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Kevlar-walled aeroacoustic wind tunnels have become important research facilities to evaluate low-noise wind turbine designs. In this paper a semi-analytical Green’s function for the transmission of sound through Kevlar walls and shear layers was developed in order to provide accurate acoustic measurements at high flow speeds. The semi-analytical Green’s function was evaluated experimentally with a monopole sound source. It was possible to find the correct source location when the Green’s function was used to formulate the steering vectors for the beamforming technique. The corrected steering vectors provided a much better source level estimation than if a free-field Green’s function was used to formulated the steering vectors, but there was a systematic overcorrection at low frequencies and an undercorrection at high frequencies. The analytical transmission loss was very sensitive to the convection velocity of the vorticity shed at the membrane apertures, which in turn depended on the aerodynamic loading of the Kevlar walls. It is likely that further fine-tuning of the formula of the convection velocity will reduce the systematic error. The experimental setup to validate the Green’s function could be improved if aerodynamic loading was applied to the walls.

Surface erosion of wind turbine blades is one of rather critical problems of the wind energy development. In this overview paper, recent studies in the mechanisms, modelling and possibilities of preventing the surface erosion of wind turbine blades are discussed. Latest research in the area of leading edge erosion (LEE) from different viewpoints, based on meteorology, aerodynamics, materials science, computational mechanics are summarized. Technologies of experimental testing of anti-erosion coatings, effect of leading edge erosion on aerodynamics of wind turbines, roughness and its evolution are discussed, as well as meteorological aspects, parameters and characteristics of precipitation, possibilities of prediction of rain and hail and regional differences. Further, computational models of LEE and directions of the development of anti-erosion coatings are presented.

We present an analytical model for assessing the aerodynamic performance of a wind turbine rotor though a different parametrization of the the classical Blade Element Momentum (BEM) model. The model is named the Radially Independent Actuator Disc model (RIAD) and it establishes an analytical relationship between the local-thrust loading and the local-power, known as the Local-Thrust-Coefficient and the Local-Power-Coefficient respectively. The model has a direct physical interpretation, showing the contribution for each of the 3 losses: wake-rotation-loss, tip-loss and viscous-loss. The gradients for RIAD is found through the use of the Complex-step-method and power optimization is used to show how easily the method can be used for rotor optimization. The main benefit of RIAD is the ease at which it can be applied for rotor optimization, and especially load constraint power optimization as it is described in Loenbaek et al. (2020). The relationship between the RIAD input and the rotor chord and twist is established and it is validate against a BEM solver.

A novel wind turbine rotor optimization methodology is presented. Using an assumption of radial independence it is possible to obtain an optimal relationship between the global power- (CP) and load-coefficient (CT, CFM) through the use of KKT-multipliers, leaving an optimization problem that can be solved at each radial station independently. It allows to solve load constraint power and Annual-Energy-Production (AEP) optimization problems where the optimization variables are only the KKT-multipliers (scalars), one for each of the constraint. For the paper two constraints, namely the thrust and blade-root-flap-moment is used, leading to two optimization variables. Applying the optimization methodology to maximize power (P) or Annual-Energy-Production (AEP) for a given thrust and blade-root-flap-moment, but without a cost-function, leads to the same overall result with the global optimum being unbounded in terms of rotor radius (R~) with a global optimum being at R~ → ∞. The increase in power and AEP is in this case ΔP = 50 % and ΔAEP = 70 %, with a baseline being the Betz-optimum rotor. With a simple cost function and with the same setup of the problem a Power-per-Cost (PpC) optimization resulted in a Power-per-Cost increase of ΔPpC = 4.2 % with a radius increase of ΔR = 7.9 % as well as a power increase of ΔP = 9.1 %. This was obtained while keeping the same flap-moment and reaching a lower thrust of ΔT = −3.8 %. The equivalent for AEP-per-Cost (AEPpC) optimization leads to an increased cost efficiency of ΔAEPpC = 2.9 % with a radius increase of a ΔR = 17 % and an AEP increase of ΔAEP = 13 %, again with the same, maximum flap-moment, while the maximum thrust is −9.0 % lower than the baseline.

