Aeroelastic stability predictions for a MW‐sized blade

Classical aeroelastic flutter instability historically has not been a driving issue in wind turbine design. In fact, rarely has this issue even been addressed in the past. Commensurately, among the wind turbines that have been built, rarely has classical flutter ever been observed. However, with the advent of larger turbines fitted with relatively softer blades, classical flutter may become a more important design consideration. In addition, innovative blade designs involving the use of aeroelastic tailoring, wherein the blade twists as it bends under the action of aerodynamic loads to shed load resulting from wind turbulence, may increase the blade's proclivity for flutter. With these considerations in mind it is prudent to revisit aeroelastic stability issues for a MW‐sized blade with and without aeroelastic tailoring. Focusing on aeroelastic stability associated with the shed wake from an individual blade turning in still air, the frequency domain technique developed by Theodorsen for predicting classical flutter in fixed wing aircraft has been adapted for use with a rotor blade. Results indicate that the predicted flutter speed of a MW‐sized blade is slightly greater than twice the operational speed of the rotor. When a moderate amount of aeroelastic tailoring is added to the blade, a modest decrease (12%) in the flutter speed is predicted. By comparison, for a smaller rotor with relatively stiff blades the predicted flutter speed is approximately six times the operating speed. When frequently used approximations to Theodorsen's method are implemented, drastic underpredictions result, which, while conservative, may adversely impact blade design. These underpredictions are also evident when this MW‐sized blade is analysed using time domain methods. Published in 2004 by John Wiley & Sons, Ltd.     

Aeroelastic instability problems for wind turbines

This paper deals with the aeroelastic instabilities that have occurred and may still occur for modern commercial wind turbines: stall‐induced vibrations for stall‐turbines, and classical flutter for pitch‐regulated turbines. A review of previous works is combined with derivations of analytical stability limits for typical blade sections that show the fundamental mechanisms of these instabilities. The risk of stall‐induced vibrations is mainly related to blade airfoil characteristics, effective direction of blade vibrations and structural damping; whereas the blade tip speed, torsional blade stiffness and chordwise position of the center of gravity along the blades are the main parameters for flutter. These instability characteristics are exemplified by aeroelastic stability analyses of different wind turbines. The review of each aeroelastic instability ends with a list of current research issues that represent unsolved aeroelastic instability problems for wind turbines. Copyright © 2007 John Wiley & Sons, Ltd.     

Accuracy of an efficient framework for structural analysis of wind turbine blades

This paper presents a novel framework for the structural design and analysis of wind turbine blades and establishes its accuracy. The framework is based on a beam model composed of two parts—a 2D finite element‐based cross‐section analysis tool and a 3D beam finite element model. The cross‐section analysis tool is able to capture the effects stemming from material anisotropy and inhomogeneity for sections of arbitrary geometry. The proposed framework is very efficient and therefore ideally suited for integration within wind turbine aeroelastic design and analysis tools. A number of benchmark examples are presented comparing the results from the proposed beam model to 3D shell and solid finite element models. The examples considered include a square prismatic beam, an entire wind turbine rotor blade and a detailed wind turbine blade cross section. Phenomena at both the blade length scale—deformation and eigenfrequencies—and cross section scale—3D material strain and stress fields—are analyzed. Furthermore, the effect of the different assumptions regarding the boundary conditions is discussed in detail. The benchmark examples show excellent agreement suggesting that the proposed framework is a highly efficient alternative to 3D finite element models for structural analysis of wind turbine blades. Copyright © 2015 John Wiley & Sons, Ltd.     

Aeroelastic analysis of a wind turbine blade using the harmonic balance method

An aeroelastic model for wind turbine blades derived from the unsteady Navier‐Stokes equations and a mode shape–based structural dynamics model are presented. For turbulent flows, the system is closed with the Spalart‐Allmaras turbulence model. The computation times for the aerodynamic solution are significantly reduced using the harmonic balance method compared to a time‐accurate solution. This model is significantly more robust than standard aeroelastic codes that rely on blade element momentum theory to determine the aerodynamic forces. Comparisons with published results for the Caradonna‐Tung rotor in hover and the classical AGARD 445.6 flutter case are provided to validate the aerodynamic model and aeroelastic model, respectively. For wind turbines, flutter of the 1.5 MW WindPACT blade is considered. The results predict that the first flapwise and edgewise modes dominate flutter at the rotor speeds considered.     

