Wind Turbine Rotor Blades Research Articles

LiDAR‐based automated UAV inspection of wind turbine rotor blades

AbstractThe global trend indicates that overall wind energy production, both onshore and offshore, will increase drastically in the next decade. Therefore, presently, much effort is focused on optimizing the operation and maintenance of wind turbines, since these are quite challenging and cost‐intensive. To aid or even completely fulfill a specific inspection task, an automated solution is proposed in this paper. The prototype is built on an M300 drone platform from DJI Technology Co. and is presented here. It requires a single, additional 2D‐LiDAR sensor mounted on an upwards frame. The proposed control and path planning algorithms have been tested in the AirSim simulation environment, as well as in local model airfields and at real onshore and offshore wind turbines. As a result, a comprehensive sequential‐phased mission is presented, which reduces the total time required for the inspection routine to approximately 14 min, representing about half the time an expert pilot may need for the same task. Additionally, a platform prototype that may be deployed on a ship's deck for a safe landing is presented. It guarantees instant adhesion upon contact and avoids unwanted drone backlash due to sudden and unexpected ship movement during the landing approach. Further work will focus mainly on additional offshore flight probes, optimizing the landing platform, and tuning the flight algorithms.

Pole-Placement-Based Calibration of an Electromagnetically Realizable Inerter-Based Vibration Absorber (IDVA) for Rotating Wind Turbine Blades

This paper deals with edgewise vibration mitigation of rotating wind turbine blades by means of inerter-based vibration absorber (IDVA), which can be realized both mechanically and electromagnetically. Introducing the electromagnetically-realizable IDVA to the blade forms a 3-degree-of-freedom (3-DOF) blade-IDVA system consisting of the rotating blade, an absorber, and a series inerter-dashpot-spring subsystem. Analytical optimal design formulas of the rotating blade-installed IDVA are then derived using a pole-placement method where the equal-modal-damping-ratio principle and the triple-root-bifurcation condition are applied. The analytical formulas show that the optimal parameters for the blade-IDVA system merely depend on the spinning speed of the rotor given the IDVA location and the absorber mass. Numerical results of the NREL 5 MW wind turbine with optimal IDVA show that optimal IDVA leads to superior performance than optimal TMD in mitigating the blade edgewise vibration and behaves nearly as same as optimal RIDTMD, along with slightly optimal damper parameters variation. This means that the inerter-dashpot-spring system can be deployed flexibly for damping edgewise vibrations of rotating blades.

Bayesian-Based Approach for the Thermographic Measurement of Flow Transition on Wind Turbine Rotor Blades

Bayesian-Based Approach for the Thermographic Measurement of Flow Transition on Wind Turbine Rotor Blades

Optimization and control strategy for wind turbine aerodynamic performance under uncertainties

Aerodynamic performance of wind turbine governs the overall energy efficiency, which has been an ever-lasting research focus in the field of wind power technology. Due to the coupling effect among the highly complex environmental and structural uncertainties, the practical aerodynamic performance may not be reliably predicted. To aggravate, this performance declines with time in service. It is of great significance to efficiently and reliably assess the impact of uncertain factors and reduce these influences on wind turbine aerodynamic performance. This paper establishes an uncertainty analysis and robustness optimization model of wind turbine aerodynamic performance considering wind speed and pitch angle error uncertainties. An approach combined the no-instrusive probabilistic collocation method is used, and the blade element momentum theory is applied to quantify influences of variable uncertainties on NREL 5 MW wind turbine aerodynamic performance. The optimization target is to reduce the sensitivity of wind turbine aerodynamic performance to uncertainties, as well as maintain capture power. The results show that the wind turbine aerodynamic and mechanical performance will be greatly affected with uncertain factors. By optimizing and adjusting wind turbine rotor speed and blade pitch angle, the wind turbine rotor power and thrust load variation can be reduced to 9.14% and 9.36%, respectively, which indeed reduces the uncertainty effects.

Textile-based piezoelectric impact sensors for fibre-reinforced plastic composites

Abstract Bird strikes on wind turbine rotor blades have long-term impacts on fibre reinforced plastic composites. To monitor and document these impacts, integrating sensors into the composite structure is necessary. However, conventional sensors introduce foreign objects that deflect the fibre orientation, weakening the composite and potentially causing damage like delamination. To address this, silver-coated glass fibre sensors have been developed and integrated into the composite structure. Impact tests confirmed the functionality of the developed sensors. These sensors seamlessly integrate into the reinforcing structure due to their textile nature and multi-layered fibre structure. Unlike conventional sensors, they do not introduce foreign objects or create mechanical weak spots. Instead, they are expected to contribute to the overall stability of the component. In summary, integrating impact sensors into fibre reinforced plastic composites is crucial for documenting bird strike damage. Conventional sensors weaken the composite, but silver-coated glass fibre sensors resolve this issue by seamlessly integrating into the structure, enhancing stability and durability.

