Blade Element Momentum Research Articles

Development of a modified BEMT model for the analysis of helical bladed vertical axis tidal turbines

Development of a modified BEMT model for the analysis of helical bladed vertical axis tidal turbines 
 Mohammad Fereidoonnezhad1, Seán Leen1, Stephen Nash1, Tomas Flanagan2, Patrick McGarry1
 1 School of Engineering, University of Galway, Galway, Ireland
 2 ÉireComposites Teo., Údarás Industrial Estate An Choill Rua, Inverin, Galway, Ireland
 E-mail: patrick.mcgarry@universityofgalway.ie 
 
 Keywords: Blade element momentum theory, Helical vertical axis tidal turbine, FEA Analysis
 Classical blade element momentum theory (BEMT) formulations are not capable of accurately simulating complex blade geometries, such as spiral or helical blade geometries. In this paper, we develop a modified BEMT model to calculate hydrodynamic forces acting on the helical vertical axis tidal turbines. Our framework accounts for curved blade geometries where the leading edge is not orthogonal to the freestream velocity, and chord vectors are not in the azimuthal-radial plane. We validate the model using experimental data for a prototype VAWT. We then perform a parametric analysis of helical bladed VATT designs. Both the helix angle of the blade and the relative orientation of the chord strongly influence the power output of a turbine. Our modified BEMT model also predicts that an increase in blade helix angle results in reduced power fluctuations. Additionally, computed fluctuations in tangential and normal forces acting on blades are shown to reduce significantly with increasing blade helix angle, suggesting a reduction in risk of fatigue failure. We also uncover a complex distribution of normal and tangential forces along the length of a helical blade. Fluctuating hydrodynamic forces computed by our modified BEMT model are input into a finite element (FE) framework to compute the stress state in a fibre reinforced composite blade material for a range of blade azimuthal positions. Analyses reveal that blade deflections are three orders of magnitude lower than the turbine radius, suggesting that sufficient structural stiffness is achieved by the blade design. Results also uncover stress concentrations in the region of strut-blade connection, revealing a higher risk of fracture and fatigue failure at this location.

Investigation of Low Order Parameters Affecting Tidal Stream Energy Resource Assessments

Tidal stream energy may have the potential to contribute to the baseline energy needs of the UK. There is a significant difference between the existing assessments of the UK’s resource, highlighting the need for a standardised model. The paper investigates some challenges of quantifying tidal stream energy. This study examines the sensitivity of resource assessments to low-order parameters, in order to inform higher order modelling and contribute to defining a standardised model. A combination of channel characteristics, turbine parameters and simulation settings formed the basis of 96+ cases to determine key parameters affecting resource assessments. Two 0-dimensional, idealised channels were modelled using 2-8 constituents from four UK sites spanning the semi-diurnal form factor range. Blockage-corrected blade element momentum theory was used to represent variable-speed, variable-pitch turbines. Three blockage designed rotors and one rotor designed for an unspecified blockage were simulated. Power capping and combined power and thrust capping were implemented.
 
 The study indicates dynamic balance is a key parameter affecting tidal dynamics when turbines are present in a channel. Variability of annual energy production due to the nodal cycle was most significant in the drag dominated channel with increasing blockage. Constituents with large amplitude and phase differences led to greater variability of the resource across the nodal cycle. Results indicate that the maximum deviation from the average annual energy production over the 18.6-year nodal cycle is +/- 8.8%. At a low form factor site, the difference between using 2 or 4 constituents was insignificant. However, for a higher form factor site, that is still semi-diurnal, the peak velocity increases by 0.32 m/s with 8 constituents compared to 2. Random Forest modelling indicates that the most important characteristic point on a variable-speed, variable-pitch turbine performance curve is the rated speed. Comparison of capping strategies highlights that combined power and thrust capping leads to an increase in energy per swept area. Combined capping decreased the variability of the resource over the nodal cycle, which is helpful to maintain a steady supply to the grid and sizing energy storage systems.