A parametric study of coupled‐mode flutter for MW‐size wind turbine blades

With the increasing size of offshore wind turbine rotors, the design criteria used for the blades may also evolve. Current offshore technology utilizes three relatively stiff blades in an upwind configuration. With the goal of minimizing the mass, there is an interest in the lightweight rotors that instead utilize two flexible blades oriented downwind. These longer blades are more flexible and thus susceptible to experience flow‐induced instability. Coupled‐mode flutter is one of the destructive aeroelastic instabilities that can occur in flexible structures subjected to aerodynamic loading. Because of variation in one of the system parameters, e.g., flow velocity, structural modes coalesce at a critical flow velocity, and coupled‐flutter occurs. In the present work, a parametric study is conducted in order to study the influence of the natural frequencies in the torsional and flapwise directions on the critical flutter speed for wind turbine blades. Three MW‐size wind turbine blades are studied using a three‐dimensional blade model, which includes coupled flapwise and torsional displacements. The results show that the three blades have very similar behavior as the system parameters vary. It is shown that the first torsional natural frequency and the ratio of the first torsional natural frequency to the first flapwise natural frequency are the most critical parameters affecting the onset of instability. Critical flutter speeds even lower than the blade rated speed can be observed for blades with low torsional natural frequencies. Copyright © 2015 John Wiley & Sons, Ltd.     

Aeroelastic design of large wind turbine blades considering damage tolerance

Modern offshore turbine blades can be designed for high fatigue life and damage tolerance to avoid excessive maintenance and therefore significantly reduce the overall cost of offshore wind power. An aeroelastic design strategy for large wind turbine blades is presented and demonstrated for a 100 m blade. High fidelity analysis techniques like 3D finite element modeling are used alongside beam models of wind turbine blades to characterize the resulting designs in terms of their aeroelastic performance as well as their ability to resist damage growth. This study considers a common damage type for wind turbine blades, the bond line failure, and explores the damage tolerance of the designs to gain insight into how to improve bond line failure through aeroelastic design. Flat‐back airfoils are also explored to improve the damage tolerance performance of trailing‐edge bond line failures. Copyright © 2016 John Wiley & Sons, Ltd.     

Viscous and Aeroelastic Effects on Wind Turbine Blades. The VISCEL Project. Part II: Aeroelastic Stability Investigations

The recent introduction of ever larger wind turbines poses new challenges with regard to understanding the mechanisms of unsteady flow–structure interaction. An important aspect of the problem is the aeroelastic stability of the wind turbine blades, especially in the case of combined flap/lead–lag vibrations in the stall regime. Given the limited experimental information available in this field, the use of CFD techniques and state‐of‐the‐art viscous flow solvers provides an invaluable alternative towards the identification of the underlying physics and the development and validation of sound engineering‐type aeroelastic models. Navier–Stokes‐based aeroelastic stability analysis of individual blade sections subjected to combined pitch/flap or flap/lead–lag motion has been attempted by the present consortium in the framework of the concluded VISCEL JOR3‐CT98‐0208 Joule III project. Copyright © 2003 John Wiley & Sons, Ltd.     

Indicial lift response function: an empirical relation for finite‐thickness airfoils,and effects on aeroelastic simulations

The aeroelastic response of wind turbines is often simulated in the time domain by using indicial response techniques. Unsteady aerodynamics in attached flow are usually based on Jones's approximation of the flat plate indicial response, although the response for finite‐thickness airfoils differs from the flat plate one. The indicial lift response of finite‐thickness airfoils is simulated with a panel code, and an empirical relation is outlined connecting the airfoil indicial response to its geometric characteristics. The effects of different indicial approximations are evaluated on a 2D profile undergoing harmonic pitching motion in the attached flow region; the resulting lift forces are compared with computational fluid dynamics (CFD) simulations. The relevance for aeroelastic simulations of a wind turbine is also evaluated, and the effects are quantified in terms of variations of equivalent fatigue loads, ultimate loads, and stability limits. The agreement with CFD computations of a 2D profile in harmonic motion is improved by the indicial function accounting for the finite‐thickness of the airfoil. Concerning the full wind turbine aeroelastic behavior, the differences between simulations on the basis of Jones's and finite‐thickness indicial response functions are rather small; Jones's flat‐plate approximation results in only slightly larger fatigue and ultimate loads, and lower stability limits. Copyright © 2012 John Wiley & Sons, Ltd.     

