Root Bending Moment Research Articles

Active aeroelastic wing application on a forward swept wing configuration

Active aeroelastic wing is introduced to benefit the forward swept wing (FSW) aeroelastic performance owing to the FSW's elaborate aeroelastic problem.In this research, a computational aeroelastic (CAE) code was developed to conduct aeroelastic simulations, which presents challenges of data interpolation and flow capturing since torsional deformations instead of bending dominate in FSW deformation and separations on FSW occur earlier due to the boundary layer accumulation. Two simulations are conducted to verify the adaptability and accuracy of CAE method on FSW. CAE simulations are conducted on a straight FSW with control surfaces in two phases. The results from first phase obtained at Mach 0.8 demonstrate that negative deflection of LE and positive deflection of TE can effectively depress the torsion angle and root-bending moment at identical lift. Deflections of LE 0°, TE +20° can reduce LE and TE wingtip displacement by 36.53% and 39.87%, respectively, resulting in 78.97% decline of torsion angle. The second-phase calculations, conducted at Mach 0.9 at different dynamic pressures, illustrate depression of FSW torsion and deformation by LE and TE downward deflection. The LE and TE control effectiveness is always beyond 1 and the control power of the FSW increases with increasing dynamic pressure.

Assessing extreme loads on a tidal turbine using focused wave groups in energetic currents

Tidal stream turbines are subject to large hydrodynamic loads, including those induced by extreme waves. Scale model testing in the laboratory plays an important role in ensuring that full scale tidal turbines are designed and operated in a manner that is appropriate for harsh ocean environments where waves and tidal currents coexist.For the first time, a fully-instrumented scaled tidal turbine is tested in short-duration focused wave groups representative of extreme environmental load cases expected at energetic tidal sites. In this paper, the subsequent variations in rotor-based loads, power and blade root bending moments are reported. These measurements are found to strongly follow the spectral and temporal form of the focused wave conditions, and peak loads and power output are found to exceed current-only values by 85% and 200% respectively. These rotor-averaged values display a high level of repeatability, demonstrating the suitability of focused waves for testing seabed-mounted tidal turbines. Extreme blade loads, which are dependent on angular position relative to wave phase, are captured through rapidly obtained repeat tests. New insight is subsequently gained into loading and response of tidal turbines in extreme sea conditions.

Blade element momentum theory for a tidal turbine

A key hydrodynamic difference between tidal current and wind turbines is the volume-flux constrained flow field in which tidal turbines operate and the resulting streamwise static pressure difference that develops in the flow passage. Blade Element Momentum (BEM) theory is extended to account analytically for the effects of blockage and the development of the static pressure difference in the flow passage and shows agreement in thrust and power predictions to within ±3% of equivalent blade resolved simulations. The confined flow BEM model is employed to study two different power capping strategies: varying the rotational speed with fixed pitch blades; and pitching the blades to feather at constant rotational speed. Pitch-to-feather achieves reduced thrust above rated flow speed which leads to a greater extractable resource than achievable with overspeed control, due to the feedback between device thrust and available tidal resource. Flow confinement is shown to reduce the flow speed at which rated power occurs, and increases the rotor loads and power below rated conditions. It is also shown that root bending moments, which affect fatigue damage rates, increase with flow confinement.

Variation of loads on a three-bladed horizontal axis tidal turbine with frequency and blade position

Sustainable and cost effective design for tidal current turbines requires knowledge of the complex nature of unsteady loads on turbine components including blades, rotor and support structure. This study investigates experimentally the variation with frequency of rotor thrust and torque loads, of streamwise root bending moment on individual blades and of loads on foundation at the bed. Comparisons between these different load spectra are also established. The impact of absolute rotor angular position on blade and rotor thrust loads is also examined. The study is based on measurements from a 1/15 scale, three-bladed, horizontal axis machine tested in a recirculating flume, in onset flows of 3% and 12% turbulence intensity. It is found that for frequencies below the rotational frequency, load spectra are correlated to spectral density of the onset flow velocity. Above the rotational frequency, loads are mainly affected by turbine operation phenomena. The tower shadowing effect is clearly identified through frequency and angular analysis. Finally, thrust loads as experienced by the rotor alone are for the first time compared with streamwise and transverse foundation loads. Higher frequency loads experienced by the tower are shown to be affected by different vortex shedding regimes associated with different regions of the wake. All the experimental measurements presented in this article can be accessed from http://dx.doi.org/10.7488/ds/2423.

