Wind Turbine Rotor Research Articles

Hybrid Testing System Development for Single Point Mooring Lines FOWT’s

Abstract To advance the development of various concepts for floating offshore wind turbines, it is imperative to have access to experimental data. These data are used in the validation of the tools for the simulation of floating turbines and also for the tuning of the models. CENER has developed a hybrid testing methodology that enhances the performance of wave basin tests for scaled models of Floating Offshore Wind Turbines (FOWTs). This approach allows us to apply forces and moments from the wind turbine to the floating platform without needing to replicate wind within the testing facility or create a complex model of the wind turbine rotor. However, when it comes to testing single point moored (SPM) platforms, a unique challenge arises. These platforms have an additional degree of freedom as they can freely rotate around the vertical axis of their mooring attachment (yaw degree of freedom). These type of platforms can present important misalignments with respect to the wind depending on the combination of wind, wave and current directions. Consequently, accurately capturing the impact of misalignment on platform dynamics, especially in yaw, is essential to assess the platform’s ability to self-align with the wind direction, a key factor known as weathervaning capacity. The X1Wind platform is an innovative floating wind system, based on a single point mooring solution with a downwind wind turbine configuration that allows it to be passively self-orientated. The concept has been extensively tested on prototypes at real sea conditions as well as on scaled models at wave basins. For the last wave basin testing campaign, X1Wind has entrusted CENER with the development of the hybrid testing methodology adapted to the requirements of SPM platforms. The testing campaign has covered different wave and wind conditions, including misalignment scenarios and has shown a good dynamic behavior of the platform on yaw. This study outlines the advancements made in the CENER Software-in-the-Loop (SiL) hybrid testing system for wave basin tests of SPM platforms. In order to correctly introduce the direction of the aerodynamic forces on the wind turbine under misaligned conditions, improvements on the software and hardware (loads actuator) of the SiL system were developed and tested. An actuator system consisting of 6 propellers was used. The numerical model that calculates the rotor aerodynamic loading were expanded to take into consideration the direction of the aerodynamic loads with respect to the platform. Additionally, the drag of the wind over the tower and other platform elements must be considered, as they can play a relevant role in the generation of the weathervaning moment. The study concludes that the use of the upgraded SiL hybrid testing system is an effective and afordable solution for the testing of scaled SPM platforms at wave basin facilities. It also summarizes the different considerations that the system has to overcome for the particularity of this type of testing application.

Wake Structures and Performance of Wind Turbine Rotor With Harmonic Surging Motions Under Laminar and Turbulent Inflows

ABSTRACTThis study presents a comprehensive numerical analysis of a full‐scale horizontal‐axis floating offshore wind turbine (FOWT) rotor subjected to harmonic surging motions under both laminar and turbulent inflow conditions. Utilizing high‐fidelity computational fluid dynamics (CFD) simulations, namely, large eddy simulation (LES) with actuator line model (ALM), this research investigates the rotor performance, wake characteristics, and wake structures of a surging FOWT in detail. The study delves into the influence of varying inflow turbulence intensities, surging settings, and their interplay on the aerodynamic performance and the wake aerodynamics of a FOWT rotor. The results show that, through employing the phase‐averaging technique, surge‐induced periodic coherent structures (SIPeCS) can be identified in the wake of all the surging cases studied, irrespective of the inflow conditions and the surging settings. Additionally, the findings show that the faster wake recovery observed in the surging cases is not caused by enhancing the instability‐induced turbulence level, a previously accepted hypothesis. Instead, the results indicate that it is due to the enhanced advection process resulting from the induction fields of SIPeCS that causes the wake to recover faster. The analysis of rotor performance shows that the time‐averaged rotor performances are affected by the intricate aerodynamics arising from the surging motions. With certain surging settings, the time‐averaged thrust and the time‐averaged power of a surging rotor are found to be simultaneously lower and higher compared with those of a fixed rotor. Furthermore, the study underscores the importance of considering both the magnitude of surging and the rate of surging simultaneously to fully characterize the hysteresis load on a surging rotor.

