7.4 - Optical Rotor-Blade Deformation Measurements using a Rotating Camera

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.

Knowledge of propeller or rotor blade behaviour under real operating conditions is crucial for optimizing the performance of a propeller or rotor system. A team of researchers, technicians and engineers from Avia Propeller, DLR, EVEKTOR and HARDsoft developed a rotating stereo camera system dedicated to in-flight blade deformation measurements. The whole system, co-rotating with the propeller at its full speed and hence exposed to high centrifugal forces and strong vibration, had been successfully tested on an EVEKTOR VUT 100 COBRA aeroplane in Kunovice (CZ) within the project AIM2—advanced in-flight measurement techniques funded by the European Commission (contract no. 266107). This paper will describe the work, starting from drawing the first sketch of the system up to performing the successful flight test. Apart from a description of the measurement hardware and the applied IPCT method, the paper will give some impressions of the flight test activities and discuss the results obtained from the measurements.

An analytical model capable of predicting the induced deformation due to the presence of bend-twist and stretch-twist elastic couplings in multi-cell closed thin walled beams with arbitrary cross-sections is presented. For various structural and material configurations, the results obtained by the developed model are compared with the results of the finite element analysis. It is shown that the developed analytical model provides reasonable accuracy in predicting induced twist. The developed model is implemented in an aero-structure simulation environment for simulation of wind turbines utilising adaptive blades. Keywords: thin-walled beam, multi-cell closed thin walled beams, adaptive blades, elastic coupling, bending-twist coupling, wind turbine composite blade 1 Introduction Fibrous composite materials have been broadly used in aeronautical and aerospace structures due to their proven advantages, such as high strength-weight ratios. One particular application of these materials is in fabricating smart and adaptive aerodynamic lifting surfaces such as wind turbine adaptive blades and aircraft smart wings. An adaptive blade acts as an open-loop controller that senses the wind velocity or rotor speed variations and adjusts its aerodynamic characteristics accordingly to improve the wind turbine performance. This self-control system can be achieved by implementing elastic coupling in the structure of the blade. In order to determine the aerodynamic performance of adaptive blades at various loading conditions, a structural analyser is required to calculate the induced deformation of the blade. Figure (1) shows the simulation environment for wind turbines utilising this type of blades. Torsional deformation of an adaptive blade is the key parameter influencing the wind turbine aerodynamic performance. The accuracy of the predicted torsional deformation is crucial in simulation and design of adaptive blades [1, 2]. A number of analytical models of anisotropic thin-walled beams have been proposed for box beams. Most of researchers employed strain energy and virtual work methods to study the static and dynamic characteristic of thin- and thick-walled beams. Chandra et al. [3] and Kim and White [4] developed analytical models for circumferentially asymmetric stiffness (CAS) and circumferentially uniform stiffness (CUS) for single-cell box beams. They considered the bending, torsional and extensional loads and included shear and warping effects. Their model was, however, limited to rectangular cross-sections. Wu et al. [5]

Typically, the characteristics of a compressor are computed using the hot geometry provided for the design working point. However, the actual shape of the compressor blade will be different due to different deformation under different operating conditions. The deformations are mainly caused by centrifugal and aerodynamic loads. This paper focuses on the impact of aerodynamic loads on blade deformation and calculated aerodynamic performance. The results show that neglecting aerodynamic forces during blade deformation computation leads to an increase in compressor mass flow rate and pressure ratio, particularly in high aspect ratio fans. Therefore, the impact of aerodynamic loads on blade deformation and aerodynamic performance cannot be neglected for high aspect ratio blades.

Health monitoring of wind turbine blades in operation using three-dimensional digital image correlation

