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EDITORIAL/SOLAR SCENERY

J. Sol. Energy Eng. 2005;127(4):437. doi:10.1115/1.2077255.
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This Special Issue on Wind Energy contains articles that evolved from papers presented at the ASME Wind Energy Symposium, held January 2005. The range of topics covered includes: resource assessment, aerodynamics, and structures.

Topics: Wind energy
Commentary by Dr. Valentin Fuster

RESEARCH PAPERS

J. Sol. Energy Eng. 2005;127(4):438-443. doi:10.1115/1.2035704.

This paper presents a numerical method for predicting the atmospheric boundary layer under stable, neutral, or unstable thermal stratifications. The flow field is described by the Reynolds’ averaged Navier-Stokes equations complemented by the kϵ turbulence model. Density variations are introduced into the momentum equation using the Boussinesq approximation, and appropriate buoyancy terms are included in the k and ϵ equations. An original expression for the closure coefficient related to the buoyancy production term is proposed in order to improve the accuracy of the simulations. The resulting mathematical model has been implemented in FLUENT . The results presented in this paper include comparisons with respect to the Monin-Obukhov similarity theory, measurements, and earlier numerical solutions based on kϵ turbulence models available in the literature. It is shown that the proposed version of the kϵ model significantly improves the accuracy of the simulations for the stable atmospheric boundary layer. In neutral and unstable thermal stratifications, it is shown that the version of the kϵ models available in the literature also produce accurate simulations.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):444-455. doi:10.1115/1.2035702.

In order to continue cost-optimization of modern large wind turbines, it is important to continuously increase the knowledge of wind field parameters relevant to design loads. This paper presents a general statistical model that offers site-specific prediction of the probability density function (PDF) of turbulence driven short-term extreme wind shear events, conditioned on the mean wind speed, for an arbitrary recurrence period. The model is based on an asymptotic expansion, and only a few and easily accessible parameters are needed as input. The model of the extreme PDF is supplemented by a model that, on a statistically consistent basis, describes the most likely spatial shape of an extreme wind shear event. Predictions from the model have been compared with results from an extreme value data analysis, based on a large number of full-scale measurements recorded with a high sampling rate. The measurements have been extracted from ”Database on Wind Characteristics” (http:∕∕www.winddata.com∕), and they refer to a site characterized by a flat homogeneous terrain. The comparison has been conducted for three different mean wind speeds in the range of 1519ms, and model predictions and experimental results are consistent, given the inevitable uncertainties associated with the model as well as with the extreme value data analysis.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):456-463. doi:10.1115/1.2037092.

Tip vortex locations have been measured in the wake of a model rotor in both axial flow and yaw using quantitative flow visualization. For each setting, the axial force coefficient has been derived, as well, from measurements. The results agree well with those previously published on the Delft University of Technology model rotor. The main interest is to determine the tip vortex pitch, wake skew angle, wake expansion, and to physically interpret the data. The results also help to validate and construct models. The tip vortex location data complement the existing skewed wake velocity data from hot-wire anemometry, making it a valuable experimental database.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):464-474. doi:10.1115/1.2035705.

The aerodynamic performance of the National Renewable Energy Laboratory (NREL) Phase VI horizontal axis wind turbine (HAWT) under yawed flow conditions is studied using a three-dimensional unsteady viscous flow analysis. Simulations have been performed for upwind cases at several wind speeds and yaw angles. Results presented include radial distribution of the normal and tangential forces, shaft torque, root flap moment, and surface pressure distributions at selected radial locations. The results are compared with the experimental data for the NREL Phase VI rotor. At low wind speeds (7ms) where the flow is fully attached, even an algebraic turbulence model based simulation gives good agreement with measurements. When the flow is massively separated (wind speed of 20ms or above), many of the computed quantities become insensitive to turbulence and transition model effects, and the calculations show overall agreement with experiments. When the flow is partially separated at wind speed above 15ms, encouraging results were obtained with a combination of the Spalart-Allmaras turbulence model and Eppler’s transition model only at high enough wind speeds.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):475-487. doi:10.1115/1.2035707.

