Unsteady Blade Element Momentum Research Articles

Numerical validation of the dynamic aerodynamic similarity criterion for floating offshore wind turbines under equivalent pitch motions

Floating offshore wind turbines, as large equipment for deep-sea wind energy utilization, must be carefully studied for their aerodynamic performances under platform motions. In experiments, the model wind turbine designed by the traditional thrust coefficient similarity is usually insufficient for unsteady aerodynamic reproductions of the prototype wind turbine. In this paper, a dynamic aerodynamic similarity criterion based on the mapping of the optimal tip speed ratio is introduced for wind tunnel model tests, and a model wind turbine is designed using the NREL 5-MW prototype wind turbine. In this case, to avoid the over-height model tower aroused in pitch motions, a method of equivalent surge motions instead of pitch motions is proposed and validated for feasibility. Then dynamic aerodynamic similarities under different pitch amplitudes and frequencies are validated by the Unsteady Blade Element Momentum theory. Results indicate that temporal and spatial characteristics and hysteresis effects of thrust and power coefficients all maintain better similarity with those of the prototype wind turbine than those of the model wind turbine designed by the traditional criterion. This paper provides a criterion with equivalent pitch motions for unsteady aerodynamic studies in wind tunnel model tests, especially the high-fidelity Hardware/Software-In-the-Loop test.

Evaluation of Model Predictions of the Unsteady Tidal Stream Resource and Turbine Fatigue Loads Relative to Multi-Point Flow Measurements at Raz Blanchard

The next stage of development of the tidal stream industry will see a focus on the deployment of tidal turbines in arrays of increasing device numbers and rated power. Successful array development requires a thorough understanding of the resource within potential deployment sites. This is predictable in terms of flow speeds, based upon tidal constituents. However, the operating environment for the turbine is more complex than the turbine experiencing a uniform flow, with turbulence, shear and wave conditions all affecting the loading on the turbine components. This study establishes the accuracy with which several alternative modelling tools predict the resource characteristics which define unsteady loading—velocity shear, turbulence and waves—and assesses the impact of the model choice on predicted damage equivalent loads. In addition, the predictions of turbulence are compared to a higher fidelity model and the occurrence of flow speeds to a Delft3D model for currents and waves. These models have been run for a specific tidal site, the Raz Blanchard, one of the major tidal stream sites in European waters. The measured resource and predicted loading are established using data collected in a recent deployment of acoustic Doppler current profilers (ADCPs) as part of the Interreg TIGER project. The conditions are measured at three locations across the site, with transverse spacing of 145.7 m and 59.3 m between each device. Turbine fatigue loading is assessed using measurements and model predictions based on an unsteady blade element momentum model applied to near-surface and near-bed deployment positions. As well as across-site spatial variation of loading, the through life loading over a 5-year period results in an 8% difference to measured loads for a near-surface turbine, using conditions purely defined from a resource model and to within 3% when using a combination of modelled shear with measured turbulence characteristics.

Dynamic analysis of offshore triceratops supporting wind turbine: Preliminary studies

Offshore triceratops, the recent innovation in deep water compliant platforms, is primarily designed to withstand lateral forces through its geometric shape. The uniqueness of the platform is the presence of the ball joints between the legs and deck, which partially restrain the transfer of rotations from the legs to the deck and vice-versa. However, displacements such as surge, sway, and heave motions are transferred, ensuring a rigid connection between the legs and the deck. Efficient operations of offshore wind turbines are more dependent on the support systems on which they are mounted. Increased stability and reduction in stress concentration in rigid connections are desirable. Nonlinear dynamic response analysis is carried out in FAST by coupling the frequency response of the platform obtained in ANSYS AQWA with that of the HydroDyn module of FAST. The current study investigates a fully coupled three-dimensional hydro-aerodynamic model of triceratops mounted with a horizontal axis wind turbine. Unsteady Blade Element Momentum theory (BEM) is used to estimate the aerodynamic loads, which encompasses the effect of wind shear using a power law and spatially coherent turbulence. In contrast, Morison equations are used to estimate the hydrodynamic loads on the platform. After the preliminary proportioning of the platform, Response Amplitude Operator (RAO) plots are drawn to illustrate the partial motion transfer between the deck and the buoyant legs. Based on the preliminary studies, it is seen that the environmental loads do not impose instability, reinforcing the dynamic stability of the platform. Frequency responses for operating and parked conditions illustrate the coupling between the degrees of freedom and the influence of the rotor motion of the wind turbine on the platform deck. Tether tension variation is assessed in all three legs for the operational sea states to check the safety standards for a compliant system to avoid tether pull-out. The presented study is prima facie to encourage the suitability of triceratops as floaters to support the wind turbine under moderate sea states. Highlights This study is focused on new-generation offshore triceratops as support system for wind turbine Prelimiary dynamic anlaysis of coupled action of the supprting system and wind turbine are presented Use of ball joints help partial isolation of the deck and restrain transfer of moment from the turbine shaft to the supporting system, which is a novelty Infleunce of rotor motion of the triceratops is illustarted to highlight the advantage of complinacy of trirceratops This study disucsses only the performance assessment and not the design perspectives