Investigation of wind turbine unsteady aerodynamics and trailing-edge noise characteristics under wake steering

In wake steering strategy, the shaft of an upstream wind turbine is yawed to deflect the wake and decrease the wake-swept area of the downstream wind turbine rotor. This intentional yaw misalignment increases the output power of the downstream wind turbine at the expense of some loss of upstream wind turbine power, consequently increasing the total wind farm power. However, the asymmetric non-uniform inflow into the downstream wind turbine due to the deflected wake causes unsteady aerodynamic loads and affects the wind turbine noise characteristics. In this study, the unsteady vortex lattice method coupled with the curled wake model is used to predict the unsteady aerodynamics of a downstream wind turbine in wake steering. Based on the unsteady aerodynamic results, the trailing-edge noise characteristics are predicted by a semi-empirical method. The impact of wake steering on trailing-edge noise level and amplitude modulation is evaluated. The results show that the asymmetric non-uniform inflow due to wake steering increases the amplitude modulation of the downstream wind turbine.

Similarities in the meandering of yawed rotor wakes

This study investigated the meandering of yawed wind turbine rotor wakes, focusing on the similarities across different yaw angle scenarios. Spectrum analysis of velocity fluctuations reveals that the meandering of the yawed rotor wake is symmetrical about the wake center, despite its skewness. The non-zero lateral force of the yawed rotor enhances meandering in the lateral direction compared to the vertical direction. However, the lateral profiles of meandering strength exhibit similarities across different yaw angle scenarios, indicating a consistent wake meandering mode. The wake meandering frequency increases with the yaw angle. A relationship involving wake meandering frequency, drag coefficient, and yaw angle is formulated for wind turbine rotor wakes under different yaw angles. This relationship is also applicable to thin plate wakes within a certain range of inclination angles/yaw angles. The present study reveals the similarity in wake meandering characteristics across different yaw angle scenarios, which is instrumental in improving our understanding of wake meandering and in developing analytical wake models for wind turbines.

Healable adhesive paste development for thick adhesive joints

A new adhesive formulation is developed, enabling assisted self-healing for wind turbine rotor blade (WTRB) applications. The properties of this epoxy based formulation are compared to those of a state-of-the-art toughened epoxy adhesive (Sikapower®-830). An optimal content of 20 wt% poly(ε-caprolactone) (PCL) microparticles and 4 wt% fumed silica (NS) in a baseline epoxy fulfilled most requirements, such as sufficient rheological yield stress, adequate mechanical properties, glass transition temperature (Tg) higher than the operation temperature range and good adhesion, although the elastic modulus remained about 85 % of the desired value. In addition, 60 % healing efficiency was achieved in fractured samples after moderate heating. This approach provides new perspectives to control damage in thick bonded joints.

The influence of the rotor spacing distance and rotating speed ratio on the power production of dual rotor wind turbines using a modified two-way interaction blade element momentum method

The present study aims to develop an accurate and fast improved blade element method (BEM) with two-way rotor-rotor interaction model which can account for two-way rotor-rotor interaction and the influence of rotor spacing while most previous BEM models for co-axis dual rotor wind turbines (DRWT) ignore the rear to front rotor interaction effect. A modified BEM model distinct from computationally intensive Computation Fluids Dynamics (CFD) model, is proposed for DRWTs. Validation of the present model is conducted by comparing it with experimental data from referenced literature for both co- and counter-rotating configuration. In our case study, for both co- and counter-rotating configurations, the power coefficient (CP) maps exhibit a double-hump shape instead of a single peak on the power output -λ curve observed in a single rotor wind turbine system. This characteristic may lead the DRWT to a local CPmax condition rather than the global CPmax while pursing the optimum operational condition. Although the counter-rotating DRWT can achieve a higher CPmax for most rotor spacing ratio (S/R) cases, it exhibits inferior power output than the single rotor for the S/R = 0.1 case. The present model can serve as a rapid and accurate model for configuring and control strategy evaluating DRWTs.

Aerodynamic characteristics of wind turbines considering the inhomogeneity and periodic incentive of wake effects