Detection of erosion damage on airfoils by means of thermographic flow visualization

Cavity-type defects on a wind turbine rotor blade due to erosion lead to an sub-optimal boundary layer flow behavior and result in a reduced annual energy production. To enable an indirect defect detection from far distances, with no contact and no wind turbine stop, the thermographic visualization of the defect wake flow is studied. Experimental investigations and complementary flow simulations on two different airfoil shapes show that the detection is possible above a critical Reynolds number. Furthermore, the wake flow depends on multiple influencing quantities such as the airfoil geometry and the associated chord Reynolds number as well as the cavity position, the cavity shape and the associated cavity Reynolds number. Adjacent cavities can also influence the wake flow behavior. For instance, the cavity position has an influence on the turbulent flow region even after the natural laminar–turbulent flow transition line, and many cavities close to each other shift the transition line towards the leading edge. With increasing aspect ratio as well as increasing inflow velocity, the wake flow turbulence and, thus, the visibility of the wake in the thermographic images becomes higher. However, even if the wake flow from the cavity to the transition line is not continuously visible, a change in the natural transition can still be observed. As a result, thermographic flow visualization enables the indirect detection of cavity-type defects on airfoils and can provide further information about the cavity features.

Aerodynamic shape optimization of wind turbine rotor blades using the continuous adjoint method

Aerodynamic shape optimization of wind turbine rotor blades using the continuous adjoint method

Mode I fracture of thick adhesively bonded GFRP composite joints for wind turbine rotor blades

This article investigates the effects of voids, joint geometry, and test conditions on the quasi-static Mode I fracture performance of thick adhesive Double Cantilever Beam (DCB) joints such as those prevailing in wind industry and shipbuilding. The specimens were made by glass fiber reinforced epoxy adherend and SikaPower®-830 epoxy adhesive in the cm thickness range. Side-grooved shape guided the crack propagation direction and assisted stable propagation, while lower cross-head displacement rates reduced the occurrence of unstable crack propagation and prevented crack deflection. Porosities, which are inevitable due to the high viscosity of the adhesive, led to unstable propagation and promoted crack path deviations. They could also decrease the apparent fracture energy release rate (SERR) since the crack surface is reduced. In conclusion, grooved DCB joints with low void content tested at low displacement rates showed stable crack propagation without significant crack path deviation. This method enables comparison with future adhesive formulations and the refinement of full-scale blade models.

Characterization of a short fiber-reinforced adhesive designed for wind turbine rotor blades

Wind turbine rotor blades are usually made of two half-shells and shear webs that are joined together by structural adhesives. The adhesive joints suffer from longitudinal strains from blade bending combined with shear strains from torsion and shear forces acting on the blade. It is thus important to provide an accurate and reliable characterization of the adhesive as a bulk material to account for cohesive failure.This work focusses on the characterization of a short fiber-reinforced adhesive used in wind turbine rotor blades under uniaxial tension, compression, and shear as well as biaxial tension-compression/shear for both static and fatigue loading. To obtain small scatter in measurement results, a specimen geometry was designed that ensures a clear maximum stress in the test section while minimizing stress concentrations in the transition between the load introduction regions and the test section. The manufacturing process was then designed in such a way that an optimum mixture of resin and hardener was obtained without the inclusion of voids. The latter was verified after manufacturing by means of micro-CT scanning for all specimens. A very extensive test campaign was then carried out in order to quantify stiffness, static strength, fatigue strength, and fatigue-related stiffness degradation. Excerpts of this test campaign are presented in this paper.

