Yawed Conditions Research Articles

Numerical Simulation of Dynamic Stall Characteristics for Wind Turbine under Yawed Condition

When a horizontal axis wind turbine rotor works under yawed condition, a cyclic variation of Angle of Attack (AOA) at blades will causes dynamic stall phenomenon and accordingly increases the fatigue load. In order to relate the yawed condition with dynamic stall characteristic, a 3D and time-accurate Computational Fluid Dynamics (CFD) is used for the simulations of flow-field and dynamic stall characteristic on the National Renewable Energy Laboratory (NREL) Phase VI wind turbine rotor at yaw 30 degrees. The local AOA and airfoil aerodynamics, i.e., lift coefficient Cl and drag coefficient Cd, are computed based on CFD results. The results show that the horizontal component of yawed inflow and unsteady flow around sections of blade will delay the formation of separation vortex and also the occurrence of stall. The dynamic load accompanied with dynamic stall phenomenon is significantly increased on blade, and the hysteresis characteristic of airfoil lift and drag is more remarkable at the inboard blade sections. The derived results are helpful to develop more reliable aerodynamic models for wind turbine design codes, and also can provide theoretical guidance for the optimal design of a wind turbine.

A new stall delay algorithm for predicting the aerodynamics loads on wind turbine blades for axial and yawed conditions

AbstractStall delay is a complicated phenomenon that has gained for many years the attention of industry and academics in the fields of helicopter and wind turbine aerodynamics. Since most of the potential flow theories still rely on the use of 2D aerofoil data for simulating loads on a rotating blade, less degree of accuracy is expected because of 3D rotational effects. In this work, a new model for correcting the 2D steady aerodynamic data for 3D effects is presented. The model can reduce the uncertainty in the blade design process and, subsequently, make wind turbines more cost‐effective. This model combines the stall delay model of Corrigan and Schillings, a modified version of an inviscid stall delay model, a new modification factor to account for the effect of the angle of attack changes and a new tip loss factor. Furthermore, the model applies the use of the separation factor of Du and Selig to evaluate the area on the rotor disc where stall delay is most prominent. The new stall delay model was embedded in a free‐wake vortex model to estimate the aerodynamic loads on the National Renewable Energy Laboratory Phase VI rotor blades consisting of the S809 aerofoil sections. The results in this study confirm the validity of the 3D corrections by the proposed new model under both axial and yawed flow conditions. Copyright © 2017 John Wiley & Sons, Ltd.

Comparative study on the wake deflection behind yawed wind turbine models

In this wind tunnel campaign, detailed wake measurements behind two different model wind turbines in yawed conditions were performed. The wake deflections were quantified by estimating the rotor-averaged available power within the wake. By using two different model wind turbines, the influence of the rotor design and turbine geometry on the wake deflection caused by a yaw misalignment of 30° could be judged. It was found that the wake deflections three rotor diameters downstream were equal while at six rotor diameters downstream insignificant differences were observed. The results compare well with previous experimental and numerical studies.

Development of a new free wake model using finite vortex element for a horizontal axis wind turbine

The treatment of rotor wake has been a critical issue in the field of the rotor aerodynamics. This paper presents a new free wake model for the unsteady analysis for a wind turbine. A blade-wake-tower interaction is major source of unsteady aerodynamic loading and noise on the wind turbine. However, this interaction can not be considered in conventional free wake model. Thus, the free wake model named Finite Vortex Element (FVE hereafter) was devised in order to consider the interaction effects. In this new free wake model, the wake-tower interaction was described by dividing one vortex filament into two vortex filaments, when the vortex filament collided with a tower. Each divided vortex filaments were remodeled to make vortex ring and horseshoe vortex to satisfy Kelvin’s circulation theorem and Helmholtz’s vortex theorem. This model was then used to predict aerodynamic load and wake geometry for the horizontal axis wind turbine. The results of the FVE model were compared with those of the conventional free wake model and the experimental results of SNU wind tunnel test and NREL wind tunnel test under various inflow velocity and yaw condition. The result of the FVE model showed better correlation with experimental data. It was certain that the tower interaction has a strong effect on the unsteady aerodynamic load of blades. Thus, the tower interaction needs to be taken into account for the unsteady load prediction. As a result, this research shows a potential of the FVE for an efficient and versatile numerical tool for unsteady loading analysis of a wind turbine.