In wind farms, the wake can lead to a loss of output power, and the uneven characteristics of wake can also exacerbate the fatigue load of downstream wind turbine (WT), affecting their operating life. To explore the impact of wake effects on downstream WT, this article proposes a new method for calculating the load of WT wake wind conditions and verifies it. Establishing a 3DJG wake model coupled with improved BEM for wind turbine aerodynamic performance calculation method. The influence of downstream WT operation phase angle and positions of 3D wake on the time-varying aerodynamic characteristics of downstream WTs was discussed from three perspectives: Blade elements on the spanwise, single blade and wind rotor. Results shown that: (1) For the blade element, calculated the shear force in the flapwise direction and the edgewise direction on each blade segment. The overall shear force of blade element in the vertical upward direction of WT blades is significantly greater than that in other directions. The non-uniformity of the wake is not sufficient to change the position of the maximum shear force on the blade. The position of the maximum shear force at each wake position is in the rear segments of the blade; (2) For the single blade, the most severe load fluctuation occurs when it is in the partial wake condition. The period of this fluctuation is 360°. When in full wake condition, the load fluctuation is relatively small, and due to the symmetry of the wake, the load fluctuation period is only 180°. The load standard deviation (SD) of the partial wake condition increases by 51 % compared to the full wake condition. (3) For the wind rotor, different wake positions have a weighty impact on the power loss of the WT rotor. Compared to the incoming shear wind, the power loss at the wake center can reach up to 54.22 %. At the wake boundary, the power loss of the WT is only 5.6 %. The non-uniformity of wake also has a periodic impact on the power output of the entire WT rotor, with the fluctuation period is 120°, and the fluctuation amplitude of no more than 2 %. This article provides a more intuitive quantitative analysis of the impact of wake on the time-varying aerodynamic characteristics of downstream WTs. It has certain reference value for the design strength verification of WTs, as well as the yaw strategy and layout design of wind farms.

Thin-walled FRP-concrete-steel tubular tower with a top mass block subjected to lateral impact loading: Experimental study and FE analysis

Thin-walled hollow steel tube towers (HSTs), frequently utilized as a wind turbine tower, are facing challenges such as corrosion and local buckling in their service life. To enhance the anti-corrosion capacity and local buckling resistance, an FRP tube and an annular concrete layer can be employed to protect the HST, thus forming a novel member: thin-walled FRP-concrete-steel tubular towers (TW-FCSTs). Using a horizontal vehicle impact system, this study investigated the impact behavior of five large-scale TW-FCSTs, each with a top mass block to mimic the rotor and nacelle of wind turbine. All specimens were designed with a large diameter (300 mm) and a large void ratio (0.73 or 0.82). The experimental results revealed that: (1) Under lateral impact loading, TW-FCSTs displayed a global flexural failure mode, accompanied by localized concavity damage; (2) The inertial effect was enlarged by the top mass block, affecting the dynamic response of TW-FCSTs; (3) the increase of steel thickness led to higher energy dissipation but lower local deformation; (4) the increase of void ratio resulted in larger local deformation but smaller lateral global displacement. Finally, based on LS-DYNA, FE models were utilized to simulate the dynamic responses of TW-FCSTs with a top mass block.

Numerical investigation of turbulent flow over a small-scale vertical axis wind turbine

The present paper aims to analyse the aerodynamics of turbulent fluid flow over a vertical axis wind turbine (VAWT). A 2D study is carried out for a two-bladed SAVONIUS wind turbine, where the rotor diameter is D=20cm. The governing equations (Unsteady Reynolds Averaged Navier Stokes URANS) are solved numerically using a CFD (computational fluid dynamics) open-source code based on the finite volume method; the SST turbulence model is applied for the system closure. An arbitrary mesh interface (AMI) technique is applied at the sliding interface boundary, which is a circular surface separating between the rotating zone and the fixed zone where the mesh remains stationary. The numerical results allow predicting the vorticity and pressure distributions over a rotating wind turbine for different tip speed ratios in order to characterize the generated wake. The velocity deficit is calculated for different positions behind the rotor to characterize the disturbance rate of the flow.

Semi-analytical solutions for steady-state bending-bending coupled forced vibrations of a rotating wind turbine blade by means of Green's functions

