Wind Turbine Rotor Research Articles

On the cyclic stability diagram of offshore wind turbine supported on multiple foundations in saturated dense sand

This study explored the issue of cyclic loading on a tripod suction bucket foundation system in silty sand through three-dimensional (3D) finite element (FE) models by developing a cyclic stability diagram, in which the response is presented in terms of the average load ratio (ALR) and the cyclic load ratio (CLR), and three regions are identified: (1) no cyclic load effects, (2) cyclic load effects, and (3) severe load effects after approximately 1000 one-way load cycles. The simulations, based on dynamic FE analysis, enabled the identification of the ‘’self-healing’’ phenomenon due to enhanced stiffness degradation of a foundation in tension by incorporating the correct estimate of the soil-foundation interface and geometric nonlinearities. In addition, an approximating expression for the calculation of the accumulated rotation of offshore wind turbine (OWT) supported on multiple foundations is provided as a function of ALR and CLR utilizing modified least-squares method (MLSM). The proposed expression leads to notable improvements in the prediction of induced rotation of OWTs such that the coefficient of determination, R2, was obtained ranging from minimum value of 0.89 to a maximum value of 0.99. This study suggests that the salient features of multiple foundations must be considered in the design process of offshore wind turbines.

Investigations on offshore wind turbine inflow modelling using numerical weather prediction coupled with local-scale computational fluid dynamics

The computational power available nowadays to industry and research paves the way to increasingly more accurate systems for the wind resource prediction. A promising approach is to support the mesoscale numerical weather prediction (NWP) with high fidelity computational fluid dynamics (CFD). This approach aims at increasing the spatial resolution of the wind prediction by not only accounting for the complex and multiphysics aspects of the atmosphere over a large geographical region, but also including the effects of the fine scale turbulence and the interaction of the wind flow with the sea surface. In this work, we test a set of model setups for both the mesoscale (NWP) and local scale (CFD) simulations employed in a multi-scale modelling framework. The method comprises a one-way coupling interface to define boundary conditions for the local scale simulation (based on the Reynolds Averaged Navier–Stokes equations) using the mesoscale wind given by the NWP system. The wind prediction in an offshore site is compared with LiDAR measurements, testing a set of mesoscale planetary boundary layer schemes, and different model choices for the local scale simulation, which include steady and unsteady approaches for simulation and boundary conditions, different turbulence closure constants, and the effect of the wave motion of the sea surface. The resulting wind is then used for the simulation of a large wind turbine, showing how a realistic wind profile and an ideal exponential law profile lead to different predictions of wind turbine rotor performance and loads.

Investigation of the Causes of Premature Rain Erosion Evolution in Rotor Blade-like GFRP Structures by Means of CT, XRM, and Active Thermography

Premature rain erosion damage development at the leading edges of wind turbine rotor blades impair the efficiency of the turbines and should be detected as early as possible. To investigate the causes of premature erosion damage and the erosion evolution, test specimens similar to the leading edge of a rotor blade were modified with different initial defects, such as voids in the coating system, and impacted with waterdrops in a rain erosion test facility. Using CT and XRM with AI-based evaluation as non-destructive measurement methods showed that premature erosion arises from the initial material defects because they represent a weak point in the material composite. In addition, thermographic investigations were carried out. As it shows results similar to the two lab-based methods, active thermography has a promising potential for future in-situ monitoring of rotor blade leading edges.

CFD modelling of a fan with a cycloidal rotor

Despite the wide possibilities of using a cycloidal rotor in the form of propulsion systems for unmanned aerial vehicles (UAV), cycloidal propellers for sea-going ships, rotors of wind turbines or sea or river cycloidal energy converters, there is practically no research on the use of this solution in the form of a cycloidal rotor fan (CRF) for HVAC (heat, ventilation & air conditioning) applications. The main features of such a machine, compared to conventional solutions, is a possibility of changing the flow direction only by changing pitch angles of the rotor blades. This study analysed two variants of the fan, first equipped with an asymmetrical CLARK Y profile, and other with a symmetrical NACA0012. Numerical simulation of cycloidal rotor fan developed in Ansys CFX was presented, that enables the simulation of fan operation. The results obtained from CFD for both variants were compared with those obtained during experimental measurements made with the use of Laser Doppler Anemometry (LDA). The comparison showed good agreement between the numerical analysis and the performed experiment. Despite the operation based on the same cycloidal regulation settings and rotation speed, the fan equipped with symmetrical blades slightly more curved the flow angle than the asymmetrical one.