A Navier-Stokes solver, CFX V5.6, is coupled with an in-house developed Vortex-Panel method for the numerical analysis of wind turbines. The Navier-Stokes zone is confined to the near-field around one wind turbine blade, the Vortex-Panel method models the entire vortex sheet of a two-bladed rotor and accounts for the far-field. This coupling methodology reduces both numerical diffusion and computational cost. The parallelized coupled solver (PCS) is applied to the NREL Phase VI rotor configuration under no-yaw conditions. Fully turbulent flow is assumed using thek-ϵandk-ωturbulence models. Results obtained are very encouraging for fully attached flow. For separated and partially stalled flow, results are in good agreement with experimental data. Discrepancies observed between the turbulence models are attributed to different prediction of the onset of separation. This is revealed by two-dimensional (2D) results of the S809 airfoil.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):488-495. doi:10.1115/1.2035706.

To further reduce the cost of wind energy, future turbine designs will continue to migrate toward lighter and more flexible structures. Thus, the accuracy and reliability of aerodynamic load prediction has become a primary consideration in turbine design codes. Dynamically stalled flows routinely generated during yawed operation are powerful and potentially destructive, as well as complex and difficult to model. As a prerequisite to aerodynamics model improvements, wind turbine dynamic stall must be characterized in detail and thoroughly understood. The current study analyzed turbine blade surface pressure data and local inflow data acquired by the NREL Unsteady Aerodynamics Experiment during the NASA Ames wind tunnel experiment. Analyses identified and characterized two key dynamic stall processes, vortex initiation and vortex convection, across a broad parameter range. Results showed that both initiation and convection exhibited pronounced three-dimensional kinematics, which responded in systematic fashion to variations in wind speed, turbine yaw angle, and radial location.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):496-502. doi:10.1115/1.2037090.

The effect of rotation has been investigated with emphasis on the impact of blade geometry on the “correction factor” in stall models. The data used came from field tests and wind tunnel experiments performed by the National Renewable Energy Laboratory and were restricted to the steady-state nonyawed conditions. Three blade layouts were available; a blade with constant chord without twist (phase II), a blade with constant chord and twist (phases III and IV), and a tapered blade with twist (phase VI). Effects due to twist and taper were determined from comparison ofcnbetween the different blade layouts. The formulation of the stall model was rewritten so that the measuredcnvalues could be used without reference to 2D airfoil performance. This enabled a direct comparison of the normal force characteristics between the four blade stations of the selected blade configurations. In particular, the correction termfused in stall models for rotational effects was analyzed. The comparison between the test results with a straight and a twisted blade showed that a relation for twist+pitch is required inf. In addition, a dependency offon the angle-of-attack was identified in the measurements and it is recommended that this dependency be incorporated in the stall models.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):503-516. doi:10.1115/1.2037094.

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 10to12ms 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.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):517-528. doi:10.1115/1.2035700.

A semiempirical acoustic generation model based on the work of Brooks, Pope, and Marcolini [NASA Reference Publication 1218 (1989)] has been developed to predict aerodynamic noise from wind turbines. The model consists of dividing the blades of the wind turbine into two-dimensional airfoil sections and predicting the total noise emission as the sum of the contribution from each blade element. Input is the local relative velocities and boundary layer parameters. These quantities are obtained by combining the model with a Blade Element Momentum (BEM) technique to predict local inflow characteristics to the blades. Boundary layer characteristics are determined from two-dimensional computations of airfoils. The model is applied to the Bonus 300 kW wind turbine at a wind speed of 8 m/s. Comparisons of total noise spectra show good agreement with experimental data.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):529-537. doi:10.1115/1.2037089.