Turbine fatigue load prediction from field measurements of waves and turbulence

The interest in this work is understanding the conditions which contribute to the cyclic loading experienced by a tidal turbine. Tidal turbines are placed in locations with high flow speeds in order to extract maximum kinetic energy. These flow speeds combined with other environmental factors can lead to a complex operating conditions. When assessing turbine loading across a site, most models use ambient flow conditions, with very few codes including the impact of waves with currents. Previous studies have analysed the influence of wave conditions from measurements on the loading, through a consideration of the turbine not operating. Although tidal conditions are bi-directional, wave conditions do vary with direction and this combination will be investigated in this study.
 Full-scale site data was gathered at Le Raz Blanchard, a potential tidal site as part of the Interreg funded TIGER project. The focus in this study is on data gathered which provides both wave and current conditions over a period of 45 days. Loading on the turbine components is assessed through an efficient blade element momentum theory method, which is combined with a synthetic turbulence model to provide a time-varying onset flow. This work determines the impact of the measured waves, and the way in which they are analysed on the turbulence characteristics in the onset flow, specifically the interaction of waves with current and influence on the loading experienced on the turbine.

Tidal Turbine Benchmarking Project: Stage I - Steady Flow Blind Predictions

This paper presents the first blind prediction stage of the Tidal Turbine Benchmarking Project being conducted and funded by the UK's EPSRC and Supergen ORE Hub. In this first stage, only steady flow conditions, at low and elevated turbulence (3.1%) levels, were considered. Prior to the blind prediction stage, a large laboratory scale experiment was conducted in which a highly instrumented 1.6m diameter tidal rotor was towed through a large towing tank in well-defined flow conditions with and without an upstream turbulence grid. Details of the test campaign and rotor design were released as part of this community blind prediction exercise. Participants were invited to use a range of engineering modelling approaches to simulate the performance and loads of the turbine. 26 submissions were received from 12 groups from across academia and industry using solution techniques ranging from blade resolved computational fluid dynamics through actuator line, boundary integral element methods, vortex methods to engineering Blade Element Momentum methods. The comparisons between experiments and blind predictions were extremely positive helping to provide validation and uncertainty estimates for the models, but also validating the experimental tests themselves. The exercise demonstrated that the experimental turbine data provides a robust data set against which researchers and design engineers can test their models and implementations to ensure robustness in their processes, helping to reduce uncertainty and provide increased confidence in engineering processes. Furthermore, the data set provides the basis by which modellers can evaluate and refine approaches.

Modelling the effects of boundary proximity on a tidal rotor using the actuator line method

For a turbine in unconstrained flow, the maximum power coefficient (CP ) is limited to 16/27 of the undisturbedkinetic energy flux through the rotor area according to work attributed to Betz, Lanchester, and Joukowsky [1].This maximum, often referred to as the Betz limit, occurs when the rotor presents the optimum resistance tothe incoming flow, imparting enough force to generate power without overly choking the flow through the rotor.However, in the context of tidal stream energy, the flow is often constrained by the seabed and the free surfacechanging the balance of optimal rotor resistance. Thus, the limit for maximum power extraction is modifiedto the form first presented by Garrett and Cummins, CP = (16/27)(1 − B)−2 [2]. The factor B, known as theblockage ratio, represents the fraction of the channel cross-section occupied by the rotor swept area, and allowsrotors operating in confined conditions to theoretically exceed the Betz limit. Subsequent theoretical work byNishino and Willden extended this model to demonstrate that constructive interference effects between closelyspaced turbines in a co-planar fence can allow efficiency increases above the Betz limit, even in an infinitely widechannel where the global blockage ratio is negligible [3]. This phenomenon, attributed to the concept of localblockage, defined as the ratio of rotor swept area to the surrounding flow passage area, has been observed byseveral studies both experimentally, and numerically using actuator disk and blade element momentum methods[4, 5, 6]. However, neither the experimental nor the numerical studies provided detail on unsteady loading effectsstemming from azimuthal variations in the flow field caused by anisotropy in the local blockage, such as whenthe rotor is not centred in the flow passage. In this study, a single tidal rotor is simulated using the actuatorline model embedded in a Reynolds-Averaged Navier-Stokes solver with varying degrees of anisotropic blockageimposed by proximity to a non-deformable upper boundary. The investigation is carried out in the contextof the Supergen ORE Unsteady Tidal Turbine Benchmarking Project using a 1.6 m rotor in a computationaldomain equivalent to the towing tank dimensions at the Qinetiq Haslar facility in which the turbine was testedexperimentally [7]. The discrete blade representation and unsteady nature of the actuator line method allowsinvestigation of variations in loads and the importance of boundary proximity by extracting the spanwise loaddistributions and local flow parameters at different positions around the azimuth. The effects of two differenttip-loss correction models on the spanwise force distributions and overall loads are investigated. Analysis of theintegrated rotor loads shows potential for an increase in the maximum CP of ∼1% with changing proximity tothe upper boundary without any detriment to the power-to-thrust ratio. However, anisotropy in the local flowpassage can result in azimuthally varying blade forces, introducing an additional source of fatigue loading tothe rotor and drive train.