Wing twisting by elastic instability: A purely passive approach

The design of morphing structures requires attaining concurrently light-weight, load-carrying and shape adaptable systems that minimize complexity, actuation requirements and part count. Exploiting the external loads to produce local stiffness variations induced by elastic instabilities offers the potential to activate shape deformations purely passively. Following this approach, we present the structural response study of wings capable of attaining compliant buckling-induced twisting deformations. The structure is designed such that a specific level of aerodynamic load triggers the elastic instability in a shear web part of the wing box. The buckling-induced variation in effective shear stiffness, resulting from the development of a stable, reversible diagonal tension field, leads to a twisting deformation. The structural behaviour is characterized by two drastically different mechanical responses, delimited by the onset of elastic instability. The influence of the buckling component design on the attainable change in spanwise angle of attack is investigated for the specific case of a finite composite wing structure under aerodynamic loads. The type of composite material, its thickness, and the fibre orientation are the considered design parameters. The buckling-induced variation in twist angle, and the corresponding aerodynamic pressure redistribution on the wing, is exploited for achieving load alleviation, reducing the global lift coefficient and decreasing the wing root bending moment.

In-blade Load Sensing on 3D Printed Wind Turbine Blades Using Trailing Edge Flaps

As wind turbines become larger the loading on the blades also increases. Controlling a section of the trailing edge of the turbine airfoil is found to reduce load fluctuations on wind turbine blades. Here a detailed experimental setup is described showing the development of a compact airfoil section capable of measuring the surface pressure, root bending moment, and controlling a TEF simultaneously and in time resolved fashion to quantity the influence of a TEF on a wind turbine. This experimental work includes a trailing edge flap that covers 20% of an S833 airfoil with a chord of 178 mm. Surface pressure and blade root strain are measured for varying angles of attack and flap angle. Coefficient of lift and moment are obtained from the 54 pressure taps. The lift and drag forces are also obtained from the strain gages at the root of the blade. A 3D printed blade section is designed and built to house the actuation and sensing on the airfoil. The trailing edge flap was tested inside a 0.61 m wind tunnel as a baseline case and the results showed how the lift, drag, and root moment on the airfoil can change for different flap angles. The coefficient of lift changed by 30% for flap angle of 20°. The entire blade with the flap will also be installed on a 3.4 diameter wind turbine to study the influence of a flap on load variation.

Investigation on the potential of individual blade control for lifetime extension

In recent years the focus of wind energy industry is on reducing levelized cost of energy by rotor upscaling. Moreover, a current topic of interest to both industry and academia is the extension of lifetime to existing wind turbines approaching the end of initial design span. Thus, the need for load alleviation technologies integrated in the design process or for retrofit purposes is becoming more relevant. One of these is individual blade pitch control, a recurring topic in research, with known advantages and weaknesses namely the pitch actuator and bearing wear. The present work suggests such a system incorporating three independent controllers with input the root bending moments on the rotating frame. The linear system used for controller design is based on black box identification of non-linear simulations and filters are used both for the input and output. Different setups of the independent blade control scheme are applied on a 10 MW reference turbine, with a large and highly flexible rotor representative of the current industrial status, under wind conditions as defined by relevant certification standards. The investigation aims on evaluating the system’s performance based on the fatigue load alleviation potential for different components as well as identifying the tradeoff for each design choice. Finally, based on basic assumptions the reductions are translated to possible life time extension for each component based on a combined operation where the new controllers are applied for a percentage of the initial 20 year lifetime.

Detailed analysis of a waked turbine using a high-resolution scanning lidar

Sandia National Laboratories and the National Renewable Energy Laboratory conducted a wake-steering field campaign at the Scaled Wind Farm Technology facility. The campaign included the use of two highly instrumented V27 wind turbines, an upstream met tower, and high-resolution wake measurements of the upstream wind turbine using a customized scanning lidar from the Technical University of Denmark (DTU). The present work investigates the impact of the upstream wake on the downstream turbine power and blade loads as the wake swept across the rotor in various waked conditions. The wake position was tracked using the DTU SpinnerLidar and synchronized to the met tower and turbine sensors. Fully and partially waked conditions reduced the power output and increased the fatigue loading on the downstream wind turbine. Partial wake impingement was found to result in a 10% increase in fatigue loading over the fully waked condition. Rotational sampling of the blade root bending moments revealed that the fatigue damage accrued during full turbine waking, was primarily caused by turbulence within the wake rather than velocity shear, while the partially waked turbine experienced a large 1-per revolution fatigue due to shear. The development of a power to fatigue load metric curve indicated the wake positions where shifting the wake has the most benefit for the waked turbine.

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.

