Root Flapwise Bending Moment Research Articles

Aeroelastic analysis of a very large wind turbine in various atmospheric stability conditions

With the growing trend towards larger wind turbine rotor diameters, the impact of wind shear on rotor performance and loads becomes increasingly significant. Atmospheric stability strongly influences wind shear, leading to higher wind shear under stable atmospheric conditions. In this study, the aeroelastic performance of the IEA 22 MW rotor is assessed under inflow conditions generated by different methods. Inflow conditions were generated using turbulence models specified in the IEC Standards and also by Large Eddy simulations. Standalone OpenFAST simulations were conducted with the respective inflow conditions.It was found that at rated and above-rated wind speeds, the time-averaged wind turbine design loads were higher in stable atmospheric conditions, in comparison to the IEC NTM inflow conditions, while the opposite held for below-rated wind speeds. Specifically, the time-averaged root flapwise bending moment and rotor thrust were found to be higher by up to 7% in stable atmospheres. However, maximum design and fatigue loads were considerably higher in the IEC NTM case due to elevated turbulence levels. Compared to the IEC NTM case, the damage equivalent root flapwise bending moment was found to be 30% to 70% lower in the different scenarios.

Hybrid-Lambda: a low-specific-rating rotor concept for offshore wind turbines

Abstract. We introduce an aerodynamic rotor concept for an offshore wind turbine which is tailored for an increased power feed-in at low wind speeds by a substantial increase in the rotor diameter while maintaining the rated power. The main objective of the conceptual design is to limit the steady-inflow loads (blade flapwise root bending moment (RBM) and thrust) to the maximum values of a reference turbine. The outer part of the blade (i.e. outer 30 % span) is designed for a higher design tip speed ratio (TSR) and a lower axial induction than the inner part. By operating at the high TSR in light winds, the slender outer part fully contributes to the increased power capture. In stronger winds the TSR is reduced and the torque generation is shifted to the inner section of the rotor. Moreover, the blade design efficiently reduces the power losses when the flapwise RBM is limited through peak shaving, below rated wind speed. This is of high importance, given the wind speed distribution at offshore sites. The characteristics of the rotor are first investigated with stationary blade element momentum simulations and further analysed with aeroelastic simulations, considering the flexibility of blades and tower to show that a structural design is feasible even for a blade of this size and complexity. The economic revenue and the cost of valued energy of the turbine are estimated and compared to the IEA 15 MW offshore reference turbine, considering a fictitious wind-speed-dependent feed-in price. Our results for the turbine concept with an increase in rotor diameter by 36 % show that the revenue can be increased by 30 % and the cost of valued energy can be reduced by 16 % compared to the reference turbine.

Gaidai-Xing reliability method validation for 10-MW floating wind turbines

In contrast to well-known bivariate statistical approach, which is known to properly forecast extreme response levels for two-dimensional systems, the research validates innovative structural reliability method, which is particularly appropriate for multi-dimensional structural responses. The disadvantage of dealing with large system dimensionality and cross-correlation across multiple dimensions is not a benefit of traditional dependability approaches that deal with time series. Since offshore constructions are built to handle extremely high wind and wave loads, understanding these severe stresses is essential, e.g. wind turbines should be built and operated with the least amount of inconvenience. In the first scenario, the blade root flapwise bending moment is examined, whereas in the second, the tower bottom fore-aft bending moment is examined. The FAST simulation program was utilized to generate the empirical bending moments for this investigation with the load instances activated at under-rated, rated, and above-rated speeds. The novel reliability approach, in contrast to conventional reliability methods, does not call for the study of a multi-dimensional reliability function in the case of numerical simulation. As demonstrated in this work, it is now possible to assess multi-degree-of-freedom nonlinear system failure probability, in the case when only limited system measurements are available.