Wind power has received significant attention in recent years as a renewable energy source. Wind turbine blades, characterized by their long lengths, are prone to damage as a result of aeroelastic instability. This paper aims to derive semi-analytical solutions for steady-state forced vibrations of rotating wind turbine blades, considering the coupling of bending, bending, and unsteady aerodynamic loads. The novelty of this work lies in the use of the Green's function method to solve differential dynamic equations with variable coefficients, which are induced by aerodynamic forces. To model a wind turbine blade, the Euler-Bernoulli beam model is employed, and Greenberg's expressions are taken into consideration. Governing equations for coupled vibrations of the blade are then determined. In order to solve the governing equations, displacement solutions are decomposed into quasi-static displacements and dynamic displacements. The Laplace transformation and Green's function methods are utilized to obtain fundamental solutions of the governing equations. Subsequently, employing the principle of superposition, Fredholm integral equations for steady-state forced vibration of a rotating wind turbine blade are derived. To spatially discretize Fredholm integral equations, compound trapezoid formulae and central finite difference approximations are employed. This discretization process leads to the formation of a system of algebraic equations. Semi-analytical solutions for coupled forced vibrations of the rotating wind turbine blade are obtained by solving the algebraic equations. In the numerical solution part, validation of proposed solutions are verified by comparing them with numerical and finite element solutions in the literature. Influences of some important physical parameters, such as the rotating velocity, the setting angle, the cone angle, the inflow ratio, and damping, on vibration responses of blades are discussed.

A Comparison of a Three Blade and Five Blade Wind Turbine in Terms of the Mechanical Properties Using the Q-Blade Software

يمكن لتوربينات الرياح المنتشرة في مزارع الرياح على نطاق المرافق أن تدعم تلبية رغبات الطاقة المستقبلية وتقليل انبعاثات ثاني أكسيد الكربون عن طريق تقليل متطلبات الطاقة من الوقود الأحفوري. مع ارتفاع درجة حرارة الهواء على مدار اليوم، تزداد سرعة الرياح بسبب تدرجات درجة الحرارة، والتي تنتج تدرجًا في الكثافة / الضغط، مما يؤدي إلى حركة الهواء التي تواجهها توربينات الرياح. اعتمادًا على تضاريس الأرض، يمكن أن تواجه الرياح وتوجه في الوديان بين التلال وفوقها حيث تتدفق وتتبع منحنيات الأرض. تنتج هذه التضاريس زيادة في سرعة الرياح في القمم والتلال. في هذا العمل، تم تصميم شفرة دوار توربينات الرياح الأفقية الصغيرة للعمل في ظل سرعة الرياح المنخفضة، باستخدام برنامج (Q-Blade). استنادًا إلى نظرية عنصر الشفرة (BEM). مع الجنيح NACA3712. تم استخدام دوار ثلاثي الشفرات ودوار بخمس شفرات بناءً على نوع التوربين وحجم الدوار لتوليد الطاقة الميكانيكية من طاقة الرياح. تم إجراء مقارنة وتحليل قوة التوربين ومعامل القدرة ومعامل عزم الدوران عند سرعة رياح منخفضة ((1m/s-8m/s وتم الحصول على نتائج دقيقة للغاية. وجد أن أفضل أداء يمكن أن يعمل فيه دوار توربيني ثلاثي الشفرات بقدرة توربينية تبلغ (955W) كما تم الحصول على قدرة التوربين ((582W للدوار خماسي الشفرات. وجد أن تصميم توربينة رياح أفقية صغيرة بخمس ريش أفضل من التوربين بثلاث ريش ومناسب للعمل في المناطق ذات سرعة الرياح المنخفضة وبكفاءة عالية مقارنة بحجم التوربين.

Boundary layer stability on a rotating wind turbine blade section

Wall-resolved large eddy simulations of the flow on a rotating wind turbine blade section are conducted to study the rotation effects on laminar-turbulent transition on the suction surface. A chord Reynolds number of 1×105 and angles of attack (AoA) of 12.8°, 4.2°, and 1.2° are considered. Simulations with and without rotation are performed for each AoA. For AoA=12.8°, rotation increases the reverse flow from 7% of the free-stream velocity in the non-rotating case to 16% of it in the rotating case in the laminar separation bubble (LSB), triggering an oblique instability mechanism in the latter, leading to a faster breakdown to small-scale turbulence. However, rotation delays transition and reattachment in 3%–4% of the chord due to the acceleration of the boundary layer upstream of the LSB, which is subject to a strong adverse pressure gradient (APG), stabilizing Tollmien–Schlichting (TS) waves. Regarding AoA=4.2° and 1.2°, rotation slightly decelerates the attached boundary layer since the APG is very mild but accelerates the separated flow downstream, stabilizing Kelvin–Helmholtz (KH) modes. This mitigates the oblique instability mechanism and slows down the breakdown of KH vortices in the rotating case. In these cases, the transition location is little affected by rotation, possibly due to a rotation-independent absolute instability. Rotation also generates a spanwise tip-flow in the LSB for AoA=4.2° and 1.2°, which is highly unstable and triggers stationary and traveling crossflow modes. Nevertheless, the amplitudes of these modes remain too low to trigger transition.