The wind tunnel with Kevlar walls is becoming more and more popular for aeroacoustic measurements, because the necessary corrections to convert aerodynamic coefficients to free field values are moderate and at the same time the signal to noise ratio of acoustic measurements are in the same order as in an open jet configuration. Yet, the technology is new and the correction methods for aerodynamic and acoustic coefficients are not well established. This paper aims on bench-marking two of the largest Kevlar-walled wind tunnels by cross validation of the measurements performed on a NACA633018 aerofoil model. The measured aerodynamic coefficients showed and excellent agreement in the attached flow regime. Maximum lift was predicted differently in the two facilities. The differences might be due to three-dimensional effects for separated flow and the fact that the aspect ration was different in the two facilities. The difference of the measured overall sound pressure levels was less ± 1 dB for attached flow conditions. However, we observed a difference in the slope of the sound pressure spectra and recommend to further investigate the acoustic correction methods.

In this paper a simple model to predict losses in annual energy production due to leading edge roughness or leading edge erosion from wind turbines is presented. Because detailed information about wind turbines not always are available, the input to this model requires only a few parameters: rated power, rotor radius, two Weibull parameters and categories of local damages. Therefore, some assumptions are made in the model. It was validated against a common implementation of a BEM code and showed good agreement in the loss of annual energy production. Also, a categorisation methods was proposed. The categories ranged between a and e corresponding to an undamaged blade and a severely damaged blade, respectively. This categorisation was made locally on the blade. The blade is divided into annular elements and each annular element is graded with a category. By summing up the annular elements on the blade, each with their categories, the total loss in energy production was predicted.

This paper describes a benchmark of four airfoils in the Poul la Cour Tunnel (PLCT). The wind tunnel, the corrections used and the method of making adapters for the airfoils are also described. Very good agreement was in general observed between the measurements in PLCT and in other high quality wind tunnels. Some deviations were seen, but they were mainly attributed to the differences in separation on the airfoil. Apart from the benchmarking, this paper also highlights the challenges in testing airfoils in general such as obtaining 2D flow on thick airfoils that inherently shows separated flow and how to make adapters for airfoils tested in other wind tunnels.

We investigate the optimal relationship between the aerodynamic power, thrust loading, and size of a wind turbine rotor when its design is constrained by a static aerodynamic load. Based on 1D-axial momentum theory, the captured power P~ for a uniformly loaded rotor can be expressed in terms of the rotor radius R and the rotor thrust coefficient CT. Common types of static Design Driving Load Constraints (DDLC), e.g. limits on permissible root-bending moment or tip deflection, may be generalized into a form that also depends on CT and R. Using these relationships to maximize P~ subject to a DDLC, shows that operating the rotor at the Betz limit (maximum CP) does not lead to the highest power capture. Rather, it is possible to improve performance with a larger rotor radius and lower CT without violating the DDLC. As an example, a rotor design driven by a tip-deflection constraints, may achieve 1.9 % extra power capture P~ compared to the baseline (Betz limit) rotor. The method is extended for optimization of rotors with respect to Annual Energy Production (AEP), where the thrust characteristics CT(V) needs to be determined together with R. This results in much higher relative potential for improvements, since the constraint limit can be met over a larger range of wind speeds. For example, a relative gain in AEP of +5.7 % is possible for a rotor design constrained by tip deflections compared with a rotor designed for optimal CP. The optimal solution for AEP leads to a thrust curve with three distinct operational regimes and so called thrust-clipping.

The present study investigates the differences in the predicted lift for a wind tunnel model from either surface pressures or side wall pressures. The investigated model is the NACA63-418 tested in the Poul la Cour tunnel. The measurements are compared with previous conducted wind tunnel campaigns for a Reynolds number of 3E6. The 3D CFD model of the wind tunnel will be used for investigating details of the flow mechanisms, and to validate assumptions and findings from the experimental data. The CFD simulations show that when the flow is attached the lift based on the side wall pressures results in almost the same lift as calculated from the surface pressures. Around the maximum lift and in the stalled region the flow is not anymore 2D. For the NACA63-418 airfoil the lift based on the side wall pressures are generally highest. The CFD simulations confirm that the method used to predict the lift from the limited side wall pressure strips in the wind tunnel is applicable.

We analyze the new Danish Poul la Cour Tunnel (PLCT) facility using 2D and 3D Computation Fluid Dynamics simulations, with the aim to address the need for wind tunnel corrections. The assessment of tunnel corrections is based on 2D free simulations representing undisturbed airfoil flow, 2D tunnel configurations and a full 3D tunnel setup using laminar/turbulent Improved Delayed Dettached Eddy Simulations. By comparing lift, drag and pressure distributions and detailled flow patterns, the influence of the 2D and 3D tunnel is evaluated. In the present study a NACA 633-418 airfoil is investigated in the tunnel.