This paper reports an investigation of the use of off-axis carbon fibers in the all-carbon spar cap of a 37-m wind turbine rotor blade to induce twist-flap coupling. Many studies have been published on the structure of wind turbine rotor blades incorporating off-axis fibers; none has studied optimizing the blade structure simultaneously considering the angle of off-axis material, the fraction of off-axis material, constraints on cross-fiber and in-plane shear strength, constraints on tip deflection, and blade cost. A parametric study has been conducted varying the angle of off-axis fibers from 5° to 25° and varying the volume fraction of off-axis fibers in the spar cap from 10% to 90%. In all configurations, the remainder of the spar cap material is 0° carbon fiber. The spar cap thickness has been adjusted in each blade to simultaneously minimize the weight of carbon material, and hence the blade cost, while satisfying constraints on carbon fiber strain and tip deflection. The study also examines the cross-fiber strain and stress and the in-plane shear stress in the 0° and off-axis carbon layers. The conclusion of this study is that the optimal angle for most cost-effectively achieving twist-flap coupling—considering constraints on fiber strain, cross-fiber strength, in-plane shear strength, and tip deflection—is closer to 7.5° than the 20° that has frequently been reported by prior researchers. As much as 90% of the spar cap carbon fibers can be rotated to 7.5° off-axis before in-plane shear strength is exceeded.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):538-543. doi:10.1115/1.2037091.

With the current trend toward larger and larger horizontal axis wind turbines, classical flutter is becoming a more critical issue. Recent studies have indicated that for a single blade turning in still air the flutter speed for a modern 35 m blade occurs at approximately twice its operating speed (2 per rev), whereas for smaller blades (5–9 m), both modern and early designs, the flutter speeds are in the range of 3.5–6 per rev. Scaling studies demonstrate that the per rev flutter speed should not change with scale. Thus, design requirements that change with increasing blade size are producing the concurrent reduction in per rev flutter speeds. In comparison with an early small blade design (5 m blade), flutter computations indicate that the non rotating modes which combine to create the flutter mode change as the blade becomes larger (i.e., for the larger blade the second flapwise mode, as opposed to the first flapwise mode for the smaller blade, combines with the first torsional mode to produce the flutter mode). For the more modern smaller blade design (9 m blade), results show that the non rotating modes that couple are similar to those of the larger blade. For the wings of fixed-wing aircraft, it is common knowledge that judicious selection of certain design parameters can increase the airspeed associated with the onset of flutter. Two parameters, the chordwise location of the center of mass and the ratio of the flapwise natural frequency to the torsional natural frequency, are especially significant. In this paper studies are performed to determine the sensitivity of the per rev flutter speed to these parameters for a 35 m wind turbine blade. Additional studies are performed to determine which structural characteristics of the blade are most significant in explaining the previously mentioned per rev flutter speed differences. As a point of interest, flutter results are also reported for two recently designed 9 m twist/coupled blades.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):544-552. doi:10.1115/1.2037093.

Postbuckling analysis of composite laminates representative of wind turbine blade substructures, utilizing the commercial finite element software ANSYS , is presented in this paper. The procedure was validated against an existing postbuckling analysis. Three shell element formulations, SHELL91, SHELL99, and SHELL181, were examined. It was found that the SHELL181 element with reduced integration should be used to avoid shear locking. The validated procedure was used to examine the variation of the buckling behavior, including postbuckling, with lamination schedule of a laminate representative of a wind turbine blade shear web. This analysis was correlated with data from a static test. A 100% postbuckling reserve in a composite structure representative of a shear web was quantified through test and analysis. The buckling behavior of the shear web was improved by modifying the lamination schedule to increase the web bending stiffness. Modifications that improved the buckling load of the structure did not always equate to improvements in the postbuckling reserve.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):553-562. doi:10.1115/1.2037108.