Preliminary performance assessment from towing tank testing of a horizontal-axis turbine

Following the Norwegian government’s goal to be a ‘low emission society’ by 2050, there is a significant push to develop offshore renewable energy; principally offshore wind, but also wave and tidal stream. The MarinLab towing tank at the Western Norway University of Applied Sciences, Norway is 50 m long with a 3.0×2.2 m section and was inaugurated in 2016 as an educational and research facility, supporting local industry to achieve a low-carbon transition.
 To advance knowledge of aero- or hydrodynamic interaction of future turbines for both wind and tidal stream energy, a model scale test-bed horizontal axis turbine has been developed. This turbine will enable testing of rotor diameters typically in the range of 500-600 mm. The turbine is instrumented with a torque-thrust sensor of 5 Nm/100 N capacity, custom-manufactured by Marin in the Netherlands. The turbine is speed regulated via a 4096 counts angular encoder connected to a 200 W Maxon EC-i brushless motor.
 Before testing new rotor geometries, a benchmarking study is being undertaken with an existing known geometry. The benchmark rotor has diameter, D=700 mm and employs the same NACA 63418 airfoil as Mycek et al. (2014), allowing comparison. Unlike Mycek et al. (2014), the nacelle has a nominal diameter of 90 mm and length 760 mm. The size of the rotor risks exceeding the capacity of the torque sensor, such that maximum tow-speed is limited to 0.8 m/s. The benchmark blades are machined from solid aluminium in a four-axis CNC milling machine, following a method similar to Payne et al. (2017). Due to machining limitations, the minimum trailing edge thickness is specified as 0.2 mm. The final surface finish is achieved by manual polishing, and the final dimensions, quantified using a Hexagon ROMER Absolute laser scanner, are found to be within ±0.2 mm accuracy. Future blade sets will typically have a smaller diameter, allowing for 3D printing and consistency of surface finish.
 A blade-element momentum model has been run with Prandtl tip and hub losses and 0° pitch (Fig. I). Lift and drag coefficients were calculated for a range of chord Reynolds numbers using XFOIL, for angles of attack between -5 to 16° and extrapolated to ±180° with Viterna’s method. Due to difficulties in estimating laminar-turbulent transition around critical Reynolds numbers, there is uncertainty in the XFOIL coefficients. At peak TSR, the chord-based Reynolds number is approximately 280k at 3/4 span. Curves for both Rec =250k (solid) and 500k (dashed) with Ncrit = 9 are presented, showing a significant drop in performance for lower Reynolds numbers, due to a collapse in Cl/Cd. Initial tests were conducted in near- zero ambient turbulence, though passive turbulence grids are available for future testing. Reasonable agreement on CT is observed when corrected for blockage according to Bahaj (2007), and further assessment will provide results on CP, tower loads and wake recovery.