This study investigates how losses in energy production from wind turbines are influenced by leading edge roughness, rotor control and wind climates using computations. The performance of a NACA633-418 airfoil with five different damage types was predicted using Computational Fluid Dynamics (CFD) with bumps at the leading edge and grooves into the leading edge. Also, one type corresponding to a repair of a leading edge with an “overbite” was investigated. These airfoil characteristics were used in rotor computations where three different wind climates and five different maximum tip speeds were investigated. The rotor computations reflected that the Annual Energy Production (AEP) increased significantly with the average wind speed on the site. They also reflected that the relative AEP losses due to leading edge damages reduced with increasing average wind speed on sites. For the low wind speed site the losses were between 1% and 4% depending on the extent and the type of the blade damage. For the high wind speed site the losses were between 0.5% and 3%. Furthermore, the bigger the extent of the damages were the bigger the losses were. Finally, it was shown that increasing the maximum tip speed increased AEP and decreased the losses.

One of the main uncertainties in airfoil and rotor design is the determination of the position of transition on the blade as this can have considerable influence on the aerodynamic load sand in particular rotor power. Even for simulations on a rotor in uniform, steady inflow the determination of the position of transition is challenging. The atmospheric inflow on a real MW rotor adds a new level of complexity and uncertainty to the transition modelling and prediction.What is the impact of the atmospheric turbulence on transition compared with transition on a blade section in wind tunnel flow ? The present work is aiming at improving our insight into this subject by analysing transition data measured in the DanAero experiment in 2009 on an industrial 2MW turbine with an 80m rotor with LM38.8 m blades. Transition position is determined from analysing spectra of high frequency (50kHz) surface pressure fluctuations from 56 flush mounted microphones at a radius of 37 m. The position of transition is correlated with measured inflow angle close to the section with transition data. A comparison of the rotor transition with a similar set-up in the LM Glasfiber wind tunnel for transition detection on a 2D blade section copy of the rotor blade, shows a much earlier transition on the pressure side of the rotor blade. On the suction side the difference is much smaller due to an already early transition on the 2D blade section in the wind tunnel, probably caused by small surface irregularities or bumps. Analysing the pressure fluctuations in the laminar boundary layer close to the leading edge which are closely correlated with the turbulent inflow it can be seen that below 300 Hz the energy level on the rotor is much higher than in the tunnel. However,above 300 Hz they are comparable. It is finally shown that the transition on the rotor on the pressure side is moving forward when the spectral energy from 100-300 Hz in the laminar boundary layer increases.

Cold tolerance of insects is arguably among the most important traits defining their geographical distribution. Even so, very little is known regarding the causes of cold injury in this species-rich group. In many insects it has been observed that cold injury coincides with a cellular depolarization caused by hypothermia and hyperkalemia that develop during chronic cold exposure. However, prior studies have been unable to determine if cold injury is caused by direct effects of hypothermia, by toxic effects of hyperkalemia, or by the depolarization that is associated with these perturbations. Here we use a fluorescent DNA-staining method to estimate cell viability of muscle and hindgut tissue from Locusta migratoria and show that the cellular injury is independent of the direct effects of hypothermia or toxic effects of hyperkalemia. Instead, we show that chill injury develops due to the associated cellular depolarization. We further hypothesized that the depolarization-induced injury was caused by opening of voltage-sensitive Ca(2+) channels, causing a Ca(2+) overload that triggers apoptotic/necrotic pathways. In accordance with this hypothesis, we show that hyperkalemic depolarization causes a marked increase in intracellular Ca(2+) levels. Furthermore, using pharmacological manipulation of intra- and extracellular Ca(2+) concentrations as well as Ca(2+) channel conductance, we demonstrate that injury is prevented if transmembrane Ca(2+) flux is prevented by removing extracellular Ca(2+) or blocking Ca(2+) influx. Together these findings demonstrate a causal relationship between cold-induced hyperkalemia, depolarization, and the development of chill injury through Ca(2+)-mediated necrosis/apoptosis.