Investigation of the Rotor Wake of Horizontal Axis Wind Turbine under Yawed Condition

The wake and the lack of existing velocity behind the wind turbine affect the energy production and the mechanical integrity of wind turbines downstream in the wind farms. This paper presents an investigation of the unsteady flow around a wind turbine under yawed condition. The simulations and experimental measures are made for the yaw angle rotor 30° and 0°. The wind velocity is 9.3 m/s and the rotation velocity rotor of the wind turbine in 1300, 1500 and 1800 rpm. The wind turbine rotor which is modeled is of a commercial wind turbine i.e. Rutland 503. The approach Improved Delayed Detached Eddy Simulation (IDDES) based on the SST turbulence model is used in the modeling of the flow. The solutions are obtained by using the solver which uses finite volume method. The particle image velocimetry (PIV) method is used in wind tunnel measurements in the experimental laboratory of the ENSAM Paris-Tech. The yawed downstream wake of the rotor is compared with that obtained by the experimental measurements. The results illustrate perfectly the development of the near and far wake of the rotor operation. It is observed that the upstream wind turbine yawed will have a positive impact on the power of the downstream turbine due the distance reduction of the downstream wake of the wind turbine. However the power losses are important for yawed wind turbine when compared with the wind turbine without yaw. The improved understanding of the unsteady environmental of the Horizontal Axis wind Turbine allows optimizing wind turbine structures and the number of wind turbines in wind farms.

Experimental and theoretical study of wind turbine wakes in yawed conditions

This work is dedicated to systematically studying and predicting the wake characteristics of a yawed wind turbine immersed in a turbulent boundary layer. To achieve this goal, wind tunnel experiments were performed to characterize the wake of a horizontal-axis wind turbine model. A high-resolution stereoscopic particle image velocimetry system was used to measure the three velocity components in the turbine wake under different yaw angles and tip-speed ratios. Moreover, power and thrust measurements were carried out to analyse the performance of the wind turbine. These detailed wind tunnel measurements were then used to perform a budget study of the continuity and Reynolds-averaged Navier–Stokes equations for the wake of a yawed turbine. This theoretical analysis revealed some notable features of the wakes of yawed turbines, such as the asymmetric distribution of the wake skew angle with respect to the wake centre. Under highly yawed conditions, the formation of a counter-rotating vortex pair in the wake cross-section as well as the vertical displacement of the wake centre were shown and analysed. Finally, this study enabled us to develop general governing equations upon which a simple and computationally inexpensive analytical model was built. The proposed model aims at predicting the wake deflection and the far-wake velocity distribution for yawed turbines. Comparisons of model predictions with the wind tunnel measurements show that this simple model can acceptably predict the velocity distribution in the far wake of a yawed turbine. Apart from the ability of the model to predict wake flows in yawed conditions, it can provide valuable physical insight on the behaviour of turbine wakes in this complex situation.

Validation and comparison of aerodynamic modelling approaches for wind turbines

The development of large capacity Floating Offshore Wind Turbines (FOWT) is an interdisciplinary challenge for the design solvers, requiring accurate modelling of both hydrodynamics, elasticity, servodynamics and aerodynamics all together. Floating platforms will induce low-frequency unsteadiness, and for large capacity turbines, the blade induced vibrations will lead to high-frequency unsteadiness. While yawed inflow conditions are still a challenge for commonly used aerodynamic methods such as the Blade Element Momentum method (BEM), the new sources of unsteadiness involved by large turbine scales and floater motions have to be tackled accurately, keeping the computational cost small enough to be compatible with design and certification purposes. In the light of this, this paper will focus on the comparison of three aerodynamic solvers based on BEM and vortex methods, on standard, yawed and unsteady inflow conditions. We will focus here on up-to-date wind tunnel experiments, such as the Unsteady Aerodynamics Experiment (UAE) database and the MexNext international project.

