Wind Turbine Aerodynamics Research Articles

Parametric CFD analysis of the taper ratio effects of a winglet on the performance of a Horizontal Axis Wind Turbine

Winglets have been used to augment the power extraction of Horizontal Axis Wind Turbines (HAWT). However, despite some understanding on the way they affect the aerodynamics of wind turbines, the influence of specific geometric parameters still requires investigations. To assist with the resolution of this problem, this paper describes the influence of taper ratio and root chord ratio of a winglet on power and thrust coefficients and annual energy output (AEO) of a HAWT. Computational Fluid Dynamics (CFD) simulations obtained the effects by numerically solving the Reynolds-Averaged Navier–Stokes (RANS) equations in steady state using the κ-ω SST turbulence model and a moving reference frame methodology. New Mexico wind turbine experimental data validated the computational results, and computational analyses of the winglets were compared with the validated simulations. The set of winglets is composed by twelve configurations, and the results reveal a higher aerodynamic efficiency with a lower taper ratio and a lower root chord ratio, suggesting winglets can be optimized for enhancing the performance of the horizontal axis wind turbines. Besides that, the AEO is sensitive to the local wind regime and the utilization of winglets is recommended for sites with a medium to high mean wind speed.

Fish-ridge wind turbine aerodynamics characteristics in Oscillating Water Column (OWC) system

This paper analyzes the fish-ridge type wind turbine performance and characteristics of energy extraction applied in a low wave Oscillating Water Column (OWC) system. This article contributes to providing a better understanding of the application of OWC and VAWT in a low wave environment. The aerodynamic characteristics of the three-blade fish-ridge turbine in an OWC chamber have been successfully investigated. CFD simulation with Reynolds-Averaged-Stokes (RANS) equations was used to obtain airspeed and air pressure contours under compressed and decompressed conditions in the turbine blades. Experiments on laboratory scale test rig also obtained data. The blade torque and turbine power coefficient at different AoA were validated through the experimental test to obtain numerical equations for the relationship between airspeeds, torque, tip speed ratio, and turbine power. The turbine design was 0.2 m long and 0.1 m wide and with an overlap ratio of 15%. The maximum tested airspeed was 20m/s. We found that the fish-ridge turbine has a homogeneous air velocity distribution and pressure due to the 15% overlap area. The maximum efficiency of the fish-ridge turbine under compressed conditions was 30% at TSR 0.9, while under decompressed conditions, the maximum efficiency reached 28% at TSR 0.6.

Dynamic Modeling and Performance Analysis of PMSG- based Variable Speed WTG: Case Study of Adama Wind Farm I, Ethiopia

In this paper, the performance of Permanent Magnet Synchronous Generator (PMSG) -based Variable Speed Wind Turbine Generator (WTG) at Adama Wind Farm I (WTG), connected to a grid is studied. To study the performance of the WTG, both machine and grid side converters are modeled and analyzed very well. On the machine side, maximum power point tracking (MPPT) for maximum energy extraction is done using the direct speed control (DSC) technique, which is linked with the optimal tip speed ratio for each wind speed value considered. On the grid side, dc-link voltage and reactive power flow to the grid are controlled. For this purpose, first, the simulation model of the system is prepared in MATLAB Simulink considering the dynamic mathematical model of the PMSG, and Wind Turbine Aerodynamic model using the user-defined function blocks. Then, the PI regulators designed for direct speed, torque (current) control, and dc-link voltage are employed in the model. Moreover, to study and analyze the behavior of the system in a variable speed operation, a wind speed starting from cut-in wind speed (3m/s) to the rated wind speed (11m/s) is applied in 4s. The simulation result of the existing system model shows that the actual values of performance variables correspond well with the analytical values of the system. In addition, the chosen control algorithms applied in the control system of the generator-side converter are hence verified.

Numerical study on the aerodynamics of an experimental wind turbine: Influence of nacelle and tower on the blades and near-wake

