Passive Load Alleviation Research Articles

Aero-structural rapid screening of new design concepts for offshore wind turbines

The development of large-scale wind turbines is challenged by loads, mass, and cost growth, and more flexible blades that impact power and tower clearance. Further, the long-duration design iterations with standard-sequential-multidisciplinary processes challenge the development of new timely and robust designs. To overcome these limitations and answer open questions of large-scale wind turbines, we develop and demonstrate an aero-structural rapid screening (ASRS) design approach. ASRS uses optimization techniques, emulates a baseline controller, produces detailed blade, tower, and monopile models, and provides fast aerodynamic, structural, and economic results. Blade deflection is introduced as a design variable to achieve load-aligned designs as a function of wind speed for passive load alleviation. To illustrate the approach, a large set of 25 MW rotors for offshore and fixed-bottom wind turbines are studied for variations in blade pre-cone, load alignment via blade deflection, and upwind vs. downwind configurations. These designs vary in number of blades, blade length, axial induction factor, and airfoil family. We demonstrate that blade deflection can be designed to maximize energy capture and minimize turbine mass. ASRS is demonstrated to identify trends and trade-offs that are useful to down-select to options to be studied further using aero-servo-elastic simulations with high-fidelity control design.

Flare folding wing tips for static and gust loads alleviation

The Passive gust load alleviation device can effectively simplify the control system by relying on the adaptive deformation of the structure. This research examines the influence of using folding wingtips during flight as a means of reducing static loads and gusty loads. Effects of hinge direction, wing-tip weight, stiffness and wing-tip centre of gravity on static and gust alleviation performance has been examined. The results show that the flare folding wing-tip can significantly reduce the static load and gust load of the aircraft. In a static aeroelastic trim analysis, a folding wingtip can increase span by 25% without increasing the wing root bending moment and decrease the trim angle of attack by 0.14°. In the gust response analysis, the maximum bending moment of wing root can be decrease by nearly 50% compared with the fixed wing-tip, only 17% higher than the baseline model without a wing-tip.

Application of Aeroelastic Tailoring for Load Alleviation on a Flying Demonstrator Wing

This article presents the application of aeroelastic tailoring in the design of wings for a flying demonstrator, as well as the validation of the design methodology with flight test results. The investigations were performed in the FLEXOP project (Flutter Free Flight Envelope Expansion for Economical Performance Improvement), funded under the Horizon 2020 framework. This project aimed at the validation of methods and tools for active flutter control, as well as at the demonstration of the potential of passive load alleviation through composite tailoring. The technologies were to be demonstrated by the design, manufacturing and flight testing of an unmanned aerial vehicle of approximately 7 m wingspan. This article addresses the work towards the load alleviation goals. The design of the primary load-carrying wing-box in this task is performed using a joint DLR–TU Delft optimization strategy. Two sets of wings are designed in order to demonstrate the potential benefits of aeroelastic tailoring—first, a reference wing in which the laminates of the wing-box members are restricted to balanced and symmetric laminates; second, a tailored wing in which the laminates are allowed to be unbalanced, hence allowing for the shear–extension and bending–torsion couplings essential for aeroelastic tailoring. Both designs are numerically optimized, then manufactured and extensively tested to validate and improve the simulation models corresponding to the wing designs. Flight tests are performed, the results of which form the basis for the validation of the applied aeroelastic tailoring approach presented in the article.

Study on mechanical properties of wind turbine blades with bend-twist coupling effect

Bend-twist coupling (BTC), a method of passive load alleviation for turbine blade, has a significant influence on blade mechanical properties. The coupling behavior between bending and twisting motions stems from the material anisotropy in blade laminate or from the unique blade geometry such as sweep, deflection, and pre-bending. The material-based approach is employed in this research to determine influence regularity of BTC design on blade mechanical properties. To begin with, the blade property analysis tool PreComp that integrates a shear flow method with the classical laminate theory is adopted to compute the cross-sectional stiffnesses of the blades with different degree of BTC effect. Next, the bend-twist coupling coefficients for the BTC blades with different fiber orientation are calculated. Finally, the remaining mechanical properties including static performance, modal characteristics and buckling limits for different BTC blades are determined by the finite element method and the influence of changes in the degree of BTC effect on all these mechanical properties is further evaluated. The results show that the BTC effect markedly influences the different mechanical properties of the blade in different patterns. The 20° BTC blade has the highest BTC coefficient and the edgewise BTC coefficient at 100 m is 2.5 times higher than the flapwise BTC coefficient, while the maximum value of the torsional modal frequency of 3.8429 Hz and the greatest critical buckling load of 4328.1 kN corresponds to the 10° BTC blade and the 15° BTC blade respectively.

