Blade Element Momentum Method Research Articles

Aerodynamic validation studies on the performance analysis of iced wind turbine blades

Ice accretion on wind turbine blades distorts blade profiles and causes degradation in the aerodynamic characteristic of the blades. In this study ice accretion on turbine blades are simulated under various icing conditions, and the resulting power losses are estimated. The Blade Element Momentum method is employed together with an ice accretion prediction methodology based on the Extended Messinger model in a parallel computing environment. The predicted iced profiles are first validated with the experimental and numerical data available in the literature. 2D flow solutions and aerodynamic loads over iced blade profiles are obtained with 3 different flow solvers of increasing fidelity; XFOIL, an open-source panel code coupled with a turbulent boundary layer model, SU2, an open-source RANS solver, and METUDES, an in-house DDES solver. The power production losses of a 30 kW wind turbine operating with iced blades are then investigated in detail. It is shown that the XFOIL-based tool developed for the performance analysis of iced wind turbines successfully predicts ice profiles on turbine blades under various icing conditions and the consequent power losses. About 20% power loss is predicted for a 30 kW wind turbine exposed to icing conditions for an hour.

Takeoff and Performance Trade-Offs of Retrofit Distributed Electric Propulsion for Urban Transport

While vertical takeoff and landing aircraft have shown promise for urban air transport, distributed electric propulsion on existing aircraft may offer immediately implementable alternatives. Distributed electric propulsion could potentially decrease takeoff distances enough to enable thousands of potential intercity runways. This conceptual study explores the effects of a retrofit of open-bladed electric propulsion units. To model and explore the design space, blade element momentum method, vortex lattice method, linear-beam finite element analysis, classical laminate theory, composite failure, empirically based blade noise modeling, motor and motor-controller mass models, and gradient-based optimization are used. With liftoff time of seconds and the safe total field length for this aircraft type undefined, this paper focused on the minimum conceptual takeoff distance. It was found that 16 propellers could reduce the takeoff distance by over 50% compared with the optimal 2-propeller case. This resulted in a conceptual minimum takeoff distance of 20.5 m to clear a 50 ft (15.24 m) obstacle. It was also found that, when decreasing the allowable noise by approximately 10 dBa, the 8-propeller case performed the best with a 43% reduction in takeoff distance compared with the optimal 2-propeller case. This resulted in a noise-restricted conceptual minimum takeoff distance of 95 m.

Comparative study on the wake description using actuator disc model with increasing level of complexity

The present work evaluates the benefits obtained when different levels of geometric and aerodynamic information of wind turbine blades are included in the actuator disc (AD) model, progressively increasing its descriptive capabilities. A novel AD model is presented, equivalent in precision to the AD coupled with the blade element momentum method, but avoiding the need to modify the CFD code by the use of a pre-calculated table. It is compared against three classical AD models, particularly aiming at the simulation of the wind farm wake interaction. The NREL-5MW reference wind turbine is adopted as test case, and configurations with single and two shifted wind turbines are analyzed.

Performance of a 3D pendulum tuned mass damper in offshore wind turbines under multiple hazards and system variations

Misaligned wind-wave and seismic loading render offshore wind turbines suffering from excessive bi-directional vibration. However, most of existing research in this field focused on unidirectional vibration mitigation, which is insufficient for research and real application. Based on the authors\' previous work (Sun and Jahangiri 2018), the present study uses a three dimensional pendulum tuned mass damper (3d-PTMD) to mitigate the nacelle structural response in the fore-aft and side-side directions under wind, wave and near-fault ground motions. An analytical model of the offshore wind turbine coupled with the 3d-PTMD is established wherein the interaction between the blades and the tower is modelled. Aerodynamic loading is computed using the Blade Element Momentum (BEM) method where the Prandtl\'s tip loss factor and the Glauert correction are considered. Wave loading is computed using Morison equation in collaboration with the strip theory. Performance of the 3d-PTMD is examined on a National Renewable Energy Lab (NREL) monopile 5 MW baseline wind turbine under misaligned wind-wave and near-fault ground motions. The robustness of the mitigation performance of the 3d-PTMD under system variations is studied. Dual linear TMDs are used for comparison. Research results show that the 3d-PTMD responds more rapidly and provides better mitigation of the bi-directional response caused by misaligned wind, wave and near-fault ground motions. Under system variations, the 3d-PTMD is found to be more robust than the dual linear TMDs to overcome the detuning effect. Moreover, the 3d-PTMD with a mass ratio of 2% can mitigate the short-term fatigue damage of the offshore wind turbine tower by up to 90%.