The nacelle and tower affect the flow through the rotor and near-wake of horizontal-axis wind turbines. For this reason, aerodynamic models of the full geometry of wind turbines are needed to gain insight to improve turbine designs. In this work, the unsteady flow around an experimental horizontal-axis wind turbine is studied using computational fluid dynamics simulations. The emphasis is put on the effects of the nacelle, tower and rotation of the blades on the induction region and near-wake. The simulations are performed using unsteady Reynolds-Averaged Navier-Stokes equations in conjunction with the shear-stress transport k-ω model. The sliding mesh method is employed to enable the rotation of the blades. The simulations were performed at the design tip speed ratio (λ=6.67) and two off-design conditions (λ = 4.17 and 10) to assess the general validity of the numerical model. The obtained results of global loads, force distribution on the blades and tower, velocity components and angle of attack are presented in detail. It is found that the tower has little effect on the rotor loads (circular pattern), but the blades have an important effect on the tower (three-leaved rose pattern). The force distribution reveals only significant fluctuations on the blades (r/R<0.75) at λ = 4.17, whereas the tower is affected in the zone exposed to the wake behind the rotor for all three wind speeds. Furthermore, the interaction between blades is evaluated in terms of velocity and angle of attack. The correspondence between the results for different rotor blade positions reveals the effect of the ground on the rotor is negligible. The Omega method is used to visualize the behaviour of the tip and root vortices, wake expansion, coherent structures and development of the near-wake. The numerical results presented here contribute to a deeper understanding of the unsteady flow around horizontal-axis wind turbines.

Rotor‐integrated modeling of wind turbine aerodynamics

AbstractA novel approach for the modeling of rotor‐integrated aerodynamic loads is suggested to answer the need for a comprehensive, insightful, and analytical actuator disc model. All the six degrees of freedom (including tangential components) are considered. It is shown that loads may be written as a quadratic form of a reduced six‐component velocity vector at the hub. The individual contributions of lift and drag, azimuthal variations, as well as blade pitching and tip losses are isolated. Errors introduced by the necessary approximations are discussed, and parametric corrections are considered. Parameter identification methods are then suggested, and the performance of the resulting calibrated analytical models is assessed. Results show that the new modeling approach is able to accurately model both the mean values of the thrust and power coefficients and their derivatives with respect to tip‐speed ratio and pitch angle across the full range of operating wind speeds. Furthermore, it is able to reconstruct the general rotor behavior with a minimal amount of information available. Tangential components are also well modeled, although they require the knowledge of airfoil properties. The model's architecture leaves room for extensions to dynamic flow, skewed flow, and azimuthal load variations.

Modeling and simulation of forces applied to the horizontal axis wind turbine rotors by the vortex method coupled with the method of the blade element

This work focuses on the study of the forces applied to the rotors of horizontal axis wind turbines. The aerodynamics of a wind turbine is regulated by the flow around the rotor, where the prediction of air loads on the rotor blades in various operational conditions and its relationship with the structural dynamics of the rotor is crucial for design purposes. One of the most important challenges in the aerodynamics of wind turbines therefore to accurately predict the forces on the blade when the blade and the wake are modeled by different approaches such as the theory of the blade element (BEM) the vortex method and CFD method which involves solving the Navier-Stokes equations. In our article, the model is used for modeling and simulate the forces applied to the rotors of wind turbines horizontal axis use the application of the method of the blade element for modeling the rotor and the vortex method of free wake modeling in order to develop a model of the rotor, which can be used to study wind farms. This model is intended to speed up the calculation, guaranteeing a good representation of aerodynamic loads exerted by the flow of wind.

Modeling and simulation of forces applied to the horizontal axis wind turbine rotors by the vortex method coupled with the method of the blade element

This article aims to study the forces applied to the rotors of horizontal axis wind turbines. The aerodynamics of a turbine are controlled by the flow around the rotor, or estimate of air charges on the rotor blades under various operating conditions and their relation to the structural dynamics of the rotor are critical for design. One of the major challenges in wind turbine aerodynamics is to predict the forces on the blade as various methods, including blade element moment theory (BEM), the approach that is naturally adapted to the simulation of the aerodynamics of wind turbines and the dynamic and models (CFD) that describes with fidelity the flow around the rotor. In our article we proposed a modeling method and a simulation of the forces applied to the horizontal axis wind rotors turbines using the application of the blade elements method to model the rotor and the vortex method of free wake modeling in order to develop a rotor model, which can be used to study wind farms. This model is intended to speed up the calculation, guaranteeing a good representation of the aerodynamic loads exerted by the wind.