In-blade angle of attack measurement and comparison with models

The torque generated by a wind turbine blade is dependent on several parameters, one of which is the angle of attack. Several models for predicting the angle of attack in yawed conditions have been proposed in the literature, but there is a lack of experimental data to use for direct validation.To address this problem, experiments were conducted at the University of Waterloo Wind Generation Research Facility using a 3.4 m diameter test turbine. A five-hole pressure probe was installed in a modular 3D printed blade and was used to measure the angle of attack, a, as a function of several parameters. Measurements were conducted at radial positions of r/R = 0.55 and 0.72 at tip speed ratios of λ = 5.0, 3.6, and 3.1. The yaw offset of the turbine was varied from -15° to +15°.Experimental results were compared directly to angle of attack values calculated using a model proposed by Morote in 2015. Modeled values were found to be in close agreement with the experimental results. The angle of attack was shown to vary cyclically in the yawed case while remaining mostly constant when aligned with the flow, as expected. The quality of results indicates the potential of the developed instrument for wind turbine measurements.

Latest results from the EU project AVATAR: Aerodynamic modelling of 10 MW wind turbines

This paper presents the most recent results from the EU project AVATAR in which aerodynamic models are improved and validated for wind turbines on a scale of 10 MW and more. Measurements on a DU 00-W-212 airfoil are presented which have been taken in the pressurized DNW-HDG wind tunnel up to a Reynolds number of 15 Million. These measurements are compared with measurements in the LM wind tunnel for Reynolds numbers of 3 and 6 Million and with calculational results. In the analysis of results special attention is paid to high Reynolds numbers effects. CFD calculations on airfoil performance showed an unexpected large scatter which eventually was reduced by paying even more attention to grid independency and domain size in relation to grid topology. Moreover calculations are presented on flow devices (leading and trailing edge flaps and vortex generators). Finally results are shown between results from 3D rotor models where a comparison is made between results from vortex wake methods and BEM methods at yawed conditions.

3-D Time-Accurate CFD Simulations of a Multi-Megawatt Slender Bladed HAWT under Yawed Inflow Conditions

In the present study numerical investigations of a generic Multi-Megawatt slender bladed Horizontal-Axis Wind Turbine (HAWT) under yawed inflow conditions were conducted. A three-dimensional URANS flow solver based on structured overlapping meshes was used. The simulations were conducted at wind speeds of 7m/sec, 11 m/sec and 15 m/sec for different yaw angles ranging from +60° to -60°. It was concluded that, for below rated wind speeds, under small yaw angles (below ±15°) the magnitudes of the blade forces are slightly increased, while under high yaw angles (above ±15°) there is a significant decrease. Moreover, the load fluctuations, for the different yaw angles, have the same frequency but different amplitude and oscillation shape. It was concluded that at the above rated wind speed of 15 m/sec, the blade aerodynamic loads are significantly affected by the yaw inflow conditions and the magnitude values of the loads are decreased with increasing yaw angle. It can be concluded that the angle of attack and the tower interference are the utmost variables affecting the yawed turbines.

CFD study on the impact of yawed inflow on loads, power and near wake of a generic wind turbine

AbstractThis article deals with the influence of yawed inflow conditions on the performance of a single generic 2.4MW wind turbine. It presents the results of studies performed at the Institute of Aerodynamics and Gas Dynamics by means of computational fluid dynamics, using a fully meshed wind turbine with all boundary layers being resolved. The block‐structured flow solver FLOWer is used; a dual‐time stepping method for temporal discretization and a second‐order Jameson–Schmidt–Turkel method for the calculation of the convective fluxes are applied. All simulations are carried out using a detached eddy simulation approach. In detail, two different wind speeds and a yaw angle range between −50° and +50° are evaluated in the paper. Based on these data, it is shown that the reduction of power output follows a cosine to the power of X function of the yaw angle. Furthermore, the growing azimuthal non‐uniformity of the load distributions with increasing yaw angle magnitude is analysed by spanwise load distributions. As a central influence on the load distributions, the advancing and retreating blade effect is identified. Moreover, the deflection of the wake as a result of the inflow is investigated, and the deflection angles are compared with a modelling approach. A connection line between wake deflection and load asymmetry is drawn. The results are of particular importance for wind park situations with downstream turbines facing the distorted inflow created from upstream ones. Copyright © 2016 John Wiley & Sons, Ltd.