Exploration of the effect of wing component post-buckling on bending-twist coupling for nonlinear wing twist

A new investigated concept for passive load alleviation is to exploit the nonlinear behavior of wing design components to trigger a deformation which reduce loads once a critical load level is reached. The necessary deformation is a torsional rotation which is supposed to reduce the angle of attack. For this target, wingbox sections are investigated regarding their nonlinear behavior with finite element analysis. Parameter studies feature anisotropic carbon fiber reinforced polymer (CFRP) layups for the skins, layups and thicknesses for spars and the presence of stringers. Results show a desired nonlinear progressive bending-torsion coupling for an unstiffened wingbox section, when the upper skin and the rear spar are modified. After modification they are allowed to buckle within the load envelope. The skin has an anisotropic layup. The rear spar needs to be thinner than the front spar. Both modifications result in progressively increasing torsional rotation of the wingbox with increasing load. Stringers are not applied because they limit the nonlinearity which is not desired for the envisioned load alleviation technique.

A passive load alleviation aircraft wing: topology optimization for maximizing nonlinear bending–torsion coupling

Aircraft wings with passive load alleviation morph their shape to a configuration where the aerodynamic forces are reduced without the use of an actuator. In our research, we exploit geometric nonlinearities of the inner wing structure to maximize load alleviation. In order to find designs with the desired properties, we propose a topology optimization approach. Passive load alleviation is achieved through bending–torsion coupling. The wing twist will reduce the angle of attack, thus lowering the aerodynamic forces. Consequently, the objective function is to maximize the torsion angle. Since shape morphing should only affect loads that exceed normal maneuvering loads, a displacement constraint is enforced, preventing torsion at lower force levels. Maximizing the displacement will lead to topologies for which the finite element solver cannot find a solution. To circumvent this, we propose adding a compliance value to the objective function. This term has a weighting function, which controls how much influence the compliance value has: after a set number of iterations, the initially high level of influence will drop. We used a geometric nonlinear finite element formulation with a linear elastic material model. The addition of an energy interpolation scheme reduces mesh distortion. We successfully applied the proposed methodology to two different test cases resembling an aircraft wing box section. These test cases illustrate the methodology’s potential for designing new geometries with the desired nonlinear behavior. We discuss what design features can be deduced and how they achieve the nonlinear structural response.

The influence of the bound vortex on the aerodynamics of curved wind turbine blades

Passive load alleviation on wind turbine blades can be achieved through geometric bend-twist coupling, for example by sweeping the blade backwards. In order to obtain the correct load distribution of a curved blade with in-plane sweep and/or out-of-plane dihedral, the influence of the blade shape on the aerodynamics must be modelled correctly. This includes the influence of the curved bound vortex, and it is especially important when designing a wind turbine blade with aeroelastic tailoring. In this paper, the background for modelling the curved bound vortex influence will be described in detail and a modified method is proposed. The proposed method of bound vorticity modelling is compared for curved and straight translating wings as well as wind turbine blades with results from a panel code and a Navier-Stokes solver. From this comparison, the advantages of the current modification with respect to the other lifting-line implementations are shown. The method proposed in the present work is general and applicable to any lifting-line like model.

Passive load alleviation on wind turbine blades from aeroelastically driven selectively compliant morphing

AbstractLoad alleviation control is highly desirable to reduce penalties associated with the added structural mass required to withstand rare load scenarios. This is particularly true for wind turbine designs incorporating long‐span blades. Implementation of compliance‐based morphing structures to modify the lift distribution passively has the potential to mitigate the impact of rare, but integrally threatening, loads on wind turbine blades while limiting the addition of actuation and sensing systems. We present a novel passive load alleviation concept based on a morphing flap exhibiting selective compliance from an embedded bistable element. A multifidelity, aeroelastic tool is used to study the shape adaptability of a morphing flap indicating that passive changes from high lift generation to load alleviation configurations can be achieved by exploiting the energy of the flow. This mechanism offers a method to reduce catastrophic peak loads potentially, thus offering the possibility to lower the overall structural weight of wind turbine blades.