Aerodynamic performance assessment of φ-type vertical axis wind turbine under pitch motion

The floating vertical axis wind turbine (VAWT) is considered as a competitive device in the utilization of offshore wind energy. However, the platform pitch motion would affect its aerodynamic behavior. In this paper, the aerodynamic performance of a floating φ-type VAWT under pitch motion is investigated by using the Improved Delayed Detached Eddy Simulation SST k−ω turbulence model. After verifying the feasibility of the numerical model, the effects of pitch motion amplitude and period on the aerodynamic characteristics were evaluated, and the impacts of these observations were elucidated. The results showed that the averaged net power coefficient increment of about 1.5%–15% could be obtained under platform pitch motions, and the fluctuation of aerodynamic loads was found to increase. Besides, the pitch motion pattern could be regarded as the combination of surge and heave motions, which explained the similarity of their effects on the wind turbine aerodynamics. Furthermore, it was found that the frequency of the peak torque coefficient would change under different periods of pitch motion, which should be noticed in the design of floating wind turbine. Finally, it was concluded that the current study provided additional information about the effect of pitch motion on wind turbine aerodynamics.

Advancement of an analytical double-Gaussian full wind turbine wake model

A recently proposed analytical wake model for a horizontal axis utility scale wind turbine is revisited, and revised and improved. The model is based upon conservation of momentum in the context of actuator disc theory, and the assumption of a distribution of the double-Gaussian type for the velocity deficit in the wake. The model is developed and improved and reveals characteristics of the wind turbine wake velocity deficit for the full wake, including the near-wake to within close proximity of the wind turbine rotor. Full 2-dimensional model fitting to lidar wake measurement data obtained from a 5 MW utility scale wind turbine is carried out for the full range of inflow wind velocities of primary interest. Such a full wind turbine wake model has the potential to facilitate analytic calculations within the wind turbine wake region, and the potential to improve understanding of wind turbine aerodynamics.

Influence of the Solidity Ratio on the Small Wind Turbine Aerodynamics

Increasing popularity of individualised electricity generation from wind by prosumers creates a strong demand for profitable and highly efficient small wind turbines. This paper investigates the influence of rotor blade solidity parameter on device efficiency in hope of determining its optimal value as a part of the development process of the GUST small wind turbine. The study involved experimental analysis in the wind tunnel and numerical simulations performed in QBlade software. Different solidities of the rotor were achieved by alteration of (1) number of blades and (2) chord distribution along the blade span. The increase of rotor solidity resulted in augmentation of the aerodynamic efficiency in both approaches. The elongation of the chord by 33% in a 3-bladed rotor resulted in a bigger power coefficient increment than addition of a 4th blade with original chord distribution. Even though the solidity was the same, the 3-bladed rotor performed better, possibly due to lower form drag. The results emphasize the importance of the rotor solidity optimization during the small wind turbine rotor development and may significantly influence overall power output.

Design and Implementation of a Wind Farm Controller Using Aerodynamics Estimated From LIDAR Scans of Wind Turbine Blades

A hierarchical Wind Farm Control (WFC) approach was previously developed that uses Power Adjusting Controllers (PACs) on each wind turbine in a wind farm. The PACs can be retrofitted to existing assets with no knowledge of, or change to, the wind turbine full envelope controller (FEC). However, knowledge of the wind turbine aerodynamics is required and is not usually directly available from the Original Equipment Manufacturer (OEM), necessitating estimation. In this letter, estimated aerodynamic properties are obtained via a scanning LIDAR that directly measures the shape of a 2.5MW commercial wind turbine's blades. The impact of the resulting aerodynamic uncertainty on the PAC tuning and the accuracy of the change in power output from the PAC at a turbine level and at a wind farm level is assessed. It is shown that it is possible to tune a stable PAC using aerodynamic information estimated via blade scanning. Although the requested turbine change in power suffers from some inaccuracy, the slow integral action at a WFC level causes the impact on the accuracy of the change in wind farm power output to be negligible. As such, the application of a WFC methodology utilising PACs without prior knowledge of the turbine aerodynamics is shown to be possible by using blade scanning to estimate the aerodynamic coefficients. Hence it is practical to retrofit the methodology to wind farms when aerodynamic information from the OEM is not available.

Numerical simulation of wind turbine wake based on extended k‐epsilon turbulence model coupling with actuator disc considering nacelle and tower

The Reynolds-averaged Navier–Stokes (RANS) method coupling with the actuator disc model (ADM) is considered as a promising numerical simulation technology of wind turbine wake, and it is widely utilised in the aerodynamics of wind turbines and optimal layout of wind farms. The k − e turbulence model is widely adopted, among the RANS-based turbulence models. However, the k − e turbulence model easily overestimates the turbulence viscosity in the wake, which results in fast recovery of wake velocity and failure in wake forecasting. In addition, ADM with the oversimplified geometrical structure ignores the effects of nacelle and tower on the wind turbine wake, which further lowers the accuracy of wake simulation. Therefore, the numerical simulation of wind turbine wake based on the extended k − e turbulence model of EI Kasmi coupling with ADM considering nacelle and tower is proposed. Comparing the results of Marchwood Engineering Laboratories (MEL) ABL wind tunnel measurements and TNO wind tunnel experiments, it has been found that the proposed model improves the simulation effect for the near wake and has a certain contribution to the wake prediction accuracy overall.