Wake structure in actuator disk models of wind turbines in yaw under uniform inflow conditions

Reducing wake losses in wind farms by deflecting the wakes through turbine yawing has been shown to be a feasible wind farm controls approach. Nonetheless, the effectiveness of yawing depends not only on the degree of wake deflection but also on the resulting shape of the wake. In this work, the deflection and morphology of wakes behind a porous disk model of a wind turbine operating in yawed conditions are studied using wind tunnel experiments and uniform inflow. First, by measuring velocity distributions at various downstream positions and comparing with prior studies, we confirm that the non-rotating porous disk wind turbine model in yaw generates realistic wake deflections. Second, we characterize the wake shape and make observations of what is termed as curled wake, displaying significant spanwise asymmetry. The wake curling observed in the experiments is also reproduced qualitatively in Large Eddy Simulations using both actuator disk and actuator line models. Results suggest that when a wind turbine is yawed for the benefit of downstream turbines, the curled shape of the wake and its asymmetry must be taken into account since it affects how much of it intersects the downstream turbines.

Prediction of Aerodynamic Loads for NREL Phase VI Wind Turbine Blade in Yawed Condition

Aerodynamic loads for a horizontal axis wind turbine of the National Renewable Energy Laboratory (NREL) Phase VI rotor in yawed condition were predicted by using the blade element momentum theorem. The classical blade element momentum theorem was complemented by several aerodynamic corrections and models including the Pitt and Peters' yaw correction, Buhl's wake correction, Prandtl's tip loss model, Du and Selig's three-dimensional (3-D) stall delay model, etc. Changes of the aerodynamic loads according to the azimuth angle acting on the span-wise location of the NREL Phase VI blade were compared with the experimental data with various yaw angles and inflow speeds. The computational flow chart for the classical blade element momentum theorem was adequately modified to accurately calculate the combined functions of additional corrections and models stated above. A successive under-relaxation technique was developed and applied to prevent possible failure during the iteration process. Changes of the angle of attack according to the azimuth angle at the specified radial location of the blade were also obtained. The proposed numerical procedure was verified, and the predicted data of aerodynamic loads for the NREL Phase VI rotor bears an extremely close resemblance to those of the experimental data.

Comparative assessment of the harmonic balance Navier–Stokes technology for horizontal and vertical axis wind turbine aerodynamics

Several important wind turbine unsteady flow regimes, such as those associated with the yawed wind condition of horizontal axis machines, and most operating conditions of all vertical axis machines, are predominantly periodic. The harmonic balance Reynolds-averaged Navier–Stokes technology for the rapid calculation of nonlinear periodic flow fields has been successfully used to greatly reduce runtimes of turbomachinery periodic flow analyses in the past fifteen years. This paper presents an objective comparative study of the performance and solution accuracy of this technology for aerodynamic analysis and design applications of horizontal and vertical axis wind turbines. The considered use cases are the periodic flow past the blade section of a utility-scale horizontal axis wind turbine rotor in yawed wind, and the periodic flow of a H-Darrieus rotor section working at a tip-speed ratio close to that of maximum power. The aforementioned comparative assessment is based on thorough parametric time-domain and harmonic balance analyses of both use cases. The paper also reports the main mathematical and numerical features of a new turbulent harmonic balance Navier–Stokes solver using Menter’s shear stress transport model for the turbulence closure. Presented results indicate that (a) typical multimegawatt horizontal axis wind turbine periodic flows can be computed by the harmonic balance solver about ten times more rapidly than by the conventional time-domain analysis, achieving the same temporal accuracy of the latter method, and (b) the harmonic balance acceleration for Darrieus rotor unsteady flow analysis is lower than for horizontal axis machines, and the harmonic balance solutions feature undesired oscillations caused by the wide harmonic content and the high-level of stall predisposition of this flow field type.