Mathematical simulation of autonomous wind electric installation with magnetoelectric generator

Purpose. Development of a mathematical model of an autonomous wind power plant based on an end-generator with a double stator and combined excitation to evaluate methods for improving the efficiency of conversion of mechanical wind energy into electricity. Methodology. The research used methods of general theory of wind power plants, methods of mathematical modeling, which are based on the numerical solution of nonlinear differential equations to evaluate methods for correcting the output power in Matlab-Simulink by modifying standard units. Findings. A numerical simulation mathematical model of an autonomous wind power plant consisting of a magnetoelectric generator with an axial magnetic flux with combined excitation and a double stator has been developed. The model was created to study the parameters and characteristics of the installation, as well as to evaluate methods and means to improve the efficiency of conversion of wind energy into electricity. According to the research, it is established that a more effective method for regulating the output power of the generator in the wind turbine is the use of additional winding for magnetization, compared with the use of additional capacity. The latter provides up to 716% increase in output power, while using the magnetizing winding can increase the output power to 3235%. The results obtained by the authors allow further developing a number of methods to increase the efficiency of conversion of mechanical energy of the wind turbine rotor into electrical energy. Originality. The mathematical model developed for the first time, in contrast to the existing ones, takes into account the presence of a double stator, an additional winding for magnetization of the magnetic system and the axial nature of the circuit of the main and additional magnetic flux. The developed model also takes into account the change in the parameters of the electric generator with axial magnetic flux when changing the parameters of the wind, the rotor of the wind turbine and the load. The model is designed to analyze the possibility of adjusting the output power of the generator when the wind speed changes. Practical value. The simulation results indicate the prospects of industrial implementation of wind power plant based on magnetoelectric generator for their use as autonomous electrical installations and as part of shunting power systems.

Rotors of Vertical-Axial Wind Turbines Assembled in Bearings and Aerodynamic Characteristics of a Blade with Unclosed Wing Profile

Abstract In world practice, traditional blades used in high-speed wind turbines, both horizontal-axial and vertical-axial, have a wing-shaped profile. However, for horizontal-axial wind turbines, blades with such a profile have a fairly narrow range of operating values of the angle of attack of the incoming air flow and a low value of the moment of pulling from place. As for vertical-axial wind turbines, the self-starting of the rotor with wing blades is completely absent and additional devices are needed to start the rotor into operation. In order to ensure the self-starting of the rotor and the operation of the wind turbine at high and low wind speeds, a new shape of the blade profile was developed, called non-closed wing profile. The concept of the development is that the blade should have a configuration in which the pulling force is involved at the beginning of the movement, and then, with the establishing of the movement, a lifting force would arise, which acquires a prevailing character in the operating mode. The article presents the results of experimental studies of the aerodynamic characteristics of the developed non-closed wing blades. One of the results obtained is to determine the effect of the thickness of the blade profile on the range of values of subcritical angles of attack of the incoming air flow and the differences between the nature and range of changes in the coefficients of lifting force and pulling force in a traditional wing blade and a blade with a non-closed wing profile. Studies of the rotor model of a vertical-axial wind turbine with non-closed wing blades have confirmed the presence of its self-starting and operability even at low wind speeds.

Dynamic Control of Non-Linearly Tapering FGM Beams

This work presents the dynamics and active vibration control of a non-uniform functionally graded beam. Thanks to the strong use of beams and specifically with non-uniform section, in the different industrial applications, such as helicopter rotor blades, wind turbines, space and marine structures, we present in this article, a comparison of linearly and non-linearly tapering beams. The FGM beam is equipped with four layers of piezoelectric materials as sensors and actuators, bonded on the upper and lower surfaces of the main structure, on different finite elements to see the influence of its location on the dynamics and active control. In this study, the Timoshenko beam’s theory combined with FEM is applied to a beam divided into a finite number of elements. Hamilton's principle is applied to generate the equation of motion. The structure is modeled analytically and numerically and the simulation results are presented at the end. The optimal LQG control with Kalman filtering is applied.