Extreme coherent gusts with direction change – probabilistic model, yaw control, and wind turbine loads

Abstract. Observations of large coherent fluctuations are used to define a probabilistic model of coherent gusts with direction change. The gust model provides the joint description of the gust rise time, amplitude, and directional changes with a 50-year return period. The observed events are from a decade of measurements from a coastal site in western Denmark, making the derived gust model site specific. In conjunction with the gust model, a yaw controller is presented in this study to investigate the load implications of the joint gust variables. These loads are compared with the design load case of the extreme coherent gust with direction change (ECD) from the IEC 61400-1 Ed.4 wind turbine safety standard. Within the framework of our site-specific gust model we find the return period of the ECD to be approximately 460 years. From the simulations we find that for gusts with a relatively long rise time the blade root flapwise bending moment, for example, can be reduced by including the considered yaw controller. From the extreme load comparison of the ECD and the modeled gusts we see that by including the variability in the gust parameters the load values from the modeled gusts are between 20 % and 74 % higher than the IEC gusts.

A novel design approach for estimation of extreme load responses of a 10-MW floating semi-submersible type wind turbine

Offshore structures are constructed to withstand extreme wind and wave-induced loads, so studying these extreme loads is vital as it allows offshore structures, e.g., wind turbines, to be designed and operated with minimal disruption. A novel statistical model that is precise and meticulous will facilitate these extreme load values to be estimated accurately. Bivariate average conditional exceedance rate (ACER2D) method was utilized in this paper. This multivariate statistical analysis is more appropriate than a univariate statistical analysis for complete structures, e.g., wind turbines, since it can extrapolate the extreme values with better accuracy. This paper uses this ACER2D method to explore a novel approach to estimating the extreme load responses of a 10-MW semi-submersible type floating wind turbine (FWT). Two cases are considered to understand the feasibility of the ACER2D on the extreme load responses. The first case analyses the blade root flap wise bending moment, while the second one analyses the tower bottom fore-aft bending moment. Based on the performance of the proposed novel method, the ACER2D method can offer better robust and precise bivariate predictions of the bending moments of the FWT.

Vortex-model-based Multi-objective Optimization of Winglets for Wind Turbines using Machine Learning

Different Design Driving Load constraints (DDLs), are explored in this work to determine under which constraints and conditions a winglet can have an added value to the wind turbine blade design. Multi-objective Bayesian optimization is used to maximize the rotor’s power production while minimizing the flapwise DDLs. Surrogate models, created using machine learning techniques such as Gaussian Processes and Bayesian Neural Networks, are used in combination with an acquisition function, to determine what designs should be evaluated by the lifting line model AWSM, with the goal to obtain designs that lie on the Pareto front of two or more objectives. The recent Bayesian Neural Networks as surrogate model were able to find the Pareto-front most effectively in this work. Furthermore, the results show that different DDL constraints led to different winglet designs, with noticeable differences between upwind and downwind winglet designs. Winglet designs were found to be able to increase power without increasing the thrust, root flapwise bending moment and flapwise bending moment at radial locations on the blade. A noticeable increase in power was found when introducing sweep to the winglet design.

Innovative aerodynamic rotor concept for demand-oriented power feed-in of offshore wind turbines

We introduce an aerodynamic rotor concept for a 15 MW offshore wind turbine which is tailored for an increased power feed-in at low wind speeds. The main objective of the conceptual design is to limit the stationary loads (blade flapwise root bending moment (RBM) and thrust) to the maximum value of the IEA 15 MW offshore reference turbine, while greatly increasing the swept rotor aera. The outer part of the blade (e.g. outer 30% of the rotor) is designed for a higher design tip speed ratio (TSR) and a lower axial induction than the inner part. By operating at the high TSR in light winds, the slender outer part fully contributes to the increased power capture. In stronger winds the TSR is reduced and the torque generation is shifted to the inner section of the rotor. Moreover, the blade is designed in a way that makes the limitation of the flapwise RBM through peak shaving aerodynamically more efficient. With static blade element momentum simulations the characteristics of the rotor are investigated and the economic revenue of the turbine is estimated, considering a wind speed dependent feed-in price. Our results show that the revenue can be increased by 30% compared to the reference turbine with an increase of rotor diameter by 36%.