Active Diagnostic Experimentation on Wind Turbine Blades with Vibration Measurements and Analysis

Abstract This paper deals with the key operational problems of wind turbosets, especially offshore, where vibrations are generated by rotor blades, as a consequence of erosive wear or icing. The primary causes of the imbalance of wind turbine rotors have been characterised, the observable symptoms of which include various forms of vibrations, transmitted from the turbine wheel to the bearing nodes of the power train components. Their identification was the result of an active diagnostic experiment, which actually entered the aerodynamic-mass imbalance of a turbine rotor into a wind power train, built as a small scale model. The recording of the observed monitoring parameters (vibration, aerodynamic, mechanical and electrical) made it possible to determine a set of symptoms (syndrome) of the deteriorated (entered) dynamic state of the entire wind turboset. This provides the basis for positive verification of the assumed concept and methodology of diagnostic testing, the constructed laboratory station and the measuring equipment used. For this reason, testing continued, taking into account the known and recognisable faults that most often occur during the operation of offshore wind turbosets. Transferring the results of this type of model research to full-size, real objects makes it possible to detect secondary (fatigue) damage to the elements transmitting torque from the wind turbine rotor to the generator early, especially the thrust bearings or gear wheel teeth.

CFD study on wake characteristics of a dual rotor wind turbine using different turbulence models

CFD study on wake characteristics of a dual rotor wind turbine using different turbulence models

Modeling and control of a novel mechatronic model of a 750 kW-FSWT with power-splitting transmission using RBF neural network: a bond graph approach

This work addresses issues that appear from wind turbines equipped with frequency converters and presents a new fixed-speed wind turbine (FSWT) concept based on a split power transmission, along with a Mechatronic Control Model. The wind turbine rotor drives the first transmission shaft, while a servomotor with variable speed powers the second input. The differential gear transmission output is linked to the electric grid through an asynchronous generator. To optimize the power extracted from the wind energy while minimizing excessive dynamic loads on the wind turbine, a mechatronic control model of a 750 kW-FSWT is applied using a proportional and integral controller (PI). The paper suggests employing neural networks and evolutionary algorithms for determining appropriate PI gains. For collective servomotor control of a 750 kW-FSWT, a radial basis function (RBF) neural networks based on PI controller is proposed. To acquire an optimum dataset for RBF training, the particle swarm optimization (PSO) evolutionary algorithm is harnessed. The robustness and effectiveness of the proposed model and controller are verified and confirmed through simulation results. Hands-on experience is conducted using the 20-sim software package.

Investigating the relationship between natural frequency and residual strength and stiffness of cross‐ply laminate under cyclic loading

AbstractPredicting the fatigue life of wind turbine rotor blades is a challenging and crucial engineering task. This study investigates the correlation between modal parameters and the degradation of residual strength and tensile modulus in the cross‐ply glass epoxy laminate [0/90]7 used in wind turbine blades under fatigue loading. The tensile and vibration characteristics were assessed, followed by constant amplitude fatigue tests at 35%, 43% and 55% of the ultimate tensile strength, with R = 0.1 and a frequency of 8 Hz. The modal analysis was performed on the cycled specimens at life fractions from 0.05 to 0.70 and residual modulus and strength were obtained. The results establish a well‐defined correlation between these residual mechanical properties and the natural frequency. Normalized residual strength, tensile modulus and natural frequency demonstrated similar behaviors during the fatigue life. An initial rapid decrease in the first tenth of the life fraction was observed, followed by minimal changes up to a life fraction of 0.7. The strong correlation between the first mode natural frequency and both the residual strength and the tensile E‐modulus provides a promising basis for developing accurate fatigue life prediction models for fiber‐reinforced composite structures. © 2024 Society of Chemical Industry.