Offshore reanalysis wind speed assessment across the wind turbine rotor layer off the United States Pacific coast

Abstract. The California Pacific coast is characterized by considerable wind resource and areas of dense population, propelling interest in offshore wind energy as the United States moves toward a sustainable and decarbonized energy future. Reanalysis models continue to serve the wind energy community in a multitude of ways, and the need for validation in locations where observations have been historically limited, such as offshore environments, is strong. The U.S. Department of Energy (DOE) owns two lidar buoys that collect wind speed observations across the wind turbine rotor layer along with meteorological and oceanographic data near the surface to characterize the wind resource. Lidar buoy data collected from recent deployments off the northern California coast near Humboldt County and the central California coast near Morro Bay allow for validation of commonly used reanalysis products. In this article, wind speeds from the Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2), the Climate Forecast System version 2 (CFSv2), the North American Regional Reanalysis (NARR), the European Centre for Medium-Range Weather Forecasts Reanalysis version 5 (ERA5), and the analysis system of the Rapid Refresh (RAP) are validated at heights within the wind turbine rotor layer ranging from 50 to 100 m. The validation results offer guidance on the performance and uncertainty associated with utilizing reanalyses for offshore wind resource characterization, providing the offshore wind energy community with information on the conditions that lead to reanalysis error. At both California coast locations, the reanalyses tend to underestimate the observed rotor-level wind resource. Occasions of large reanalysis error occur in conjunction with stable atmospheric conditions, wind speeds associated with peak turbine power production (> 10 m s−1), and mischaracterization of the diurnal wind speed cycle in summer months.

Identifying the flap side‐edge noise contribution of a wind turbine blade section with an adaptive trailing edge

AbstractActive trailing‐edge technology is a promising application for localized load alleviation of large‐diameter wind turbine rotors, accomplished using one or more control surfaces in the rotor blade's outer region. This work focuses on identifying noise contributions from the flap side‐edge and the trailing edge in a laboratory condition. Measurements were conducted in the Acoustic Wind Tunnel Braunschweig (AWB) at the German Aerospace Center's (DLR) Braunschweig site. The small‐scale model has a span of 1,200 mm and a chord length of 300 mm. The control surface, a plain flap, has a span of 400 mm and a chord length of 90 mm. Far‐field noise was measured using a phased‐microphone array for various flow speeds, angles of attack, and flap deflection angles. Due to the size of the model and assumed closeness of the sound sources, two noise reduction addons were installed interchangeably: trailing‐edge brush and flap side‐edge porous foam for sound source identification. Analysis of the far‐field noise reveals that, while changes to the flap deflection angle alter the far‐field noise spectra, the trailing‐edge noise remains the predominant noise source at deflection angles and . No additional noise level was observed from the flap side edge within the measurable frequency range at these angles. The flap side‐edge noise has an increased role for frequency larger than 2 kHz for the larger flap deflection angles of and .

Experimental investigation on the dynamic response of an innovative semi-submersible floating wind turbine with aquaculture cages

Floating offshore wind turbines integrating with other functional devices can not only produce green electricity but also realize three-dimensional utilization of ocean space. Therefore, an innovative concept of semi-submersible floating wind turbine with aquaculture cages is proposed here. Correspondingly, its dynamical behaviors are required to be examined and investigated while various oceanic conditions are considered, including but not limited to an irregular wave condition and combined wind and wave conditions. Hence, a series of model tests in 1:40 scale is carried out in Shanghai Jiao Tong University Ocean Engineering Basin. Firstly, the scaling law and experimental setup are interpreted in detail. Calibrations of the wind generator and wave makers are carefully performed to ensure the accuracy and reliability of model tests. Then the dynamic responses of the new floating wind-aquaculture platform (FWAP) are studied experimentally, including the periods, damping, motions, accelerations and tower top loads. The results show that the existence of nets mainly has a damping effect on the dynamics of the new FWAP under different sea states. The motions, accelerations and tower top loads can be significantly increased when the FWAP undergoes 50-year extreme sea state. When the new FWAP is subjected to combined wind and wave, more complicated dynamic responses can be detected due to the rotation of wind turbine. The results can serve as a reference basis for the improvement of the new FWAP.