Aeroelastic analysis of a floating offshore wind turbine in platform‐induced surge motion using a fully coupled CFD‐MBD method

Modern offshore wind turbines are susceptible to blade deformation because of their increased size and the recent trend of installing these turbines on floating platforms in deep sea. In this paper, an aeroelastic analysis tool for floating offshore wind turbines is presented by coupling a high‐fidelity computational fluid dynamics (CFD) solver with a general purpose multibody dynamics code, which is capable of modelling flexible bodies based on the nonlinear beam theory. With the tool developed, we demonstrated its applications to the NREL 5 MW offshore wind turbine with aeroelastic blades. The impacts of blade flexibility and platform‐induced surge motion on wind turbine aerodynamics and structural responses are studied and illustrated by the CFD results of the flow field, force, and wake structure. Results are compared with data obtained from the engineering tool FAST v8.     

Aeroelastic validation and Bayesian updating of a downwind wind turbine

Downwind wind turbine blades are subjected to tower wake forcing at every rotation, which can lead to structural fatigue. Accurate characterisation of the unsteady aeroelastic forces in the blade design phase requires detailed representation of the aerodynamics, leading to computationally expensive simulation codes, which lead to intractable uncertainty analysis and Bayesian updating. In this paper, a framework is developed to tackle this problem. Full, detailed aeroelastic model of an experimental wind turbine system based on 3‐D Reynolds‐averaged Navier‐Stokes is developed, considering all structural components including nacelle and tower. This model is validated against experimental measurements of rotating blades, and a detailed aeroelastic characterisation is presented. Aerodynamic forces from prescribed forced‐motion simulations are used to train a time‐domain autoregressive with exogenous input (ARX) model with a localised forcing term, which provides accurate and cheap aeroelastic forces. Employing ARX, prior uncertainties in the structural and rotational parameters of the wind turbine are introduced and propagated to obtain probabilistic estimates of the aeroelastic characteristics. Finally, the experimental validation data are used in a Bayesian framework to update the structural and rotational parameters of the system and thereby reduce uncertainty in the aeroelastic characteristics.     

风力机叶片动态气弹变形及其对整机性能的影响

基于风力机整机刚柔耦合模型,文章提出了一种叶片动态气弹扭转变形分析的新方法。该方法采用SIMPACK和AeroDyn软件联合数值仿真对风力机在几种恶劣风况下进行动力学分析,通过对分析结果的变换处理,进而得到叶片在复杂工况下的动态气弹变形数据。采用该方法,重点分析了叶片气弹扭转变形对风力机气动功率及气弹稳定性的影响。该方法为大型风电叶片的气弹特性评价以及气弹剪裁设计提供了一种新的技术手段。     

Effect of steady deflections on the aeroelastic stability of a turbine blade

This paper deals with effects of geometric non‐linearities on the aeroelastic stability of a steady‐state deflected blade. Today, wind turbine blades are long and slender structures that can have a considerable steady‐state deflection which affects the dynamic behaviour of the blade. The flapwise blade deflection causes the edgewise blade motion to couple to torsional blade motion and thereby to the aerodynamics through the angle of attack. The analysis shows that in the worst case for this particular blade, the edgewise damping can be decreased by half. Copyright © 2010 John Wiley & Sons, Ltd.     

Flap/lead–lag aeroelastic stability of wind turbine blades

A numerical tool for investigating the aeroelastic stability of a single wind turbine blade subjected to combined flap/lead–lag motion is presented. Its development is motivated by recent concern about destructive edgewise vibrations of modern stall‐controlled blades. The stability tool employs a finite element formulation to discretize in space the structural and aerodynamic governing equations. Unsteady aerodynamics is considered by means of the extended ONERA lift and drag models. The mathematical form of these models allows for a combined treatment of dynamics and aerodynamics through the introduction of a so‐called ‘aeroelastic beam element’. This is an extended two‐node beam element having both deformation and aerodynamic degrees of freedom. Several linear and non‐linear versions of the stability tool are available, differing in the way that instantaneous lift and/or drag is treated. In the linear case, stability is investigated through eigenvalue analysis. Time domain integration is employed for non‐linear stability analysis. Results are presented and discussed for a 17 m stall‐controlled blade. Copyright © 2001 John Wiley & Sons, Ltd.     