Performance evaluation of a blade-mounted LiDAR with dynamic versus fixed parameters through feedback-feedforward individual pitch and trailing edge flap control

This paper investigates combined feedback-feedforward control of a wind turbine using a blade-mounted telescope LiDAR sensor to measure the inflow wind speed. The objective of this paper is to evaluate the controller performance with fixed telescope parameters compared to the wind speed dependent dynamically changing telescope parameters. First the telescope parameters for both cases are determined, then the robust feedback controllers are extended with an inverse-based feedforward individual pitch and trailing edge flap controller to alleviate the 1P and 2P loads of the flapwise blade root bending moments. A minor performance degradation is observed with the fixed telescope parameters in comparison to the dynamic telescope parameters.

Influence of atmospheric stability on the load spectra of wind turbines at alpha ventus

Dependence between fatigue loads on a wind turbine located in an offshore wind farm and atmospheric stability was analysed by using measurement data of alpha ventus. An investigation of the fatigue load spectra of the wind turbines was performed enabling a detailed evaluation besides the damage equivalent load. Therefore, load spectra of a turbine in free-stream and a turbine experiencing wake inflow conditions were compared. Atmospheric stability was taken into account by means of the Bulk Richardson number. It was shown, how atmospheric stability affects load spectra of tower base and blade root bending moment. Hereby, it was found that few events of high fatigue loading increase the damage equivalent load (DEL) substantially, especially in unstable conditions.

Design and Analysis of Wind Turbine Rotors Using Hinged Structures and Rods

Light weight and high stiffness are key design factors in ensuring cost effectiveness and reliability of wind turbines, especially for the inboard region of the rotor blades. In this study, several novel designs were developed to improve the mechanical performance of the rotor. Experiments were performed on an isolated blade incorporating the new features of a hinged structure and rods. The results validated the effectiveness of these features at alleviating the root-bending moment of the blade under varying wind loads and enhancing the stiffness of the blade. A numerical investigation was carried out to further examine the bending moment distribution, shear and axial force, and rod tension of these novel rotor designs under uniform loads. Longitudinal geometrical variations of the blade were considered in the model. Results showed that two designs realized a favorable bending moment distribution and improved the modal frequencies of the edgewise modes: bisymmetrical rods on a single-hinged structure and interveined symmetrical rods on a cantilevered structure. However, these designs have different deformation mechanisms. In addition, the first group of edgewise modal frequencies of these two designs were improved compared with the traditional rotor design. Their potential values in the application to the design of a lightweight, high-stiffness, and reliable wind turbine rotor were discussed.

Global Optimization of Wing Aerodynamic Optimization Case Exhibiting Multimodality

An investigation into an aerodynamic optimization benchmark case that exhibits multimodality is presented using a global optimization approach. The recently suggested case 6 of the AIAA Aerodynamic Design Optimization Discussion Group involves the drag minimization of a rectangular NACA0012 wing subject to lift and the root bending moment, as well as other geometric constraints. In this paper, optimization of that case using a state-of-the-art constrained population-based global optimization framework is presented. Shape control is achieved via a hierarchical application of the radial basis function domain element approach. Three different fidelities of optimization cases are performed to investigate the multimodality of various aspects of the full problem, with four independent runs of each case being performed. When considering chord optimization, the optimizer converges to two independently identifiable minima with very similar performances. When adding dihedral and sweep variation, further multimodality is introduced, although this multimodality is reasonably well defined, with minima including forward and rearward sweeps, and an upward and a downward winglet. Introducing thickness variables leads to a highly multimodal problem due to the coupling between the chord and thickness variables. The theoretical minimum drag for this case is 24.7 counts, which a number of the runs gets close to achieving.

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.

Trailed vorticity modeling for aeroelastic wind turbine simulations in standstill

Abstract. Current fast aeroelastic wind turbine codes suitable for certification lack an induction model for standstill conditions. A trailed vorticity model previously used as an addition to a blade element momentum theory based aerodynamic model in normal operation has been extended to allow computing the induced velocities in standstill. The model is validated against analytical results for an elliptical wing in constant inflow and against standstill measurements from the NREL/NASA Phase VI unsteady experiment. The extended model obtains good results in the case of the elliptical wing but underpredicts the steady loading for the Phase VI blade in attached flow. The prediction of the dynamic force coefficient loops from the Phase VI experiment is improved by the trailed vorticity modeling in both attached flow and stall in most cases. The exception is the tangential force coefficient in stall, where the codes and measurements deviate and no clear improvement is visible. This article also contains aeroelastic simulations of the DTU 10 MW reference turbine in standstill at turbulent inflow with a fixed and idling rotor. The influence of the trailed vorticity modeling on the extreme flapwise blade root bending moment is found to be small.

