Performance Of Wind Turbine Rotors Research Articles

Effect of Induction and Blade Elasticity Modelling on Wind Turbine Rotor Performance Predictions

This study investigates the impact of blade induction modelling on the accuracy of wind turbine rotor aeroelastic predictions. It extends the capabilities of AEOLIAN (AErOeLastic sImulAtioN), a Fluid Structure Interaction (FSI) solver based on Blade Element Momentum Theory (BEMT) coupled with a Lumped Mass approach to represent the blade structure. Herein, AEOLIAN’s analytical wake induction engineering model is replaced with the outcomes of a physically-consistent three-dimensional Free-Vortex Wake (FVW) formulation initially employed in AeroROTOR. This versatile aeroelastic simulation tool is implemented within the framework of MATLAB Simulink/Simscape-Multibody©, a modular environment suitable for industry analysts, researchers, and academic users focusing on wind turbine aero-servo-elastic applications. Furthermore, it serves to lay the groundwork for the development of advanced control laws for multi-megawatt rotors, fostering innovation in the design and optimization of the next-generation wind turbines. The presented analyses focus on predicting the aeroelastic behavior of the bottom-fixed NREL 5MW rotor in uniform axial flow over the operating range, complemented by more detailed investigations at the rated condition undergoing inflow with/without wind misalignment (yaw). The study on key performance parameters is conducted by comparing with the higher-fidelity data from available Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) coupled with CFD.

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.

Effects of Atmospheric Icing on Performance of Controlled Wind Turbine

Icing deteriorates the performance of wind turbine rotors by changing the blade airfoils’ shapes. It decreases the lift, increases the drag, and subsequently causes power production losses and load increase on turbines’ structures. In the present study, the effects of atmospheric icing on the performance of a controlled large-scale wind turbine is estimated through simulations. To achieve the target, the MS (Mustafa Sahin) Bladed Wind Turbine Simulation Model is used for the analyses of the National Renewable Energy Laboratory (NREL) 5 MW turbine with and without iced blades. Icing modeling is realized based on its main characteristics and its effects on blade aerodynamics. Turbine performance estimations are carried out at various uniform wind speeds between cut-in and cut-out wind speeds and are presented in terms of various turbine parameters such as power, thrust force, blade pitch angle, and rotor speed. Simulation evaluations show that even a light ice accretion along the blades varies the turbine characteristics and dynamics, changes the cut-in and rated wind speeds, and affects the aforementioned turbine parameters differently in the below and above rated regions.

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

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

Assessment of actuator disc models in predicting radial flow and wake expansion

Navier-Stokes actuator disc models have become a mature methodology for investigating wind turbine rotor performance with numerous articles published annually making use of this approach. Despite their popularity, their ability to predict near wake expansion remains questionable. The objective of this paper is to analyse the predictive ability of actuator disc models and compare results with other popular types of codes. The methodology employs the use of an actuator disc Computational Fluid Dynamics approach to model an actuator disc and a real (finite bladed) turbine case. Results are validated with existing experimental data. In addition, results from an actuator line model with and without tip corrections and a 3D vortex panel method are presented to aid the discussion. Results show that all models give a poor wake expansion prediction particularly in the inboard to mid-board areas. A good prediction is found in the outboard regions. In addition, contrary to the well known positive effects of tip corrections on load prediction, this work shows that this does not bring any particular benefit on wake expansion prediction. The conclusions from this work help to guide the use of actuator disc models in more complex flow scenarios including floating offshore wind turbine analysis.

Assessing wind turbine energy losses due to blade leading edge erosion cavities with parametric CAD and 3D CFD

Wind turbine leading edge erosion is a complex installation site-dependent process that spoils the aerodynamic performance of wind turbine rotors. This gradual damage process often starts with the formation of pits and gouges leading ultimately to skin delamination. This study demonstrates the application of open source parametric CAD functionalities for the generation of blade geometries with leading edge erosion damage consisting of pits and gouges. This capability is key to the development of high-fidelity computational aerodynamics frameworks for both advancing knowledge on eroded blade aerodynamics, and quantifying energy losses due to erosion. The considered test case is an offshore 5 MW turbine featuring leading edge pit and gouge damage in the outboard part of its blades. The power and loads of the nominal and damaged turbines are determined by means of a blade element momentum theory code using airfoil force data obtained with 3D Navier-Stokes computational fluid dynamics. An annual energy loss between about 1 and 2.5 percent of the nominal annual energy yield is encountered for the considered leading edge damages. The benefits of adaptive power control strategies for mitigating erosion-induced energy losses are also highlighted.