Coupled dynamic analysis of horizontal axis floating offshore wind turbines with a spar buoy floater

Coupled dynamic analysis of a floating offshore wind turbine (FOWT) is predicted with in-house ITU-WAVE computational tool. The hydrodynamic parameters are approximated with time marching of boundary integral equation whilst aerodynamic parameters are solved with unsteady blade element momentum method. In addition, forces on FOWT due to mooring lines are predicted with quasi-static analysis whilst hydrodynamic viscous effects are included with Morison equation. FOWT’s blades are considered as an elastic structure whilst tower is considered as a rigid structure. The effects of steady wind speed on surge motion spectrum decrease the spectrum amplitude over wave frequency ranges, but this effect is not significant. The duration of time domain simulation plays significant role in the region of surge and pitch resonant frequencies. The numerical results of in-house ITU-WAVE computational code for eigenfrequencies of blades, aerodynamics and hydrodynamics parameters are validated against other numerical results which shows satisfactory agreements.

Comparison of different fidelity aerodynamic solvers on the IEA 10 MW turbine including novel tip extension geometries

Lifting-line based solvers could supersede the blade element momentum (BEM) method as the industry standard in the near future as rotor sizes of modern wind turbines and computational resources continue to increase. A comparison study between both methods is presented where the IEA 10 MW wind turbine is evaluated in the aero-servo-elastic simulation tool QBlade, comparing the lifting-line free vortex wake method to an unsteady blade element momentum solver. Besides the baseline rotor of the IEA 10 MW turbine, the comparison includes several blade tip extensions, including a swept and a dihedral geometry, to further differentiate capabilities between both methods in aerodynamically complex flow fields. As a reference serve results from equivalent simulations performed with multiple fidelity solvers of the simulation tool HAWC2. Results of a rigid load case demonstrate considerable improvement regarding the aerodynamic accuracy of lifting-line based methods in below rated conditions over BEM codes. The aerodynamic loads on geometrically complex tip extensions in steady conditions prove good capabilities of lower fidelity codes to predict out-of-plane bend shapes but also clear limitations regarding loads on swept geometries. A quasi-steady aeroelastic load case further demonstrates the capability of both QBlade codes to produce comparable results to similar fidelity solvers of the HAWC2 tool on an integral level. A detailed comparison in the time domain shows a larger dependency of the results on the type of structural solver that is used in contrast to the fidelity level of the aerodynamic method.

Linear and non-linear wave theories coupled with a modified BEM model for quantifying dynamic loads of a tidal stream turbine

Dynamic loads resulted from wave-current interaction is a serious problem facing tidal current turbines (TCTs), which may lead to catastrophic results. The main issue in such environments is the accelerated fatigue, which likely results from the cyclic loading especially in the axial direction. For investigating the current effects on the wave field and evaluating the resulted dynamic loads, a modified Blade Element Momentum (BEM) model was presented and coupled with linear and non-linear wave theories.Firstly, a validation study has been performed between the present modified BEM model coupled with linear wave theory and previous numerical and experimental results. A previously introduced dynamic inflow model was reemployed in the present BEM model during this validation to assess its applicability and accuracy. Very good agreements were seen between the present model and the experimental data at different wave properties and operational conditions, which emphasized the precision of the present unsteady BEM model and its corrections for further investigations.Comparing with other operational conditions, the intrinsic wave period and wave height had significant effects on the resulted hydrodynamic loading ranges of both the blade root bending moments. By the use of linear wave theory, the resulted out-of-plane bending moment was under predicted. Linear wave and second-order models had similar bending moments at all wave and flow conditions. Due to considering the current effects on wave properties, Dean's, third-order and fifth-order models had dynamic loadings differed in phase, frequency and amplitude from the experimental data. Fifth-order model was finally considered the best choice for simulating the wave-current velocity at different wave conditions and immersion depths.An additional complete time series runs were performed to assess the effect of dynamic stall correction inclusion in the present BEM model on the resulted hydrodynamic loads. It had minor effects on the bending moments, while had noticeable influence on the turbine thrust.