A numerical study on the hydrodynamics of a floating tidal rotor under the combined effects of currents and waves

This work examines the hydrodynamics of a 20 m diameter axial-flow tidal rotor supported by a catamaran-style floating platform. Using a time-domain seakeeping model of the float, coupled with a dynamic model of the rotor based on blade-element momentum theory, the floating tidal turbine was analysed under the combined effects of following waves and currents. The rotor loads were analysed in scenarios with and without platform motions, starting from equivalent initial conditions. While the results show that mean power and thrust are not significantly affected, thrust and power fluctuations are substantial for the rotor under waves with and without platform motions. When platform motions were considered, amplification and reduction of load fluctuations were observed at different wave periods. These effects are associated with the phase interactions between waves and platform motion response. The reductions in thrust and power fluctuations at certain ranges of wave periods suggest that platform motions do not necessarily have an adverse impact on the operation of floating tidal rotors and could potentially be exploited to reduce fatigue damage and improve the quality of power delivery. The amplification of transient loads, on the contrary, suggests that consideration is required when designing floating systems to avoid potentially damaging effects.

BEM Turbine Model and PID Control System of a Floating Hybrid Wind and Current Turbines Integrated Generator System

This is a new installment in the series of publications that describe the mathematical modeling of the Floating Hybrid Generator Systems Simulator (FHYGSYS) tool. This work presents an improved mathematical model of the turbines of the floating hybrid system—consisting of an “OC3-Hywind” wind turbine and two marine current turbines—presented by the authors in previous publications. In this third installment, the modeling of the three turbines of the floating hybrid system is described using the Blade Element Momentum (BEM) theory. This modeling allows one to replace the one based on the One-Dimensional theory used in previous installments. For the operation of modeling with BEM, it has been considered necessary to implement a continuous feedback control system. In this case, two PID (proportional–integral–derivative) controllers have been implemented in each of the turbines. The first controls the torque on the turbine generator and the second controls the collective pitch angle of the blades. The results obtained are presented and validated through a code-to-code comparison with simulations carried out with FASTv8 under the same conditions and with the operating results of marine current turbines that exist in the literature. This improvement in the mathematical model offers the possibility of implementing other types of controllers that allow for the testing of different strategies of the floating hybrid control system, with the aim of maximizing energy production while ensuring the structural stability of the floating hybrid system.

Wind farm power maximization based on analytical sensitivity model considering wake effect

To reduce the wake effect on wind energy capture, it is necessary to optimize the operating parameters of each wind turbine (WT) in the wind farm (WF) to improve the overall wind power. Considering the complex coupling characteristics of wind energy captured by upstream and downstream WTs caused by the wake effect, existing optimization methods for the WF usually adopt intelligent algorithms or numerical sensitivity-based search algorithms. However, most of the existing studies overlook the exploration of the physical mechanism underlying the wake influence on WF power, and the optimization speed still needs to be improved. To this end, in this paper, the analytical sensitivity model of WF power is formulated with the adjustable parameter - the pitch angle which is a key factor for the wake effect. On this basis, the improvement mechanism of how the WF power can be improved by changing the wake effect through the pitch angle is analyzed using the Blade Element Momentum theory, which can be used to guide and accelerate the optimization process. Furthermore, the WF power maximization method based on the analytical sensitivity model is proposed, and its effectiveness and advancement are verified based on SimWindFarm simulation under different operation scenarios.

Comparative Study of the Blade Number and Airfoil Profile Impacts on the Twist/Chord Distribution of a Small Wind Turbine Blade

The blade number and airfoil profile effects on the blade shape of a small horizontal-axis wind turbine (SHWT) were investigated. For this purpose, the NACA4412, SG6042, and SG6043 airfoils, as well as 2, 3, and 4 blades, were considered. Then, two optimization processes were used: first, the blades were designed to maximize the power coefficient ( C p ), and then a multiobjective optimization that included both maximizing C p and maximizing the starting torque ( Q s ) was employed. The differential evolution (DE) algorithm was employed to perform the optimization, and the blade element momentum aerodynamic approach was used to conduct the relevant computations. Also, to ensure the performance of the optimal blades, the computational fluid dynamics method was employed as well. The findings revealed that regardless of the number of blades and the type of airfoil, raising the twist angle ( θ p ) and chord length ( c ) along the radial direction of the blade, especially at the root part, helps increase the Q s . It was observed that increasing the number of blades does not have a significant effect on the θ p distribution of the selected airfoils, but the c of the blades fitted with all three airfoils decreases. Regardless of the number of blades, while the geometry of blades utilizing the NACA4412 and SG6042 airfoils are close to each other, the blade with the SG6043 airfoil has the shortest c , which reduces the generated Q s of blades fitted with this airfoil. The results also establish that by increasing the number of blades from 2 to 3, the power coefficient ( C p ) of the blades fitted with all three airfoils increases, but by further increasing the number of blades from 3 to 4, the change in C p completely depends on the airfoil profile.

