Estimating design reliability of composite rotor blades under ultimate loading

Structural analysis for designing rotor blades with integral actuators

The paper describes a computational simulation approach for durability, damage tolerance (D&DT) and reliability of composite wind turbine blade structures in presence of uncertainties in material properties. This computer-based prediction methodology combines composite mechanics with finite element analysis, damage and fracture tracking capability, probabilistic analysis and a robust design algorithm to reduce weight of turbine bales without loss in structural durability and reliability. A composite turbine blade was first assessed with finite element based multi-scale progressive failure analysis to determine failure modes and locations as well as the fracture load. Analysis D&DT results were validated with static test performed at Sandia National Laboratories. The work was followed by detailed weight analysis to identify contribution of various materials to the overall weight of the blade. The methodology ensured that certain types of failure modes, such as delamination progression, are contained to reduce risk to the structure. Probabilistic analysis indicated that composite shear strength has a great influence of the blade ultimate load under static loading. Weight was reduced by 12% with robust design without loss in reliability or D&DT. It was achieved by replacing a small volume of key materials with foam.

Dynamic Model-based Saddle-point Approximation for Reliability and Reliability-based Sensitivity Analysis

This report presents the study of fluid-structure interaction (FSI) of a 5kW horizontal axis wind turbines (HAWT). The motivation of the current investigation is to address the FSI problem of HAWT in the whole flow field analysis with hybrid turbulence model method which is used to find out the flow and structure characteristics of wind turbine in operation under different tip speed ratio (TSR). The target of current work is to obtain detailed information of flow field and structure response simultaneously using the finite element analysis (FEA) method as well as providing new reliable and accurate approach which could be quickly and economically employed in wind turbine FSI problem analysis. In this report, a detailed and comprehensive literature review related to FSI problem of wind turbine has been presented and includes aerodynamic, wind turbine theory, computational fluid dynamic, fluid structure interaction and aeroelastic stability. A 5kW wind turbine is designed according to blade element momentum (BEM) method and a 3D wind turbine modeling is generated in SolidWorks. The 3D wind turbine modeling is imported into FEA software by coupling with the structure domain and the fluid domain to achieve FSI analysis and different TSR are applied to the computational fluid dynamics (CFD) analysis to gain the wind turbine operation information under different scenarios. The simulation results are investigated to reveal the influence of the FSI phenomenon on wind turbine performance. Analysis of wind turbine aeroelastic stability is conducted to prevent blade from experiencing flutter problem. A multiple objective optimization method for wind turbine design including aerodynamic and structure has been developed to improve wind turbine performance. The flow field analysis results showed that the strength of vortex at the tip and root of the blade increases as wind speed increases when wind turbine is under operating conditions. With wind speed of 10m⁄s, most of the flow over the lower surface remains attached and the wind turbine blade was running under design condition. As the wind speed increases, flow separation emerges. When the wind speed increasing to 16m⁄s, the wind turbine blade works under deep stall condition. The wind turbine power coefficient drops continuously from 0.46 to 0.22, as wind speed increased from 10m⁄s to 20m⁄s. The comparison of wind turbine power and torque coefficient under different CFD configuration shows the advantage of whole flow field analysis with hybrid turbulence model. The structural analysis results demonstrated that most of the pressure force acts at a region around the 1⁄4 chord from the leading edge of the blade under operating condition and the displacement of blade tip in flap direction is bigger than those in the other directions. Modal analysis results deduced that flap is the main type of vibration in first and second modes. The difference of natural frequency between single blade and blade rotor becomes larger as vibration order increases. The dynamic performances of wind turbine blade and rotor are totally different and show the whole flow field method is better than the one third domain method. The wind turbine aeroelastic analysis results illustrate that the bending frequency and torsion frequency are around 23.94Hz and 38.46Hz respectively under TSR=4 and both move towards 30Hz under TSR=1.7. When TSR is smaller than 1.7 the wind turbine will experience flutter due to the overlapping of bending and torsion frequencies. The multiple objectives optimization method takes the aerodynamic performance and blade loading into account to gain better design parameter. The comparison between initial and optimal design shows that optimized method could provide higher electrical energy around 1073kW•h per annum under rated wind speed. The optimized wind turbine critical flutter wind speed will increase from initial design speed of 35m/s to 38m/s under optimization design.

This paper presents a dynamic response analysis of the blade of horizontal axis wind turbines using finite element method. The blade is treated as a thin-walled beam based on the classical lamination theory, and accounts for arbitrary material layup and non-linear anisotropic fibre-reinforced composites. Applying the proposed method,A 29 m rotor blade, previously reported in specialized literature, was chosen as a case study the dynamic behaviour built in a FEM software tool. It is developed to predict natural frequencies and corresponding vibration modes in rotating blade in-plane and out-of-plane. Numerical results are serve as a design tool for the large composite wind turbine blade structure required during design and optimization.