Comparison of different cross-sectional approaches for the structural design and optimization of composite wind turbine blades based on beam models

Abstract. During the preliminary design phase of wind turbine blades, the evaluation of many design candidates in a short period of time plays an important role. Computationally efficient methods for the structural analysis that correctly predict stiffness matrix entries for beam models including the (bend–twist) coupling terms are thus needed. The present paper provides an extended overview of available approaches and shows their abilities to fulfill the requirements for the composite design of rotor blades with respect to accuracy and computational efficiency. Three cross-sectional theories are selected and implemented to compare the prediction quality of the cross-sectional coupling stiffness terms and the stress distribution based on different multi-cell test cross-sections. The cross-sectional results are compared with the 2D finite element code BECAS and are discussed in the context of accuracy and computational efficiency. The analytical solution performing best shows very small deviations in the stiffness matrix entries compared to BECAS (below 1 % in the majority of test cases). It achieved a better resolution of the stress distribution and a computation time that is more than an order of magnitude smaller using the same spatial discretization. The deviations of the stress distributions are below 10 % for most test cases. The analytical solution can thus be rated as a feasible approach for a beam-based pre-design of wind turbine rotor blades.

Initial Study of the Onsite Measurement of Flow Sensors on Turbine Blades (SOTB).

This paper presents a new framework using MEMS flow sensors on turbine blades (SOTB) to investigate unsteady flow features of a rotating wind turbine. Self-heating flow sensors were implemented by the U18 complementary metal-oxide semiconductor (CMOS) MEMS foundry provided by Taiwan Semiconductor Research Institute (TSRI). Flow sensor chips with a size of 1.5 mm × 1.5 mm were parylene-coated, output via a wireless data acquisition system (WDAQ), and mounted at the root, middle and tip of a 1.2 m diameter semi-rigid turbine blade of a 400 W horizontal axis wind turbine (HAWT). The instantaneous angles of attack (AOAs) of the SOTB were found to be 46~62°, much higher than the general stall AOA of 15°, but were accurate considering the normal detection of the flow sensors. The computational fluid dynamics (CFD) simulation of the HAWT was also compared with the SOTB output. The onsite measurement herein revealed that the 3D secondary flow increment, mostly obvious near the middle part of the turbine blades, degraded both the sensor and the turbine performance and initially justified the onsite measurement application.

Harnessing the power of thermal imagery and visual inspection- a mean for reliable damage detection of wind turbine rotor blades

Generation of green electricity as part of the energy transition is leading to a growing market in the wind energy sector all over the world. Maintenance and inspection are key to the reliability, safety and efficiency of wind turbines, the regular maintenance of rotor blades focuses on damage such as erosion on the leading edge of the profile, delamination and thermal cracks due to lightning strikes. To date, visual inspection by technicians (climbers) has been the state of the art and it is time consuming besides posing safety risk for themselves. Recently, drone-based inspections using visual cameras have become more common, enabling fast, reliable and cost-effective inspections. However, no internal damage to the rotor blades can be detected during such an inspection. Thermography is a recognised method for detecting damage beneath the surface of an object, which has been promoted and further developed at BAM for years. To enhance the accuracy and reliability of wind turbine blade inspection, the fusion of thermal and visual data has emerged as a promising technique. Here, a drone-based multisensory system is being presented that combines thermal imagery, high-resolution visual images and a three-dimensional image obtained from 3D Laser scanner to represent a comprehensive view of the blades' internal and external conditions. Besides this, flight data obtained from IMU, GPS and LiDAR sensors will be used for enhancing the inspection data. Thus, fusion of images enables a more detailed analysis of potential defects and this not only improves the overall effectiveness of the inspection process but also provides windmill operators a new dimensions for data analysis and decision making.

Continuum approach of nonlinear dynamic analysis of a rotating wind turbine blade considering hub motion

This paper reports the nonlinear dynamics of a large wind turbine blade considering the hub motion. The investigation is based on a continuum mathematical model, which is derived using the extended Hamilton principle. The proposed model is validated via comparison with both published results and numerical simulations obtained from open source software. The steady state amplitude curves as well as the dominating frequencies of the modal forces are compared and analyzed with discussions on the effects of the hub motion amplitude, yaw angle and rotational frequency of the blade. Comparison of dominant frequencies of modal forces shows that the skewed inflow can strengthen the periodic excitation whose frequency equals to the rotating frequency of the blade. The steady state amplitude curves show that vibration of the edge-wise mode is more significant than that of the flap-wise mode when under the hub motion excitation. Moreover, internal resonance between the first three modes is also observed. The skewed inflow has a significant effect on the first mode in terms of vibration amplitude and the most unfavorable direction of hub motion. Finally, recommendation on how to avoid the resonance and internal resonance of the blades under skewed inflow are given.