Configurable dual rotor wind turbine model based on BEM method: Co-rotating and counter-rotating comparison

This article proposes a Blade Element Momentum (BEM) theory based model applied to dual rotors wind turbines. Dual rotor wind turbines studied consist of two rotors mounted coaxially to recover the flow from the first rotor. These rotors can turn in the same (co-rotating) or in opposite directions (counter-rotating). The aim of the model is to be fast, accurate and configurable in order to be used as design tool or for optimization purposes. The model considers axial and tangential induction influences of the first rotor on the second one and the influence of the distance between them. The model has been validated by comparison with experimental data from the literature and shows that the counter-rotating configuration has a power coefficient 4.6% higher than the co-rotating for the same turbine geometrical setting. Besides, in our case study, the counter-rotating power coefficient can be in average on different wind speeds 10.6% higher compare to the single rotor.

A cost-effective CNN-BEM coupling framework for design optimization of horizontal axis tidal turbine blades

The paper proposes a novel cost-effective framework that combines deep learning convolutional neural network (CNN) and blade element momentum (BEM) models for optimizing the performance of three-dimensional (3D) horizontal axis tidal turbines (HATTs). The framework employs signed distance function (SDF) to reconstruct the three-dimensional blade geometry, utilizes CNN to identify the hydrodynamic performance of each blade section, and ultimately predicts the performance of HATT using BEM. On top of the new CNN-BEM model, the rotor blade geometrical optimization by multi-objective non-dominated sorting genetic algorithm (NSGA-II) is carried out to obtain a better trade-off solution with the maximal power coefficient of turbine and minimal hydrodynamic load exerted on the blades. The results show that the CNN-BEM model has good agreement with experimental data and reduces prediction time by 46.7% compared to the conventional Xfoil-BEM model, while reducing general optimization time by 20.1%. The new model's cost-efficiency allows for a better trade-off solution with reduced hydrodynamic load while maintaining the power coefficient. Thus, the proposed model has the capability to deliver both accurate and fast prediction and optimization of HATT performance, making it a valuable tool for guiding the design of tidal turbines.

On the estimation of the angle of attack for vertical axis wind turbines

PurposeThis paper aims to investigate the problem of estimating the angle of attack (AoA) and relative velocity for vertical axis wind turbine (VAWT) blades from computational fluid dynamics data.Design/methodology/approachTwo methods are implemented as function objects within the OpenFOAM framework for estimating the blade’s AoA and relative velocity. For the numerical analysis of the flow around and through the VAWT, 2 D unsteady Reynolds-averaged Navier–Stokes (URANS) simulations are carried out and validated against experimental data.FindingsTo gain a better understanding of the complex flow features encountered by VAWT blades, the determination of the AoA is crucial. Relying on the geometrically-derived AoA may lead to wrong conclusions about blade aerodynamics.Practical implicationsThis study can lead to the development of more robust optimization techniques for enhancing the variable-pitch control mechanism of VAWT blades and improving low-order models based on the blade element momentum theory.Originality/valueAssessment of the reliability of AoA and relative velocity estimation methods for VAWT’ blades at low-Reynolds numbers using URANS turbulence models in the context of dynamic stall and blade–vortex interactions.

