Modeling dynamic stall on wind turbine blades under rotationally augmented flow fields

This paper presents an investigation of two well‐known aerodynamic phenomena, rotational augmentation and dynamic stall, together in the inboard parts of wind turbine blades. This analysis is carried out using the following: (1) the National Renewable Energy Laboratory's Unsteady Aerodynamics Experiment Phase VI experimental data, including constant as well as continuously pitching blade conditions during axial operation; (2) data from unsteady delayed detached eddy simulations (DDES) carried out using the Technical University of Denmark's in‐house flow solver Ellipsys3D; and (3) data from a reduced order dynamic stall model that uses rotationally augmented steady‐state polars obtained from steady Phase VI experimental sequences, instead of the traditional two‐dimensional, non‐rotating data. The aim of this work is twofold. First, the blade loads estimated by the DDES simulations are compared with three select cases of the N‐sequence experimental data, which serves as a validation of the DDES method. Results show reasonable agreement between the two data in two out of three cases studied. Second, the dynamic time series of the lift and the moment polars obtained from the experiments are compared with those from the dynamic stall model. This allowed the differences between the stall phenomenon on the inboard parts of harmonically pitching blades on a rotating wind turbine and the classic dynamic stall representation in two‐dimensional flow to be investigated. Results indicated a good qualitative agreement between the model and the experimental data in many cases, which suggests that the current two‐dimensional dynamic stall model as used in blade element momentum‐based aeroelastic codes may provide a reasonably accurate representation of three‐dimensional rotor aerodynamics when used in combination with a robust rotational augmentation model. Copyright © 2015 John Wiley & Sons, Ltd.     

Aerodynamic response of an airfoil section undergoing pitch motion and trailing edge flap deflection: a comparison of simulation methods

The study presents and compares aerodynamic simulations for an airfoil section with an adaptive trailing edge flap, which deflects following a smooth deformation shape. The simulations are carried out with three substantially different methods: a Reynolds‐averaged Navier–Stokes solver, a viscous–inviscid interaction method and an engineering dynamic stall model suitable for implementation in aeroelastic codes based on blade element momentum theory. The aerodynamic integral forces and pitching moment coefficients are first determined in steady conditions, at angles of attack spanning from attached flow to separated conditions and accounting for the effects of flap deflection; the steady results from the Navier–Stokes solver and the viscous–inviscid interaction method are used as input data for the simpler dynamic stall model. The paper characterizes then the dynamics of the unsteady forces and moments generated by the airfoil undergoing harmonic pitching motions and harmonic flap deflections. The unsteady aerodynamic coefficients exhibit significant variations over the corresponding steady‐state values. The dynamic characteristics of the unsteady response are predicted with an excellent agreement among the investigated methods at attached flow conditions, both for airfoil pitching and flap deflection. At high angles of attack, where flow separation is encountered, the methods still depict similar overall dynamics, but larger discrepancies are reported, especially for the simpler engineering method. Copyright © 2014 John Wiley & Sons, Ltd.     

对一种动态失速模型的改进及精度研究

基于现有的Hopf分岔法动态失速模型（Hopf bifurcation model）,引入Wagner函数计算其附着流下等效攻角,对原模型的边界层再附着项进行一定修改,使新模型可表示为状态空间的形式,并为原模型补齐了对于阻力和力矩系数的建模。相比于常见的ONERA和Leishman-Beddoes动态失速模型,新模型在附着流下拥有与解析理论最一致的幅值及相位特性;分离流下,新模型在大部分情况下的计算精度优于常见模型,且能更好地捕捉初级失速涡和深度失速下出现的多级失速涡现象。其中对轻度失速的分析表明,各动态失速模型在轻度失速下的环量项建模仍具有一定的提升空间。     

考虑动态失速与风轮支架损失的H型垂直轴风力机自由尾迹模型构建

采用基于升力线理论的自由尾迹模拟模型,耦合了Leishman-Beddoes（LB）动态失速模型和模拟支架损失的半经验公式,编制H型垂直轴风轮计算程序,对不同直径、不同实度的风轮进行气动性能模拟。为了检测模型对H型风轮输出计算的准确性,选取2种不同实度（分别为0.13和0.48）的H型风力机进行了对比分析。结果表明,当尖速比小于4时,动态失速影响较为明显。考虑支架损失的计算结果明显优于未考虑支架损失的计算结果,支架损失总体上随尖速比的增大而增大,2台风力机对应的最大支架损失分别高达60%和35.5%。考虑动态失速效应和支架损失的计算模型能较为准确地估算风轮实度较小的H型风轮的输出,但对于实度较大的H型风轮,其计算准确度有待进一步提高。     