The effect of shear web on the dynamic response of a wind turbine blade subjected to actual dynamic load

The advent of wind turbines has enabled mankind to utilize renewable energy sources for the development of power. The blade being the most crucial part and the design of the same remains a challenge since it is subjected to dynamic loads due to the rotation of the blade along with unsteady wind velocity. The prediction of the dynamic wind loads acting on the blade is a difficult task and thus this has been analyzed in the present work. Two different approaches have been proposed to predict accurately the variation of the wind loads acting on the rotor using the unsteady blade element momentum theory. The effect of gravity has also been accounted for in computing the response of the structure. The effect of the position of shear web and the number of shear webs on the response of the structure has also been analyzed in the present work.

Time domain flutter analysis of bend-twist coupled large composite wind turbine blades: a parametric study

This article presents flutter prediction of composite wind turbine blades through time domain aeroelastic analysis using unsteady blade element momentum (BEM) theory based aerodynamic model. Comparison of the flutter speeds obtained in the present study with those previously obtained through Eigen solution based approach using Theodorsen’s theory based aerodynamic model for several different lamination sequences and blade configuration has been presented. It has been observed that both the approaches predict highest flutter speed at the same blade configuration and lamination sequence. However, flutter limit obtained through the present approach is less as compared to that obtained through Eigen solution based approach.

Accuracy and consistency of CFD and engineering models for simulating vertical axis wind turbine loads

Abstract The present work is intended to assess the ability of state-of-the-art approaches with various fidelity levels for accurate load predictions on vertical axis wind turbines (VAWT). The assessments are conducted by employing the Double-Multiple-Streamtube (DMS), Improved-DMS (IDMS), Unsteady Blade Element Momentum (UBEM), Vortex Model and fully resolved computational fluid dynamics (CFD) approaches. For the later case, three different codes are employed, namely FLOWer, TAU and Ansys Fluent. Three different turbines from low up to high rotor solidity (0.23, 0.53 and 1.325) are selected as the case studies. The prediction results are compared with experimental data at various operating ranges in terms of integral and azimuthal loads. The studies demonstrate that there is consistent agreement between engineering models at lightly loaded cases for the power curve prediction. The discrepancy at high tip speed ratio (λ) is caused by wake expansion, unsteady and decambering effects. In contrast, CFD predictions hardly show consistent power prediction but deliver accurate thrust values.

Dynamic loading characterization of a horizontal axis tidal current turbine

In the present work, a parameter study across a range of intrinsic wave periods, wave heights, current velocities, hub immersion depths, yaw misalignments and tip speed ratios on the dynamic behavior of blade root bending moments has been performed. For evaluating the dynamic loading, a modified unsteady Blade Element Momentum (BEM) model was prepared and coupled with linear and non-linear wave theories. Firstly, a validation study with previous experimental and numerical work was performed. Fifth-order model gave more realistic results taking in consideration the effects of current on wave properties. So, the parameter study calculations were performed using the fifth-order model. The effect of varying wave period, wave height and current velocity with and without the presence of yaw misalignment range on the response of root bending moments has been studied. In addition, the effects of tidal immersion depth variation at different flow conditions have been investigated. Also, the bending moments’ behavior due to the change of tip speed ratio has been studied.Through investigating the effects of wave height, wave period and current velocity variation in the presence of yaw misalignment range, it has been observed that the in-plane bending moments reduction over their median loading have a greater reduction at high wave heights and steep waves respectively. Furthermore, yaw misalignment variation became more influential at higher velocities, while the in-plane bending moments reduction to average loadings had greater values at low velocities. Due to wave height and wave period increase, the maximum loading ranges of 233% (more than the double) of the median out-of-plane bending moment and 146% of the median in-plane bending moment were observed. The submergence ratio reduction seemed to have minor effects even with increasing the current velocity, while had a higher influence at increased wave heights. Also, the excessive increase in the turbine rotational speed has caused a significant increase in the in-plane bending moment frequency oscillations, which in turn affects the quality of the electricity production.