Low-speed, low induction multi-blade rotor for energy efficient small wind turbines

To promote the deployment of small wind turbines (SWTs), thorough understanding of design parameter implications is essential. In-depth research is required to comprehend the influence of design TSR on off-design wind speed performance of multi-blade SWTs. In-house built blade element momentum algorithm was employed, which considered Reynolds number dependence of aerodynamic coefficients and correlated well with experimental results. Peak power coefficients were produced for 0.9, 1.5, 2 and 3 m diameter S826 rotors with blade numbers from 2 to 12 and design TSR of 2–10. Remarkably, regardless of diameter, greatest CP,max values were achieved around design TSR of 4. For peak efficiency, smaller the diameter, narrower the blade number and design TSR range. High-speed rotors have wider TSR range for high power coefficient. Yet, it was shown that operating TSR of low-speed rotors deviates less from design TSR as wind speed varies. It was revealed that low-speed (with a threshold design TSR of 3), low-induction multi-blade rotors provide high CP,max, better efficiency at off-design, shorter starting time and lower wind speed than three-bladed high-speed rotor. A small boost in operational TSR was found to effectively mitigate loss in off-design performance. These are key features to maximize energy harvesting.

Actuator disk theory and blade element momentum theory for the force-driven turbine

The Actuator Disk Theory and the Blade Element Momentum Theory (BEMT) are widely applied in the field of tidal and wind turbine design. The current BEMT turbine design method is based on the assumption of achieving the Betz limit when the axial induction factor (a) reaches 13. But this only applies to turbines driving by a constant velocity, i.e. the velocity-driven turbine. This paper introduces a new type of turbine, namely the force-driven turbine which by its name is driven by a constant force.This paper then expanded the actuator disk theory for the force-driven turbine and identified the relationship between the axial induction factor with the power and the energy yield of the force-driven turbine. According to the relationship, this paper proposes a new BEMT turbine design method for the preliminary design of the force-driven turbine. Then, a case study is conducted to demonstrate and verify the developed method. The case study shows the newly developed method can be used to quickly and effectively identify the optimum design for force-driven turbines.

Novel High-Precision and Efficient Momentum Source Method

Effective and reliable slipstream numerical simulation methods are important for propeller-driven aircraft design. This paper presents a high-precision and efficient momentum source method (HPE-MSM) based on a novel actuator disk load prediction model established by the frozen rotor method and blade element momentum theory. The simulation results of two benchmark test cases of an isolated propeller and a typical turboprop airliner show that the accuracy of the HPE-MSM proposed in this paper is close to the time-averaged unsteady results of the sliding mesh method (SMM), with a maximum error of 5% in aircraft lift and drag coefficients compared with experimental values. Meanwhile, the calculation efficiency of the HPE-MSM is comparable to quasi-steady methods, with only about 3.4% of the computation resources of the SMM in the whole aircraft simulation. This novel approach achieves high-precision and efficient simulation of the slipstream, which has the potential to improve the design level of propeller-driven aircraft.

Fully coupled aero-hydrodynamic modelling of floating offshore wind turbines in nonlinear waves using a direct time-domain approach

In standard operating conditions, a floating offshore wind turbine (FOWT) suffers from the combined action of nonlinear waves and wind. It is important to explore the effect of higher-order hydrodynamic loads induced by the nonlinear waves on its motion response. In this work, a fully coupled aero-hydrodynamic analysis method for FOWT is developed. The hydrodynamic part is based on the nonlinear potential flow theory and the perturbation approach, using a higher-order boundary element method in the time domain. Blade element momentum theory is applied to evaluate the aerodynamic load on the turbine rotor. The mooring system is modelled by the catenary theory. The developed method is verified against the published data of the NREL's 5-MW baseline wind turbine supported by the DeepCwind semi-submersible platform. The verifications are conducted stepwise in the following ways, including hydrodynamic wave excitation load, free-decay response, the load-displacement relationship of the mooring line, the steady-state response of the wind turbine, and fully coupled aero-hydrodynamic interaction. It is found that the nonlinear effect mainly influences the surge motion. The coupled configuration has little effect on the linear wave force and platform displacement but notably impacts the nonlinear wave force and displacement. The FOWT motion is found to be critical in changing the wave field, the maximum wave elevation position around the platform, and the nonlinear wave force acting on the floating platform. The results illustrate that the traditional indirect time-domain method using the Quadratic Transfer Function (QTF) based on assumed response cannot appropriately evaluate the nonlinear wave force of a FOWT in coupled motions. The results also reveal that nonlinear wave hydrodynamics is necessary to understand the motion response of a fully coupled FOWT.