A stochastic model for the simulation of wind turbine blades in static stall

The aim of this work is to improve aeroelastic simulation codes by accounting for the unsteady aerodynamic forces that a blade experiences in static stall. A model based on a spectral representation of the aerodynamic lift force is defined. The drag and pitching moment are derived using a conditional simulation technique for stochastic processes. The input data for the model can be collected either from measurements or from numerical results from a Computational Fluid Dynamics code for airfoil sections at constant angles of attack. An analysis of such data is provided, which helps to determine the characteristics of stall. The model is applied to wind turbine rotor cases, including the stand still condition, and results are compared to experimental data. Copyright © 2009 John Wiley & Sons, Ltd.     

Dynamic stall modelling of the S809 aerofoil and comparison with experiments

An analysis of dynamic stall for the S809 aerofoil has been performed in conjunction with the Leishman–Beddoes dynamic stall model that was modified for wind turbine applications. Numerical predictions of the lift, drag and pitching moment coefficients were compared with measurements obtained for an oscillating S809 aerofoil at various reduced frequencies, mean angles of attack and angle of attack amplitudes. It was found that the results using the modified model were in good agreement with the experimental data. Hysteresis in the aerodynamic coefficients was captured well, although the drag coefficient was slightly underpredicted in the deep stall flow regime. Validation against the experimental data showed overall good agreement. The mathematical structure of the model is such that it can be readily incorporated into a comprehensive analysis code for a wind turbine. Copyright © 2006 John Wiley & Sons, Ltd.     

New vortex‐lift and tangential‐force models for HAWT aerodynamic load prediction

Horizontal axis wind turbines (HAWTs) experience three‐dimensional rotational and unsteady aerodynamic phenomena at the rotor blades sections. These highly unsteady three‐dimensional effects have a dramatic impact on the aerodynamic load distributions on the blades, in particular, when they occur at high angles of attack due to stall delay and dynamic stall. Unfortunately, there is no complete understanding of the flow physics yet at these unsteady 3D flow conditions, and hence, the existing published theoretical models are often incapable of modelling the impact on the turbine response realistically. The purpose of this paper is to provide an insight on the combined influence of the stall delay and dynamic stall on the blade load history of wind turbines in controlled and uncontrolled conditions. New dynamic stall vortex and nonlinear tangential force coefficient modules, which integrally take into account the three dimensional rotational effect, are also proposed in this paper. This module along with the unsteady influence of turbulent wind speed and tower shadow is implemented in a blade element momentum (BEM) model to estimate the aerodynamic loads on a rotating blade more accurately. This work presents an important step to help modelling the combined influence of the stall delay and dynamic stall on the load history of the rotating wind turbine blades which is vital to have lighter turbine blades and improved wind turbine design systems.     

Validation of the Beddoes–Leishman dynamic stall model for horizontal axis wind turbines using MEXICO data

The aim of this study is to assess the load predicting capability of a classical Beddoes–Leishman dynamic stall model in a horizontal axis wind turbine environment, in the presence of yaw misalignment. The dynamic stall model was tailored to the horizontal axis wind turbine environment and validated against unsteady thick airfoil data. Subsequently, the dynamic stall model was implemented in a blade element‐momentum code for yawed flow, and the results were compared with aerodynamic measurements obtained in the MEXICO (Model Rotor Experiments under Controlled Conditions) project on a wind turbine rotor placed in a large scale wind tunnel. In general, reasonable to good agreement was found between the blade element‐momentum model and MEXICO data. When large yaw misalignments were imposed, poor agreement was found in the downstroke of the movement between the model and the experiment. Still, over a revolution, the maximum normal force coefficient predicted was always within 8% of experimental data at the inboard stations, which is encouraging especially when blade fatigue calculations are being considered. Copyright © 2012 John Wiley & Sons, Ltd.     

