Aeroelastic Simulations Research Articles

Flap-wise loads reduction of rotor blades by embedded flap-wise absorbers

ABSTRACTElastomeric and Fluidlastic®absorbers are proposed to be embedded in blade cavity to reduce troublesome second harmonic flap-wise blade loads of rigid rotors. A simple model of a blade with a flap-wise dynamic absorber is derived to investigate centrifugal force effects on the absorber's rotating frequency. The system's dynamic equations of motion indicate that centrifugal force has no effect on absorber's frequency under rotation. Aeroelastic simulation of the coupled rotor and absorber system based on an elastic beam model in generalised force formulation is used to explore load reduction extent and dynamic absorber behaviour. At a forward speed of 200 km/h, the elastomeric absorber reduces the second harmonic flap-wise root-bending moment by 88.8% for unacceptably large stroke; the Fluidlastic®absorber reduces the moment by 78.5% with a stroke 4.26% of blade chord length. Results indicate that placing an embedded Fluidlastic®absorber in the blade's flap-wise direction has potential for reducing load in different flight states with relatively small strokes. Increasing tuning port area ratio or Fluidlastic®absorber mass can distinctly decrease strokes without increasing loads. However, deviation from normal rotor speed significantly lowers Fluidlastic®absorber performance.

In-plane self-excitation of two-bladed horizontal axis wind turbine rotors due to blade elasticity and gravity

The two-bladed rotor is one of the promising concepts to emerge from the study of super large wind turbines. However, the rotor is prone to generating larger vibrations compared with conventional three-bladed rotors. In particular, in-plane vibration is hard to avoid because its damping is small. Furthermore, blades are becoming more flexible as wind turbines are getting larger. In-plane self-excitation of a 10-MW wind turbine with a two-bladed rotor was studied in this article through aero-elastic simulations. This study shows that even if the blade deformations are almost the same, large rotor in-plane self-excitation does not occur in a three-bladed rotor; however, it does occur in a two-bladed rotor. The self-excitation was shown to be caused by a combination of blade in-plane elasticity and gravity. Furthermore, the mechanism was theoretically demonstrated through simplified models that showed a mass and a spring.

Nonlinear PI control for variable pitch wind turbine

Wind turbine uses a pitch angle controller to reduce the power captured above the rated wind speed and release the mechanical stress of the drive train. This paper investigates a nonlinear PI (N-PI) based pitch angle controller, by designing an extended-order state and perturbation observer to estimate and compensate unknown time-varying nonlinearities and disturbances. The proposed N-PI does not require the accurate model and uses only one set of PI parameters to provide a global optimal performance under wind speed changes. Simulation verification is based on a simplified two-mass wind turbine model and a detailed aero-elastic wind turbine simulator (FAST), respectively. Simulation results show that the N-PI controller can provide better dynamic performances of power regulation, load stress reduction and actuator usage, comparing with the conventional PI and gain-scheduled PI controller, and better robustness against of model uncertainties than feedback linearization control.

Impact of a wind turbine on turbulence: Un-freezing turbulence by means of a simple vortex particle approach

A vortex particle representation of turbulent fields is devised in order to address the following questions: Does a wind turbine affect the statistics of the incoming turbulence? Should this imply a change in the way turbulence boxes are used in wind turbine aero-elastic simulations? Is it acceptable to neglect the influence of the wake and the wind turbine on the turbulent inflow? Is there evidence to justify the extra cost of a method capable of including these effects correctly? To this end, a unified vorticity representation of the flow is used: the wind turbine model is represented by a bound vorticity lifting line while the turbine wake vorticity and the turbulence vorticity are projected onto vortex particles. In the present work the rotor blades are stiff leaving aero-elastic interactions for future work. Inflow turbulence is generated with the model of Mann and converted to vortex particles that are inserted at the inlet of the computational domain. First the quality of the reconstructed turbulent flow field is evaluated and then the wind turbine is added in the simulations. The lack of a driving-force to sustain turbulence is found to give a progressive decay of turbulence away from the insertion point. The presence of the wind turbine and its wake is found to have insignificant effect on upstream turbulence. Finally, the mean velocity profiles in the wake are found to be in good agreement with both lidar measurements and CFD simulations.