The aim of this work is to investigate and characterize the high-frequency surface pressure fluctuations on a full-scale wind turbine blade and in particular the influence of the atmospheric turbulence. As these fluctuations are highly correlated to the sources of both turbulent inflow noise and trailing edge noise, recognized to be the two main sources of noise from wind turbines, this work contributes to a more detailed insight into noise from wind turbines. The study comprises analysis and interpretation of measurement data that were acquired during an experimental campaign involving a 2 MW wind turbine with a 80 m diameter rotor as well as measurements of an airfoil section tested in a wind tunnel. The turbine was extensively equipped in order to monitor the local inflow onto the rotating blades. Further a section of the 38 m long blade was instrumented with 50 microphones flush-mounted relative to the blade surface. The measurements of surface pressure spectra are compared with the results of two engineering models for trailing edge noise and for turbulent inflow noise. The measured pressure fluctuations are related to the local inflow angle and are also compared to measurements in a wind tunnel on a copy of the blade section of the full scale blade. Computational Fluid Dynamics calculations were conducted to investigate the influence of the inflow conditions on the airfoil and blade sections aerodynamics and aeroacoustics. Comparisons between measurement data and model results show the influence of atmospheric turbulence. The different noise generation mechanisms can be identified and the influence of various parameters can be consistently reproduced by the models.

Wind turbine multidisciplinary design optimization is currently the focus of several investigations because it has showed potential in reducing the cost of energy. This design approach requires fast methods to evaluate wind turbine loads with a sufficiently high level of fidelity. This paper presents a method to estimate wind turbine fatigue damage suited for optimization design applications. The method utilizes a high-order linear wind turbine model. The model comprehends a detailed description of the wind turbine and the controller. The fatigue is computed with a spectral method applied to power spectral densities of wind turbine sensor responses to turbulent wind. In this paper, the model is validated both in time domain and frequency domain with a nonlinear aeroservoelastic model. The approach is compared quantitatively against fatigue damage obtained from the power spectra of time series evaluated with nonlinear aeroservoelastic simulations and qualitatively against rainflow counting. Results are presented for three cases: load evaluation at normal operation in the full wind speed range, load change evaluation due to two different controller tunings at normal operation at three different wind speeds above rated and load dependency on the number of turbulence seeds used for their evaluation. For the full-range normal operation, the maximum difference between the two frequency domain-based estimates of the tower base lateral fatigue moments is 36%, whereas the differences for the other sensors are less than 15%. For the load variation evaluation, the maximum difference of the tower base longitudinal bending moment variation is 22%. Such large difference occurs only when the change in controller tuning has a low effect on the loads. Furthermore, results show that loads evaluated with the presented method are less dependent on the turbulent wind realization; therefore, less turbulence seeds are required compared with time-domain simulations to remove the dependency on the wind realization used to estimate loads. Copyright © 2015 John Wiley & Sons, Ltd.

Impact fatigue caused by collision with rain droplets, hail stones and other airborne particles, also known as leading-edge erosion, is a severe problem for wind turbine blades. Each impact on the leading edge adds an increment to the accumulated damage in the material. After a number of impacts the leading-edge material will crack. This paper presents and supports the hypothesis that the vast majority of the damage accumulated in the leading edge is imposed at extreme precipitation condition events, which occur during a very small fraction of the turbine's operation life. By reducing the tip speed of the blades during these events, the service life of the leading edges significantly increases from a few years to the full expected lifetime of the wind turbine. This life extension may cost a negligible reduction in annual energy production (AEP) in the worst case, and in the best case a significant increase in AEP will be achieved.

Cold tolerance of insects is arguably among the most important traits defining their geographical distribution. Even so, very little is known regarding the causes of cold injury in this species-rich group. In many insects it has been observed that cold injury coincides with a cellular depolarization caused by hypothermia and hyperkalemia that develop during chronic cold exposure. However, prior studies have been unable to determine if cold injury is caused by direct effects of hypothermia, by toxic effects of hyperkalemia, or by the depolarization that is associated with these perturbations. Here we use a fluorescent DNA-staining method to estimate cell viability of muscle and hindgut tissue from Locusta migratoria and show that the cellular injury is independent of the direct effects of hypothermia or toxic effects of hyperkalemia. Instead, we show that chill injury develops due to the associated cellular depolarization. We further hypothesized that the depolarization-induced injury was caused by opening of voltage-sensitive Ca2+ channels, causing a Ca2+ overload that triggers apoptotic/necrotic pathways. In accordance with this hypothesis, we show that hyperkalemic depolarization causes a marked increase in intracellular Ca2+ levels. Furthermore, using pharmacological manipulation of intra- and extracellular Ca2+ concentrations as well as Ca2+ channel conductance, we demonstrate that injury is prevented if transmembrane Ca2+ flux is prevented by removing extracellular Ca2+ or blocking Ca2+ influx. Together these findings demonstrate a causal relationship between cold-induced hyperkalemia, depolarization, and the development of chill injury through Ca2+-mediated necrosis/apoptosis.