Digital Image Correlation became one of the dominant optical full-field techniques used in experimental mechanics. It does not require sophisticated equipment to evaluate the full displacement vector field of the specimen under test with great accuracy. In this paper, we would like to put the reader's attention to the DIC technique as a solution that may identify the genuine character of the load applied to the object under test. Our experience with testing wind turbine blades using DIC proves that possibility. Full Text: PDF References E.M.C Jones, M.A. Iadicola, "A good practises guide for digital image correlation" International Digital Image Correlation Society, (2018), CrossRef B. Pan, "Digital image correlation for surface deformation measurement: historical developments, recent advances and future goals", Meas. Sci. Technol. 29, 082001 (2018), CrossRef B. Pan, L.P. Yu, Q.B. Zhang, "Review of single-camera stereo-digital image correlation techniques for full-field 3D shape and deformation measurement", Sci. China Technol. Sci. 61, 2 (2018), CrossRef S. Sharma, N. Iniyan Thiruselvam, S.J. Subramanian, G.S. Kumar, "Computation of strains from stereo digital image correlation using principal component analysis", Meas. Sci. Technol. 32, 105201 (2021), CrossRef IEC 61400-5:2020 Wind energy generation systems - Part 5: Wind turbine, DirectLink IEC 61400-23:2014 Wind turbines - Part 23: Full-scale structural testing of rotor blades, DirectLink

The current paper presents approaches to evaluate the equivalent modal viscous damping ratios for a wind turbine tower, using Fourier and wavelet analysis based linear regression. In the Fourier analysis, the estimated damping ratios are constant, but in the wavelet analysis, temporally varying damping ratios using a time-segmented least squares approach can be identified. The estimation of time varying damping is important in assessing the risk of negative aeroelastic damping in wind turbines. In absence of experimental data, the proposed approaches have been illustrated on numerically simulated response obtained from a simple wind turbine model. The model used for generating the response time history data for the wind turbine tower is composed of a flexible tower and rotor blade system, inter-connected using a substructuring technique, which facilitates the blade/tower coupling. A model order reduction technique is first used to model each of the two substructures (tower/nacelle and rotor system) as single-degree-of-freedom systems (DOF), allowing both systems to be later coupled together to form an equivalent two-DOF coupled tower/blade wind turbine tower model with equivalent viscous damping. A wind-induced forced vibration analysis of the coupled system is then carried out using artificially generated wind drag time histories. Two identification procedures based on (a) Fourier and (b) wavelet analysis, and linear regression, are used to solve the inverse problem for evaluating the first- and second-equivalent modal damping ratios of the coupled system. For the numerical example presented in order to demonstrate the applicability of the proposed approaches, good agreement was observed between the originally specified modal damping ratios and the subsequently estimated values. The estimation of damping using wavelet analysis techniques can be applied to situations where damping may vary with time. The proposed approaches can be used without any limitation on response data obtained for more complex models of turbines and loadings or on experimentally measured data from wind turbines.

Fault Diagnosis and Fault Tolerant Control with Application on a Wind Turbine Low Speed Shaft Encoder

In the present wind energy research, Darrieus-type vertical-axis wind turbines are increasingly appreciated, especially in small installations. In particular, H-shaped turbines can provide attractive spaces for novel design solutions, aimed at reducing the visual impact of the rotors and then at improving their degree of integration with several installation contexts (e.g. a built environment); moreover, novel small rotors are thought to be able in the near future to make large use also of new and cheaper materials (e.g. plastic or light alloys) which could notably reduce the final cost of the produced energy. As a consequence, the structural analysis of the rotors becomes more and more important: a continuous check between the design solutions needed to maximize the aerodynamic performance of the blades and the structural constraints must be provided. In addition, the requirements of international standards for certification encourage the development of proper numerical tools, possibly with low computational costs for the manufacturers. In this study, the structural analysis of a novel three-helix-bladed Darrieus turbine is presented; the turbine is a real industrial machine almost entirely produced with plastic with a new complex aesthetic design. In detail, a structural modeling based on beam elements has been developed and assessed in comparison to more complex models, as it was thought to provide a notable reduction of the computational cost of the simulations with an acceptable decrease of the accuracy. Moreover, the 1D structural model was exploited to verify the capabilities of a novel software, able to verify the dynamic response of the wind turbine in real functioning, i.e. with mechanical loads and interactions with the wind flow. Benefits and drawbacks of the proposed modeling approach are finally discussed by analyzing both the calculation time and the accuracy of the simulations.