Design clustering of offshore wind turbines using probabilistic fatigue load estimation

In large offshore wind farms fatigue loads on support structures can vary significantly due to differences and uncertainties in site conditions, making it necessary to optimize design clustering. An efficient probabilistic fatigue load estimation method for monopile foundations was implemented using Monte-Carlo simulations. Verification of frequency domain analysis for wave loads and scaling approaches for wind loads with time domain aero-elastic simulations lead to 95% accuracy on equivalent bending moments at mudline and tower bottom. The computational speed is in the order of 100 times faster than typical time domain tools. The model is applied to calculate location specific fatigue loads that can be used in deterministic and probabilistic design clustering. Results for an example wind farm with 150 turbines in 30–40 m water depth show a maximum load difference of 25%. Smart clustering using discrete optimization algorithms leads to a design load reduction of up to 13% compared to designs based on only the highest loaded turbine position. The proposed tool improves industry-standard clustering and provides a basis for design optimization and uncertainty analysis in large wind farms.

Aeroelastic deployable wing simulation considering rotation hinge joint based on flexible multibody dynamics

Morphing wings have been developed by several organizations for a variety of applications including the changing of flight ability while in the air and reducing the amount of space required to store an aircraft. One such example of morphing wings is the deployable wing that is expected to be used for Mars exploration. When designing wings, aeroelastic simulation is important to prevent the occurrence of destructive phenomena while the wing is in use. Flutter and divergence are typical issues to be addressed. However, it has been difficult to simulate the aeroelastic motion of deployable wings because of the significant differences between these deployable wings and conventional designs. The most apparent difference is the kinematic constraints of deployment, typically a hinge joint. These constraints lead not only to deformation but also to rigid body rotation. This research provides a novel method of overcoming the difficulties associated with handling these kinematic constraints. The proposed method utilizes flexible multibody dynamics and absolute nodal coordinate formulation to describe the dynamic motion of a deployable wing. This paper presents the simulation of the rigid body rotation around the kinematic constraints as induced by the aeroelasticity. The practicality of the proposed method is confirmed.

A comprehensive investigation of trailing edge damage in a wind turbine rotor blade

Wind turbine rotor blades are sophisticated, multipart, lightweight structures whose aeroelasticity-driven geometrical complexity and high strength-to-mass utilization lend themselves to the application of glass-fibre or carbon-fibre composite materials. Most manufacturing techniques involve separate production of the multi-material subcomponents of which a blade is comprised and which are commonly joined through adhesives. Adhesive joints are known to represent a weak link in the structural integrity of blades, where particularly, the trailing-edge joint is notorious for its susceptibility to damage. Empiricism tells that adhesive joints in blades often do not fulfil their expected lifetime, leading to considerable expenses because of repair or blade replacement. Owing to the complicated structural behaviour—in conjunction with the complex loading situation—literature about the root causes for adhesive joint failure in blades is scarce. This paper presents a comprehensive numerical investigation of energy release rates at the tip of a transversely oriented crack in the trailing edge of a 34m long blade for a 1.5MW wind turbine. First, results of a non-linear finite element analysis of a 3D blade model, compared with experimental data of a blade test conducted at Danmarks Tekniske Universitet (DTU) Wind Energy (Department of Wind Energy, Technical University of Denmark), showed to be in good agreement. Subsequently, the effects of geometrical non-linear cross-section deformation and trailing-edge wave formation on the energy release rates were investigated based on realistic aeroelastic load simulations. The paper concludes with a discussion about critical loading directions that trigger two different non-linear deformation mechanisms and their potential impact on adhesive trailing-edge joint failure. Copyright © 2016 John Wiley & Sons, Ltd.

Dynamic Load Alleviation in Wake Vortex Encounters

This paper introduces an integrated approach for flexible-aircraft time-domain aeroelastic simulation and controller design suitable for wake encounter situations. The dynamic response of the vehicle, which may be subject to large wing deformations in trimmed flight, is described by a geometrically nonlinear finite-element model. The aerodynamics are modeled using the unsteady vortex lattice method and include the arbitrary time-domain downwash distributions of a wake encounter. A consistent linearization in the structural degrees of freedom enables the use of balancing methods to reduce the problem size while retaining the nonlinear terms in the rigid-body equations. Numerical studies on a high-altitude, long-endurance aircraft demonstrate the reduced-order modeling approach for load calculations in wake vortex encounters over a large parameter space. Closed-loop results finally explore the potential of combining feedforward/feedback control and distributed control surfaces to obtain significant load reductions.