Three-dimensional unsteady aerodynamics of a H-shaped vertical axis wind turbine over the full operating range

Vertical-axis wind turbines have great potential for harvesting energy with high conversion efficiency both in small-scale distributed installations and in the emerging floating offshore technology; nevertheless, their complex unsteady flow field makes design and optimization very challenging. This paper documents an extensive CFD study on a H-shaped VAWT over the entire range of operation. First, a direct 2D-3D comparison of the prediction capability of CFD models over the entire turbine operational curve is presented, highlighting the range of operation in which 3D effects become crucial and, instead, the range of TSR for which a 2D approximation is acceptable. The simulations are systematically compared to a wide and accurate experimental data set of wind-tunnel experiments in controlled conditions. The combination of CFD results with measured data allowed to assess the turbine performance and time-resolved velocity measurements in the wake. Furthermore, the paper proposes a totally novel approach in analysing the computed non-periodic unsteadiness in the wake. Finally, a tracking of the unsteady tip vortex is also proposed, with remarkable similarity to experimental results documented in the literature. This fact suggests that URANS discretization permits to approximate the dynamic and evolution of the tip vortex, at least in the near wake region.

URANS simulations of a horizontal axis wind turbine under stall condition using Reynolds stress turbulence models

Over the last decade, a dramatic increase in the size of commercial wind turbines is noticeable. The optimal structural design and reliable fatigue-life prediction of these large-scale structures depend on the accurate modeling of the turbulent flow around the rotors. The present paper aims at extending the knowledge of the turbulent flow characteristics around horizontal axis wind turbines and assessing the performance of several URANS turbulence models, principally Reynolds stress turbulence (RST) models, in predicting the wind turbine aerodynamics under stall condition. The simulations are performed using the NREL phase VI wind turbine over a wide range of wind speeds. Three RST models and the linear and quadratic variants of the k−ω SST turbulence model are examined. The results demonstrate that the RST models generally provide more reliable predictions. An evaluation of the Boussinesq hypothesis questions the applicability of the turbulence models employing this hypothesis to the wind turbine aerodynamic problems. Finally, the elliptic blending RST model, which is one of the latest formulations of the RST models, appears to provide the best trade-off between accuracy and computational cost.

Influence of limiting the projection region on coarse Large Eddy Simulation-Actuator Line Model simulations

Wind energy has developed worldwide, becoming a mature technology. Nevertheless, major advances regarding wind turbine aerodynamics and control as well as wind farm control and its integration in the power grid are being foreseen in the near future. To accomplish that, different simulation tools have emerged. The Actuator Line Model (ALM) in the frame of Large Eddy Simulation method has been used to study different related topics, from wake stability to wind turbine interaction and wind farm control, and it is considered the state of the art to simulate with high fidelity the wind flow through wind turbines and wind farms. Despite that, the ALM results, particularly loads and power production, have shown to be dependent on the numerical setup, like the projection function and its smearing parameter. The present paper aims to contribute to the latter by showing the influence of limiting the projection region. It is found that projecting the aerodynamic forces onto a cylinder that confines the rotor disk improves the computed power and loads compared to a Blade Element Momentum code and affects the wake development.

Assessment of weak compressibility in actuator line simulations of wind turbine wakes

The trend of increasing rotor diameters and tip-speeds has brought about concerns of non-negligible compressibility effects in wind turbine aerodynamics. The investigation of such effects on wakes is particularly difficult when using actuator line models (ALM). This is because crucial regions of the flow, i.e. the direct vicinity of the blade, are not simulated but represented by body forces. To separately assess the impact of compressibility on the wake and the ALM itself, we conduct large-eddy simulations (LES) where the forces of the ALM are prescribed and based on the local sampled velocity (standard procedure), respectively. The LES are based on the weakly-compressible Lattice Boltzmann Method (LBM). Further to the comparison of (near-)incompressible to compressible simulations we investigate cases with artificially increased compressibility. This is commonly done in weakly-compressible approaches to reduce the computational demand. The investigation with prescribed forces shows that compressibility effects in the wake flow are negligible. Small differences in the wake velocity (of max. 1%) are found to be related to local compressibility effects in the direct vicinity of the ALM. Most significantly, compressibility is found to affect the sampled velocity and thereby accuracy of the ALM.