Active rotor coning for a 25 MW downwind offshore wind turbine

A two-bladed downwind turbine system was upscaled from 13.2 MW to 25 MW by redesigning aerodynamics, structures, and controls. In particular, three 25-MW rotors were developed, and the final version is a fully redesigned model of the original rotor. Despite their radically large sizes, it was found that these 25-MW turbine rotors satisfy this limited set of structural design drivers at the rated condition and that larger blade lengths are possible with conewise load-alignment. In addition, flapwise morphing (varying the cone angle with a wind-speed schedule) was investigated to minimize mean and fluctuating blade root bending loads using steady inflow proxies for the maximum and lifetime damage equivalent load moments. Compared to the fixed coned rotor case, morphing can provide an Annual Energy Production (AEP) increase of 6%, and the maximum blade root flapwise bending moment increases 21% (still under the constraint, i.e., 10% of the ultimate moments) as a trade-off. The resulting series of 25-MW rotors can be a valuable baseline for further development and assessment of ultra-large-scale wind turbines.

Parameter varying control of wind turbine smart rotor for structural load mitigation

This study investigates the Linear Parameter Varying (LPV) control of a Horizontal Axis Wind Turbine (HAWT) smart rotor in order to alleviate the structural loads. The smart rotor blades are equipped with actuators called Deformable Trailing Edge Flap (DTEF). These flaps benefit from a continuous airfoil deformation on the trailing edge in order to change the aerodynamic properties of blades. The objective of using a flap is to reduce the amplitude of load fluctuations in high wind speeds to decrease the fatigue loads on the wind turbine components and increase their working life. Here, the aerodynamic behavior of NACA 64-418 airfoil in the presence of DTEF deflection is studied numerically. The airfoil with 10% chord deformation is mounted on 5 MW NREL reference wind turbine blades in 67% to 95% of its length relative to the blade root. The nonlinear aeroelastic FAST code is used to determine the effects of DTEFs on structural load mitigation. Consequently, the modeling of nonlinear wind turbine system using LPV approach is discussed including the angle of DTEFs as a control input. A robust LPV controller has been designed using Linear Matrix Inequality (LMI) solution along the operating trajectory to guarantee the internal stability and H∞ robust performance of the closed-loop system in the presence of modeling uncertainties as well as wind turbulence. In sum, the idea of this study is the aerodynamic modeling of the wind turbine equipped with a smart rotor, designing an LPV controller and comparing the results with the conventional PI gain-scheduled controller. The principal objective is to reduce the amplitude of blade root flapwise bending moment oscillations about its mean values along the operating trajectory. As a summary of the results, a reduction in blade root flapwise bending moment fatigue load has been observed by 33% based on Damage Equivalent Load (DEL) calculation.

Analysis of turbulence models fitted to site, and their impact on the response of a bottom-fixed wind turbine

This study compares a wind field recommended by the wind turbine design standards to more realistic wind fields based on measurements. The widely used Mann spectral tensor model with inputs recommended by the standard, is compared to FitMann, the Mann model with inputs fitted to measurements and TIMESR; using measured time series combined with the Davenport coherence model. The Mann model produces too low energy levels at the lowest frequencies of the wind spectra, while the wind spectra generated by FitMann approaches the measured values. TIMESR reproduces the measured spectral values at all frequencies. The different models give similar vertical coherence, while the Mann and FitMann models give lower horizontal coherence than TIMESR. Investigating the wind loads on a bottom-fixed 10-MW wind turbine, the spectra for the tower bottom fore-aft and blade root flapwise bending moment follow the shape of the wind spectra closely at low frequencies. The low-frequency range is important for the blade root and in particular the tower bottom bending moment. Thus, the TIMESR model, followed by FitMann, is assumed to give the most accurate fatigue estimates. For the specific situation analysed in this study, the FitMann model gives only 18 and 5 % lower estimates than TIMESR of the tower bottom and blade root damage equivalent bending moments, while the Mann model gives 27 % and 12 % lower estimates. The tower top yaw and fore-aft bending moments depend on the wind coherence. For the specific situation analysed in this study, the FitMann model gives 9 and 5 % higher estimates of the tower top yaw and tower top damage equivalent (bending) moments compared to TIMESR, while the Mann model gives 23 % and 18 % higher estimates. Since only measurements of the vertical coherence are available, it is not clear which model is superior for the tower top moments. However, the importance of a proper coherence model is documented.