EFFICIENT CFD/CSD COUPLING METHODS FOR AEROELASTIC APPLICATIONS

A fast aeroelastic numerical simulation method using CFD/CSD coupling are developed. Generally, aeroelastic numerical simulation costs much time and significant hardware resources with CFD/CSD coupling. In this paper, dynamic grid method, full implicit scheme, parallel technology and improved coupling method are researched for efficiency simulation. An improved Delaunay graph mapping method is proposed for efficient dynamic grid deform. Hybrid grid finite volume method is used to solve unsteady flow fields. The dual time stepping method based on parallel implicit scheme is used in temporal discretization for efficiency simulation. An approximate system of linear equations is solved by the GMRES algorithm with a LU-SGS preconditioner. This method leads to a significant increase in performance over the explicit and LU-SGS implicit methods. A modification of LU-SGS is proposed to improve the parallel performance. Parallel computing overs a very effective way to improve our productivity in doing CFD/CFD coupling analysis. Improved loose coupling method is an efficiency way over the loose coupling method and tight coupling method. 3D wing's aeroelastic phenomenon is simulated by solving Reynolds-averaged Navier–Stokes equations using improved loose coupling method. The flutter boundary is calculated and agrees well with experimental data. The transonic hole is very clear in numerical simulation results.

風力発電システムの独立翼ピッチ操作による翼荷重ゲインスケジューリング制御

A gain-scheduled control approach of blade loads using individual blade pitch manipulation is developed to reduce variations in aerodynamic loads of wind turbines during rotation. The motivation of the study is that the dynamic characteristics of the blade loads depend on the wind speed and blade pitch. The developed approach consists of the gain-scheduled feedback control (PI action) of the rotor speed using collective blade pitch and generator torque manipulations and the gain-scheduled feedback control (P action) of the d- and q-axis blade loads using individual blade pitch manipulation. The gain-scheduling strategy for the blade load control is developed through a numerical analysis of the NREL 5MW-onshore wind turbine-generator system using the aeroelastic simulation model (FAST) and the observed high wind speed data. The sensitive analysis of the proportional gain for the blade load control reveals that the stability limit gain decreases with the increase in the wind speed. The gain-scheduling strategy based on the stability limit gain reduces the variation in the blade loads during rotation as compared with the individual blade pitch manipulation using the fixed gain as well as the collective blade pitch manipulation without the blade load control.

Assessment of wind turbine structural integrity using response surface methodology

In site suitability assessment of wind turbines, it is often the case that one or several wind climate parameters exceed the reference values for the wind turbine design class. In such cases, the IEC 61400-1 requires a load calculation based on the site specific wind climate conditions in order to document the structural integrity. This load calculation demands a significant number of aero-elastic simulations which are time consuming to perform and require expert knowledge. In this paper it is investigated, to which extent the site specific loads can be determined based on a response surface methodology (RSM).Two response surfaces are presented, formulated based on Taylor approximation and Central Composite Design. For each RSM, the model uncertainty is estimated for different combinations of the wind climate parameters, along with the statistical uncertainty introduced by a limited number of random seeds. The results show that fatigue loads during power production, in general, can be assessed accurately using both RSMs. However, central composite design introduces the smallest model uncertainty. For ultimate loads resulting from extreme turbulent inflow, a larger model uncertainty is introduced. Central composite design leads, again, to the lowest model uncertainty. The statistical uncertainty related to the number of aero-elastic simulations is modelled for each RSM using bootstrapping. In general, the statistical uncertainty related to the number of random seeds is larger than the model uncertainty related to the RSMs.

Rotor anisotropy as a blade damage indicator for wind turbine structural health monitoring systems

Structural damage of a rotor blade causes structural anisotropy of the rotor. In rotor dynamic, the anisotropy affects the symmetry of the rotor mode shapes, and the latter can be utilized to detect the blade damage. The mode shape symmetry can be characterized by relative blades’ magnitude and phase. The study examines the potential use of these parameters as rotor damage indicators.Firstly the indicators are studied analytically using a simple 6 degrees-of-freedom model of a rotating rotor. Floquet analysis is used due to the time periodic nature of the considered system. Floquet analysis allows one to perform analytical modal decomposition of the system and study the sensitivity of the damage indicators to the amount of damage. Secondly, operational modal analysis (OMA) is involved to extract the same damage indicators from simulated experimental data, which was synthesized via numerical simulations.Finally, the same procedure was applied to operating Vestas V27 wind turbine, first using the simulated experimental data obtained by using aeroelastic simulation code HAWC2 and then using the data acquired during the measurement campaign on a real wind turbine.The study demonstrates that the proposed damage indicators are significantly more sensitive than the commonly used changes in natural frequency, and in contrast to the latter, can also pinpoint the faulty blade. It is also demonstrated that these indicators can be derived from blades vibration data obtained from real life experiment.