An improved actuator disc model for the numerical prediction of the far-wake region of a horizontal axis wind turbine and its performance

Actuator disc models are frequently used to provide a semi-analytical approach to estimating aerodynamic loads on rotary blades. The basic idea is to distribute the aerodynamic loads on a virtual rotating disc instead of simulating the actual rotating blade. These loads are then imposed to represent the source terms of the Navier-Stokes equations, which can be solved numerically using the computational fluid dynamic methods. The thickness of the actuator disk grid is one important factor considerably affecting calculations of the wind turbine rotor. Past researches generally considered the idea of fixed grid thickness exerting along the blade in their actuator disk modeling. However, this study introduces an innovative or improved actuator disk model, which takes into account the real blade thickness of a wind turbine in the computational fluid dynamic simulations. This novel actuator disk model is then incorporated into a finite-volume solver. This solver uses the second-order central and upwind schemes to approximate the diffusive and advective fluxes at the cell faces, respectively. The k-ε turbulence model is used to close the turbulence closure problem. The NREL 5-MW wind turbine blade is chosen as a benchmark test to evaluate the results of the newly developed solver for the numerical predictions of the wind turbine rotor. The current study reveals that there is a specific fixed grid thickness value resulting in the most accurate predictions though with a much slower convergence rate. This specific value really depends on the chosen test cases and varies from one case to another. Its exact value may be found via try-and-error procedures, which would be computationally very expensive and time-consuming. Alternatively, this study introduces an improved actuator disk model capable of reinstating the real blade thickness in calculations. Subsequently, the wind turbine rotor performance is predicted numerically using both the improved and classic actuator disk models applying various fixed grid thickness values along the blade. The results demonstrate that the accuracy of the classic actuator disk model strongly depends on the grid thickness magnitude. On the contrary, the newly improved actuator disk model results in some solutions, which are as accurate as those of the most accurate classic actuator disk model, while suitably speeding up the convergence rate.

On the effect of blade deformations on the aerodynamic performance of wind turbine rotors subjected to yawed inflow

This work is aimed to investigate the effects of elastic blade deformations on the aerodynamics of large wind turbine rotors subjected to yawed inflow. Due to the increasing rotor size and advanced light weight blade designs, significant blade deflections can be observed on a regular basis for modern wind turbines. However, especially for complex flow situations like yawed inflow, the role of blade deformations is still not completely understood. In this paper, numerical simulations are conducted on the DTU 10 MW reference wind turbine to gain a better understanding of the involved phenomena. Results are obtained by two numerical methods of different fidelity. First, by the aero-elastic simulation tool FAST, which is based on the low fidelity Blade Element Momentum Theory (BEM) and makes use of common skewed wake correction models. Secondly, by a high-fidelity framework which couples the open-source Computational Fluid Dynamics (CFD) toolbox OpenFOAM with the in-house geometrically non-linear beam solver BeamFOAM. The evaluation of the results is based on the analysis of azimuthal variations of the sectional forces along the blade span and reveals a generally good agreement between the used numerical approach in terms of force amplitudes and variation phases. However, especially for cases of larger yaw angles, the BEM models clearly over-predict the variation amplitudes of the forces up to 40% in the outer blade part.

Streamwise vortex generator for separation reduction on wind turbine rotors

PurposeThe purpose of this paper is to describe numerical investigations focused on the reduction of separation and the aerodynamic enhancement of wind turbine blades by a rod vortex generator (RVG).Design/methodology/approachA flow modelling approach through the use of a Reynolds-averaged Navier–Stokes solver is used. The numerical tools are validated with experimental data for the NREL Phase VI rotor and the S809 aerofoil. The effect of rod vortex generator’s (RVG) configuration on aerofoil aerodynamic performance, flow structure and separation is analysed. RVGs’ chordwise locations and spanwise distance are considered, and the optimum configuration of the RVG is applied to the wind turbine rotor.FindingsResults show that streamwise vortices created by RVGs lead to modification of flow structure in boundary layer. As a result, the implementation of RVGs on aerofoil has proven to decrease the flow separation and enhance the aerodynamic performance of aerofoils. The effect on flow structure and aerodynamic performance has shown to be dependent on dimensions, chordwise location and spanwise distribution of rods. The implementation of devices with the optimum configuration has shown to increase aerodynamic performance and to significantly reduce separation for selected conditions. Application of rods to the wind turbine rotor has proven to avoid the spanwise penetration of flow separation where applied, leading to reduction of flow separation and to aerodynamic enhancement.Originality/valueThe proposed RVGs have shown potential to enhance the aerodynamic performance of wind turbine rotors and profiles, making devices an alternative solution to the classical vortex generators for wind turbine applications.