Design of a Horizontal Axis Tidal Turbine for Less Energetic Current Velocity Profiles

Existing installations of tidal-stream turbines are undertaken in energetic sites with flow speeds greater than 2 m/s. Sites with lower velocities will produce far less power and may not be as economically viable when using “conventional” tidal turbine designs. However, designing turbines for these less energetic conditions may improve the global viability of tidal technology. Lower hydrodynamic loads are expected, allowing for cost reduction through downsizing and using cheaper materials. This work presents a design methodology for low-solidity high tip-speed ratio turbines aimed to operate at less energetic flows with velocities less than 1.5 m/s. Turbines operating under representative real-site conditions in Mexico and the Philippines are evaluated using a quasi-unsteady blade element momentum method. Blade geometry alterations are undertaken using a scaling factor applied to chord and twist distributions. A parametric filtering and multi-objective decision model is used to select the optimum design among the generated blade variations. It was found that the low-solidity high tip-speed ratio blades lead to a slight power drop of less than 8.5% when compared to the “conventional” blade geometries. Nonetheless, an increase in rotational speed, reaching a tip-speed ratio (TSR) of 7.75, combined with huge reduction in the torque requirement of as much as 30% paves the way for reduced costs from generator downsizing and simplified power take-off mechanisms.

Numerical investigations into the idealized diurnal cycle of atmospheric boundary layer and its impact on wind turbine's power performance

The power generated by a wind turbine largely depends on the properties of wind resource. In this work, wind characteristics in different regimes occurring throughout the idealized diurnal cycle and its impact on wind turbine's power performance are investigated systematically by means of large-eddy simulation (LES), and blade element momentum method (BEM), respectively. Through a precursor simulation of the atmospheric boundary layer (ABL) over a homogenous surface throughout a day, it is found that the resulting shapes of wind profiles (including wind speed, wind direction and turbulence level) vary significantly at different time periods, induced by distinct stabilities of the atmosphere. The simulated wind field data are then applied to a NREL 5 MW wind turbine for its power evaluation. Due to variabilities in wind shear and turbulence, the equivalent (disk-averaged) wind speed is introduced for power prediction. It is found that the magnitude and fluctuation of turbine's diurnal power are closely related to the atmospheric stability. In general, the average power production is higher under convective conditions during the day than under stable conditions at night, with a difference approaching 24.4%. This indicates that wind energy resource assessment will close to reality and benefit from increased accuracy if atmospheric stability impacts are considered for turbine's power predictions.

Analysis of Wind Turbine Using QBlade Software

Owing to the fast development in the energy field, the demands are increasing to improve energy efficiency and lifetime of wind turbine. The wind blades are considered as the most important and expensive part in the wind system. Therefore, it’s important to understand deeply the behaviour of turbine blades. In this research paper, full details were presented to analyze and optimize the behaviour and performance of the blade of the small horizontal axis wind turbine (less than 1 KW). QBlade software was used to simulate the wind turbine blade during the working conditions. The mathematical formulations which used in QBlade software were based on the Blade Element Momentum method (BEM). It was studied deeply the effect of design parameters (Twist Angle and Chord length) on the behaviour and performance of the wind turbine. It was used SG6043 airfoil for 10 different sections of 1.17 m blade length. The obtained results were of high accuracy, and it was proved that the QBlade software is reliable to analyze the blades of wind turbine. The paper exhibits the necessary steps to build and optimize the blade of wind turbine, in addition to the features and advantages of the software.

Wind Turbine Aerodynamic Modeling in Icing Condition: Three-Dimensional RANS-CFD Versus Blade Element Momentum Method

Icing limits the performance of wind turbines in cold climates. The prediction of the aerodynamic performance losses and their distribution due to ice accretion is essential. Blade element momentum (BEM) is the basis of blade structural studies. The accuracy and limitations of this method in icing condition are assessed in the present study. To this purpose, a computational study on the aerodynamic performance of the full-scale NREL 5 MW rotor is performed. Three-dimensional (3D) steady Reynolds-averaged Navier–Stokes (RANS) simulations are performed for both clean and iced blade, as well as BEM calculations using two-dimensional (2D) computational fluid dynamics (CFD) sectional airfoil data. The total power calculated by the BEM method is in close agreement with the 3D CFD results for the clean blade. There is a 4% deviation, while it is underestimated by 28% for the iced one. The load distribution along the clean blade span differs between both methods. Load loss due to the ice, predicted by 3D CFD, is 32% in extracted power and the main loss occurs at the regions where the ice horn height exceeds 8% of the chord length.