A demonstration of the use of Proper Orthogonal Decomposition (POD) is presented for the identification of energetic modes that characterize the spatial random field describing the inflow turbulence experienced by a wind turbine. POD techniques are efficient because a limited number of such modes can often describe the preferred turbulence spatial patterns and they can be empirically developed using data from spatial arrays of sensed input/excitation. In this study, for demonstration purposes, rather than use field data, POD modes are derived by employing the covariance matrix estimated from simulations of the spatial inflow turbulence field based on standard spectral models. The efficiency of the method in deriving reduced-order representations of the along-wind turbulence field is investigated by studying the rate of convergence (to total energy in the turbulence field) that results from the use of different numbers of POD modes, and by comparing the frequency content of reconstructed fields derived from the modes. The National Wind Technology Center’s Advanced Research Turbine (ART) is employed in the examples presented, where both inflow turbulence and turbine response are studied with low-order representations based on a limited number of inflow POD modes. Results suggest that a small number of energetic modes can recover the low-frequency energy in the inflow turbulence field as well as in the turbine response measures studied. At higher frequencies, a larger number of modes are required to accurately describe the inflow turbulence. Blade turbine response variance and extremes, however, can be approximated by a comparably smaller number of modes due to diminished influence of higher frequencies.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):563-569. doi:10.1115/1.2047589.

Mandell have recently presented an updated constant-life diagram (CLD) for a fiberglass composite that is a typical wind turbine blade material. Their formulation uses the MSU/DOE fatigue data base to develop a CLD with detailed S-N information at 13 R-values. This diagram is the most detailed to date, and it includes several loading conditions that have been poorly represented in earlier studies. Sutherland and Mandell have used this formulation to analyze typical loads data from operating wind farms and the failure of coupons subjected to spectral loading. The detailed CLD used in these analyses requires a significant investment in materials testing that is usually outside the bounds of typical design standards for wind turbine blades. Thus, the question has become: How many S-N curves are required for the construction of a CLD that is sufficient for an “accurate” prediction of equivalent fatigue loads and service lifetimes? To answer this question, the load data from two operating wind turbines and the failure of coupons tested using the WISPERX spectra are analyzed using a nonlinear damage model. For the analysis, the predicted service lifetimes that are based on the CLD constructed from 13 R-values are compared to the predictions for CLDs constructed with fewer R-values. The results illustrate the optimum number of R-values is 5 with them concentrated between R-values of 2 and 0.5, or 2 and 0.7.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):570-580. doi:10.1115/1.2037107.

This paper addresses the feasibility of using innovative vacuum infused anionic polyamide-6 (PA-6) thermoplastic composites for MW-size wind turbine blades structures. To compare the performance of this fully recyclable material against commonly used less sustainable thermoset blade materials in a baseline structural MW-size blade configuration (box-spar/skins), four different blade composite material options were investigated: Glass/epoxy, carbon/epoxy, glass/PA-6, and carbon/PA-6. Blade characteristics such as weight, costs, and natural frequencies were compared for rotor blades ranging between 32.5 and 75m in length, designed according to both stress and tip deflection criteria. Results showed that the PA-6 blades have similar weights and natural frequencies when compared to their epoxy counterpart. For glass fiber blades, a 10% reduction in material cost can be expected when using PA-6 rather than epoxy while carbon fiber blades costs were found to be similar. Considering manufacturing, processing temperatures of PA-6 are significantly higher than for epoxy systems; however, the associated cost increase is expected to be compensated for by a reduction in infusion and curing time.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2005;127(4):581-587. doi:10.1115/1.2047590.

Traditional wind turbines are commonly equipped with induction generators because they are inexpensive, rugged, and require very little maintenance. Unfortunately, induction generators require reactive power from the grid to operate; capacitor compensation is often used. Because the level of required reactive power varies with the output power, the capacitor compensation must be adjusted as the output power varies. The interactions among the wind turbine, the power network, and the capacitor compensation are important aspects of wind generation that may result in self-excitation and higher harmonic content in the output current. This paper examines the factors that control these phenomena and gives some guidelines on how they can be controlled or eliminated.

Commentary by Dr. Valentin Fuster

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