Simulation of an airfoil with a deformable flap applicable in wind turbine structural load reduction

Flow over an airfoil equipped with Deformable Trailing Edge Flap (DTEF) has been numerically studied in a two-dimensional steady-state condition with various angles of attack. The airfoil is NACA 64-418, and the flap angle is defined by changing camber-line geometry at 10% chord length from the trailing edge. It has been shown that the direction of the flap deflection has significant impacts on aerodynamic behaviors, which provides an extra means to adjust wind turbine structural loads. Simulations have been conducted with aerodynamic-aeroelastic FAST code in the form of an open-loop control scheme to determine the DTEF's performance in a wind turbine. The wind turbine behavior has been plotted and compared for various flap angles. The load-variation ranges of the wind turbine have been identified, which help determine their sensitivity to flap changes. Finally, an open-loop control circuit is aimed at reducing the amplitude of oscillations of the blade root flapwise bending moment.

Comparative study of short-term extreme responses and fatigue damages of a floating wind turbine using two different blade models

In this work, two different blade structural models are used to estimate the blade deformations and the global structural responses of a 10MW floating offshore wind turbine (FOWT). One model is based on the Euler-Bernoulli beam theory and it is solved by the linear normal mode superposition method. The other model is based on the geometry exact beam theory (GEBT) which can consider the full geometric nonlinearity and large deformation. The control equations of GEBT are discretized by Legendre spectral finite elements. The aero-hydro-servo-elastic fully coupled numerical simulations are conducted in the open-source analysis tool OpenFAST to explore the feasibility of the two different structural models for modeling large scale wind turbine blades. Both the steady-state and dynamic results show that power generation and thrust on rotor are similar for the different blade models. There is a small difference in the results of the blade pitch angle and flapwise and edgewise blade root bending moment at high wind speeds due to the lack of torsion degree of freedom in the mode-based method. The difference between the two models is mainly reflected in the prediction of blade tip deformations. The one-hour short-term extreme blade root bending moments and the damage equivalent fatigue loads at blade root are both compared based on the two models. For edgewise bending moment, the extreme value of GEBT model is found at cut-out wind speed, whereas the linear beam model predicts the extreme value around rated wind speed. For the flapwise bending moment, the extreme value is captured around the rated wind speed for both of the two models, but GEBT model presents a larger value. As for fatigue loads, the short-term 1 Hz damage equivalent loads calculated based on the linear beam model are smaller than GEBT model at almost all load cases for both edgewise and flapwise root bending moment, which implies that the linear beam model may underestimate the life time fatigue damage at blade root.

Rotor Loads Reduction by Dynamically Extendable Chord

Dynamically extendable blade chord sections show promise for reducing helicopter rotor loads. A rotor model based on an elastic beam concept, and capable of predicting helicopter power, is used. A stiff in-plane four-bladed rotor with a shape similar to the UH-60A rotor is used as a baseline for comparisons. For the control of the 4/rev (per revolution) vertical hub force, it is not beneficial to actuate the extendable chord at hover and low-speed flight. At a high speed of , the extendable chord, with a width of 10% rotor radius and responded to an extension magnitude of 10% chord length, obtained a maximum force reduction of 89.4%. The magnitude of the dynamic chord needs to be optimized according to the flight state. The performance can be enhanced by increasing the extension or width of the dynamic chord. The dynamically extendable chord was not suitable for reducing the 2/rev blade flapwise root bending moment. However, a 3/rev dynamic chord extension showed great potential in reducing the 3/rev flapwise root bending moment and the 4/rev rotor rolling and pitching moments simultaneously. The effectiveness of a 5/rev dynamic chord in reducing the 4/rev rotor rolling or pitching moment degraded significantly as compared with a 3/rev actuation.