Hybrid vortex simulations of wind turbines using a three‐dimensional viscous–inviscid panel method

AbstractA hybrid filament‐mesh vortex method is proposed and validated to predict the aerodynamic performance of wind turbine rotors and to simulate the resulting wake. Its novelty consists of using a hybrid method to accurately simulate the wake downstream of the wind turbine while reducing the computational time used by the method. The proposed method uses a hybrid approach, where the near wake is resolved by using vortex filaments, which carry the vorticity shed by the trailing edge of the blades. The interaction of the vortex filaments in the near vicinity of the wind turbine is evaluated using a direct calculation, whereas the contribution from the large downstream wake is calculated using a mesh‐based method. The hybrid method is first validated in detail against the well‐known MEXICO experiment, using the direct filament method as a comparison. The second part of the validation includes a study of the influence of the time‐integration scheme used for evolving the wake in time, aeroelastic simulations of the National Renewable Energy Laboratory 5 MW wind turbine and an analysis of the central processing unit time showing the gains of using the hybrid filament‐mesh method. Copyright © 2017 John Wiley & Sons, Ltd.

On the influence of airfoil deviations on the aerodynamic performance of wind turbine rotors

The manufacture of large wind turbine rotor blades is a difficult task that still involves a certain degree of manual labor. Due to the complexity, airfoil deviations between the design airfoils and the manufactured blade are certain to arise. Presently, the understanding of the impact of manufacturing uncertainties on the aerodynamic performance is still incomplete. The present work analyzes the influence of a series of airfoil deviations likely to occur during manufacturing by means of Computational Fluid Dynamics and the aeroelastic code FAST. The average power production of the NREL 5MW wind turbine is used to evaluate the different airfoil deviations. Analyzed deviations include: Mold tilt towards the leading and trailing edge, thick bond lines, thick bond lines with cantilever correction, backward facing steps and airfoil waviness. The most severe influences are observed for mold tilt towards the leading and thick bond lines. By applying the cantilever correction, the influence of thick bond lines is almost compensated. Airfoil waviness is very dependent on amplitude height and the location along the surface of the airfoil. Increased influence is observed for backward facing steps, once they are high enough to trigger boundary layer transition close to the leading edge.

Improved blade element momentum theory for wind turbine aerodynamic computations

Blade element momentum (BEM) theory is widely used in aerodynamic performance predictions and design applications for wind turbines. However, the classic BEM method is not quite accurate which often tends to under-predict the aerodynamic forces near root and over-predict its performance near tip. The reliability of the aerodynamic calculations and design optimizations is greatly reduced due to this problem. To improve the momentum theory, in this paper the influence of pressure drop due to wake rotation and the effect of radial velocity at the rotor disc in the momentum theory are considered. Thus the axial induction factor in far downstream is not simply twice of the induction factor at disc. To calculate the performance of wind turbine rotors, the improved momentum theory is considered together with both Glauert’s tip correction and Shen’s tip correction. Numerical tests have been performed for the MEXICO rotor. Results show that the improved BEM theory gives a better prediction than the classic BEM method, especially in the blade tip region, when comparing to the MEXICO measurements.

BEM Simulation and Performance Analysis of a Small Wind Turbine Rotor

The blade element momentum (BEM) method is a popular tool for predicting the performance of wind turbine rotors. This study investigated the impact of including factors such as tip loss, hub loss and drag coefficients in BEM simulations of a Bergey XL.1 small wind turbine. The Bergey XL.1 has constant chord, untwisted blades that are challenging to simulate owing to the large variation in angle of attack along the blade during operation. Methods of including post-stall airfoil characteristics, and three wake approaches (Buhl, Glauert and Wilson-Walker) were also examined. BEM simulations were consistent with test data from a Bergey XL.1 collected using a vehicle-based platform. Including tip losses, drag coefficients and wake effects in the BEM simulation had a significant impact on predicted performance, while the effect of including hub loss was negligible. The results illustrate that BEM methods can predict the performance of small wind turbine rotors.

Application of Circulation Controlled Blades for Vertical Axis Wind Turbines

The blades of a vertical axis wind turbine (VAWT) rotor see an inconsistent angle of attack through its rotation. Consequently, VAWT blades generally use symmetrical aerofoils with a lower lift-to-drag ratio than cambered aerofoils tailored to maximise horizontal axis wind turbine rotor performance. This paper considers the feasibility of circulation controlled (CC) VAWT blades, using a tangential air jet to provide lift and therefore power augmentation. However CC blade sections require a higher trailing-edge thickness than conventional sections giving rise to additional base drag. The choice of design parameters is a compromise between lift augmentation, additional base drag as well as the power required to pump the air jet. Although CC technology has been investigated for many years, particularly for aerospace applications, few researchers have considered VAWT applications. This paper considers the feasibility of the technology, using Computational Fluid Dynamics to evaluate a baseline CC aerofoil with different trailing-edge ellipse shapes. Lift and drag increments due to CC are considered within a momentum based turbine model to determine net power production. The study found that for modest momentum coefficients significant net power augmentation can be achieved with a relatively simple aerofoil geometry if blowing is controlled through the blades rotation.