Aerodynamic loads calculation and analysis for large scale wind turbine based on combining BEM modified theory with dynamic stall model

The aerodynamic loads for MW scale horizontal-axis wind turbines are calculated and analyzed in the established coordinate systems which are used to describe the wind turbine. In this paper, the blade element momentum (BEM) theory is employed and some corrections, such as Prandtl and Buhl models, are carried out. Based on the B-L semi-empirical dynamic stall (DS) model, a new modified DS model for NACA63-4xx airfoil is adopted. Then, by combing BEM modified theory with DS model, a set of calculation method of aerodynamic loads for large scale wind turbines is proposed, in which some influence factors such as wind shear, tower, tower and blade vibration are considered. The research results show that the presented dynamic stall model is good enough for engineering purpose; the aerodynamic loads are influenced by many factors such as tower shadow, wind shear, dynamic stall, tower and blade vibration, etc, with different degree; the single blade endures periodical changing loads but the variations of the rotor shaft power caused by the total aerodynamic torque in edgewise direction are very small. The presented study approach of aerodynamic loads calculation and analysis is of the university, and helpful for thorough research of loads reduction on large scale wind turbines.     

A vortex panel model for the simulation of the wake flow past a vertical axis wind turbine in dynamic stall

A 2D vortex panel model with a viscous boundary layer formulation has been developed for the numerical simulation of a vertical axis wind turbine (VAWT), including the operation in dynamic stall. The model uses the ‘double wake’ concept to reproduce the main features of the unsteady separated flow, including the formation and shedding of strong vortical structures and the wake–blade interaction. The potential flow equations are solved together with the integral boundary layer equations by using a semi‐inverse iterative algorithm. A new criterion for the reattachment of the boundary layer during the downstroke of a dynamically stalled aerofoil is implemented. The model has been validated against experimental data of steady aerofoils and pitching aerofoils in dynamic stall at high and low Reynolds numbers (Re = 1.5 × 10⁶ and Re = 5 × 10⁴). For the low Reynolds number case, time‐resolved 2D particle image velocimetry (PIV) measurements have been performed on a pitching NACA 0012 aerofoil in dynamic stall. The PIV vorticity fields past the oscillating aerofoil are used to test the model capability of capturing the formation, growth and release of the strong leading edge vortex that characterizes the dynamic stall. Furthermore, the forces extracted from the PIV velocity fields are compared with the predicted ones for a quantitative validation of the model. Finally, the model is applied to the computation of the wake flow past a VAWT in dynamic stall; the predicted vorticity fields and forces are in good agreement with phase‐locked PIV data and CFD‐DES available in the literature. Copyright © 2012 John Wiley & Sons, Ltd.     

Integrating a new flow separation model and the effects of the vortex shedding for improved dynamic stall predictions using the Beddoes–Leishman method

This paper aims at improving dynamic stall predictions on the S809 aerofoil under 2D flow conditions. The method is based on the well‐known Beddoes–Leishman model; however, a new flow separation model and a noise generator are integrated to improve the predictions in the load fluctuations, including those induced by vortex shedding on the aerofoil upper surface. The flow separation model was derived from a unique approach based on the combined use of unsteady aerodynamic loads measurements, the Beddoes–Leishman model and a trial‐and‐error technique. The new flow separation model and random noise generator were integrated in the Beddoes–Leishman model through a new solution algorithm. The numerical predictions of the unsteady lift and drag coefficients were then compared with the Ohio State University measurements for the oscillating S809 aerofoil at several reduced frequencies and angles of attack. The results using the proposed models showed improved correlation with the experimental data. Hysteresis loops for the aerodynamic coefficients are in good agreement with measurements. Copyright © 2016 John Wiley & Sons, Ltd.     

A combined aeroelastic‐aeroacoustic model for wind turbine noise: verification and analysis of field measurements

In this paper, semi‐empirical engineering models for the three main wind turbine aerodynamic noise sources, namely, turbulent inflow, trailing edge and stall noise, are introduced. They are implemented into the in‐house aeroelastic code HAWC2 commonly used for wind turbine load calculations and design. The results of the combined aeroelastic and aeroacoustic model are compared with field noise measurements of a 500 kW wind turbine. Model and experimental data are in fairly good agreement in terms of noise levels and directivity. The combined model allows separating the various noise sources and highlights a number of mechanisms that are difficult to differentiate when only the overall noise from a wind turbine is measured. Copyright © 2017 John Wiley & Sons, Ltd.     