Effect of Platform Motion on Aerodynamic Performance and Aeroelastic Behavior of Floating Offshore Wind Turbine Blades

In the present study, a numerical framework for predicting the aerodynamic performance and the aeroelastic behavior of floating offshore wind turbine rotor blades involving platform motion was developed. For this purpose, the aerodynamic and structural analyses were conducted simultaneously in a tightly coupled manner by exchanging the information about the aerodynamic loads and the elastic blade deformations at every time step. The elastic behavior of the turbine rotor blades was described by adopting a structural model based on the Euler-Bernoulli beam. The aerodynamic loads by the rotor blades were evaluated by adopting a blade element momentum theory. The numerical simulations were conducted when the platform of the wind turbine independently moves in each of the six degrees-of-freedom directions consisting of heave, sway, surge, roll, pitch, and yaw. It was observed that flexible blades exhibit complicated vibratory behaviors when they are excited by the aerodynamic, inertia, and gravitational forces simultaneously. It was found that the load variation caused by the platform surge or pitch motion has a significant influence on the flapwise and torsional deformations of the rotor blades. The torsional deformation mainly occurs in the nose-down direction, and results in a reduction of the aerodynamic loads. It was also found that the flapwise root bending moment is mainly influenced by the platform surge and pitch motions. On the other hand, the edgewise bending moment is mostly dictated by the gravitational force, but is not affected much by the platform motion.

An investigation of the impact of wind speed and turbulence on small wind turbine operation and fatigue loads

This paper investigates the operation and loading of a 5 kW HAWT using the aeroelastic code FAST. Wind data from built environment site at Port Kennedy (PK) and from a flat terrain site in Östergarnsholm (OG), are analysed and compared with IEC 61400-2. The longitudinal turbulence intensity (TIu) in the PK wind field was 22%; which was higher than the estimated value in IEC 61400-2 Normal Turbulence Model. The TI in the flat terrain (OG) was below 18% for all mean wind speeds. The selected wind conditions from the two locations were used as input in FAST simulation to investigate the performance and loading of the turbine. The elevated turbulence in PK wind fields increased the output rotor power which was more than that predicted by the standard. Similarly, PK wind field also showed higher blade root flapwise bending moment resulting into twice as much damage load on the turbine blades due to large short-term fluctuations in both wind speed and direction. This value for OG was below the standard's prediction. We observe that the current IEC standard seems inadequate for urban siting of SWTs and requires modification for more reliable deployment in turbulent sites.

Blade Element Momentum Study of Rotor Aerodynamic Performance and Loading for Active and Passive Microjet Systems

This study investigates the performance of microjets for load reduction on the NREL-5 MW wind turbine and identifies optimal system parameters. Microjets provide blowing normal to the blade surface and can rapidly increase or decrease lift on a blade section, enabling a wind turbine to respond to local, short-term changes in wind condition. As wind turbine rotors become larger, control methods that act on a single blade or blade section are increasingly necessary to reduce critical fatigue and extreme loads. However, microjets require power to operate, and thus, it is crucial that the fatigue reduction justifies any energy input to the system. To examine the potential for fatigue reduction of a range of potential microjet system configurations, a blade element momentum (BEM) code and a flow energy solver were used to estimate the energy input and the change in primary fatigue metrics. A parametric analysis was conducted to identify the optimal spanwise position and length of the microjets over a range of air mass flow rates. Both active and passive air supply methods were considered. A passive microjet system applied to the NREL 5-MW rotor produced a 3.7% reduction in the maximum flapwise root bending moment (FRBM). The reduction in the peak bending moment increased to 6.0% with a 5 kPa blower that consumes approximately 0.1% of the turbine output power. The most effective configurations placed microjets between the blade midspan to three-quarters span. Load reduction was achieved for both active and passive modes of air supply to the microjet system.