Impact of shell structure stiffness on aero-structural coupling in wind turbine rotor blades

Wind turbine rotor blades are heavily loaded composite structures that experience a mixture of aerodynamic, inertial, gravitational, and gyroscopic forces during their operation life. Due to the high loads, the cross-sections of the blades are subjected to in-plane and out-of-plane deformations. The out-of-plane deformations are referred to as shear warping while the in-plane deformations are also called blade breathing. Blade breathing depends on the magnitude of the mechanical loads, which are expressed by means of internal forces and moments, and the stiffness of the blade shell.In this work, the relationships between in-plane cross-sectional deformations and internal loads are investigated. For the quantification of the deformation, a reference blade is studied via 3D finite shell element simulations for different loading scenarios. The cross-section of interest is located at the radial position of maximum chord. To compare the shape of the cross-sections in the undeformed and the deformed configurations, a procedure is proposed to relate the positions of nodes associated with the cross-section of interest in both configurations to a joint coordinate system. The shape of the deformed cross-section is then extracted and compared with the undeformed configuration. The comparison is executed for the individual internal forces and moments, namely flapwise and edgewise bending moments, normal force, shear forces, and torsion moment, respectively. The deformation patterns are discussed and it is addressed how these may influence the aerodynamic behavior of the cross-section under consideration.

Residual stresses in thick composites considering through-thickness temperature gradients during manufacturing

In the composite materials of wind turbine rotor blades, residual stresses form during manufacturing due to the different thermo-chemo-mechanical behaviors of the fibers and the matrix. These residual stresses decrease the ultimate and fatigue strength of the blade because the failure stress and cycles to failure are reached at lower levels of external loading. It is thus of utmost importance to consider residual stresses in the design stage to obtain reliable blade designs.This work deals with the prediction of residual stresses in a thick composite representative for a spar cap in a wind turbine rotor blade. A model is formulated for the thermo-chemo-mechanically coupled problem. An emphasis is given on the non-uniform and time-variable temperature distribution across the thickness of the composite. The different sources of residual stresses (e.g., chemical shrinkage, non-uniform distribution of different thermal expansion coefficients of fibers and matrix, exotherm, etc.) are analyzed, quantified, and compared. The impact of the time-dependent temperature distribution on the formation of residual stresses is highlighted by means of a representative numerical example.

Quantification of process-induced effects on fatigue life of short-glass-fiber-filled adhesive used in wind turbine rotor blades

Cracks in the adhesive joints of wind turbine blades can often cause severe structural blade damage. Effects induced by the manufacturing process can have a significant impact on the cycle number toward crack initiation in the bonding material. This research compared two experimental series of neat adhesive, dog-bone-shaped specimens with different degrees of cure, fiber orientation, and porosity content, which were subjected to static and cyclic tension-tension loading. To enhance the comparability of the two coupon series, the experimental data was normalized. Normalization factors were introduced to compensate for the different properties of the two series. These factors were derived from correction models. After normalization, the S-N values of the two data series were in good agreement in the high-cycle fatigue regime. In the very-low-cycle fatigue regime, however, there was still a lack of agreement in some cases, including the static strength. Moreover, the failure mechanisms were investigated by analyzing images from the fracture surfaces and dedicated samples using X-ray microscopy. The correction models presented can be applied to estimate the design strength of adhesives. This knowledge can then be used to make rotor blade structures more resistant to crack initiation.

Prediction of crack growth at trailing edge bondlines of a wind turbine rotor blade for the assessment of remaining service life

Debonding along trailing-edge joint of a wind turbine rotor blade is one of the often reported failure modes during the rotor blade maintenance inspections. This study focuses on buckling-induced debonding along the trailing edge under in-plane compressive loads due to the gravity forces on the rotor blade. With a priori known debond crack length, an analytical model is used for predicting debond growth rate and assessing the time that takes the crack to reach a critical length. For the growth rate prediction, required input parameters are size of the initial debonding, laminate and adhesive layer thicknesses, elastic and interfacial fracture mechanics properties, gravitational load distribution and position of the initial debonding. The damage model is implemented in excel spreadsheet tool. The predictions of the analytical model were compared with the results from 3D finite element simulations and the difference between the results (energy release rate) was 8%. This study demonstrates how damage specific tools can be developed for predicting remaining useful life of rotor blades.

Design of an element test specimen for fatigue delamination growth of a thick laminate

Following the observation of multiple delaminations growing from a tunnelling crack in the spar cap of a wind turbine rotor blade subjected to cyclic loading, an element test specimen is proposed. Design of the element test specimen to study fatigue delamination of a thick laminate is presented. Complexities inherent to a full structure e.g., curvatures, ply drops, manufacturing defects, etc. are removed to study the fatigue delamination damage mechanism. The element test specimen is made of unidirectional layers with two embedded artificial delamination cracks connected by a tunnelling crack. In this paper, analytical and numerical (2D) models are used to design the element test specimen.