Unsteady aerodynamic performance analysis of an airborne wind turbine under load varying conditions at high altitude

The blade air loads of the Airborne Wind Turbine (AWT) may be significantly influenced by the unsteady flow-field at high altitude. Under these susceptible situations, the rotor needs in-depth considerations with respect to transient aspects. The present study focuses on the unsteady aerodynamic performance of stand-alone rotor under the influence of wind shear, yawed, and tilted configurations. The Computational Fluid Dynamics (CFD) transient simulations based on the sliding mesh approach are carried out for analyzing the unsteady behavior of the rotor by lifting the rotor assembly at an airborne altitude from the ground. All simulations are conducted at optimal operating conditions of wind speed and tip speed ratio. In-house Unsteady Blade Element Momentum (UBEM) code using the wind shear, dynamic stall, dynamic wake, and yaw /tilt model are applied to acquire an empirical assessment in terms of rotor torque, thrust, and power coefficient. Finally, the CFD simulated results are compared against the performance curves generated by the UBEM model and acceptable agreement is found within 4.1% deviation at peak operational conditions. The results further reveal that the time-varying aerodynamic loads on the rotor blade gradually achieve steady behavior after three rotation periods. Meanwhile, unique similarities are found in yawed and tilted inflow cases and a 10.7% loss in power coefficient is observed due to rotor yawed inflows. Additionally, in the absence of tower shadows, the strong flow interactions around the rotor blade impose a positive impact on power output. Despite the complexity of unsteady flow phenomena, this present study would be helpful to design a more proficient airborne rotor under the varying load conditions.

Effect of platform disturbance on the performance of offshore wind turbine under pitch control

AbstractThe aerodynamic performance of offshore floating wind turbines (OFWTs) is more complicated than onshore wind turbines due to 6‐degree of freedom (DOF) motion of the floating platform. In the current study, the aerodynamic analysis of a horizontal‐axis floating offshore wind turbine is performed with the aim of studying the effects of floating platform movement on the aerodynamic characteristics of the turbine in the presence of a pitch angle control system. The National Renewable Energy Laboratory (NREL) 5‐MW offshore wind turbine is selected as the baseline wind turbine. For this sake, the unsteady blade element momentum method with dynamic stall and dynamic inflow models have been employed to obtain the unsteady aerodynamic loads. The baseline pitch angle control system is assumed to be coupled with the aerodynamic model to maintain the rated condition of the wind turbine and also to approach a closer model of wind turbine. In case of pitching motion input, the reduction of mean power coefficient for tip speed ratios (TSRs) less that 7 is expected by an amount of 16% to 20% at pitch amplitude of 2° and frequency of 0.1 Hz. For high TSRs, the trend is reverse with respect to fixed‐platform case. The mean thrust coefficient is reduced for almost all range of TSRs with maximum loss of 37%. Moreover, the mean control pitch angle that is an index of control system effort is increased. The results also represent the importance of considering the pitch control system for aerodynamic analysis of disturbed OFWT.

Aeroelastic stability analysis of a wind turbine blade section with trailing edge flap using a flexible unsteady blade elements momentum theory

This study aims to develop an unsteady aerodynamic model and blade element momentum theory to analyze the aeroelasticity of a rotating airfoil with trailing edge flap of a wind turbine blade. The Hamiltonian principle serves to define the equations of motion. Theodorsen and Loewy aerodynamic models alone and together with blade element momentum (BEM) theory are applied to form aeroelastic equation of motion. Theodorsen or Loewy aerodynamics together with BEM theory present a new flexible unsteady blade element momentum aerodynamic model. Verification of developed aeroelastic equation motion is conducted on a fixed and a rotating airfoil successfully. Flutter boundary results imply the superiority of proposed aerodynamic model. Besides, we investigated theoretical effects of pitch and flap stiffness changes on flutter. The derived results demonstrate that pitch stiffness is the dominant mode on flutter boundary of the rotating blade section with a control surface.