This work is concerned with the experimental study of airfoil stall and the modelling of stall noise. Using pressure taps and high-frequency surface pressure microphones flush-mounted on airfoils measured in wind tunnels and on an operating wind turbine blade, the characteristics of stall are analyzed. This study shows that the main quantities of interest, namely convection velocity, spatial correlation and surface pressure spectra, can be scaled highlighting the universal nature of stall independently of airfoil shapes and flow conditions, although within a certain range of experimental conditions. Two main regimes for the scaling of the correlation lengths and the surface pressure spectra, depending on the Reynolds number of the flow, can be distinguished. These results are used to develop a model for the surface pressure spectra within the detached flow region valid for Reynolds numbers ranging from 1 × 10 6 to 6 × 10 6 . Subsequently, this model is used to derive a model for stall noise. Modelled noise spectra are compared with experimental data measured in anechoic wind tunnels with reasonably satisfactory agreement.

A full-scale test was performed on a Vestas V27 wind turbine equipped with one active 70 cm long trailing edge flap on one of its 13 m long blades. Active load reduction could be observed in spite of the limited spanwise coverage of the single active trailing edge flap. A frequency-weighted model predictive control was tested successfully on this demonstrator turbine. An average flapwise blade root load reduction of 14% was achieved during a 38 minute test, and a reduction of 20% of the amplitude of the 1P loads was measured. A system identification test was also performed, and an identified linear model, from trailing edge flap angle to flapwise blade root moment, was derived and compared with the linear analytical model used in the model predictive control design model. Flex5 simulations run with the same model predictive control showed a good correlation between the simulations and the measurements in terms of flapwise blade root moment spectral densities, in spite of significant differences between the identified linear model and the model predictive control design model. Copyright © 2013 John Wiley & Sons, Ltd.

The blade element momentum (BEM) method is widely used for calculating the quasi-steady aerodynamics of horizontal axis wind turbines. Recently, the BEM method has been expanded to include corrections for wake expansion and the pressure due to wake rotation (BEMcor), and more accurate solutions can now be obtained in the blade root and tip sections. It is expected that this will lead to small changes in optimum blade designs. In this work, BEMcor has been implemented, and the spanwise load distribution has been optimized to find the highest possible power production. For comparison, optimizations have been carried out using BEM as well. Validation of BEMcor shows good agreement with the flow calculated using an advanced actuator disk method. The maximum power was found at a tip speed ratio of 7 using BEMcor, and this is lower than the optimum tip speed ratio of 8 found for BEM. The difference is primarily caused by the positive effect of wake rotation, which locally causes the efficiency to exceed the Betz limit. Wake expansion has a negative effect, which is most important at high tip speed ratios. It was further found that by using BEMcor, it is possible to obtain a 5% reduction in flap bending moment when compared with BEM. In short, BEMcor allows fast aerodynamic calculations and optimizations with a much higher degree of accuracy than the traditional BEM model. Copyright © 2011 John Wiley & Sons, Ltd.

Design loads on turbines are normally simulated with an aeroelastic model using an engineering BEM type model with the turbulent inflow generated with a turbulence model like the Mann model. There are several fundamental uncertainties in this approach, e.g. how well the unsteady induction in response to the turbulent flow is computed. However, within the last few years full 3D CFD rotor computations with turbulent inflow have been performed which can provide detailed insight into this complex load response. In the present work we present computations with the EllipSys3D solver on the 80m diameter NM80 turbine used in the DANAERO project where surface pressure measurements at four radial positions were conducted. The aerodynamic loads integrated from the pressure distributions have been derived and compared with computations by the aeroelastic code HAWC2. Overall a very good correlation is found by comparing PSD spectra of the measured sectional blade forces with HAWC2 simulations using specific flow input from the meteorology mast at six heights. In another comparison using purely turbulent inflow for the simulations on the NM80 rotor some deviations between the force spectra are found between EllipSys3D results and HAWC2 simulations at the inboard part of the blade and at high frequencies.