Different functionalized chord and twist distributions in aerodynamic design of HAWTs

In this study, blade element momentum (BEM) theory has been used to calculate power output and performance of 100 kW and 1 MW horizontal axis wind turbine. Riso‐A1‐21 airfoils are considered as utilized section along the blade span. Generally, Riso family of airfoils is appropriate for wind turbine application. In order to improve the accuracy of turbine performance prediction, lots of corrections have been applied on the BEM method. The aim of this research study is proposing a new procedure to introduce the chord and twist distributions by fitting different types of functions on them. Accordingly, 48 functions are selected as possible distributions for chord and twist angle profiles. These functions are selected in such a way that their behavior is close to the trend of their classical distributions. The utilized design method in this study is optimum in terms of performance and also making distribution of chord and twist functional which is more appropriate for manufacturing process. Finally, the effect of using best functions on blade aerodynamic coefficients has been studied. Results show that despite the changing chord and twist distributions from their classical trends, the aerodynamic coefficients are approximately similar to those in the classical design except in some region of blade. © 2019 American Institute of Chemical Engineers Environ Prog, 38:e13108, 2019

Design and Optimization of Composite Offshore Wind Turbine Blades

In order to obtain an optimal design of composite offshore wind turbine blade, take into account all the structural properties and the limiting conditions applied as close as possible to real cases. This work is divided into two stages: the aerodynamic design and the structural design. The optimal blade structural configuration was determined through a parametric study by using a finite element method. The skin thickness, thickness and width of the spar flange, and thickness, location, and length of the front and rear spar web were varied until design criteria were satisfied. The purpose of this article is to provide the designer with all the tools required to model and optimize the blades. The aerodynamic performance has been covered in this study using blade element momentum (BEM) method to calculate the loads applied to the turbine blade during service and extreme stormy conditions, and the finite element analysis was performed by using abaqus code to predict the most critical damage behavior and to apprehend and obtain knowledge of the complex structural behavior of wind turbine blades. The approach developed based on the nonlinear finite element analysis using mean values for the material properties and the failure criteria of Hashin to predict failure modes in large structures and to identify the sensitive zones.

Effect of Rotor Thrust on the Average Tower Drag of Downwind Turbines

A new analysis method to calculate the rotor-induced average tower drag of downwind turbines in the blade element momentum (BEM) method was developed in this study. The method involves two parts: calculation of the wind speed distribution using computational fluid dynamics, with the rotor modeled as a uniform loaded actuator disc, and calculation of the tower drag via the strip theory. The latter calculation considers two parameters, that is, the decrease in wind speed and the pressure gradient caused by the rotor thrust. The present method was validated by a wind tunnel test. Unlike the former BEM, which assumes the tower drag to be constant, the results obtained by the proposed method demonstrate much better agreement with the results of the wind tunnel test, with an accuracy of 30%.

Impact of Blade Flexibility on Wind Turbine Loads and Pitch Settings

Along with the upscaling tendency, lighter and so more flexible wind turbine blades are introduced for reducing material and manufacturing costs. The flexible blade deforms under aerodynamic loads and in turn affects the flow field, arising the aeroelastic problems. In this paper, the impacts of blade flexibility on the wind turbine loads, power production, and pitch actions are discussed. An advanced aeroelastic model is developed for the study. A free wake vortex lattice model instead of the traditionally used blade element momentum (BEM) method is used to calculate the aerodynamic loads, and a geometrically exact beam theory is adopted to compute the blade structural dynamics. The flap, lead-lag bending, and torsion degrees-of-freedom (DOFs) are all included and nonlinear effects due to large deflections are considered. The National Renewable Energy Laboratory (NREL) 5 MW reference wind turbine is analyzed. It is found that the blade torsion deformations are significantly affected by both the aerodynamic torsion moment and the sectional aerodynamic center offset with respect to the blade elastic axis. Simulation results further show that the largest bending deflection of the blade occurs at the rated wind speed, while the torsion deformation in toward-feather direction continuously increases along with the above-rated wind speed. A significant reduction of the rotor power is observed especially at large wind speed when considering the blade flexibility, which is proved mainly due to the blade torsion deformations instead of the pure-bending deflections. Lower pitch angle settings are found required to maintain the constant rotor power at above-rated wind speeds.