Dominant Frequency Extraction for Operational Underwater Sound of Offshore Wind Turbines Using Adaptive Stochastic Resonance

Underwater sound generated by the rapidly increasing offshore wind farms worldwide greatly affects the underwater soundscape and may cause long-term cumulative effects on sound-sensitive marine organisms. However, its analysis and impact assessment are heavily interfered with by underwater ambient noise. In this study, an adaptive stochastic resonance method is proposed to extract the dominant frequency of wind turbine operational sound when heavy noise is present. In particular, a time–frequency–amplitude fusion index was proposed to guide the parameter tuning of an adaptive stochastic resonance system, and an equilibrium optimizer based on the physical dynamic source–sink principle was adopted to optimize the parameter-tuning process. The results from the simulation and field data showed that the dominant frequency of operational sound was extracted adaptively. For field data with wind speeds of 4.13–6.15 m/s (at 90 m hub height), the extracted dominant frequency varied with wind speed between 90 and 107 Hz, and it was highly correlated with the wind turbine rotor speed monitored synchronously in the air, with a correlation coefficient of 0.985. Compared to other existing methods, our method has a higher output signal-to-noise ratio and a shorter running time.

Investigation of Offshore Wind Turbine Structure Deflection Using Experimental Work, Numerical, and Theoretical Approach

Electrical energy from offshore wind turbines is an important source of clean, renewable energy production. One of the reasons for the low efficiency of wind turbines is the change in the flow angle of attack, denoted by the symbol (α), as a result of the deflection of the structure. The study aims to know the values of the maximum deflection of the proposed structure under the environmental conditions of the Arabian Gulf water area. Our research adopted a novel approach to extracting results; the characteristics of sea waves were extracted from the experimental work after fixing five sea waves, knowing the displacement of the top of the structure, and using the numerical approach in the ANSYS-Fluent program to know the average wind and wave forces. Two simulations were performed. The first included the five cases of sea wave characteristics without rotating wind turbine blades. The second was for the fifth case only with the wind turbine blades rotating at a speed of (20.5 rpm), assuming that the structure was exposed to a constant wind speed (12 m/s) for the two simulations. The study also included obtaining the maximum deflection value of the structure. Then, the equations of the theoretical approach were developed based on the Euler-Bernoulli bending moment equation, and the forces extracted from the simulations were entered into the theoretical equations to extract the maximum deflection values of the structure. Reading the experimental work resulted in the highest displacement of the top of the structure in the fifth case (0.178 m). The result of the second simulation had the highest value of the structure deflection (0.201 m). In comparison, its value came in the theoretical approach (0.160 m), which adopted the forces of the second simulation.

On the relationship between turbine thrust and near-wake velocity and vorticity

Vortical impulse theory is used to investigate the relationship between turbine thrust and the near-wake velocity and vorticity fields. Three different hypotheses regarding the near-wake structure allow the derivation of novel expressions for the thrust on a steadily rotating wind turbine, and these are tested using stereoscopic particle-image velocimetry (PIV) data acquired just behind a rotor in a water channel. When one assumes that vortex lines and streamlines are aligned in a rotor-fixed frame of reference, one obtains a PIV-based thrust estimate that fails even to capture the trend of the directly measured thrust, and this failure is attributed to an implicit assumption that most of the generated thrust does useful work. When one neglects the axial gradients of radial velocity, the PIV-based thrust estimate captures the measured thrust trend, but underpredicts its magnitude by approximately $33\,\%$ . The third and most promising physical proposition treats the trailing vortices as purely ‘rolling’ structures that exhibit zero-strain rate in their cores, with the corresponding thrust estimates in close agreement with direct thrust measurements. This best-performing expression appears as a correction to the classical thrust expression from momentum theory, possessing additional squared-velocity terms that can account for the high-thrust regime of turbine operation that is typically addressed empirically.