A decoupling control strategy for wind turbine blades equipped with active flow controllers

The use of active controls has shown to be of substantial help in supporting the increasing size of wind turbines by reducing peak stresses and fatigue loads. In this respect, this paper proposes the use of intuitive frequency‐based control strategies for reducing loads in wind turbine blades equipped with multi‐input multi‐output (MIMO) active flow controllers. For that purpose, a loop‐shaping approach is considered for analysing the dynamic of actively controlled wind turbine blades. Preliminary aeroelastic simulations are carried out to validate the results. It is shown that the MIMO vibration control problem can effectively be decomposed into a number of decoupled single‐input single‐output control problems because of the strong correlation between the dominant aeroelastic blade dynamics and actuator deployments. As a result, it is demonstrated that classical single‐input single‐output control systems can perform as efficiently as MIMO controllers for damping the aeroelastic dynamics of wind turbine blades. Copyright © 2016 John Wiley & Sons, Ltd.     

Innovative design approaches for large wind turbine blades

A preliminary design study of an advanced 50 m blade for utility wind turbines is presented and discussed. The effort was part of the Department of Energy WindPACT Blade System Design Study with the goal to investigate and evaluate design and manufacturing issues for wind turbine blades in the 1–10 MW size range. Two different blade designs are considered and compared in this article. The first is a fibreglass design, while the second design selectively incorporates carbon fibre in the main structural elements. The addition of carbon results in modest cost increases and provides significant benefits, particularly with respect to blade deflection. The structural efficiency of both designs was maximized by tailoring the thickness of the blade cross‐sections to simplify the construction of the internal members. Inboard the blades incorporate thick blunt trailing edge aerofoils (flatback aerofoils), while outboard more conventional sharp trailing edge high‐lift aerofoils are used. The outboard section chord lengths were adjusted to yield the least complex and costly internal blade structure. A significant portion of blade weight is related to the root buildup and metal hardware for typical root attachment designs. The results show that increasing the number of studs has a positive effect on total weight, because it reduces the required root laminate thickness. The aerodynamic performance of the blade aerofoils was predicted using computational techniques that properly simulate blunt trailing edge flows. The performance of the rotor was predicted assuming both clean and soiled blade surface conditions. The rotor is shown to provide excellent performance at a weight significantly lower than that of current rotors of this size. Copyright © 2004 John Wiley & Sons, Ltd.     

Damping of edgewise vibration in wind turbine blades by means of circular liquid dampers

This paper proposes a new type of passive vibration control damper for controlling edgewise vibrations of wind turbine blades. The damper is a variant of the liquid column damper and is termed as a circular liquid column damper (CLCD). Rotating wind turbine blades generally experience a large centrifugal acceleration. This centrifugal acceleration makes the use of this kind of oscillatory liquid damper feasible with a small mass ratio to effectively suppress edgewise vibrations. A reduced 2‐DOF non‐linear model is used for tuning the CLCD attached to a rotating wind turbine blade, ignoring the coupling between the blade and the tower. The performance of the damper is evaluated under various rotational speeds of the rotor. A special case in which the rotational speed is so small that the gravity dominates the motion of the liquid is also investigated. Further, the legitimacy of the decoupled optimization is verified by incorporating the optimized damper into a more sophisticated 13‐DOF aeroelastic wind turbine model with due consideration to the coupled blade‐tower‐drivetrain vibrations of the wind turbine as well as a pitch controller. The numerical results from the illustrations on a 5 and a 10MW wind turbine machine indicate that the CLCD at an optimal tuning can effectively suppress the dynamic response of wind turbine blades. Copyright © 2015 John Wiley & Sons, Ltd.     

