Wind Turbine Blade Section Research Articles

Active load control of wind turbine blade section with trailing edge flap: Wind tunnel testing

In order to reduce the cost of energy, the rotor diameter of wind turbine has been continuously increased. Fatigue load becomes more important as wind turbines increase in size. The reduction of fatigue load can result in increased component lifetimes, reduced maintenance requirements, and an overall lower cost of energy. In this article, a load reduction analysis and experiment are performed to show the load reduction abilities of an active aerodynamic load control device, namely trailing edge flaps, in the operation of large wind turbines. Through numerical simulations that use trailing edge flaps as active aerodynamic load control devices on wind turbines, a 30%–50% reduction in the standard deviation oscillations of root bending moment from the mean value was achieved. To validate the active load control analysis and show the potential of the trailing edge flap for load reduction, an active load control experiment of the wind turbine blade was performed. The experimental model was developed using a non-rotating blade equipped with a trailing edge flap and load-cell sensors to facilitate feedback control. A controller is based on the proportional–derivative control in order to minimize the blade root load variation in the flapwise direction. Through the experiment, a 20%–30% reduction in the standard deviation oscillations of blade root load from the mean value was achieved.

풍력 블레이드의 결빙에 의한 공력특성 및 성능 변화

추운 기후에서 운영되는 풍력터빈 시스템의 표면에 발생한 결빙으로 인하여 공기역학적 성능이 크게 저하될 수 있다. 이러한 결빙은 양력감소 및 항력증가를 야기하고, 발전효율에 부정적인 영향을 미치게 된다. 이로 인하여 풍력발전기의 성능저하 또는 과부하, 무게중심의 변화에 따른 과도진동, 결빙파편이 지상으로 떨어질 경우의 안전성 문제, 계기의 결빙으로 인한 계기 측정오차, 최악의 경우 풍력 시스템 정지 등의 문제가 발생한다. 본 연구에서는 결빙증식이 풍력 발전기의 공력특성에 미치는 영향을 CFD 기법을 이용해 분석하였다. 또한 결빙증식 결과를 바탕으로 BEM 기법을 적용시켜 삼차원 블레이드에 대한 공력성능을 계산하였다. 결빙의 두께는 상대적인 속도차이에 의해 블레이드 중심에서 끝단으로 갈수록 증가함을 알 수 있었고, 공기의 속도가 결빙증식에 미치는 주요 인자임을 확인하였다. A significant degradation in the aerodynamic performance of wind turbine system can occur by ice accretion on the surface of blades operated in cold climate. The ice accretion can result in performance loss, overloading due to delayed stall, excessive vibration associated with mass imbalance, ice shedding, instrumental measurement errors, and, in worst case, wind turbine system shutdown. In this study, the effects of ice accretions on the aerodynamic characteristics of wind turbine blade sections are investigated on the basis of modern CFD method. In addition, the computational results are used to predict the performance of three-dimensional wind turbine blade system through the blade element momentum method. It is shown that the thickness of ice accretion increases from the root to the tip and the effects of icing conditions such as relative wind velocity play significant role in the shape of ice accretion.

Effects of nonuniform incident velocity on a dynamic wind turbine airfoil

AbstractFor further insight into the performance of a horizontal axis wind turbine blade section under yaw loads, a 2D numerical simulation of a pitching S809 airfoil under dynamic stall with an unsteady incident velocity is presented. The streamwise incident velocity and pitch angle of incidence oscillated with the same frequency but with a range of phase differences, − π ≤ Φ ≤ π. Changing Φ caused variation of the results, which can be highlighted as significantly augmented and dramatically damped dynamic stall loads, both increasing and decreasing trends for vortex growth time during Φ increase, a shifted location of the maximum loads and a change in the order of the vortex pair circulation in each cycle. The results showed strong dependency on the velocity and acceleration of the freestream during dynamic stall, which categorized the results in four individual subdomains with different behaviors. Copyright © 2014 John Wiley & Sons, Ltd.

Updating Finite Element Model of a Wind Turbine Blade Section Using Experimental Modal Analysis Results

This paper presents selected results and aspects of the multidisciplinary and interdisciplinary research oriented for the experimental and numerical study of the structural dynamics of a bend-twist coupled full scale section of a wind turbine blade structure. The main goal of the conducted research is to validate finite element model of the modified wind turbine blade section mounted in the flexible support structure accordingly to the experimental results. Bend-twist coupling was implemented by adding angled unidirectional layers on the suction and pressure side of the blade. Dynamic test and simulations were performed on a section of a full scale wind turbine blade provided by Vestas Wind Systems A/S. The numerical results are compared to the experimental measurements and the discrepancies are assessed by natural frequency difference and modal assurance criterion. Based on sensitivity analysis, set of model parameters was selected for the model updating process. Design of experiment and response surface method was implemented to find values of model parameters yielding results closest to the experimental. The updated finite element model is producing results more consistent with the measurement outcomes.