INVESTIGATION OF THE PERFORMANCE CHANGES OF THE SAVONIUS WIND TURBINE ROTORS WITH THE SAME FRONT VIEW AREA OF LOOK BY CHANGE OF THE ASPECT RATIO

In this study, Savonius type wind turbine rotors with the same front view area were produced with 3D printers by changing their aspect ratios, and then both experimental and numerical analyzes were carried out according to them attaching end plate and without end plate situations. The wind tunnel in the university was used in the experimental analysis. For numerical analysis, rotors and wind tunnels modeled in CAD program were analyzed in Ansys Fluent program. The analyzes made are for imaging purposes only, and the results in the experimental analyzes are accepted as correct. The reduction of aspect ratios in the uncapped analyzes decreased the overall efficiency, but the opposite effect was observed when the analyzes were repeated with the end plate. In addition, while negative pressure was observed in the rotors made without cover (bare case) in the digital images, this situation was not seen in the analyzes made with the end plate.

Noise simulations of flap devices for wind turbine rotors

AbstractThe rotor of a large diameter wind turbine experiences more substantial and more dynamic loads due to the fluctuating and heterogeneous wind field. The project SmartBlades 2.0 investigated rotor blade design concepts that alleviate aerodynamic loading using active and passive mechanisms. The present work evaluates the acoustics of the two load alleviating concepts separately, an inboard slat and an outboard flap, using the Fast Random Particle Mesh/Fast Multipole Code for Acoustic Shielding (FRPM/FMCAS) numerical prediction toolchain developed at DLR with input from the averaged flow field from RANS. The numerical tools produce a comparable flap side‐edge noise spectrum with that of the measurement conducted in the Acoustic Wind Tunnel Braunschweig (AWB). The validated FRPM/FMCAS was then used to analyze the self‐noise from a slat at the inboard section of a rotor blade with a 44.45 m radius and compared with that from the outboard trailing edge. Furthermore, the rotational effect of the rotor was included in the post‐processing to emulate the noise observed at ground level. The findings show an increase in the slat's overall sound pressure level and a maximum radiation upwind of the wind turbine for the case with the largest wind speed that represents the off‐design condition. In operational conditions, the slat adds at most 2 dB to the overall sound pressure level. The toolchain evaluates wind turbine noise with conventional or unconventional blade design, and the problem can be scaled up for a full‐scale analysis. As such, the tools presented can be used to design low‐noise wind turbines efficiently.

Experimental and numerical investigation of the yaw moment of a downwind coned wind turbine rotor

AbstractThe yaw moment of wind turbine rotors has never been in the focus of wind turbine aerodynamics. With the increasing activities in the development of support structures for Floating Offshore Wind Turbines (FOWT), which passively align with the wind, the interest has shifted, as an accurate determination of the yaw moment is a crucial issue for a successful design of such power plants. A downwind coned rotor is a promising option to increase the yaw moment and therefore the self‐alignment capability of a passively yawing FOWT. Unfortunately, experimental and numerical studies on the estimation of the yaw moment of wind turbine rotors are rare. This is especially the case for downwind coned rotors. The aim of the present work is to provide reliable knowledge in this field. For this purpose, an extensive experimental and numerical study is carried out to determine the yaw moment of a downwind coned rotor. The results obtained from measurements in the wind tunnel are compared to those of simulations using a high fidelity RANS method and a blade element momentum theory (BEMT) method. BEMT is widely applied and can be considered as state of the art for predicting aerodynamic loads on FOWTs. However, the basic assumptions of BEMT do not account for a realistic influence of the skewed wake, so that the application of a correction method is necessary. In this work, the frequently used wake skew correction method based on Pitt and Peters is utilised and its influence on the calculation of the yaw moment is investigated. It is shown that this correction method yields a significant overprediction of the yaw moment in comparison to the measurements and consequently even impairs the quality of the simulation in this case. In contrast to this, the wake‐resolving RANS method is capable of reproducing the measurements with reasonable accuracy and provides valuable insight into the role of the lateral force for the measurement of the yaw moment.