Adaptive State Feedback Pitch Angle Control of Wind Turbines for Speed Regulation and Blades Loadings Alleviation

This research article presents an adaptive state feedback blades pitch control of wind turbines for speed regulation and structural blades loadings mitigation in high wind speeds. The proposed control is a combination of a state feedback control and an integrator. It enables to improve the generator speed tracking, as well as to reduce the flap-wise bending moment at the root of the blades. The motivation principal of the proposed controller is that its structure allows surpassing the trade-off between the control objectives. The control scheme is tested on the NREL’s FAST 5 MW baseline wind turbine model in MATLAB/Simulink under different wind speed profiles and its performances are compared with those of the baseline gain scheduling proportional integral (GSPI) control and the disturbance accommodation control (DAC). The simulation results enable to conclude that the proposed controller has good performances in term of generator speed tracking as well as blade root flap-wise bending moment fluctuations reduction. Moreover, the controller is tested to be robust under system uncertainties.

Load reduction based on a stochastic disturbance observer for a 5 MW IPC wind turbine

Control and operation systems of wind turbines must primarily ensure the fully automatic operation of wind turbines in a constantly changing environment. Economic efficiency charges the control system to ensure that the highest possible efficiency is achieved and the mechanical loads caused by disturbances are minimized. The ability of an observer, in this case a Kalman filter (Kf), to estimate non-measurable states from a set of measurements using a model of the plant suggests the idea of extending the model of the plant by a model of the disturbance. Disturbance states thus can be reconstructed and an easy-to-determine quasi-disturbance-feedforward controller can be used to reject them. This method is called Disturbance-Accommodating Control (DAC). In this paper, Dryden’s turbulence model-which shapes a white noise signal via a form filter to meet spectrum conditions - and an inverse notch filter to model the rotational sampling effect are used for each blade, in contrary to the hitherto used deterministic disturbance models or the simple random walk models for stochastic turbulence. Measurement- and model-uncertainties are described as uncorrelated white noise. With this approach, the requirements of the Kf derivation are met and quantitative measures for the Kf process noise covariance matrix are available especially for the disturbance. The simplified tuning process and the high potential for load reduction are demonstrated for the NREL 5 MW Wind turbine. The reduction by a factor of 4.4 of the standard deviation of the flapwise root bending moment shows the high potential of this stochastic DAC approach. A parameter study to determine the influence of the turbulence spectrum bandwidth and to identify the dependency of the stochastic DAC approach on uncertainties of the process noise covariance matrix was performed. The study shows that the Kf is robust against a wide spectrum of parameter variations. Only if the time constant of the Dryden filter is significantly reduced, the performance is decreased.

Design and simulation of Macro-Fiber composite based serrated microflap for wind turbine blade fatigue load reduction

A Macro-Fiber Composite (MFC) based active serrated microflap is designed in this research for wind turbine blades. Its fatigue load reduction potential is evaluated in normal operating conditions. The force and displacement output of the MFC-based actuator are simulated using a bimorph beam model. The work done by the aerodynamic, centripetal and gravitational forces acting on the microflap were calculated to determine the required capacity of the MFC-based actuator. MFC-based actuators with a lever mechanical linkage are designed to achieve the required force and displacement to activate the microflap. A feedback control scheme is designed to control the microflap during operation. Through an aerodynamic-aeroelastic time marching simulation with the designed control scheme, the time responses of the wind turbine blades are obtained. The fatigue analysis shows that the serrated microflap can reduce the standard deviation of the blade root flapwise bending moment and the fatigue damage equivalent loads.

Large Wind Turbine Rotor Design using an Aero-Elastic / Free-Wake Panel Coupling Code

Despite the advances in computing resources in the recent years, the majority of large wind-turbine rotor design problems still rely on aero-elastic codes that use blade element momentum (BEM) approaches to model the rotor aerodynamics. The present work describes an approach to wind-turbine rotor design by incorporating a higher-fidelity free-wake panel aero-elastic coupling code called MIRAS-FLEX. The optimization procedure includes a series of design load cases and a simple structural design code. Due to the heavy MIRAS-FLEX computations, a surrogate-modeling approach is applied to mitigate the overall computational cost of the optimization. Improvements in cost of energy, annual energy production, maximum flap-wise root bending moment, and blade mass were obtained for the NREL 5MW baseline wind turbine.