A study on the influence of the numerical scheme on the accuracy of blade-resolved simulations employed to evaluate the performance of the NREL 5 MW wind turbine rotor in full scale

We present a numerical investigation on the influence of the numerical arrangement in the accuracy of the NREL 5 MW wind turbine rotor performance evaluation in full scale, using a Computational Fluid Dynamics (CFD) methodology which employs the Finite Volume Method (FVM) implemented in the OpenFOAM software. The nonlinearity of the Navier–Stokes equation, which requires special treatment and still represents a subject under intense debate in the scientific community, is tackled by using two different discretization schemes for computing the convective term. In addition, since the advection scheme numerically interacts with the turbulence closure model employed in the numerical simulations, we considered the URANS k-ω SST and DES k-ω SST models to perform the blade-resolved simulations. A hybrid central/upwind scheme, namely the LUST scheme, was considered for discretizing the convective term for both URANS and DES simulations, while the second order accurate upwind given by the LUD scheme was considered only in the URANS simulation. Considering the LUST scheme, an investigation on the temporal and spatial discretization was performed for both turbulence closure models. The performance of the NREL 5 MW rotor for offshore application in full scale was assessed in terms of power production, generated thrust, and forces distribution along the blade span, for the operation condition of optimal wind-power conversion efficiency. We provide detailed information regarding the flow features and computational cost, and verified the results from each case by comparing the CFD results against values obtained using the blade element momentum theory implemented in OpenFAST. For the spatial discretization considered in one of the meshes, the LUD scheme showed low accuracy in the results, being more susceptible to the grid influence for the URANS simulations compared to LUST, while the URANS-LUST and DES-LUST approaches were both suitable options for all the meshes investigated. The DES-LUST approach was less affected by the variation in the size of the time step employed. Additionally, finer flow structures were captured with the DES-LUST simulations at an affordable computational cost.

Design and Fully Coupled Dynamic Response Analysis of a New Floating Offshore Wind Platform

Floating offshore wind platform (FOWP) has become the economically favored option for supporting wind turbines in deep waters. It is urgent to propose new concept designs for FOWPs that can be effectively deployed. Additionally, the extensive use of steel in such platforms significantly escalates costs, necessitating the optimization of steel utilization. Motivated by these challenges, a V-shaped floating semi-submersible platform equipped with NREL 5 MW wind turbine is designed and analyzed based on the potential flow theory and the blade element momentum theory. Fully coupled time-domain simulations are conducted using the F2A program, which couples NREL FAST and ANSYS AQWA via a Dynamic Link Library (DLL), to compare the hydrodynamic performance and stability of the V-shaped floating platform with the original triangle-shaped model of “Fuyao”. Various sea conditions have been considered, including combined wind-wave action and wind-wave-current action at different incidence angles. The results show that the V-shaped floating platform has better economic and hydrodynamic performance (e.g., a reduction of 40.4% and 12.9%, respectively, in pitch and yaw motions, and a 17.4% reduction in maximum mooring tension), but lower stability than its triangle-shaped counterpart.

Fatigue analysis of wind turbine blade for various materials

A 300 kW horizontal axis wind turbine with S809 airfoil is considered for predicting the fatigue life. The wind turbine blade is designed using Blade Element Momentum (BEM) method. The blade geometries such as chord and pitch angle distributions are determined accordingly the blade is modelled using CREO software. The method of analysis is done by considering both static and transient conditions with varying boundary conditions and different loading pressures. The objective of this work is to identify the fatigue life cycles of the wind turbine blade for various composite materials (Shokrieh and Rafiee, 2016) under various operating conditions. Modelled blade is analysed at both static and transient conditions by considering various composite materials as wind turbine blade material. Simple cyclic loading conditions at stress ratio, R = −1 and R ≠ −1 are considered for the transient analysis. Static analysis is done by taking a single cut-out velocity of 12 m/s. Steady loading and response conditions are assumed; that is, the loads and the structure's response are assumed to vary slowly with respect to time. The fatigue life of the wind turbine is calculated for Glass fibre reinforced plastics-glass fibre, Carbon fibre, Technora fibre, Kevlar fibre and E-glass materials and thereby suggesting the best material having high fatigue life, less damage factor and good safety factor.