Gust and Ground Loads Integration for Aircraft Landing Loads Prediction

A methodology is described to couple unsteady aerodynamic loads with flexible bodies in multibody simulations. A representative civil jet aircraft aeroelastic model is used to evaluate the structural loads due to the combination of gust and ground loads. The implementation of unsteady aerodynamic loads is validated by comparing the results for the wing root bending moment and tip displacement with Nastran transient aeroelastic analyses for a series of gust lengths of a linear free–free aircraft model. The main objective of the paper is to prove that, with the implementation of a time domain formulation for the unsteady aerodynamic loads, a multibody simulation can be used to predict the aircraft aeroelastic loads that are comparable with traditional finite element-based methods. However, the use of a multibody simulation software offers the simulations to other maneuvers such as landings and the use of nonlinear and active structures.

Training requirements of a neural network used for fatigue load estimation of offshore wind turbines

To estimate fatigue loads of wind turbines via neural networks, costly load measurements are needed for training. Thus, our aim was to assess the minimum needed size of the training sample. We focused on the prediction of flapwise blade root bending moments with a neural network of eight inputs. Next to statistical testing of the training sample size, their representativeness compared to a one-year measurement as well as seasonal effects were investigated. Our results showed that training samples of about 2016 records of 10-minute statistics are representative and enable a reliable prediction independent from seasonal effects.

Load reduction of wind turbines using trailing edge flaps

Horizontal axis wind turbines have become an attractive renewable energy source due to their low carbon foot print. It was the aim of this study to reduce fatigue inducing variations of the blade root bending moment and thus enable the construction of lighter structures which further reduces the carbon footprint. Within the Smart Blades project the IWT-7.5-164 wind turbine has been used to investigate the feasibility of using trailing edge flaps as active control mechanism for load reduction. The analysis has been conducted using the German Aerospace Center’s in-house comprehensive rotor simulation code S4 to simulate the aerodynamics as well as the structural deformation. For a tilted rotor without flaps the blade root bending moment acting perpendicular to the rotor plane has been observed to undergo a characteristic cyclic variation due to the tilt and gravity. Adding trailing edge flaps with a constant deflection alters the mean bending moment of the cyclic variation. A following study addressed the effect of using a sinusoidal flapping motion for a range of flapping amplitudes and phases. As a result a configuration has been identified that eliminates the cyclic variation of the blade root bending moment.

Optimal threshold selection in the POT method for extreme value prediction of the dynamic responses of a Spar-type floating wind turbine

This paper concerns the calculation of the short term extreme structural responses of a Spar-type floating wind turbine by applying the peak-over-threshold (POT) method. A new approach is proposed for finding an optimum threshold value by incorporating a declustering algorithm into the POT method. To select the clusters and check the Poisson character of the number of clusters we use the concept of a dispersion index. Then, the method of maximum product of spacing is utilized to estimate the parameters in the Generalized Pareto distribution of the largest values in all the selected clusters over the optimum threshold value. As examples of calculations, optimum threshold values of the out-of-plane and in-plane blade root bending moments and the tower – Spar interface bending moments of the NREL 5-MW OC3-Hywind floating wind turbine have been obtained by utilizing the new approach. The extreme response probability plot based on an optimum threshold value has been compared with the probability plot based on an empirical threshold value, and the accuracy and efficiency of the new approach have been convincingly validated. Goodness-of-fits plots have also been included in this article for testing the accuracy of the proposed new approach. Finally, in order to make a fair comparison, we have also extrapolated the short-term extreme values using the different POT methods and compared them with the reference values obtained directly from a large number of time-domain simulations.

Prediction and Experimental Evaluation of Planar Wing Spanloads for Minimum Drag

An evaluation and analysis of optimum wing spanloads was conducted by comparing wing performance values obtained from theoretical models and experimental wind-tunnel measurements. Three wing spanloads were designed for a prescribed lift and wing root bending moment using a Lagrange multiplier optimization method, with and without incorporating a representative viscous effect. Wind-tunnel models were constructed for each spanload, and experiments were conducted to determine the relative difference in drag measured as changes were made to the design constraints. The results indicate that, for a fixed lift and wing root bending moment, significant decreases in drag from an elliptically loaded wing are achievable by using a higher aspect ratio with a nonelliptic spanload. Under the fixed design constraints, a minimum drag wing was produced by balancing increases in profile drag and decreases in induced drag associated with a planar wing-tip extension. Experimental five-hole probe wake survey measurements were used to determine the spanwise lift distributions of the wing models, which were used to verify the conformity of the experimental spanloads to the predicted values.