Horizontal Axis Wind Turbine Blade Aerodynamics in Experiments and Modeling

Turbine aerodynamics remains a challenging and crucial research area for wind energy. Blade aerodynamic forces responsible for power production must be augmented to maximize energy capture. At the same time, adverse aerodynamic loads that fatigue turbine components need to be mitigated to extend machine service life. Successful resolution of these conflicting demands and continued cost of energy reduction require accurate blade aerodynamic models. This, in turn, depends on clear physical understanding and reliable numerical modeling of rotational augmentation and dynamic stall, the two phenomena principally responsible for amplified turbine blade aerodynamic loads. The current work examines full-scale turbine blade aerodynamic measurements and current modeling methodologies to better understand the physical and numerical attributes that determine model performance     

Wake modelling of a VAWT in dynamic stall: impact on the prediction of flow and induction fields

Dynamic stall is a relevant phenomenon in the design and operation of a vertical axis wind turbine (VAWT) as it impacts loading, control and wake dynamics. Although streamtube models and single‐wake vortex models are commonly used for VAWT simulation, they either do not explicitly simulate the distribution of vorticity in the wake (streamtube models) or simplify it into a single‐wake release point (single‐wake vortex models). This can lead to inaccurate predictions of the vorticity distribution and wake dynamics, and therefore of the induction field, rotor loading and wake development, including wake mixing and re‐energizing. In this work, we use a double‐wake panel model developed for the simulation of dynamic stall in a VAWT to analyse (i) what is the flow field in dynamic stall, including the induction field, (ii) what is the error due to assuming a simplified wake, in both vorticity distribution and induction and (iii) how an incorrect simulation of the vorticity distribution can affect the prediction of the dynamics of the near and far wake. The results demonstrate that for mild separation (tip speed ratio λ≥3), single‐wake models can produce acceptable results. However, for lower tip speed ratios (λ < 3), the inaccuracy in the prediction of loads, induction field and vorticity distribution becomes significant because of an inadequate representation of the wake dynamics. These results imply that using lower order models can lead to inaccurate estimations of loads, performance and power control requirements at low tip speed ratios. Copyright © 2014 John Wiley & Sons, Ltd.     

Simulating dynamic stall in a two‐dimensional vertical‐axis wind turbine: verification and validation with particle image velocimetry data

The implementation of wind energy conversion systems in the built environment has renewed the interest and the research on Vertical Axis Wind Turbines (VAWTs). The VAWT has an inherent unsteady aerodynamic behavior due to the variation of angle of attack and perceived velocity with azimuth angle. The phenomenon of dynamic stall is then an intrinsic effect of the operation at low tip speed ratios, impacting both loads and power. The complexity of the problem and the need for new design approaches for VAWTs for the built environment have driven the authors to focus this research on the CFD modeling of VAWTs on:

• Comparing the results between commonly used turbulence models: Unsteady Reynolds Averaged Navier‐Stokes – URANS (Spalart‐Allmaras and k‐?) and large eddy models (Large Eddy Simulation and Detached Eddy Simulation).
• Verifying the sensitivity of the model to its grid refinement (space and time).
• Evaluating the suitability of using Particle Image Velocimetry (PIV) experimental data for model validation.

The current work investigates the impact of accurately modeling the separated shed wake resulting from dynamic stall, and the importance of validation of the flow field rather than validation with only load data. The structure and magnitude of the wake are validated with PIV results, and it demonstrated that the accuracy of the different models in simulating a correct wake structure has a large impact in loads.     

PIV measurements and CFD simulation of the performance and flow physics and of a small‐scale vertical axis wind turbine

The aerodynamics generated by a small small‐scale vertical axis wind turbine are illustrated in detail as a NACA0022 rotor blade carries out a complete rotation at three tip speed ratios. These aerodynamic details are then linked to the wind turbine performance. This is achieved by using detailed experimental measurements of performance and near‐blade particle image velocimetry (PIV) and also by using a two‐dimensional Reynolds‐averaged Navier–Stokes‐based computational fluid dynamics (CFD) model. Uniquely, therefore, the CFD model is validated against both PIV visualizations and performance measurements. At low tip speed ratios ( λ = 2), the flow field is dominated by large‐scale stalling behaviour as shown in both the experimental results and simulations. The onset of stall appears to be different between the experiment and simulation, with the simulation showing a gradual separation progressing forward from the trailing edge, while the experiment shows a more sudden leading‐edge roll‐up. Overall, similar scales of vortices are shed at a similar rate in both the experimental results and simulations. The most significant CFD–PIV differences are observed in predicting flow re‐attachment. At a higher tip speed ratio ( λ = 3), the flow separates slightly later than in the previous condition, and as occurs in the lower tip speed ratio, the main differences between the experiment and the simulation are in the flow re‐attachment process, specifically that the simulations predicts a delay in the process. At a tip speed ratio of 4, smaller predicted flow separation in the latter stages of the upwind part of the rotation is the main difference in comparison to the experiment. Copyright © 2013 John Wiley & Sons, Ltd.     