Laminar‐turbulent flow simulation for wind turbine profiles using the γ– transition model

ABSTRACTThe accurate prediction of the laminar‐turbulence transition process is fundamental in predicting the aerodynamic performance of wind turbine profiles. Fully turbulent flow simulations have been shown to over‐predict the aerodynamic performance and thereby negatively impacting the design of airfoils in flow regimes where the possible presence of laminar flow could be exploited to improve the performance of wind turbine rotors. Correlation‐based transition modelling offers a fully computational fluid dynamics compatible approach, where the model integrates completely with the existing turbulence model, allows for the prediction of various transition mechanisms, is applicable to three‐dimensional flows and compatible to adjoint‐based design optimization frameworks. The present paper addresses several modifications necessary for a robust transition model and investigates the accuracy of the model for a wide range of angles of attack and Reynolds numbers, which are necessary for a thorough validation of the correlation‐based transition model for wind turbine profiles. The transition model was employed to predict the transition locations; and an assessment of the various transition mechanisms, Reynolds number effects, sectional characteristics and aerodynamic performance for the NLF(1)‐0416 and S809 airfoils is presented with comparisons to experimental data and numerical solutions. Copyright © 2013 John Wiley & Sons, Ltd.

Prediction of active flow control performance on airfoils and wings

A numerical investigation of active flow control, which can offer significant improvements to aircraft wing, helicopter and wind-turbine rotor performance by suppressing detrimental effects of separated flow, is presented. Numerical simulations of pulsating jet flow control applied on airfoils and wings at low speed, high Reynolds number turbulent flow and fixed angles of incidence are carried out. Pulsating jet active flow control is applied as a surface boundary condition and the flow is time-dependent. It was found that fine grid resolution is required to capture the pulsating jet and its interaction with the boundary layer. Efficient, implicit, time-accurate numerical methods for unsteady RANS of incompressible flow are used to overcome the stringent stability limitations imposed by small grid spacing. A widely tested one-equation turbulence model is used for the prediction of the complex, unsteady flowfields. It is found that active flow control can enhance aerodynamic performance by reducing the adverse effects of separated flow. The effect of pulsation frequency, and jet exit velocity on flow control is investigated.

Numerical Investigations of Dynamic Stall Active Control for Incompressible and Compressible Flows

Active flow control can offer significant improvements to aircraft, helicopter, and wind-turbine rotor performance by suppressing detrimental effects of separated flow and dynamic stall. Numerical simulations of pulsating jet flow control of oscillating airfoil dynamic stall is performed for high Reynolds number turbulent flows. Currently available efficient and accurate numerical methods for the Navier-Stokes equations and advanced turbulence models are used for the prediction of the complex, unsteady flowfields generated by the pulsating jets. Both incompressible and compressible flow conditions are considered. It is found that pulsating jet flow control can significantly reduce the adverse effects of dynamic stall. The effect of the jet location on dynamic stall characteristics is investigated.

Phenomenological models for post-stall airfoil characteristics of horizontal-axis wind turbines

There exist significant differences in airfoil aerodynamic characteristics between wind tunnel test data (2-Dimensional flow) and field test data (3-D dimensional flow). Through the Combined Experiment Program (CEP), efforts have been made to investigate proper approaches to analyze such differences so that it enables us to incorporate the results into wind turbine rotor performance codes to assist in optimal design of a horizontal axis wind turbine. As a step towards a more indepth, theoretical analysis of various physical effects under field conditions, we first analyze the difference in airfoil characteristics from phenomenological point of view by developing correlations from experimental data. The correlations for the difference in lift coefficient and drag coefficient between 2-dimensional and 3-dimensional flow are developed to form a basis and are suggested to be modified and included in the PROP code.

Design criteria for passive pitch control of wind turbines using self-twisting blades

SYNOPSIS A simple model of blade twisting was used in conjunction with a wind turbine rotor performance model to predict the ability of a self-twisting blade to regulate the speed of a small wind turbine. The results were used in order to establish guidelines for some of the design choices for such a turbine, particularly with regard to blade shape.

The Effect of Hub Fairings on Wind Turbine Rotor Performance

Hub fairings or spinners are frequently suggested for wind turbines for reasons of aesthetics or performance. While hub fairings rarely, if ever, decrease the appearance of a wind turbine, the effects of a nose fairing may actually decrease rather than increase wind turbine rotor performance. Analysis of hub fairings effects on wind turbine performance is presented.