Aerostructural Tradeoffs for Tow-Steered Composite Wings

The adoption of conventional “black metal” composites in aircraft structures has lead to an improvement in performance over previous metallic designs. With the maturation of automated fiber placement, the design of unconventional composites featuring spatially varying tow orientations has become possible, but it is not yet clear how to utilize this new design freedom optimally. Wing design optimization studies are performed for three different material technologies: aluminum alloy, conventional carbon-fiber-reinforced composite, and tow-steered carbon-fiber-reinforced composite. To explore the design trades of these materials, Pareto fronts for the competing objectives of fuel burn and structural weight are generated through gradient-based optimization. These two objectives serve as surrogates for the direct operating cost and acquisition cost of the aircraft, respectively. All optimized wings took advantage of some amount of passive load alleviation at the high-lift maneuver condition. Although the performance increase of tow-steered composite wings was not as dramatic as the performance increase of conventional composite relative to aluminum, the improvement was still significant. This work provides a methodology that can be used to explore tradeoffs in tow-steered wing designs and could be extended to other structural technologies.

Design and Optimization of an Aeroservoelastic Wind Tunnel Model

Through the combination of passive and active load alleviation techniques, this paper presents the design, optimization, manufacturing, and update of a flexible composite wind tunnel model. In a first step, starting from the specification of an adequate wing and trailing edge flap geometry, passive, static aeroelastic stiffness optimizations for various objective functions have been performed. The second optimization step comprised a discretization of the continuous stiffness distributions, resulting in manufacturable stacking sequences. In order to determine which of the objective functions investigated in the passive structural optimization most efficiently complemented the projected active control schemes, the condensed modal finite element models were integrated in an aeroelastic model, involving a dedicated gust load alleviation controller. The most promising design was selected for manufacturing. The finite element representation could be updated to conform to the measured eigenfrequencies, based on the dynamic identification of the model. Eventually, a wind tunnel test campaign was conducted in November 2018 and results have been examined in separate reports.

High-fidelity aerostructural optimization of tow-steered composite wings

Over the past several decades, composites have increasingly become the material of choice in modern aircraft structural design. This is primarily due to the high stiffness- and strength-to-weight ratios offered by conventional composites when compared to metals. Unconventional composite designs, such as tow-steered composites, have demonstrated potential for further expanding these advantages. Unlike their conventional composite counterparts, tow-steered composites feature layers with spatially varying fiber orientations. When applied to wing design, tow-steered composites offer an increase in design freedom at the cost of higher design complexity, making them ideal candidates for design optimization. We develop a methodology for the aerostructural design optimization of tow-steered composite wings using high-fidelity physics models. We also quantify the benefits of this new technology by performing a fuel burn minimization for both a tow-steered and a conventional composite wing design. This assessment is done for the undeflected Common Research Model, which is representative of a twin-aisle transport aircraft. We find improvements of up to 2.4% in fuel burn and 24% in wing weight relative to the optimized conventional composite design. We show that this improvement is due to a combination of improved passive aeroelastic tailoring and local strength tailoring in high-stress regions in the tow-steered structure. For a higher-aspect-ratio (13.5) wing design, we find improvements of up to 1.5% and 14% in fuel savings and wing weight, respectively. To better understand the effect of aspect ratio on tow-steered wing design, we perform an optimization study where the aspect ratio is varied between 7.5 and 13.5. We found that there are diminishing returns in the benefit of tow steering as the aspect ratio is increased because less passive load alleviation is possible.

Fast trailed and bound vorticity modeling of swept wind turbine blades

Passive load alleviation can be achieved through geometric bend-twist coupling, for example, by sweeping the blade backwards. The influence of the blade sweep on the trailing vorticity and bound vorticity is not modelled in the current fast aeroelastic wind turbine codes suitable for certification. A near wake trailed vorticity model which was coupled with a blade element momentum theory based aerodynamic model has been modified to take into account the blade sweep. The extended model is compared with the original near wake model, a blade element momentum (BEM) model and full rotor computational fluid dynamics (CFD) results for the modified IEA 10MW reference wind turbines. The steady-state loadings calculated from the extended model are in better agreements with CFD compared to the original model and the BEM for four different swept blades. It is also shown that the influence of the blade sweep on normal loading is not correctly modelled by BEM and this error will be inherited to the near wake model results. Thus, further modification to BEM will likely improve the predicted normal loading for swept blades, even if no near wake model is used.