风力机振荡翼型动态失速特性的CFD研究

为采用CFD方法研究风力机振荡翼型出现的流动动态失速现象,首先依据实验结果,对比分析了Baldwin-Lomax、Johnson-King、K-ωSST、S-A和K-ω5种紊流模型计算结果,结果表明:K-ωSST模型由于考虑到分离流动的非平衡作用,针对动态失速的计算精度较高。通过对NACA0012翼型振荡运动的流场进行计算,详细分析了振荡运动过程中翼型表面分离涡的形成、放大直至造成翼型失速以及失速区涡结构。     

CFD simulations of the MEXICO rotor

The wake behind a wind turbine model is investigated using Computational Fluid Dynamics (CFD), and results are compared with measurements. The turbine investigated is the three‐bladed test rotor (D = 4.5 m) used in the Model Experiments in Controlled Conditions (MEXICO) wind tunnel experiment. During the MEXICO experiment, particle image velocimetry measurements of the induction upstream and downstream of the rotor were performed for different operating conditions, giving a unique dataset to verify theoretical models and CFD models. The present paper first describes the efforts in reproducing the experimental results using the Reynold‐Averaged Navier‐Stokes method. Second, three‐dimensional airfoil characteristics are extracted that allow simulations with simpler wake models. Copyright © 2011 John Wiley & Sons, Ltd.     

Comparison of pitching and plunging effects on the surface pressure variation of a wind turbine blade section

Numerous experiments were conducted on an oscillating airfoil in a subsonic wind tunnel. The experiments involved measuring the surface pressure distribution when the model oscillated in two types of motion, pitch and plunge, at three different Reynolds numbers, 0.42, 0.63 and 0.84 million, and over a range of reduced frequencies, k = 0.03–0.09. The unsteady aerodynamic loads were calculated from the surface pressure measurements, 64 ports, along the chord for both upper and lower surfaces of the model. Particular emphasis was placed on the effects of different types of motion on the unsteady pressure distribution of the airfoil at pre‐stall, near‐stall and post‐stall conditions. It was found that variations of the pressure distribution and aerodynamic loads with angle of attack were strongly sensitive to the displacement, oscillation frequency and mean angle of attack. The width of the hysteresis loop, position of the ‘figure‐8 shape’ and slope of the pressure coefficient curve are influenced by both types of motion, pitch and plunge. The main difference between plunging and pitching motions is due to the presence of the pitch rate for the pitching motion case, which was absent in the plunging case. Pitch rate had the strongest influence on pressure data in the near‐stall and post‐stall conditions. The trend of increasing the width of the hysteresis loops of lift coefficients with changing reduced frequency was different in two motions in the pre‐stall and post‐stall regions. The aerodynamic damping was greater for the pitching case than for the plunging one at higher reduced frequencies due to the existence of the pitch rate in the pitching oscillation, which was reversed at lower reduced frequencies. In the near‐stall region, at higher reduced frequency, the dynamic stall angle for the pitching oscillation increased while for the plunging one the effect was minimal. Increasing the oscillation amplitude was more effective for the plunging motion than for the pitching one. The effects of surface grit roughness on the pressure signature for both types of motion were also investigated. Applying the surface roughness near the leading edge affected the performance of the airfoil significantly. Copyright © 2008 John Wiley & Sons, Ltd.     

On the S809 airfoil's unsteady aerodynamic characteristics

The S809 airfoil dynamic characteristics, which are based on the airfoil dynamic tests at the University of Glasgow, are presented in this paper. The airfoil tests include static, ramp‐type (ramp‐up and ramp‐down) and oscillatory motions at Reynolds numbers of 1.0 × 10⁶ and 1.5 × 10⁶ with and without the sand‐tripped leading edge. This study aimed to explore the unsteady aerodynamic features of the S809 airfoil, such as the progression of separation from leading edge to trailing edge, the large trailing edge separation before stalling, the stall onset inception and the re‐attachment convection, and to provide some useful data for tuning/refining the semi‐empirical dynamic stall (DS) models, such as the Leishman–Beddoes DS model or its variations. Experimental results show that the S809 airfoil has a complicated DS process that renders this airfoil a challenge for any modeller of the unsteady airloads. The leading‐edge roughness has small effect on the static features, but significantly invokes earlier stall onset inception under dynamic conditions, while has small influence on the convective phase of the re‐establishment of fully attached flow. Copyright © 2009 John Wiley & Sons, Ltd.