Benchmarking different fidelities in wind turbine aerodynamics under yaw

This paper analyses the aerodynamics of wind turbines under yaw with different modelling fidelities (BEM, BEM with skewed wake model, UVLM and LES-AL). First of all, models are compared in a zero-yaw case to demonstrate their accuracy in prediction of out-of-plane loads and the discrepancy of UVLM in the in-plane loads due to the lack of viscous drag. Secondly, the yaw aerodynamics are described through the advancing/retreating and skewed wake effects, which are appropriately captured by UVLM and LES-AL and lead to an incorrect prediction of the location of maximum and minimum loading along a revolution by BEM. Further, when a skew-wake model is included in BEM, it predicts the correct locations but exhibits overly large loading variations. These predictions are consistent for all yaw angles studied (γ = 10° − 30°). All solvers predict similar decrease of root-bending moments, rotor power and thrust coefficients up to a yaw angle of 10°. However, at larger yaw angles, BEM overpredicts this decrease of coefficients with the yaw angle due to the unsuccessful performance of yaw corrections as opposed to UVLM that inherently accounts for three-dimensional effects. This study demonstrates the need to use computational models that can account for three-dimensional effects in the computation of aerodynamic loads for yaw angles above 10°.

Modeling the static and dynamic characteristics of a wind turbine airfoil and validation with experimental data

Dynamic stall modeling remains one of the most difficult topic in wind turbine aerodynamics despite its long research effort in the past. In the present studies, comprehensive investigations involving CFD, engineering models and experimental data were performed within the joint collaborative work DSWind. Both static and dynamic characteristics of a pitching wind turbine airfoil under a turbulent boundary layer flow were considered. The results indicate that optimized time-accurate CFD calculations can serve as reference data for verifying the engineering models, which can be potentially used for tuning the models under different air flow conditions.

Analysis aerodynamics diffuser-augmented wind turbines

A complete and consistent, one-dimensional momentum theory is derived for the aero-dynamics of slotted diffuser-augmented wind turbine (sDAWT). This theory does not require empirical relations. It results in a set of three-parameter relations for aerodynamic characteristics such as power coefficient Cp , rotor resistance k, rotor axial force coefficient CT , axial induction factor α, wake-expansion factor β, duct axial force coefficient CT,duct and slot mass flux . The theory predicts that the maximum achievable power coefficient Cp of (s)DAWT’s increases monotonically with increasing β, surpassing the Betz limit of open-rotor wind turbines (ORWT’s), already for modest (>2) β’s. The slot of an sDAWT feeds outside air into the diffuser, which for given β decreases the flow through the rotor and therewith Cp . However, the flow through the slot delays the onset of flow separation in the diffuser, increasing the maximum achievable β and therewith the power coefficient of sDAWT’s beyond that of DAWT’s.Based on a vortex model of the (s)DAWT, an expression is derived for the velocity induced at the rotor plane by the diffuser and for the corresponding circulation of the diffuser.The derived three-parameter relations for sDAWT’s reduce to two-parameter relations for DAWT’s and the familiar one-parameter relations for ORWT’s.

Actuator grid method for turbulence generation applied to yawed wind turbines

The aerodynamics of yawed wind turbine wakes remains a major investigation topic, especially for the use of yaw angle in control strategies. Large-Eddy Simulations are employed here to study the influence of yaw and inflow conditions on the prediction of the wind turbine wake structure. A single wind turbine setup with different yaw angles and three different inflow conditions is investigated and discussed with respect to experimental data. The wind turbine blades are modeled using the actuator line method (ALM) while tower and nacelle are represented with a body-fitted unstructured mesh. The high levels of upstream turbulence, experimentally generated by turbulence grids, are emulated here with oscillating ALM. Such approach demonstrates to be highly predictive on the turbulent flow characteristics compared to the emptied wind tunnel experimental data. Results with turbine show good agreement to the experiment data with only a slight overestimation on the magnitude of the wake deflection due to yaw. When compared to a deflection model, the confinement of the wind tunnel is highlighted. The radial and azimuthal time-averaged angle of attack exhibits a high probability of dynamic stall near the hub for a yawed turbine. These results show large discrepancies on the blade loading, highlighting the need for improved and specific models.