An efficient blade sweep correction model for blade element momentum theory

AbstractThis article proposes an efficient correction model that enables the extension of the blade element momentum method (BEM) for swept blades. Standard BEM algorithms, assuming a straight blade in the rotor plane, cannot account for the changes in the induction system introduced by blade sweep. The proposed extension corrects the axial induction regarding two aspects: the azimuthal displacement of the trailed vorticity system and the induction of the curved bound vortex on itself. The extended algorithm requires little additional processing work and maintains BEM's streamtube independent approach. The proposed correction model is applied to simulations of swept blade geometries based on the IEA 15 MW reference wind turbine. Results show good agreement with lifting line simulations that inherently can account for the swept blade geometry.Blade sweep couples bending and torsion deformations by curving the blade axis in the inplane direction. As such, it can be used to passively alleviate loads and, thus, aeroelastically tailor wind turbine blades. The implementation of aeroelastic tailoring techniques, and the aeroelastic analysis in general, becomes increasingly significant with the size of wind turbine rotors continually rising. Due to its low computing complexity, BEM remains a crucial tool in the aerodynamic and aeroelastic analysis of wind turbine rotors. Thus, the proposed correction model contributes to a fast and accurate evaluation of swept blade designs.

Optimal gust wind energy capture using critical tracking frequency for wind turbines

AbstractGusty wind in urban areas contains significant energy potential that can be harnessed, but the existing wind turbine rotor speed control systems based on continuous wind speed tracking have a noticeable response delay as compared to the duration of the common short gusts, inducing a significant deficit in harnessed excess energy content (EEC) from gusty wind. This work scrutinizes the energy content distribution among the gusty wind components and their differential impacts on the control response to identify factors to improve response timeliness. Cross‐correlation spectrum analysis between the wind turbine response and the local wind spectrum was used to identify the critical tracking frequency that can significantly reduce the average response delay of wind turbines in gusty wind conditions while maximizing the gust energy harvesting ability was developed and validated by various time‐domain simulation results. Combining the critical tracking frequency with the theoretical estimation of the EEC of a wind turbine, the expected EEC harnessing ability of a wind turbine can be estimated before installation. For the sample Savonius wind turbine in this work, the response delay of the wind turbine was shortened by 65%, leading to 25% more harnessed EEC from gusty wind.

Coordinated Rotor Speed and Pitch Angle Control of Wind Turbines for Accurate and Efficient Frequency Response

This paper presents a wind turbine (WT) control strategy to deliver frequency response (FR) that is similar to conventional synchronous generators, i.e., to deliver accurate and efficient FR. This is achieved by the coordination of WT’s rotor speed control (RSC) and pitch angle control (PAC), wherein the RSC is the summation of the proposed deloading power point tracking curves and FR signal; the PAC is kept as adjusting its control law according to the WT rotor speed. Through the above coordination, the strategy pre-reserves a portion of wind power and releasing it during underfrequency events. Compared with existing methods, the strategy has two significant advantages: 1) it does not need the switching of WT controls according to the real-time wind speed (WS), nor the identification of the boundaries among high, medium and low WSs according to the required reserved power, ultimately reducing the complexity of applying the strategy to practice; 2) its active power response during FR period is independent of the deviated rotor speed and is equal to the FR signal, ultimately enabling the WT to provide accurate and efficient FR. The simulations focusing on the grid frequency side demonstrated the advantages of the strategy over existing methods.

Determination of as-built properties of fiber reinforced polymers in a wind turbine blade using scanning electron and high-resolution X-ray microscopy

The fiber volume fraction (FVF) and porosity in fiber reinforced polymers (FRPs) depends strongly on the manufacturing process. These parameters influence the mechanical properties and thus the performance of an FRP. For this research, an epoxy-pre-impregnated glass FRP was investigated to determine the FVF and matrix mass fraction taking into consideration all material constituents, including sizing, stitching thread, as well as the porosity, the area density and the fiber orientations of each lamina, the filament fiber diameter, and the inter-laminar void size and shape. Therefore, samples from a commercially manufactured wind turbine rotor blade were experimentally investigated using scanning electron (SEM) and high-resolution X-ray microscopy (micro-CT), as well as a standardized calcination method and geometric measurements. Post-processing techniques such as thresholding and edge detection were used to analyze the images. There was good FVF agreement between SEM and the method of calcination. Micro-void cross-sectional shapes were well captured by SEM while meso- and macro-voids were volumetrically resolved with a reproducible void size distribution for two sample volumes by micro-CT.