An airplane jig shape design method based on high fidelity aeroelastic simulation

This paper develops a jig-shape design method for the elastic wing of the commercial aircraft with high aspect ratio. Two optimization steps are involved in the design method to optimize the jig shape of the elastic wing with the user-desired cruise aerodynamic performance. The real aeroelastic simulation becomes the fundamental of the design procedure. Structural mesh based CFD solver has been applied to simulate the transonic flow with high fidelity. Finite element based structural model of wing with complex configuration is constructed, and the flexibility matrix is evaluated to predict the deflection of the wing. The high-efficient mesh deformation technique and the data exchange interface between CFD and CSD are implemented, which improves the efficiency and accuracy of current approach. The jig shape design of a baseline commercial aircraft is presented, and the numerical simulation shows that the design method is strong enough to obtain the optimized jig shape efficiently.

A numerical investigation on galloping of an inclined square cylinder in a smooth flow

The phenomenon of inclined square cylinder galloping in a smooth flow is studied numerically using three-dimensional aeroelastic simulations. In keeping with the published experimental data, simulations are conducted at a reduced velocity of 16. The galloping oscillations found in experiments are successfully reproduced. The aerodynamic moments exerted on inclined cylinders are discussed in detail. It is confirmed that Karman vortex shedding has no impact on the galloping oscillation of an inclined cylinder. The hysteresis loop of the aerodynamic moment is found to differ for forward and backward cylinder inclinations. Further study reveals the shape evolution of the hysteresis loop when the cylinder is in forced simple harmonic motion with a varying amplitude. The preliminary results of this investigation show that a cylinder inclined backward gains energy from the flow more easily than a cylinder inclined forward, resulting in a larger galloping amplitude for the former cylinder. This helps to explain published experimental results.

Assessing the Importance of Geometric Nonlinear Effects in the Prediction of Wind Turbine Blade Loads

As the size of commercial wind turbines increases, new blade designs become more flexible in order to comply with the requirement for reduced weights. In normal operation conditions, flexible blades undergo large bending deflections, which exceed 10% of their radius, while significant torsion angles toward the tip of the blade are obtained, which potentially affect performance and stability. In the present paper, the effects on the loads of a wind turbine from structural nonlinearities induced by large deflections of the blades are assessed, based on simulations carried out for the NREL 5 MW wind turbine. Two nonlinear beam models, a second order (2nd order) model and a multibody model that both account for geometric nonlinear structural effects, are compared to a first order beam (1st order) model. Deflections and loads produced by finite element method based aero-elastic simulations using these three models show that the bending–torsion coupling is the main nonlinear effect that drives differences on loads. The main effect on fatigue loads is the over 100% increase of the torsion moment, having obvious implications on the design of the pitch bearings. In addition, nonlinearity leads to a clear shift in the frequencies of the second edgewise modes.

An investigation on wind turbine resonant vibrations

AbstractWind turbine resonant vibrations are investigated based on aeroelastic simulations both in frequency and time domain. The investigation focuses on three different aspects: the need of a precise modeling when a wind turbine is operating close to resonant conditions; the importance of estimating wind turbine loads also at low turbulence intensity wind conditions to identify the presence of resonances; and the wind turbine response because of external excitations. In the first analysis, three different wind turbine models are analysed with respect to the frequency and damping of the aeroelastic modes. Fatigue loads on the same models are then investigated with two different turbulence intensities to analyse the wind turbine response. In the second analysis, a wind turbine model is excited with an external force. This analysis helps in identifying the modes that might be excited, and therefore, the frequencies at which minimal excitation should be present during operations. The study shows that significant edgewise blade vibrations can occur on modern wind turbines even if the aeroelastic damping of the edgewise modes is positive. When operating close to resonant conditions, small differences in the modeling can have a large influence on the vibration level. The edgewise vibrations are less visible in high turbulent conditions. Using simulations with low‐level turbulence intensity will ease this identification and could avoid a redesign. Furthermore, depending on the external excitation, different aeroelastic modes can be excited. The investigation is performed using aeroelastic models corresponding to a 1.5 MW class wind turbine with slight variations in blade properties. Copyright © 2015 John Wiley & Sons, Ltd.