Reliability analysis of 15MW horizontal axis wind turbine rotor blades using fluid-structure interaction simulation and adaptive kriging model

Over the course of the last four decades, the rotor diameter of Horizontal Axis Wind Turbines (HAWTs) has undergone a substantial increase, expanding from 15 m (30 kW) to an impressive 240 m (15MW), primarily aimed at enhancing their power generation capacity. This growth in blade swept area, however, gives rise to heightened loads, stresses and deflections, imposing more rigorous demands on the structural robustness of these components. To prevent sudden failure and to plan effective inspection, maintenance, and repair activities, it is vital to estimate the reliability of the rotor blades by considering all the forces (aerodynamic and structural dynamics) acting on them over the turbine's lifespan. This research proposes a comprehensive methodology that seamlessly combines fluid-structure interaction (FSI) simulation, Kriging model/algorithm and Adaptive Kriging Monte Carlo Simulation (AKMCS) to assess the reliability of the HAWT rotor blades. Firstly, high-fidelity FSI simulations are performed to investigate the dynamic response of the rotor blade under varying wind conditions. Recognizing the computationally intensive nature and time-consuming aspects of FSI simulations, a judicious approach involves harnessing an economical Kriging model as a surrogate. This surrogate model adeptly predicts blade deflection along its length, utilizing training and testing data derived from FSI simulations. Impressively, the Kriging model predicts blade deflection 400 times faster than the FSI simulations, showcasing its enhanced efficiency. The optimized surrogate model is then used to estimate the flap wise blade tip deflection for one million wind speed samples generated using Weibull distribution. Thereafter, to evaluate the reliability of the blades, statistical modeling using methods such as Monte Carlo Simulation (MCS), AKMCS is performed. The results demonstrate the faster convergence of AKMCS requiring only 21 samples, as opposed to 1 million samples for MCS with minimal reduction in the precision of the estimated probability of failure (Pf) and reliability index (β). Demonstrated on the backdrop of an IEA-15MW offshore reference WT rotor blade, the proposed methodology underscores its potential to be seamlessly incorporated into the creation of WT digital twins, due to its near real-time predictive capabilities for Pf and β assessments.

Coupled free vibration analysis of rotating non-uniform cantilever beams by an element-wise Ritz method using local hierarchical functions

The coupled free vibration behaviour of rotating non-uniform cantilever beams is investigated by an efficient and accurate element-wise Ritz method using Bardell’s polynomials as local hierarchical functions. Both bending-twisting and bending-extension coupling is included in the beam model. The same hierarchical set can be suitably modified to satisfy the essential boundary conditions and represent the stretching, bending and rotation of the beam. The global matrices are obtained by minimizing the Lagrangian functional. The inter-element compatibility conditions are satisfied, and a condensation procedure is used to obtain the condensed matrices for the eigenvalue problem. The procedure is validated with examples from published studies and various case studies are presented to demonstrate the accuracy and convergence of the proposed method. A parametric study is presented to show the effect of various parameters on the free vibration response of rotating non-uniform beams. The frequency veering phenomenon is observed during the study. The proposed approach provides a simple and efficient approach for analyzing the free vibrations of rotating structures such as wind turbine rotor blades, helicopter blades, propellers, and structures with strong discontinuities, especially in the preliminary design stage.

An Acoustic Emission IoT Device for Wind Turbine Rotor Blade Condition Monitoring

This paper discusses the development and implementation of a wind turbine rotor blade condition monitoring system based on the acoustic emission non-destructive method. The condition monitoring system’s core is an acoustic emission IoT device capable of detecting active damage on the blades using micro-electro-mechanical acoustic sensors. The signals detected by the sensors are pre-processed in the IoT device and transmitted, via a Wi-Fi access point in the wind turbine nacelle, or via cellular network, to a datadriven application residing in the cloud. The data processing is completed in the cloud, where the information extracted is used to calculate proprietary quantities that are displayed and tracked. These proprietary quantities are related to the structural health of the wind turbine rotor blades and are used to rank the blades in a wind farm according to their condition. This ranking helps the wind farm site operator to prioritize inspection and plan maintenance. This condition monitoring system based on the acoustic emission IoT device has been implemented in several wind farms across North America and Europe.