Combining Unsteady Blade Pressure Measurements and a Free-Wake Vortex Model to Investigate the Cycle-to-Cycle Variations in Wind Turbine Aerodynamic Blade Loads in Yaw

Prediction of the unsteady aerodynamic flow phenomenon on wind turbines is challenging and still subject to considerable uncertainty. Under yawed rotor conditions, the wind turbine blades are subjected to unsteady flow conditions as a result of the blade advancing and retreating effect and the development of a skewed vortical wake created downstream of the rotor plane. Blade surface pressure measurements conducted on the NREL Phase VI rotor in yawed conditions have shown that dynamic stall causes the wind turbine blades to experience significant cycle-to-cycle variations in aerodynamic loading. These effects were observed even though the rotor was subjected to a fixed speed and a uniform and steady wind flow. This phenomenon is not normally predicted by existing dynamic stall models integrated in wind turbine design codes. This paper couples blade pressure measurements from the NREL Phase VI rotor to a free-wake vortex model to derive the angle of attack time series at the different blade sections over multiple rotor rotations and three different yaw angles. Through the adopted approach it was possible to investigate how the rotor self-induced aerodynamic load fluctuations influence the unsteady variations in the blade angles of attack and induced velocities. The hysteresis loops for the normal and tangential load coefficients plotted against the angle of attack were plotted over multiple rotor revolutions. Although cycle-to-cycle variations in the angles of attack at the different blade radial locations and azimuth positions are found to be relatively small, the corresponding variations in the normal and tangential load coefficients may be significant. Following a statistical analysis, it was concluded that the load coefficients follow a normal distribution at the majority of blade azimuth angles and radial locations. The results of this study provide further insight on how existing engineering models for dynamic stall may be improved through the integration of stochastic models to be able to account for the cycle-to-cycle variability in the unsteady wind turbine blade loads under yawed conditions.

Effect of turbulence on power performance of a Horizontal Axis Wind Turbine in yawed and no-yawed flow conditions

The purpose of this study was to investigate the effect of turbulence on the power performance at the yawed and no-yawed flow conditions, focusing on the turbulence intensities and boundary layer in wind tunnel experiments. In this study, UMY02-T01-26 airfoil was developed, based on the body type of large aquatic animals. Turbulence inflow was generated by an active turbulence grid. Boundary layer was simulated by a boundary layer tape attached on the rotor surface. Furthermore, the fluctuations of power coefficient against the tip speed ratio were examined during rotation. From the experiments, it could be found that the power coefficients decreased in the case of the extremely low turbulence intensity of TI = 0.50%. However, the power coefficients had a significant improvement in the high turbulence intensity of TI = 10.0%. Moreover, in the case of the yaw angle of ϕ = 30°, the power coefficients showed larger values than those of ϕ = 0° at the low tip speed ratio ranges of 1.5 < λ < 2.5. Therefore, the developed wind turbine model could exhibit sufficient performance at the Reynolds numbers of Re = 1.5 and 2.0 × 105 in the natural wind condition.

No Significant Cognitive Changes Following Season Of High School Soccer

BACKGROUND: While concussions and repeated head impacts in football have received increased attention and scrutiny; soccer athletes are repeatedly exposed to head impacts during routine play. Limited research however, has been completed among this population, especially at the high school level. PURPOSE: A pilot investigation to evaluate the relationship between a season of soccer participation and cognitive function among male high school athletes. METHODS: Six male high school soccer players (ages 15.8 ± 0.8yrs, height 178.0 ± 7.8cm, and weight 65.3 ± 8.6kg) were assessed prior to and following the 2015 soccer season. Participants played in 18 games and 32 practices, including 2 scrimmages, over the season. Each participant signed an Institutional Review Board approved assent form and proper parental consent forms were obtained. Participants completed a 22-item Symptom checklist, Satisfaction with Life Scale (SWLS), Brief Symptom Inventory 18 (BSI-18), King-Devick, AXON computerized cognitive task, and Head Rehab virtual reality tasks assessing Balance, Spatial Memory, and Reaction Time. Paired t-tests were calculated. Significance was noted at p 0.05). Head Rehab composite scores showed no significant difference pre to postseason for spatial memory or reaction time. Significant improvement in balance was observed for the roll condition (p = 0.03), while no changes were observed for the stationary, pitch, or yaw conditions. CONCLUSION: This preliminary study found no significant cognitive declines following a season of high school soccer. Future research should address male and female athletes in a larger cohort where head impact exposure is directly monitored. This research was supported by The National Institutes of Health: National Institute of Neurological Disorders and Stroke (3R15NS081691-01S1).