This contribution presents modal testing of a 2‐MW wind turbine on a 100‐m tubular tower with a 93‐m rotor developed by W2E Wind to Energy GmbH. This research is part of the DYNAWIND project of the University of Rostock and W2E. Beside classical modal analysis schemes, this contribution mainly focusses on the application of operational modal analysis techniques to a wind turbine. Specific problems are addressed, and hints for modal testing on wind turbines are given. Furthermore, an effective measurement setup is proposed for identification of the modal parameters of a wind turbine. The measurement campaign is divided in two parts. First, a measurement campaign using 8 sensor positions on a rotor blade was done while the rotor is lying on ground. Second, a detailed measurement campaign was done on the entire wind turbine with the rotor locked in Y position using 61 sensor positions on the tower, the mainframe, the gearbox, the generator, and the low‐voltage unit. While the rotor blade was tested by classical and operational modal analysis techniques, the entire wind turbine was tested by operational modal analysis techniques only. The mode shapes and eigenfrequencies of the wind turbine identified within the measurement campaigns are within the expected range of the design values of the wind turbine. But in contrast, the damping ratios differ strongly from those given in guidelines and literature. Furthermore, a strong influence of aerodynamic damping compared to structural damping is observed for the first tower mode even for a parked wind turbine.

This paper focuses on a reliability analysis of an offshore wind turbine support structure which is part of an assessment and monitoring framework for wind turbines in operation. The reliability analysis builds upon structural, loading, limit state and uncertainty models comprising design, production and erection data. This model basis facilitates the reliability analysis of the ultimate, the fatigue and the serviceability limit states utilizing stochastic finite elements. The complexity of the individual models dictates an efficient solution scheme for the reliability analysis. Such an algorithm is developed in the present paper consisting of an adaptive response surface algorithm and an importance sampling Monte-Carlo algorithm. The response surface algorithm is based on predetermined experimental designs and facilitates the adjustment of design parameters for an optimized prediction variance in the design point region. Approaches for the consideration of multiple design points and the augmentation of the design for reduction of the prediction variance are introduced. In this paper, a reliability analysis for a tripod support structure of a Multibrid M5000 wind turbine is performed. A comparison with the target reliabilities specified in DIN EN 1990 (2002) shows that the requirements are fully met. However, the consideration of system reliability leads to the conclusion that at the end of the service life there is a significant probability of fatigue damages. The quantification of the reliability for the individual structural components for all limit states facilitates an identification of sensitive components. The results of this study can support the targeted application of monitoring systems, the optimization of the support structures and additionally highlight the need for criteria to the systems reliability.

Additional investigation of the optimization of cross-sectional properties for rotor blades was performed with methodology based on the Genetic Algorithm (GA). In previous work by the authors, a multilevel optimization was conducted using gradient-based methods that are appropriate for continuous variables. In contrast to that work, here optimization is conducted with the main focus of assessing the impact of discrete variables that are inherent to the problem at both local and global levels. The GA was modified based on existing routines in MATLAB to make the code suitable for the methodology presented and to utilize interfaces with the multi-body dynamics program DYMORE, used for global analysis, and the cross-sectional analysis program VABS, used at the local level. At the first exploratory phase of the optimization, values of discrete variables are determined using the GA along with several promising regions for global minima that serve as starting points for the second phase optimization, which deals only with continuous variables. Furthermore, if more than one optimal design exists, further analysis is performed to pick the best design from among candidate configurations. ptimization of the cross-sectional layout of composite rotor blades is a challenging problem because of the large number of design parameters and the wide-ranging impact of those parameters on overall vehicle characteristics. The availability of high-fidelity structural beam analysis tools naturally allows a hierarchical decomposition of the problem that separates the local (cross-sectional) sub-problem from the global optimization. By encapsulating the intricate details of the cross-sectional design into a relatively small number of equivalent beam properties, one can significantly reduce the overall complexity of the optimization. An additional advantage of such a decoupled approach is that it follows a venerable tradition of analyzing rotorcraft systems by splitting three-dimensional (3D) problems into separate sub-problems over the crosssection (2D) and span (1D). Modern computer tools for analyzing composite rotor blades with arbitrary cross-sectional configurations such as VABS 1, 2 provide a fast and accurate means of assessing both static and dynamic structural properties of rotor blades in terms of an equivalent beam models. Calculation of the cross-sectional stiffness matrix is conducted using a finite element discretization. Results of this calculation also include a set of cross-sectional stress/strain recovery relations. Once the equivalent cross-sectional stiffnesses are obtained, a beam analysis can be conducted very rapidly, for various loading and boundary conditions as appropriate. Results from the 1D analysis can then be fed back into 2D recovering relations to obtain detailed stress and strain components over the cross-section. The overall analysis possesses accuracy comparable to that of 3D finite element methods with a reduction in computational cost by two to three orders of magnitude. 3 Optimization based on a parametric cross-sectional model was considered in Ref. 4. The layout was sufficiently realistic to allow optimization of a baseline cross-section in which the ply angles, thicknesses, and spar location were varied in order to place the shear center in a specified location while holding the crosssectional stiffnesses within a specified tolerance. This work was focused on the lower level of optimization and was continued with a still higher level of realism in Ref. 5.