Aeroelastic Control of a Wind Turbine Blade Using Microtabs Based on UA97W300-10 Airfoil

The unsteady flow over a rotating wind turbine blade is described by the Beddoes-Leishman dynamic stall model. Based on the asymmetric airfoil UA97W300-10, a wind turbine blade section with unsteady aerodynamic loads is modeled as an aeroelastic system for this vibration suppression study. Aeroelastic stability of the system is analyzed by open-loop tests to identify the flutter speed. With consideration of wind turbine operations, control strategies for flutter suppression are made separately in each operation region and implemented by a designed Adaptive Controller using microtab devices, which are positioned at the trailing edge of the airfoil with deployment at the maximum height of 5% of chord length. The control effects of the Adaptive Control and microtabs are investigated by closed-loop tests. The stability of the Adaptive Controller is proved using a new Adaptive Stability Theorem, which is also illustrated by examples.

Vortex shedding from a wind turbine blade section at high angles of attack

The unsteady flow around a stationary two-dimensional wind turbine blade section (NREL S809) has been simulated using unsteady RANS with the SST turbulence model at Re=106 and high angles of attack. Vortex shedding frequency non-dimensionalised by the projected area of the blade is in the range 0.11<St<0.16. The fluctuating coefficients at the harmonics of the fundamental vortex shedding frequency for lift, drag and pitching moment are presented. The lift force and the pitching moment (calculated with respect to the mid-chord) are dominated by the fundamental vortex shedding frequency, but significant contributions at the higher harmonics are also present. For the drag the second harmonic is as significant as the fundamental frequency, with the third and fourth harmonics contributing to a lesser extent. From the results, it is inferred that the camber of the airfoil does not substantially affect the force coefficients, suggesting that the results presented may be generally applicable to turbine blades of different geometries.

Effect of leading-edge roughness on boundary layer transition of an oscillating airfoil

A series of experiments were conducted to study the effect of leading-edge roughness on the state of the boundary layer of a wind turbine blade section using multiple hot-film sensors. The experiments involved static and dynamic tests, where airfoil motion was of plunging type oscillation. The application of surface grit roughness simulates surface irregularities that occur on the wind turbine blades.The measurements showed that increasing the angle of attack results in movement of transition locations toward the leading edge. Surface roughness moved the transition point toward the leading edge and caused early trailing edge turbulent separation, which resulted in reducing the effectiveness of the airfoil. Boundary layer instability frequencies were dominated through the transition.

외부 오염물 증착에 의한 풍력 터빈 날개 단면의 공력 성능 저하 예측

In the present study, the effects of contaminant accumulation and surface roughness on the aerodynamic performance of wind turbine blade sections were numerically investigated by using a flow solver based on unstructured meshes. The turbulent flow over the rough surface was modeled by a modified <TEX>${\kappa}-{\omega}$</TEX> SST turbulence model. The calculations were made for the NREL S809 airfoil with varying contaminant sizes and positions at several angles of attack. It was found that as the contaminant size increases, the degradation of the airfoil performance becomes more significant, and this trend is further amplified near the stall condition. When the contaminant is located at the upper surface near the leading edge, the loss in the aerodynamic performance of the blade section becomes more critical. It was also found that the surface roughness leads to a significant reduction of lift, in addition to increased drag.

Calculation of effective section stiffness properties for wind turbine blades using homogenization

The structural behavior of wind turbine blades can be accurately modeled using beam models. The accuracy in the predictions using beam models depends on the degree of accuracy in the prediction of the effective section properties. A simple and efficient method compared with full 3D analysis is developed for the prediction of effective section properties for prismatic slender members of any arbitrary cross-section shape. The technique, which is based on the superposition principle, is developed to include the shear deformation modes in predicting the effective stiffness properties. A homogenization scheme based on strain energy equivalence is employed for calculating the effective properties. The method can accurately predict the full stiffness matrix including the off-diagonal terms. Also a procedure for finding the locations of the weighted centroid and the shear center is presented. The results for four different cross-section shapes including a wind turbine blade section are validated using an existing analysis technique, Variational Asymptotic Beam Sectional Analysis. Copyright © 2012 John Wiley & Sons, Ltd.