Experimental validation of a ducted wind turbine design strategy

Abstract. A synergistic design strategy for ducted horizontal axis wind turbines (DWTs), utilizing the numerical solution of a ducted actuator disk system as the input condition for a modified blade element momentum method, is presented. Computational results of the ducted disk have shown that the incoming flow field for a DWT differs substantially from that of a conventional open rotor. The rotor plane velocity is increased in the ducted flow field, and, more importantly, the axial velocity component varies radially. An experimental full-scale 2.5 m rotor and duct were designed, using this numerical strategy, and tested at the University of Waterloo's wind turbine test facility. Experimental results indicated a very good correlation of the data with the numerical predictions, namely a doubling of the power output at a given velocity, suggesting that the numerical strategy can provide a means for a scalable design methodology.

Hydrodynamic Assessment of a Dual-Rotor Horizontal Axis Marine Current Turbine

The hydrodynamic performance of a dual-rotor horizontal axis marine turbine (HAMCT) is investigated for the power gain in operating the rear rotor without blade-pitch control. This kind of turbine can be advantageous for a rectilinear tidal current of reversing directions, where each rotor blade is optimally fixed-pitched towards its upstream velocity. The blade element momentum (BEM) method is coupled with the Park wake model. A generic three-blade turbine is shown to gain up to 20% in the coefficient of power CP as relative to the front rotor CP when operating the rear rotor at the same tip speed ratio (TSR) as the front one, gaining overall CP up to 0.55. Analytic model is derived to backup the estimate of power gain. Plots for turbine performance variation with TSR and profile hydrodynamic efficiency are given, and analysed for lab and small-medium size turbines.

Experimental validation of analytical wake and downstream turbine performance modelling

Wake effects in wind farms can cause significant power losses. In order to reduce these losses layout and control optimization can be applied. For this purpose, simple and fast prediction tools for the wake flow are needed. In the first part of this work, five analytical wind turbine wake models are compared to small-scale turbine wind tunnel measurements. The measurements are conducted at several downstream distances, varying the ambient turbulence intensity and upstream turbine blade pitch angle. Furthermore, an adjustment of a recently developed wake model is proposed. Subsequently, the adjusted model is found to perform best throughout all test cases. In the second part, the performance of an aligned downstream turbine is modelled based on the predicted wake flow using a Blade Element Momentum method with guaranteed convergence. In order to consider the non-uniform inflow velocity a mean-blade-element-velocity method is developed. Moreover, a blockage effect correction is applied. A comparison to wind tunnel measurement data shows that the wake velocity as well as the combined power of two aligned turbines are fairly well predicted. Additionally, the presented analytical framework of wake and downstream turbine performance modelling proposes several model improvements for state-of-the art wind farm simulation tools.

Floating Ice Load Reduction of Offshore Wind Turbines by Two Approaches

The offshore wind turbines (OWTs) constructed at the northern sea areas under cold climate are frequently subjected to floating ice loads. It is imperative to reduce the damage owing to the floating ice with some appropriate approaches. The purpose of this paper is to ascertain the effectiveness of the tuned mass damper (TMD) and the ice-breaking cone for reducing floating ice loads on OWTs. The National Renewable Energy Laboratory's (NREL) 5 MW OWT, which is treated as a multibody system with rigid and flexible parts, is adopted as the example model here. The multiple loads taken into consideration in the fully coupled simulation include floating ice and turbulent wind. The aerodynamic load acting on the blades is solved by the blade element momentum method based on a full-field turbulent wind farm generated by the Kaimal spectrum. The Matlock model and the Ralston model are adopted for evaluating the floating ice loads on the cylindrical and conical structures, respectively. The TMD system in the nacelle and the ice-breaking cone on the tower at the mean sea level are the two load reduction approaches of concern in this paper. A weak aeroelastic simulation of the OWT model is conducted. The solution of flexibility effectiveness depends on some accurate mode shapes by the linear modal representation. Finally, Kane's method is used for predicting the motion of the whole OWT. The relevant results reveal some positive effectiveness of the TMD system and the ice-breaking cone for reducing the floating ice load. The displacement of tower top decreases significantly with the utilization of the two approaches. The TMD system has a better performance for the side-side displacement than the fore-aft displacement. By switching the ice failure mode from crushing to bending, the ice-breaking cone reduces the floating load more effectively than the TMD system. It affects equally significantly the fore-aft and side-side displacements.