Abstract. The measurement of deformation and vibration of wind turbine rotor blades in field tests is a substantial part of the validation of aeroelastic codes. This becomes highly important for modern rotors as the rotor size increases, which comes along with structural changes, resulting in very high flexibility and coupling between different vibration modes. However, performing full-scale field measurements for rotor blade deformation is not trivial and requires high temporal and spatial resolution. A promising deformation measurement technique is based on an optical method called digital image correlation (DIC). Recently, DIC measurements on a Siemens Gamesa SWT-4.0-130 test turbine were performed on the tip of all blades in combination with marker tracking at the hub for the first time with synchronised measurement of the inflow conditions by a ground-based lidar. As the turbine was additionally equipped with strain gauges in the blade root of all blades, the DIC results can be directly compared to the actual prevailing loads to validate the measurement method. In the end, an example for a comparison of the measured deformations and torsion with aeroelastic simulations is shown in the time and frequency domain. All in all, DIC shows very good agreement with comparative measurements and simulations, which shows that it is a suitable method for measurement of deformation and torsion of multi-megawatt wind turbine rotor blades.

Composite wind turbine blades may experience interlaminar damage, including adhesive failure, cracking, and interlaminar fracture failure, from manufacturing or external fatigue load. Among these, the adhesive failure of adhesive joints is critical. Therefore, it is important to identify the failure mechanism in the adhesive joints between the spar cap–shear web and trailing edge of composite blades. We calculated fracture toughness through Mode I, Mode II, and Mixed-mode tests for quantitative analysis of adhesive joints. Then, to select a modeling method for realizing the damage generated in the blade, the method was verified from the specimen level. A damage model was constructed, considering contact conditions of the spar cap–shear web and trailing edge for an NREL 5MW wind turbine blade. Finally, a damage model based on cohesive zone modeling was used to analyze the progressive failure behavior of debonding at adhesive joints according to the external force applied to the blade.

The increasing use of onshore wind energy is leading to an increased deployment of wind turbines in structurally rich habitats such as forests. Forest-affiliated bats, in turn, are at risk of colliding with the rotor blades. Due to the legal protection of bats in Europe, it is imperative to restrict the operation of wind turbines to periods of low bat activity to avoid collisions. However, bats have also been observed to avoid wind turbines over several hundred meters distance, indicating a displacement that cannot solely be explained by modifications to the habitat. This avoidance suggests a displacement of bats by indirect factors related to wind turbine operation, e.g., wake turbulences and noise emissions. Therefore, we investigated whether the activity of forest-affiliated bats is influenced by operation mode (on/off) under variable wind conditions along transects from 80 to 450 m distance to wind turbines. We divided recordings by foraging guild, i.e., either narrow-space (Myotis, Plecotus), edge-space (Pipistrellus, Barbastella), or open-space foraging bats (Nyctalus, Eptesicus, Vespertilio), and analyzed the effects of wind turbine operation and wind speed on the recorded bat guild activity with mixed effects models. The acoustic activity of narrow-space foraging bats decreased by 77% with increasing wind speed when wind turbines were operating, while bat activity remained unaffected by wind speed when turbines were not operating. This was neither observed for open-space foraging bats nor for edge-space foraging bats, and neither wind turbine operation nor wind speed (ranging between 0 – 4 m/s at 10 m height above ground) were found to affect bat activity when considered alone. Wind turbine noise emissions are known to increase with rotor speed and consequently, wind speed, thus presenting a likely explanation for the interactive negative effect of turbine operation and wind speed specifically on noise-sensitive narrow-space foraging bats. To understand potential ecological long-term consequences for bat populations in forest areas with wind turbines and to design effective conservation measures, future research should focus on disentangling the effects of different disturbances related to turbine operation.

Horizontal-axis wind turbines are sophisticated technical constructions that require modern design methods for further improvement and higher economic output. Thorough understanding of the whole turbine and its components is indispensable for this aim. The technological development in the last decades indicates the progress in this area and shows the potential of optimizations due to the improvement of rotor blade aerodynamics by additional aerodynamic devices. In this project the effect of an aeroelastic flap mounted on the trailing edge of a rotor blade on energy yield was investigated. For this purpose a flexible flap of fiberglass reinforced plastic for the profile at 70% of the rotor blade span has been designed for the trailing edge and analyzed using a 2-way fluid-structure-interaction simulation. In the first step the simulation is reduced to a two-dimensional investigation using the cross-section at 70% diameter of the rotor blade. Compared to an unmodified profile, the aeroelastic flap increases the lift-to-drag-ratio of the profile in an angle of attack range between 3.5° to 9.5°. The aeroelastic flap has a positive effect on the glide ratio, similar to a non-elastic, static flap.

Estimation of operational parameters of the counter-rotating wind turbine with artificial neural networks

Optimal design and sensitivity analysis of airfoil-shaped rotor blade for Savonius wind turbine by using response surface methodology

The twin beam composite rotor blade