Assessment of a comprehensive aeroelastic tool for horizontal‐axis wind turbine rotor analysis

This paper presents the development of a computational aeroelastic tool for the analysis of performance, response and stability of horizontal‐axis wind turbines. A nonlinear beam model for blades structural dynamics is coupled with a state‐space model for unsteady sectional aerodynamic loads, including dynamic stall effects. Several computational fluid dynamics structural dynamics coupling approaches are investigated to take into account rotor wake inflow influence on downwash, all based on a Boundary Element Method for the solution of incompressible, potential, attached flows. Sectional steady aerodynamic coefficients are extended to high angles of attack in order to characterize wind turbine operations in deep stall regimes. The Galerkin method is applied to the resulting aeroelastic differential system. In this context, a novel approach for the spatial integration of additional aerodynamic states, related to wake vorticity and dynamic stall, is introduced and assessed. Steady‐periodic blade responses are evaluated by a harmonic balance approach, whilst a standard eigenproblem is solved for aeroelastic stability analyses. Drawbacks and potentialities of the proposed model are investigated through numerical and experimental comparisons, with particular attention to rotor blades unsteady aerodynamic modelling issues. Copyright © 2016 John Wiley & Sons, Ltd.     

Dynamic analysis tool development for swept wind turbine blades

Because of their aeroelastic behavior, swept wind turbine blades offer the potential to increase energy capture and lower fatigue loads. This article describes work to develop a dynamic analysis code for swept wind turbine blades. This work was an outgrowth of a U.S. Department of Energy contract on swept blades, where the authors used the Adams? dynamic software (MSC Software Corporation, Santa Ana, CA, USA). The new code is based on the National Renewable Energy Laboratory's FAST code and allows for lower cost analysis and faster computation times for swept blades. The additions to the FAST code include the geometry and mode shapes required for the bending and twisting motion of the swept blade. In addition, a finite element program to determine mode shapes for the swept blade was developed. Comparisons of results obtained with the new code and analytical solutions for a curved cantilever beam show good agreement in local torsional deflections. Comparisons with field data obtained for a 750 kW wind turbine with swept blades were complicated by uncertainties in the test wind speed and turbine controller settings.Copyright © 2012 John Wiley & Sons, Ltd.     

Aerodynamic characteristics of wind turbine blades with a sinusoidal leading edge

The aerodynamic characteristics of a kind of bionic wind turbine blades with a sinusoidal leading edge have been investigated in this paper based on a three‐dimensional Reynolds‐averaged Navier–Stokes simulation. The calculated results show that compared with a straight leading‐edge blade, the new‐type blade has a great improvement in shaft torque at high wind speeds. The localized vortices shedding from the leading‐edge tubercles, which can generate a much greater peak of the leading‐edge suction pressure than that from the straight leading‐edge case, are the physical essentials to enhance the wavy blade's aerodynamic performances as the blade goes into stall. In particular, the outboard segment from the 60%R station to the blade tip is the key region for wavy leading‐edge blades to improve the aerodynamic characteristics at high‐speed inflows. In this key region, a wavy blade can obtain a greater power output as the wavelength l and the waveheight δ increase. The present numerical results also show that the wavy leading‐edge shape is unfavorable for a wind turbine blade under the design conditions (e.g., at the rated wind speed). At these conditions, an early boundary‐layer separation as a result of the geometric disturbances of the leading‐edge tubercles will inevitably result in a visible shaft‐torque reduction in the wavy‐blade cases. Anyway, the wavy blades still tend to generate a more robust power output as a whole from 10 to 20 m s ^?1 than the original NREL phase‐VI blade. Copyright © 2011 John Wiley & Sons, Ltd.     

Aeroelastic multidisciplinary design optimization of a swept wind turbine blade

Mitigating loads on a wind turbine rotor can reduce the cost of energy. Sweeping blades produces a structural coupling between flapwise bending and torsion, which can be used for load alleviation purposes. A multidisciplinary design optimization (MDO) problem is formulated including the blade sweep as a design variable. A multifidelity approach is used to confront the crucial effects of structural coupling on the estimation of the loads. During the MDO, ultimate and damage equivalent loads are estimated using steady‐state and frequency‐domain–based models, respectively. The final designs are verified against time‐domain full design load basis aeroelastic simulations to ensure that they comply with the constraints. A 10‐MW wind turbine blade is optimized by minimizing a cost function that includes mass and blade root flapwise fatigue loading. The design space is subjected to constraints that represent all the necessary requirements for standard design of wind turbines. Simultaneous aerodynamic and structural optimization is performed with and without sweep as a design variable. When sweep is included in the MDO process, further minimization of the cost function can be obtained. To show this achievement, a set of optimized straight blade designs is compared to a set of optimized swept blade designs. Relative to the respective optimized straight designs, the blade mass of the swept blades is reduced of an extra 2% to 3% and the blade root flapwise fatigue damage equivalent load by a further 8%.