Aero-elastic wind turbine design with active flaps for AEP maximization

Abstract. In optimal wind turbine design, there is a compromise between maximizing the energy producing forces and minimizing the absolute peak loads carried by the structures. Active flaps are an attractive strategy because they give engineers greater freedom to vary the aerodynamic forces under any condition. Flaps can be used in a variety of different ways (i.e. reducing fatigue, peak loads), but this article focuses on how quasi-static actuation as a function of mean wind speed can be used for annual energy production (AEP) maximization. Numerical design optimization of the DTU 10 MW reference wind turbine (RWT), with the HAWTOpt2 framework, was used to both find the optimal flap control strategy and the optimal turbine designs. The research shows that active flaps can provide a 1 % gain in AEP for aero-structurally optimized blades in both add-on (i.e. the flap is added after the blade is designed) and integrated (i.e. the blade design and flap angle is optimized together) solutions. The results show that flaps are complementary to passive load alleviation because they provide high-order alleviation, where passive strategies only provide linear alleviation with respect to average wind speed. However, the changing loading from the flaps further complicates the design of torsionally active blades; thus, integrated design methods are needed to design these systems.

Study of a C-wing configuration for passive drag and load alleviation

Non-planar wing configurations are often hypothesised as a means for improving the aerodynamic efficiency of large transport aircraft; C-wings may have the ability to exploit and unify drag reduction, aeroelasticity, and dynamics and control but their capacity to do so is ambiguous. The purpose of this work is to provide an experimental demonstration with the aim of verifying the C-wing configurations practical application. Thus, the main objective of this investigation is to quantify the C-wing’s ability for drag and load alleviation relative to a planar wing of equivalent wingspan, lift, and root bending moment at Re=1.5×106. Surface clay flow visualisations have been used to provide insight into the flow over the wing surface. Aerodynamic performance metrics show that despite the C-wing operating with a 19.1% higher wing wetted area, a peak total drag reduction of 9.5% at α=6° is achieved in addition to a 1.1% reduction in the wing root bending moment for equivalent lift. Force platform measurements in combination with laser vibrometry enabled a detailed understanding of the vibrational characteristics between the model and the wind tunnel. It is shown that the C-wing can passively attenuate buffet induced vibrations of the main-wing by up to 68.6% whilst simultaneously reducing total drag without a significant increase in wing weight or root bending moment.

Design of a wind turbine swept blade through extensive load analysis

The main focus of this work is to offer an extensive investigation regarding the use of backward swept blades for passive load alleviation on wind turbines. Sweeping blades backward produces a structural coupling between flapwise bending towards the tower and torsion towards feathering. This coupling mitigates loads on the wind turbine structure due to a decrease in the angle of attack. The load alleviation can be achieved by changing the blade geometry according to three parameters: starting point for the change of shape along the blade span, blade tip sweep, and blade forward sweep. A parametric study is carried out on a 10 MW wind turbine with the purpose of outlining the relation between load variations and three geometric parameters used to introduce passive control on wind turbine blades. The objective is to estimate and analyze extreme and fatigue loads, formulating suggestions for the design of a wind turbine that employs backward swept blades. From the investigation, it is concluded that mildly and purely backward swept shapes are the best option because they allow the wind turbine to achieve load alleviations without a large increase of the blade root torsional extreme and life-time equivalent fatigue moment. The efficacy of the design procedure provided with this work is proved through its application on a 5 MW wind turbine design.

Design of an Aeroelastically Tailored 10 MW Wind Turbine Rotor

This work presents an integrated multidisciplinary wind turbine optimization framework utilizing state-of-the-art aeroelastic and strutural tools, capable of simultaneous design of the outer geometry and internal structure of the blade. The framework is utilized to design a 10 MW rotor constrained not to exceed the design loads of an existing reference wind turbine. The results show that through combined geometric tailoring of the internal structure and aerodynamic shape of the blade it is possible to achieve significant passive load alleviation that allows for a 9% longer blade with an increase in AEP of 8.7%, without increasing blade mass and without significant increases in ultimate and fatigue loads on the hub and tower.