Effect of Plunging Oscillation on an Offshore Wind Turbine Blade Section

Effect of Plunging Oscillation on an Offshore Wind Turbine Blade Section

Numerical investigation of rotational augmentation for S822 wind turbine airfoil

ABSTRACTDirect numerical simulations were carried out for an S822 wind turbine blade section at a chord Reynolds number of Re = 100, 000 and an angle of attack of α = 5°. Results for a stationary non‐rotating blade section compare favorably with wind tunnel data by the University of Illinois at Urbana‐Champaign and XFoil predictions. By adding volume forcing terms to the right‐hand side of the Navier–Stokes equations, the Coriolis and centrifugal accelerations resulting from blade rotation are modeled in the blade section simulations. Blade rotation is shown to delay separation especially near the hub, resulting in a lift increase of up to 100% and a drag reduction. The simulations provide insight into a physical mechanism that offers an explanation for the lift increase observed for rotating blade sections when compared with stationary blade sections, which is commonly referred to as rotational augmentation. Rotation is shown to lead to a radial velocity component toward the blade tip in areas where the velocity is substantially different from its free‐stream value, such as near the stagnation point and especially in the separated flow region, and to the appearance of stationary and traveling crossflow vortices. A linear stability theory analysis that compares favorably with the simulation data provides proof that the primary instabilities are of a mixed type, including both a two‐dimensional mode (Tollmien–Schlichting and Kelvin–Helmholtz type) and a stationary and unsteady crossflow mode. The crossflow instabilities accelerate transition, leading to separation delay, lift increase and drag reduction. This effect is very pronounced at 20% blade radius and still present at 80% radius. Because periodicity conditions were applied in the spanwise direction, the present results provide an explanation for rotational augmentation that is not based on the transfer of fluid from the inboard region toward the blade tip (‘centrifugal pumping’). For the low Reynolds number conditions considered here, crossflow instabilities, which destabilize the flow leading to earlier transition and a separation delay, may contribute to rotational augmentation. Copyright © 2012 John Wiley &amp; Sons, Ltd.

High accuracy mobile robot positioning using external large volume metrology instruments

A method of accurately controlling the position of a mobile robot using an external large volume metrology (LVM) instrument is presented in this article. By utilising an LVM instrument such as a laser tracker or indoor GPS (iGPS) in mobile robot navigation, many of the most difficult problems in mobile robot navigation can be simplified or avoided. Using the real-time position information from the laser tracker, a very simple navigation algorithm, and a low cost robot, 5 mm repeatability was achieved over a volume of 30 m radius. A surface digitisation scan of a wind turbine blade section was also demonstrated, illustrating possible applications of the method for manufacturing processes. Further, iGPS guidance of a small KUKA omni-directional robot has been demonstrated, and a full scale prototype system is being developed in cooperation with KUKA Robotics, UK.

Experimental Study of the Plunging Motion with Unsteady Wind Tunnel Wall Interference Effects

A series of experiments were carried out to investigate unsteady behavior of the flow field as well as the boundary layer of an airfoil oscillating in plunging-type motion in a subsonic wind tunnel. The measurements involved surface-mounted hot films complimented with surface pressure. In addition, wind tunnel wall pressure distribution was acquired to furnish a baseline for the wall interference corrections. The airfoil is the section of a 660-kW wind turbine blade. The experiments were conducted at a Reynolds number of 0.42 million, and over two reduced frequencies of k = 0.06 and 0.085, at prestall, nearstall, and poststall regions. 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. The plunging displacements were transformed into the equivalent angle of attack. The surface hot-film measurements provided information about the boundary-layer events. The boundary-layer transition occurred via a laminar separation bubble. Variations of the surface pressure coefficients and aerodynamic loads with the equivalent angle of attack showed strong sensitivity to the reduced frequency and the mean angles of attack. The wall pressure distribution was affected by the model oscillation especially the region underneath the model.