Comparing Three Aerodynamic Models for Predicting the Thrust and Power Characteristics of a Yawed Floating Wind Turbine Rotor

This study compares the time-varying rotor thrust and shaft power characteristics of a yawed floating offshore wind turbine (FOWT) predicted by three different open-source aerodynamic models. These models involve the blade-element-momentum (BEM) and the general dynamic wake (GDW) methods implemented in the design code fast developed by NREL, and a higher fidelity free-wake vortex model (FWVM) that is capable of modeling the unsteady skewed helical wake development of the yawed rotor. The study is based on the NREL 5 MW baseline rotor installed on the MIT tension-leg platform (TLP) operating with different rotor yaw angles and under regular sea wave conditions. Both the undisturbed wind speed and rotor speed are maintained constant throughout the analysis, though different sea wave heights and periods are considered. Initially, the motions of the FOWT under both axial and yawed rotor conditions are estimated in a time domain using fast. These motions are then prescribed to winds, an open-source FWVM developed by the University of Massachusetts Amherst, to determine the aerodynamic rotor thrust and power as a function of time. Both TLP surge and pitch motions are noted to impact the rotor thrust and power characteristics considerably. The three models have consistently shown that the TLP motion exhibits a negligible impact on the time-averaged rotor shaft thrust and power of the yawed rotor. On the other hand, the cyclic component of rotor thrust and power are found to be significantly influenced by the wave state at all yaw angles. Significant discrepancies between the predictions for this cyclic component from the three models are observed, suggesting the need of further research through experimental validation to ensure more reliable aerodynamics models are developed for floating wind turbine design software packages.

Influence of blade deformation and yawed inflow on performance of a horizontal axis tidal stream turbine

For a better design of tidal stream turbines operated in off-design conditions, analyses considering the effects of blade deformation and yawed inflow conditions are necessary. The flow load causes deformation of the blade, and the deformation affects the turbine performance in return. Also, a yawed inflow influences the performance of the turbine. As a validation study, a computational fluid dynamics (CFD) simulation was carried out to predict the performance of a horizontal axis tidal stream turbine (HATST) with rigid blades. The numerical uncertainty for the turbine performance with blade deformation and a yawed inflow was evaluated using the concept of the grid convergence index (GCI). A fluid–structure interaction (FSI) analysis was carried out to estimate the performance of a turbine with flexible composite blades, with the results then compared to those of an analysis with rigid blades. The influence of yawed inflow conditions on the turbine performance was investigated and found to be important in relation to power predictions in the design stages.

Aerodynamic loads and aeroelastic responses of large wind turbine tower-blade coupled structure in yaw condition

An effective method to calculate aerodynamic loads and aeroelastic responses of large wind turbine tower-blade coupled structures in yaw condition is proposed. By a case study on a 5 MW large wind turbine, the finite element model of the wind turbine tower-blade coupled structure is established to obtain the modal information. The harmonic superposition method and modified blade-element momentum theory are used to calculate aerodynamic loads in yaw condition, in which the wind shear, tower shadow, tower-blade modal and aerodynamic interactions, and rotational effects are fully taken into account. The mode superposition method is used to calculate kinetic equation of wind turbine tower-blade coupled structure in time domain. The induced velocity and dynamic loads are updated through iterative loop, and the aeroelastic responses of large wind turbine tower-blade coupled system are then obtained. For completeness, the yaw effect and aeroelastic effect on aerodynamic loads and wind-induced responses are discussed in detail based on the calculating results.