Aeroelastic large eddy simulations using vortex methods: unfrozen turbulent and sheared inflow

Vortex particles methods are applied to the aeroelastic simulation of a wind turbine in sheared and turbulent inflow. The possibility to perform large-eddy simulations of turbulence with the effect of the shear vorticity is demonstrated for the first time in vortex methods simulations. Most vortex methods formulation of shear, including segment formulations, assume a frozen shear. It is here shown that these formulations omit two source terms in the vorticity equation. The current paper also present unfrozen simulation of shear. The infinite support of the shear vorticity is accounted for using a novel approach relying on a Neumann to Dirichlet map. The interaction of the sheared vorticity with the wind turbine is shown to have an important impact on the wake shape. The obtained wake shape are closer to the one obtained using traditional computational fluid dynamics: Results with unfrozen shear do not have the severe upward motion of the wake observed in vortex methods simulation with frozen shear. The interaction of the shear and turbulence vorticity is shown to reduce the turbulence decay otherwise observed. The vortex code implemented is coupled to an aeroelastic code and examples of aeroelastic simulations under sheared and turbulent inflow are presented.

Importance Sampling for Reliability Evaluation With Stochastic Simulation Models

Importance sampling has been used to improve the efficiency of simulations where the simulation output is uniquely determined, given a fixed input. We extend the theory of importance sampling to estimate a system’s reliability with stochastic simulations. Thanks to the advance of computing power, stochastic simulation models are employed in many applications to represent a complex system behavior. A stochastic simulation model generates stochastic outputs at the same input. Given a budget constraint on total simulation replications, we develop a new approach, which we call stochastic importance sampling, which efficiently uses stochastic simulations with unknown output distribution. Specifically, we derive the optimal importance sampling density and allocation procedure that minimize the variance of an estimator. Application to a computationally intensive aeroelastic wind turbine simulation demonstrates the benefits of the proposed approach. Supplementary materials for this article are available online.

On the Importance of Nonlinear Aeroelasticity and Energy Efficiency in Design of Flying Wing Aircraft

Energy efficiency plays important role in aeroelastic design of flying wing aircraft and may be attained by use of lightweight structures as well as solar energy. NATASHA (Nonlinear Aeroelastic Trim And Stability of HALE Aircraft) is a newly developed computer program which uses a nonlinear composite beam theory that eliminates the difficulties in aeroelastic simulations of flexible high-aspect-ratio wings which undergoes large deformation, as well as the singularities due to finite rotations. NATASHA has shown that proper engine placement could significantly increase the aeroelastic flight envelope which typically leads to more flexible and lighter aircraft. The areas of minimum kinetic energy for the lower frequency modes are in accordance with the zones with maximum flutter speed and have the potential to save computational effort. Another aspect of energy efficiency for High Altitude, Long Endurance (HALE) drones stems from needing to minimize energy consumption because of limitations on the source of energy, that is, solar power. NATASHA is capable of simulating the aeroelastic passive morphing maneuver (i.e., morphing without relying on actuators) and at as near zero energy cost as possible of the aircraft so as the solar panels installed on the wing are in maximum exposure to sun during different time of the day.

Hierarchical modeling and optimization of camber morphing airfoil

A variable camber morphing airfoil with compliant ribs and flexible composite skins is studied in a hierarchical modeling framework. The key requirements of a variable camber morphing airfoil are the flexible skin, compliant internal structure and a lightweight actuation system. A biologically inspired, internal structure is adapted to produce continuous camber morphing. The conflicting requirements of the morphing skin, namely low in-plane stiffness and high out-of-plane bending stiffness, are met by designing the composite with curvilinear fiber paths. A coupled aeroelastic simulation and optimization of the proposed 2-D morphing airfoil is computationally complex. Hierarchical computational models of the morphing airfoil are used to decouple the complaint ribs and airfoil skin. In the first level of hierarchy, a fluid-structure interaction is performed with the homogenized beam model of the compliant rib structure and 2-D panel method. In the second level, a finite element model of the camber morphing skin is developed with representative boundary conditions. A multi-objective optimization framework is developed to find the optimal curvilinear fiber paths of the morphing skin to meet the airfoil geometric shape and actuation requirements of the first level. The optimal results show that a significant camber variation and significant changes in aerodynamic properties can be achieved with the compliant skin and airfoil structure considered in this study. The hierarchical modeling framework of the camber morphing airfoil discussed in this paper enhances theoretical understanding of each sub-system and reduces the computational costs.