Determination of local flow conditions and preliminary investigation on the validity of the PNoise code for large-size wind turbine

This paper summarizes the determination of more realistic local Reynolds and Mach flow numbers at the blades of a large-diameter, horizontal-axis wind turbine designed specifically for this purpose with the aid of the blade element momentum method, and the subsequent effort to extend the validation range of the modified-BPM airfoil trailing edge noise method against a single high Reynolds number acoustic dataset available. The validation extension effort proved unfruitful, and the reasons are discussed in detail in relation to the dataset employed. After coupling the BPM model to a hybrid boundary layer solver, the resultant modified method, called PNoise, was embedded into the TU Berlin (HFI) wind turbine design, open code environment, QBlade, available under General Public License. During the time period past since the release of the first integrated version (v0.95), the open-source code has been downloaded by users more than 20,000 times, prompting the trial for further validation of the proposed method. Although the validation extension is not sanctioned based on this preliminary investigation, the calculation of more realistic flow conditions over the blades of current utility-size wind turbines could be helpful to other researchers. Also, this effort highlighted the significant uncertainties associated with methods employed to obtain acoustic spectra from one specific aeroacoustic wind tunnel, especially when the acoustic signal obtained experimentally is later subjected to transforming algorithms. The findings also stress the fact that more reliable experimental data is needed under high Reynolds numbers (notice: all Reynolds numbers described in the text are local, chord-based Reynolds numbers.) in order to support TE and other airfoil self-noise model development and validation.

Study of the Hydrodynamic Behavior of Horizontal Axis Tidal Turbines Based on the Blade Element Momentum Theory and Numerical Simulations

The aim of this study is to predict the power output and the power coefficient for various tidal turbine rotors, with four different airfoils, in particular, the Eppler E603, the NACA 653-618, the NREL's S816 and the NREL's S831 airfoils. The first step of the study was the development of a user-friendly application, based on the Blade Element Momentum (BEM) theory, for the calculation of the above-mentioned quantities and the optimum chord and twist distributions of the blade for each case. Parameters such as the airfoil characteristics, the number of blades, the rotor diameter, the tidal current velocity and the tip speed ratio (TSR) were examined. From the results it was obvious that as the tidal current velocity and the rotor diameter increase, the power output increases. Furthermore, the power coefficients for all the examined airfoils were high enough, although the S831 airfoil was found to have the best performance. Because the BEM method can’t predict the three-dimensional phenomena that occur in the real flow, a commercial Computational Fluid Dynamics (CFD) code was used in order to obtain further knowledge of the flow field over the rotor with the S831 airfoil, which was found to have the highest performance.

Rotor wake engineering models for aeroelastic applications

Results from the EU AVATAR project have indicated vortex methods generally to be in good agreement with computational fluid dynamics simulations for resolving rotor aerodynamic forces in a variety of operational and inflow conditions. Limitations of the more crude blade element momentum method, most widely used in industry, have been exposed. Although not as expensive as computational fluid dynamics, the application of vortex methods to resolve rotor aerodynamics in wind turbine design and loads calculations is still hindered by its computational burden. An increase in computer power and parallelization has helped over the decades, but an efficiency boost is required to make the last step. The effect of simplifying the far wake description on predicted loads as well as computational effort is investigated for a variety of load cases. Results indicate that dynamic and time averaged loading characteristics are preserved when the mid to far wake grid resolution is reduced. The applied wake reduction is a promising technique leading to desktop design load calculations using vortex wake methods.

An actuator disk method with tip‐loss correction based on local effective upstream velocities

AbstractThis work aims at assessing the performance of a tip‐loss correction for advanced actuator disk (AD) methods coupled to large eddy simulation and making this correction possible in a wind farm configuration. The classical Glauert tip‐loss factor, commonly used in the blade element momentum method, is added here to correct the tip and the root induced velocities at the rotor. However, it requires a reference upstream velocity, which is problematic to define in complex flows, such as in wind farms. A methodology is proposed here to infer an effective upstream velocity local to each disk element, based on the one‐dimensional momentum theory and using only the local data at the rotor. This estimation is verified through a set of simulations, leading to good results in spite of the crude assumptions of the one‐dimensional momentum theory. This AD supplemented with the tip‐loss correction is compared with a high fidelity vortex particle‐mesh method, through the simulations in uniform wind of a constant circulation wind turbine and of a more realistic machine, the NREL‐5MW rotor. The results show that the AD behavior is clearly improved by the addition of a tip‐loss factor and the potential errors on the effective upstream velocity estimation have a moderate impact on the tip‐loss correction.