A comprehensive investigation of the Blade Element Momentum (BEM) model using detailed numerical simulations with an axis symmetric actuator disc (AD) model has been carried out. The present implementation of the BEM model is in a version where exactly the same input in the form of non-dimensional axial and tangential load coefficients can be used for the BEM model as for the numerical AD model. At a rotor disc loading corresponding to maximum power coefficient, we found close correlation between the AD and BEM model as concerns the integral value of the power coefficient. However, locally along the blade radius, we found considerable deviations with the general tendency, that the BEM model underestimates the power coefficient on the inboard part of the rotor and overestimates the coefficient on the outboard part. A closer investigation of the deviations showed that underestimation of the power coefficient on the inboard part could be ascribed to the pressure variation in the rotating wake not taken into account in the BEM model. We further found that the overestimation of the power coefficient on the outboard part of the rotor is due to the expansion of the flow causing a non-uniform induction although the loading is uniform. Based on the findings we derived two small engineering sub-models to be included in the BEM model to account for the physical mechanisms causing the deviations. Finally, the influence of using the corrected BEM model, BEMcor on two rotor designs is presented. Copyright © 2009 John Wiley & Sons, Ltd.

The present work contains a deformable trailing edge flap controller integrated in a numerically simulated modern, variablespeed, pitch-regulated megawatt (MW)-size wind turbine. The aeroservoelastic multi-body code HAWC2 acts as a component in the control loop design. At the core of the proposed controller, all unsteady loads are divided by frequency content. Blade pitching and generator moment react to low-frequency excitations, whereas flaps deal with high-frequency excitations. The present work should be regarded as an investigation into the fatigue load reduction potential when applying trailing edge flaps on a wind turbine blade rather than a conclusive control design with traditional issues like stability and robustness fully investigated. Recent works have shown that the fatigue load reduction by use of trailing edge flaps may be greater than for traditional pitch control methods. By enabling the trailing edge to move independently and quickly along the spanwise position of the blade, local small flutuations in the aerodynamic forces can be alleviated by deformation of the airfoil flap. Strain gauges are used as input for the flap controller, and the effect of placing strain gauges at various radial positions on the blade is investigated. An optimization routine minimizes blade root fatigue loads. Calculations are based on the 5 MW reference wind turbine part of the UpWind project primarily with a mean turbulent wind speed close to rated power. A fatigue load reduction of 25% in the blade root moment was obtained for a continuous 6.3 m long flap. Copyright © 2009 John Wiley & Sons, Ltd.

A wind tunnel test of the wind turbine airfoil Riso-B1-18 equipped with an Active Trailing Edge Flap (ATEF) was carried out. The ATEF was 9% of the total chord, made of piezo electric actuators attached to the trailing edge of a non-deformable airfoil and actuated using an (electric) amplifier. The airfoil was tested at Re = 1.66 × 106. Steady state and dynamic tests were carried out with prescribed deflections of the ATEF. The steady state tests showed that deflecting the ATEF towards the pressure side (positive β) translated the lift curve to higher lift values and deflecting the ATEF towards the suction side (negative β) translated the lift curve to lower lift values. Testing the airfoil for a step change of the ATEF from β = −3.0 to +1.8 showed that the obtainable Δcl was 0.10 to 0.13 in the linear part of the lift curve. Modeling the step response with an indicial function formulation showed that the time constant in the step change and in sinusoidal deflections in dimensionless terms was T0* = 0.6. Testing the ability of the ATEF to cancel out the load variations for an airfoil in sinusoidal pitch motion of ΔAOA = ±0.7 showed that it was possible to reduce the amplitude with around 80% from Δcl = 0.148 to Δcl = 0.032. Copyright © 2009 John Wiley & Sons, Ltd.

The present work contains an extension of the Beddoes–Leishman-type dynamic stall model. In this work, a deformable trailing-edge flap has been added to the dynamic stall model. The model predicts the unsteady aerodynamic forces and moments on an airfoil section undergoing arbitrary motion in heave, lead-lag, pitch, trailing-edge flapping. In the linear region, the model reduces to the inviscid model, which includes the aerodynamic effect of a thin airfoil with a deformable camberline in inviscid flow. Therefore, the proposed model can be considered a crossover between the work of Gaunaa for the attached flow region and Hansen et al. The model is compared qualitatively to wind tunnel measurements of a Riso/ B1-18 blade section equipped with deformable trailing-edge flap devices in the form of piezoelectric devices. Copyright © 2009 John Wiley & Sons, Ltd.