Improved order of magnitude estimate of rotational effects on HAWTs

This work proposes an improved order of magnitude estimate methodology of rotational effects on wind turbines, by applying the partial pressure field concept to the case of rotating wind turbine blades, which allows isolating acceleration that rises as a result of the rotation of the blades; then an order of magnitude scaling is performed to estimate the partial pressure due to rotation for both fully attached flow and massively separated flow. In the case of fully attached flow, it was found that rotational effects are negligible; while in the case of massively separated flow, rotational effects were found to be important. The pressure reduction was found to scale linearly with the aspect ratio inside the separated boundary layer, while it scales quadratically with the aspect ratio in the outer inviscid flow. A new estimate of the Rossby number is proposed, which is found to be the inverse of the pressure reduction coefficient estimate in the outer inviscid flow. Then, a semi-empirical model of the pressure reduction is proposed based on the order of magnitude estimate in the outer inviscid flow. The proposed semi-empirical model is then compared against the reference data of three wind turbines, namely, MEXICO, NREL phase VI, and AVATAR 10 MW rotors; along with the already known centrifugal pumping model. The results showed that the centrifugal pumping alone, inside the separated boundary layer, is not sufficient to produce the large force augmentation observed in experimental data. In contrast, the improved semi-empirical model provided a good agreement with experimental data, thanks to the quadratic scaling of the aspect ratio. And, the new estimate of the Rossby number proved to be advantageous over the existing Rossby number estimate in terms of the dependency on the angle of attack. Furthermore, the tangential and drag force coefficients were found to be strongly dependent on the airfoil type and its corresponding pressure shape. In contrast, the normal and lift force coefficients were found to be less dependent on the airfoil type. The proposed semi-empirical model is still strongly empirical, future work needs to account for the complex effect of airfoil type and its consequent interaction with the outer inviscid flow through the boundary layer.

On the failure behavior of a GFRP composite cylinder under Brazier crushing force – An experimental and computational study

The induced ovalization or out-of-plane deformation in the circular and transition airfoils of modern MegaWatt (MW) size wind turbine rotor blades is caused by the combined effect of the applied far-field bending loads and the local structural curvature. In general, the phenomenon of the ovalization or out-of-plane deformation of the circular elastic cross sections under the applied bending load or the internal/external pressure is termed the Brazier effect [1 2]. Considering the poor mechanical performance in the out-of-plane direction of the typically used laminated glass composites for wind turbine rotor blade construction, the current research work aims to develop a thoroughly validated computational methodology to understand the ovalization (out-of-plane deformation) induced failure behavior in circular cross-sections located between the root and transition region of a rotor blade, using a substructure level testing and modeling under representative load conditions. To this end, detailed manufacturing, testing, and a computational modeling campaign is devised and executed at different length scales starting from coupon to cylinder (substructure) level. The required elastic, strength, and fracture properties for the cylinder progressive damage analysis are evaluated using coupon-level experiments following various ASTM standards.The developed numerical methodology is thoroughly validated using the substructure level experimental load–displacement curves, strain, and damage profiles. Detailed experimental and numerical studies reveal that the induced ovalization leads to a non-linear relation between the applied crushing force and the local radius of curvature until the unstable collapse of the cylinder. Based on the thorough local stress and damage analysis, it is evident that along with the dominant interlaminar shear stress (ILSS), the presence of the interlaminar tensile (ILT) stress component is the key to the delamination induced collapse of the cylinder.

Defect detection and classification of offshore wind turbine rotor blades

ABSTRACT With increasing demand for electricity, wind turbines have gained significant attention from the public. Offshore wind power generation has emerged as a popular choice due to its potential to reduce power transmission losses and minimal impact on humans and organisms. However, it also presents challenges in detecting defects in Wind Turbine Rotor Blades (WTRB) and repairing them. To address this issue, this paper proposes an improved wavelet-based S-U-Net network to denoise WTRB images followed by a weakly supervised CNN method for removing background parts that could affect defective feature extraction. Defective features are then extracted using VGG16 network on a deep learning server platform, while an enhanced Particle Swarm Optimization (PSO) algorithm combined with K-means is used to classify defect features of WTRBs. Experimental results demonstrate that classification accuracy of unlabelled blade defect datasets improved significantly from 62.6% using only Kmeans clustering method up to 96.4% using our proposed algorithm approach. This study applies improved PSO and Kmeans algorithms towards offshore Wind Turbine Rotor Blade condition monitoring with precise detection test results enabling early-stage detection of Wind Turbine Rotor Blades defects leading to timely repairs.