Stereo photogrammetry measurements of the deformation of simple, flat-plate, clipped delta wings that were undergoing aeroelastic oscillations in transonic flow are described. The measurements were made in High Reynolds number Channel 2 at NASA Ames Research Center. Two types of aeroelastic responses were measured: 1) highly damped responses in which the tip of the model was deflected and then released at subcritical dynamic pressures; and 2) responses excited by perturbations in the flow as the dynamic pressure was slowly increased until the model became unstable. The model was imaged by three synchronized black-and-white video cameras, and the data were recorded on videotape. Correlated images from two of these cameras were used to estimate the space coordinates of reference marks on the model at each instant, and the motion of each mark was estimated by fitting the deflection data with a damped sinusoid. The resulting estimates of the frequency and damping of the first-bending-mode oscillations compared favorably with strain-gauge measurements. A large increase in the phase difference between motions of the leading and the trailing edges was observed just before the models became unstable.

The vertical axis wind turbine (VAWT) is considered an important device to utilize the renewable and sustainable wind energy. However, the relatively lower power coefficient has hampered its development. Therefore, this paper attempts to investigate the effect of swept blade employed in a VAWT on the enhancement in aerodynamic performance. A series of swept blades were studied in a small VAWT rotor, and the RANS SST k-ω turbulence model was utilized to simulate the flow field. The numerical model was validated against experimental data, and the aerodynamic performance was investigated with respect to force coefficients, vorticity distribution, static pressure distribution, and wind turbine wake, respectively. The results indicated that the swept blade could effectively increase power outputs by about 20% for the wind turbine. By reducing the drag coefficient, a high lift-drag ratio was achieved. And this configuration prevented the blade from suffering severe dynamic stall. Besides, the swept blade changed the distribution of low wind speed area in the wind turbine wake, which should be considered in the wind farm. It was concluded that this work provided a new way for the practical design and optimization of wind turbine.

Application of videometric technique to deformation measurement for large-scale composite wind turbine blade

Helicopter's blade during its rotation accomplishes a complex spatial movement and deformation. This report is devoted to Blade Deformation Measuring System (BDMS) which was developed to measure parameters of the real-scaled blade movement and its bending-twisting deformation. This system operates under videogrammetry principles. The basic principles of registration of video-images and processing of results of measurements are stated. Brief description of the system design is given. The results of the laboratory investigations of system features are discussed as well. This system was tested during experimental investigations of blade deformations of the real upper rotor of Kamov-26 helicopter with radius 6.5 m on the VP-5 rotor test facility in TsAGI's T-101 wind tunnel. The tests have confirmed serviceability of system and have shown opportunity of twist deformation measurements of the blade with a root-mean-square error lower 3 - 6 angular minutes and bend deformations measurements with an error of 0.3 - 0.6 mm. The technique of calibration of Blade Deformation Measurement System and appropriate software had been developed and tested.

The need for energy and electricity has been increasing globally, and this means more power is required from the power plants. Power plants, however, will then continue harming the earth because of the greenhouse gasses produced while generating energies that contribute to global warming. Using renewable sources to produce clean energies is one of the sustainable methods to deal with such challenges. Wind energy is one of the renewable sources, which is accessible anywhere on earth, creating green energy. Wind turbines are mainly categorized into Horizontal Axis Wind Turbines (HAWT) and Vertical Axis Wind Turbines (VAWT). This paper firstly presents a general comparison between the HAWTs and VAWTs. Then, it presents mathematical modelling for the aerodynamic factors of HAWT and Darrieus VAWT to assist the researchers to understand some key design aspects of wind turbines, such as lift/drag ratio, tip speed ratio, power coefficient, and torque coefficient. Also, this paper presents a review of the aerodynamic performance of the recent VAWT designs to help researchers to identify and choose the best model among the Savonius and Darrieus rotors for further development or designing a new model at different wind conditions. This comparison review shows that for a large scale HAWT upwind 3 bladed wind turbines are the most optimum. The helical Savonius rotors perform better by having positive torque coefficient at all azimuth angles. Moreover, helical Darrieus was found to produce lesser noise and suitable for conventional areas. hybrid Savonius-Darrieus rotors can solve the self-starting challenge of the VAWTs, and they are suitable at low wind speeds. At last, this review shows some of the recent hybrid Savonius-Darrieus rotors which would help to solve the low efficiency of Savonius rotor and self-starting challenge of Darrieus rotors.

Nonlinear aeroelastic response of wind turbines using Simo-Vu-Quoc rods