The design of a three-bladed wind turbine rotor is described, where the main focus has been highest possible mechanical power coefficient, CP, at a single operational condition. Structural, as well as off-design, issues are not considered, leading to a purely theoretical design for investigating maximum aerodynamic efficiency.The rotor is designed assuming constant induction for most of the blade span, but near the tip region, a constant load is assumed instead. The rotor design is obtained using an actuator disc model, and is subsequently verified using both a free-wake lifting line method and a full three-dimensional Navier-Stokes solver.Excellent agreement is obtained using the three models. Global CP reaches a value of slightly above 0.51, while global thrust coefficient CT is 0.87. The local power coefficient C-p increases to slightly above the Betz limit on the inner part of the rotor; the local thrust coefficient C, increases to a value above 1.1. This agrees well with the theory of de Vries, which states that including the effect of the low pressure behind the centre of the rotor stemming from the increased rotation, both C-p and C-t will increase towards the root. Towards the tip, both C-p and C-t decrease due to tip corrections as well as drag. Udgivelsesdato: Sep

This paper presents an investigation of the potential for reduction of fluctuating loads on wind turbine blades with the use of flaplike deflectable trailing edges. More specifically, the aeroelastic response of an elastically mounted airfoil section with a deflectable trailing edge is investigated. This is done by coupling a model for the aerodynamic forces on a deforming airfoil with a linear spring/damper model for the elastic deformation of a rigid airfoil to which the forces associated with the deflection of the trailing edge are added. The analysis showed that when the airfoil experienced a wind step from 10to12m∕s the standard deviation of the normal force could be reduced by up to 85% when the flap was controlled by the reading of the airfoil flapwise position and velocity, while reductions of up to 95% could be obtained when the flap was controlled by the reading of the angle of attack. When the airfoil experienced a turbulent wind field, the standard deviation of the normal force could be reduced by 81% for control based on measured angle of attack. The maximum reduction using a combination of flapwise position and velocity was 75%. The maximum deflection of the trailing edge geometry was, in all the considered cases, small enough to justify the use of a potential flow code for calculation of the aerodynamic forces. Calculations showed that the effect of a time lag in the actuators and sensors may drastically reduce the efficiency of the control algorithm. Likewise, the effect of a low maximum actuation velocity reduces the efficiency of the control algorithm. The analysis of the two-dimensional (2D) aeroservoelastic system shown in this paper indicates that the potential of using trailing edge flaps for reduction of fluctuating loads is significant.

As an essential ingredient in the blade element momentum theory, the tip loss effect of rotors plays an important role in the prediction of wind turbine performance. Various tip loss corrections based on the Prandtl tip loss function are analysed in the article. Comparisons with measurements and theoretical analyses show that existing tip loss correction models are inconsistent and fail to predict correctly the physical behaviour in the proximity of the tip. A new tip loss correction model is proposed that remedies the inconsistency. Comparisons between numerical and experimental data show that the new model results in much better predictions of the loading in the tip region. Copyright © 2005 John Wiley & Sons, Ltd.

This paper presents the wind turbine airfoil development at Riso. The design method is described together with our target characteristics for wind turbine airfoils. The use of the CFD code Ellipsys2D for prediction of final target characteristics is described together with the VELUX wind tunnel testing setup. Three airfoil families were developed; Riso-A1, Riso-P and Riso-B1. The Riso-A1 airfoil family was developed for rotors of 600 kW and larger. Wind tunnel testing and field testing showed that this airfoil family is well suited for stall and active stall control. However, sensitivity to roughness was higher than expected. Field tests of a 600 kW active stall wind turbine showed an estimated reduction in blade fatigue loading of up to 15% at the same annual energy yield and at the same time reduced blade weight and blade solidity. The Riso-P airfoils were developed to replace the Riso-A1 airfoils for use on pitch controlled wind turbines. Improved design objectives should reduce the sensitivity to roughness, but measurements are not yet available. The Riso-B1 airfoil family was developed for variable speed operation with pitch control of large megawatt sized rotors. Wind tunnel testing verified the high maximum lift for these airfoils, and the airfoils were found to be very insensitive to leading edge roughness. Performance with vortex generators and Gurney flaps in combination was found to be attractive for the blade root part. Field testing of a 1·5 MW rotor is in progress. Copyright © 2004 John Wiley & Sons, Ltd.