depend on vorticity conservation laws, in contrast to the axial and radial components. For a rotor and wing all components are governed by vorticity conservation.