Predicting the stochastic aerodynamic loads on blades of two yawed downwind hawts in uncontrolled conditions using a bem algorithm

Accurate predictions of the cycle-to-cycle variations in aerodynamic loads on wind turbine blades are important for estimating the life cycle costs of new wind power plants. Physical modelling of wind turbine systems is usually based on the average values of the aerodynamic parameters in the design process which would eliminate some important physical phenomena associated with the unsteady flows over the rotating blades. This paper presents a new algorithm implemented in a Blade Element Momentum (BEM) model to predict the cycle-to-cycle variations in the aerodynamic loads over multiple rotor rotations (cycles) for yawed rotors operating in natural flow conditions. In this approach, simulation theory developed by the Pacific Northwest Laboratories (PNL) is used to generate input turbulent wind speed time series on the whole rotor disc of two three-bladed downwind horizontal axis wind turbine (NREL Phase II and III) rotors. The unsteady influences of the dynamic stall kinematics and tower shadow are also considered in the current algorithm along with the phenomenon of stall delay. Predictions from the unsteady BEM model are compared to measured results and conclusions are drawn about the ability of the model to accurately simulate blade response to turbulence.

Nonlinear Aeroelastic Analysis of Large Wind Turbines Under Turbulent Wind Conditions

We investigate in this Paper the nonlinear aeroelastic response of large horizontal axis wind turbines under a stochastically simulated turbulent wind field, which is instantaneously sampled by the rotating and deforming blades of the rotor. The aeroelastic code combines the unsteady blade element momentum theory, equipped with several important corrections, with the displacement-based nonlinear finite element implementation of the geometrically exact beam theory. The structural model is both shearable and extensible and does not place any restrictions on the magnitudes of the configuration variables or those of the resulting one-dimensional strain measures. The aeroelastic coupling takes into account the beam axis deformations, elastic twist, and vibration-induced velocities. The three-component, three-dimensional turbulent wind field is obtained using the Mann spectrum, from which the instantaneously sampled wind speeds are extracted by spatiotemporal interpolation in the spirit of Taylor’s frozen turbulence hypothesis. The structural and unsteady aerodynamic analysis components of the aeroelastic code are validated using benchmark examples. A fully coupled nonlinear aeroelastic simulation is carried out for the National Renewable Energy Laboratory’s 5 MW research wind turbine. The influence of three-dimensional instantaneous sampling, transverse (lateral and vertical) components of turbulence, vertical wind shear, and flow unsteadiness on the aeroelastic response under turbulence is examined along with the effects of coupling parameters including the vibration-induced velocities and elastic twist of the rotor blades. The dominant frequency contributions in key aeroelastic response variables due to each of these effects are identified.

Loading and Blade Deflection of a Tidal Turbine in Waves

A coupled numerical model has been developed and validated to study the fluid–structural interaction responses of a three-bladed tidal turbine in aligned waves and current. The unsteady blade element momentum (BEM) theory was combined with modal analysis for hydro-elastic calculation. Both the loading and deflection of the blade were studied. The dynamic loading on the blade due to structural deformation was much smaller than the wave-induced loading under linear wave conditions for the given condition. The linear response amplitude operators (RAOs) of the loads and the blade tip deflections were obtained and used to predict the linear responses. Although both sum- and difference-frequency responses can be identified from time domain simulations, the wave-induced load and the deflection of the blade are dominated by the first-order contributions. The maximum deflection of the blade tip could reach 1.3 m (203% of the means) in the flapwise direction and 0.35 m (210% of the mean) in the edgewise direction with a wave peak period of 11.3 s and a significant wave height of 5.5 m.