Design of a Morphing Wingtip

An initial design of a morphing wingtip for a regional jet aircraft is developed and evaluated. The adaptive wingtip concept is based upon a chiral-type internal structure, enabling controlled cant angle orientation, camber, and twist throughout the flight envelope. A baseline turbofan aircraft configuration model is used as the benchmark to assess the device. Computational fluid dynamics based aerodynamics are used to evaluate the required design configurations for the device at different points across the flight envelope in terms of lift/drag and bending moment distribution along the span, complemented by panel-method-based gust load computations. Detailed studies are performed to show how the chiral structure can facilitate the required shape changes in twist, camber, and cant. Actuator requirements and limitations are assessed, along with an evaluation of the aerodynamic gains from the inclusion of the device versus power and weight penalties. For a typical mission, it was found that savings of around 2% in fuel weight are possible using the morphing wingtip device. A similar reduction in weight due to passive gust load alleviation is also possible with a slight change of configuration.

Aerodynamic load control in horizontal axis wind turbines with combined aeroelastic tailoring and trailing‐edge flaps

ABSTRACTThis paper presents an aeroservoelastic modeling approach to investigate dynamic load alleviation in large wind turbines with composite blades and trailing‐edge aerodynamic surfaces. The tower and rotating blades are modeled using geometrically non‐linear composite beams and linearized about reference rotating conditions with potentially arbitrarily large structural displacements. The aerodynamics of the rotor are represented using a linearized unsteady vortex lattice method, and the resulting aeroelastic system is written in a state‐space description that is both convenient for model reductions and control design. A linear model of a single blade is then used to design an regulator, capable of providing load reductions of up to 13% in closed loop on the full wind turbine non‐linear aeroelastic model. When combined with passive load alleviation through aeroelastic tailoring, dynamic loads can be further reduced to 35%. While the separate use of active flap controls and passive mechanisms for load alleviation has been well‐studied, an integrated approach involving the two mechanisms has yet to be fully explored and is the focus of this paper. Finally, the possibility of exploiting torsional stiffness for active load alleviation on turbine blades is also considered. Copyright © 2014 John Wiley & Sons, Ltd.

On Extension–Shearing coupled laminates

The definitive list of Extension–Shearing coupled composite laminates with up to 21 plies is derived. The listings comprise of individual stacking sequences of entirely non-symmetric laminates, which are characterised in terms of angle- and cross-ply sub-sequence relationships as well as the blend-ratio of unbalanced angle-plies. Dimensionless parameters, including lamination parameters, are provided, from which the extensional and bending stiffness terms are readily calculated. Because this new class of coupled non-symmetric laminate possesses in-plane coupling behaviour only it can also be manufactured flat under a standard elevated temperature curing process. Such laminates can be configured to produce Bending–Twisting coupling in wing-box type structures, which can be exploited to great effect in the design for passive load alleviation in wind-turbine blades, or for aero-elastic compliance in fixed wing aircraft or helicopter rotor-blades. It should be recognised that similar behaviour can also be achieved using less sophisticated designs, such as applying off-axis material alignment to otherwise balanced and symmetric laminates or by using un-balanced and symmetric designs, but additional forms of coupling behaviour arise in these cases, leading to detrimental effects on both stiffness and strength, which are demonstrated though comparisons of the structural response of competing laminate designs.

Design, manufacturing and characterization of aero-elastically scaled wind turbine blades for testing active and passive load alleviation techniques within a ABL wind tunnel

In the research described in this paper, a scaled wind turbine model featuring individual pitch control (IPC) capabilities, and equipped with aero-elastically scaled blades featuring passive load reduction capabilities (bend-twist coupling, BTC), was constructed to investigate, by means of wind tunnel testing, the load alleviation potential of BTC and its synergy with active load reduction techniques. The paper mainly focus on the design of the aero-elastic blades and their dynamic and static structural characterization. The experimental results highlight that manufactured blades show desired bend-twist coupling behavior and are a first milestone toward their testing in the wind tunnel.