Evaluation of active flow control applied to wind turbine blade section

A feasibility study for implementing active flow control (AFC) methods to improve the performance of wind turbines was performed. The experimental effort investigated the impact of zero-mass-flux (ZMF) piezofluidic actuators attempting to control boundary layer separation from thick airfoils that are suitable for wind turbine rotor blades. It was demonstrated that the ZMF actuators can replace passive vortex generators that are commonly used for boundary layer separation delay, without the inherent drag penalty that the passive devices impose. It has been shown that ZMF fluidic actuators are suitable for flow control in wind turbine application due to the fact that they are adjustable for wider Reynolds number range, while vortex generators are tuned to perform well in one design point. It was demonstrated that AFC can effectively double the maximum lift of this airfoil at low Reynolds numbers. A possible application is a significant reduction of the turbine start-up velocity. It was also found that even for a contaminated blade, AFC is capable to delay the stall and decrease the drag using low energy expenditure, therefore restoring and even surpassing the clean airfoil performance. The effectiveness of the AFC method was examined using a newly defined aerodynamic figure of merit. Various scaling options for collapsing the effect of the excitation magnitude on the lift alternation due to the activation of zero-mass-flux periodic excitation for boundary layer separation control are proposed and examined using experimental data.

Design of a new urban wind turbine airfoil using a pressure-load inverse method

This paper presents the design methodology of a new wind turbine airfoil that achieves high performance in urban environment by increasing the maximum lift. For this purpose, an inverse method was applied to obtain a new wind turbine blade section with constant pressure-load along the chord, at the design inlet angle. In comparison with conventional blade section designs, the new airfoil has increased maximum lift, reduced leading edge suction peak and controlled soft-stall behaviour, due to a reduction of the adverse pressure gradient on the suction side. Wind tunnel experimental results confirmed the computational results.

RESEARCH OF THE ROTATIONAL EFFECTS ON THE BOUNDARY LAYER OF WIND TURBINE BLADES

The flow field past the rotating blade of a horizontal axial wind turbine has been modeled with a full 3–D steady–RANS approach. Flow computations have been performed using the commercial finite–volume solver Fluent. The NREL phase VI wind turbine blade sections from the 3–D rotating geometry were chosen and the corresponding 2–D flow computations have been carried out for comparison with different angles of attack and in stalled conditions. The simulation results are analyzed. The main features of the boundary layer flow are described, for both the rotating blade and the corresponding 2–D profiles. Computed pressure distributions and aerodynamic coefficients show evidence of less lift losses after separation in the 3–D rotating case, mostly for the inward sections of the blade and the highest angles of attack, which is in agreement with the literature.

Model-Based Control of SMA Actuators with a Recurrent Neural Network in the Shape Control of an Airfoil

This paper describes the development of a neural network control for shape memory alloy (SMA) actuators as well as the control tests. Precise control of SMA actuators that are integrated in the composite structure is difficult because of the hysteretic behaviour of SMA and time delays in the control system. The weakness of the static model of SMA is that it does not take into account the actuator dynamics. The NARMA-L2 neural network model enables us to solve the inverse dynamic model of the nonlinear discrete-time dynamical system. In this study the NARMA-L2 model of the airfoil (a cross section of wind turbine blade) was created using Matlab Simulink software and neural networks. Data for network training was measured from the specimen. The performance of the constituted NARMA-L2 controller was tested in control tests. According to the control tests the NARMA-L2 controller is well suited for controlling embedded SMA actuators. The constituted controller enabled fast and stable actuation in step responses and it was also able to compensate for changes in cooling conditions.

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

AbstractNumerous 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 &amp; Sons, Ltd.

Shape Control of a FRP Airfoil Structure Using SMA-Actuators and Optical Fiber Sensors

The aim of this work was to design a control system for the shape memory alloy (SMA) actuator operated airfoil (a cross section of wind turbine blade). Design of SMA control is focused on the reliable operation of the SMA actuators. The actuator should follow the targeted shape accurately and without too much delay. Another objective is to avoid overheating which is the most critical damage to the structure. SMA actuator shape control is in principle possible to do with any position control method, but the specific properties of the SMA actuators, like the hysteresis, the first cycle effect and the long term changes, need to be taken into account. In this work, a wing profile prototype was measured using optical fiber sensors and traditional strain gauges. Also, external laser sensors were used to measure displacements of upper/lower surface and trailing edge. Shape change was obtained by embedding SMA wire actuators into fiber reinforced polymer (FRP) composite structure. SMA actuators were laminated in such way that bending of trailing edge is always downwards. Actuators are activated with Joule heating and the temperature is measured with integrated thermocouples and optical fiber temperature sensors. As a result, this work gave information about the usability of optical fibers sensors in active FRP composite structures. Measurements also give information about the efficiency of SMA actuators in shape control of relatively stiff FRP structures.

The Comparison of Form Drag and Profile Dragof a Wind Turbine Blade Section in Pitching Oscillation

The Comparison of Form Drag and Profile Dragof a Wind Turbine Blade Section in Pitching Oscillation