A fast stochastic solution method for the Blade Element Momentum equations for long‐term load assessment

AbstractUnsteady power output and long‐term loads (extreme and fatigue) drive wind turbine design. However, these loads are difficult to include in optimization loops and are typically only assessed in a post‐optimization load analysis or via reduced‐order methods. Both alternatives yield suboptimal results. The reason for this difficulty lays in the deterministic approaches to long‐term loads assessment. To model the statistics of lifetime loads they require the analysis of many unsteady load cases, generated from many different random seeds—a computationally expensive procedure. In this paper, we present an alternative: a stochastic solution for the unsteady aerodynamic loads based on a projection of the unsteady Blade Element Momentum (BEM) equations onto a stochastic space spanned by chaos exponentials. This approach is similar to the increasingly popular polynomial chaos expansion, but with 2 major differences. First, the BEM equations constitute a random process, varying in time, while previous polynomial chaos expansion methods were concerned with random parameters (ie, random but constant in time or initial values). Second, a new, more efficient basis (the exponential chaos) is used. This new stochastic method enables us to obtain unsteady long‐term loads much faster, enabling unsteady loads to become accessible inside wind turbine optimization loops. In this paper we derive the stochastic BEM solution and present the most relevant results showing the accuracy of the new method.

Hydrodynamic performance prediction of a tidal current turbine operating in non-uniform inflow conditions

A detailed understanding of the hydrodynamics of a tidal current turbine is paramount to the further development and adaptation of tidal current energy. Hydrodynamic modelling of such systems assists with reducing the cost of energy through accurate performance prediction enabling design refinement. Blade element momentum theory offers an efficient modelling technique to compute the hydrodynamic performance of a tidal current turbine. In this work, a corrected mathematical model, based on unsteady blade element momentum theory, for the application of tidal current turbines is presented. The mathematical model is compared with experimental data found in the literature, showing excellent agreement. Particular attention is given to the hydrodynamic performance of a tidal current turbine subjected to tidal current shear and yaw misalignment. The results of this study indicate that tidal current shear and yaw misalignment have a significant impact on the hydrodynamic performance of a tidal current turbine. Importantly, the hydrodynamic model presented can be used as an efficient design and optimisation tool for tidal current turbine blades. Moreover, the mathematical model presented can be combined with an economic model to assess the techno-economic performance of a tidal current turbine operating in actual site conditions.

Aerodynamic Interference Effect of Huge Wind Turbine Blades With Periodic Surge Motions Using Overset Grid-Based Computational Fluid Dynamics Approach

The accurate prediction of unsteady aerodynamic performance and loads, for floating offshore wind turbines (FOWTs), is still questionable because several conventional methods widely used for this purpose are applied in ways that violate the theoretical assumptions of their original formulation. The major objective of the present study is to investigate the unsteady aerodynamic effects for the rotating blade due to the periodic surge motions of an FOWT. This work was conducted using several numerical approaches, particularly unsteady computational fluid dynamics (CFD) with an overset grid-based approach. The unsteady aerodynamic effects that occur when an FOWT is subjected to the surge motion of its floating support platform is assumed as a sinusoidal function. The present CFD simulation based on an overset grid approach provides a sophisticated numerical model on complex flows around the rotating blades simultaneously having the platform surge motion. In addition, an in-house unsteady blade element momentum (UBEM) and the fast (fatigue, aerodynamic, structure, and turbulence) codes are also applied as conventional approaches. The unsteady aerodynamic performances and loads of the rotating blade are shown to be changed considerably depending on the amplitude and frequency of the platform surge motion. The results for the flow interaction phenomena between the oscillating motions of the rotating wind turbine blades and the generated blade-tip vortices are presented and investigated in detail.

The platform pitching motion of floating offshore wind turbine: A preliminary unsteady aerodynamic analysis

The flow-field around the rotor blades of an FOWT may be significantly influenced by the six rigid-body motions of the floating platform via the blade–wake interaction. Therefore, the accurate prediction of unsteady aerodynamic load which is calculated by many conventional numerical approaches is still questionable for an FOWT. In this study, the periodic pitching motion of the rotating turbine blades due to the floating platform motion is considered to investigate the effects of vortex–wake–blade interaction for the aerodynamic performance of an FOWT. The unsteady computational fluid dynamics (CFD) simulations based on the dynamic mesh technique were applied for analyzing the pitching motion of wind turbine due to the platform motion. The in-house unsteady blade element momentum code using the direct local relative velocity approach was also applied to simulate the unsteady aerodynamic performance. The equivalent average velocity approach which simplifies the relative velocity contribution due to the platform motion was proposed and incorporated to the in-house code. It is shown that the unsteady aerodynamic loads of the floating offshore wind turbine become sensitively changed due to the variation of frequency and amplitude of the platform motion. Additionally, there are strong flow interaction phenomena between the rotating blades with oscillating motions and generated blade-tip vortices.