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Review Article

J. Energy Resour. Technol. 2019;141(5):050801-050801-10. doi:10.1115/1.4042643.

Momentum models or streamtube models represent one of the fundamental approaches in modeling the aerodynamics of straight bladed vertical axis wind turbine (SB-VAWT) of Darrieus type. They are based on momentum (actuator disk) theory and widely used in performance evaluation of VAWTs. In this short review, the authors have strived to compile the basic momentum models that have been widely assumed in the literature for design and performance estimation of SB-VAWTs of Darrieus type. A comprehensive demonstration of the formulation needed for the implantation of these models is also proposed. Three streamtube models are investigated in this paper, namely, the single streamtube (SST), the multiple streamtube (MST), and the double multiple streamtube (DMST) models. Each of these models has it merits and demerits which are also thoroughly discussed in this review.

Commentary by Dr. Valentin Fuster

Research Papers: Alternative Energy Sources

J. Energy Resour. Technol. 2019;141(5):051201-051201-12. doi:10.1115/1.4041735.

The elliptical-bladed Savonius wind turbine rotor has become a subject of interest because of its better energy capturing capability. Hitherto, the basic parameters of this rotor such as overlap ratio, aspect ratio, and number of blades have been studied and optimized numerically. Most of these studies estimated the torque and power coefficients (CT and CP) at given flow conditions. However, the two important aerodynamic forces, viz., the lift and the drag, acting on the elliptical-bladed rotor have not been studied. This calls for a deeper investigation into the effect of these forces on the rotor performance to arrive at a suitable design configuration. In view of this, at the outset, two-dimensional (2D) unsteady simulations are conducted to find the instantaneous lift and drag forces acting on an elliptical-bladed rotor at a Reynolds number (Re) = 0.892 × 105. The shear stress transport (SST) k–ω turbulence model is used for solving the unsteady Reynolds averaged Navier–Stokes equations. The three-dimensional (3D) unsteady simulations are then performed which are then followed by the wind tunnel experiments. The drag and lift coefficients (CD and CL) are analyzed for 0–360 deg rotation of rotor with an increment of 1 deg. The total pressure, velocity magnitude, and turbulence intensity contours are obtained at various angles of rotor rotation. For the elliptical-bladed rotor, the average CD, CL, and CP, from 3D simulation, are found to be 1.31, 0.48, and 0.26, respectively. The average CP for the 2D elliptical profile is found to be 0.34, whereas the wind tunnel experiments demonstrate CP to be 0.19.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051202-051202-10. doi:10.1115/1.4041544.

Wind energy has had a major impact on the generation of renewable energy. While most research and development focuses on large, utility-scale wind turbines, a new application is in the field of small wind turbines for the urban environment. A major design challenge for urban wind turbines is the noise generated during operation. This study examines the power production and the noise generated by two small-scale wind turbines tested in a small wind tunnel. Both rotors were designed using the blade-element momentum theory using either the NREL S823 or the Eppler 216 airfoils. Point noise measurements were taken using a microphone at three locations downstream of the turbine: 16% of the diameter (two chord lengths), 50% of the diameter, and 75% of the diameter. At each location downstream of the turbine, a vertical traverse was performed to analyze the sound pressure level (SPL) from the tip of the turbine blades down to the hub. The rotor designed with the Eppler 216 airfoil showed a 9% increase in power production and decrease of up to 7 dB(A).

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051203-051203-10. doi:10.1115/1.4042235.
OPEN ACCESS

A multirotor system (MRS) is defined as containing more than one rotor in a single structure. MRSs have a great potential as a wind turbine system, saving mass and cost, and showing scale ability. The shrouded wind turbine with brimmed diffuser-augmented wind turbines (B-DAWT) has demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about 2–5 compared with a bare wind turbine. In the present research, B-DAWTs are used in a multirotor system. The power output performance of MRSs using two and three B-DAWTs in a variety of configurations has been investigated in the previous works. In the present study, the aerodynamics of an MRS with five B-DAWTs, spaced in close vicinity in the same vertical plane normal to a uniform flow, has been analyzed. Power output increases of up to 21% in average for a five-rotor MRS configuration are achieved in comparison to that for the stand-alone configuration. Thus, when B-DAWTs are employed as the unit of a MRS, the total power output is remarkably increased. As the number of units for an MRS is increased from two to five, the increase in power output becomes larger and larger. This is because that the gap flows between B-DAWTs in a MRS are accelerated and cause lowered pressure regions due to vortex interaction behind the brimmed diffusers. Thus, a MRS with more B-DAWTs can draw more wind into turbines showing higher power output.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051204-051204-8. doi:10.1115/1.4042414.

In order to obtain an optimal design of composite offshore wind turbine blade, take into account all the structural properties and the limiting conditions applied as close as possible to real cases. This work is divided into two stages: the aerodynamic design and the structural design. The optimal blade structural configuration was determined through a parametric study by using a finite element method. The skin thickness, thickness and width of the spar flange, and thickness, location, and length of the front and rear spar web were varied until design criteria were satisfied. The purpose of this article is to provide the designer with all the tools required to model and optimize the blades. The aerodynamic performance has been covered in this study using blade element momentum (BEM) method to calculate the loads applied to the turbine blade during service and extreme stormy conditions, and the finite element analysis was performed by using abaqus code to predict the most critical damage behavior and to apprehend and obtain knowledge of the complex structural behavior of wind turbine blades. The approach developed based on the nonlinear finite element analysis using mean values for the material properties and the failure criteria of Hashin to predict failure modes in large structures and to identify the sensitive zones.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051205-051205-9. doi:10.1115/1.4042450.

Wind turbine upgrades have recently been spreading in the wind energy industry for optimizing the efficiency of the wind kinetic energy conversion. These interventions have material and labor costs; therefore, it is fundamental to estimate the production improvement realistically. Furthermore, the retrofitting of the wind turbines sited in complex environments might exacerbate the stress conditions to which those are subjected and consequently might affect the residual life. In this work, a two-step upgrade on a multimegawatt wind turbine is considered from a wind farm sited in complex terrain. First, vortex generators and passive flow control devices have been installed. Second, the management of the revolutions per minute has been optimized. In this work, a general method is formulated for assessing the wind turbine power upgrades using operational data. The method is based on the study of the residuals between the measured power output and a judicious model of the power output itself, before and after the upgrade. Therefore, properly selecting the model is fundamental. For this reason, an automatic feature selection algorithm is adopted, based on the stepwise multivariate regression. This allows identifying the most meaningful input variables for a multivariate linear model whose target is the power of the upgraded wind turbine. For the test case of interest, the adopted upgrade is estimated to increase the annual energy production to 2.6 ± 0.1%. The aerodynamic and control upgrades are estimated to be 1.8% and 0.8%, respectively, of the production improvement.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051206-051206-9. doi:10.1115/1.4042529.

Several studies on wind energy have been conducted to find possible solutions to power issues related to the variable nature of the wind. One of the most promising seems to be the application of sinusoidal modifications (tubercles) on the leading edge of wind turbine blades. In the present work, a systematic study on the effects of different tubercle configurations on NREL phase VI wind turbine performance is conducted. A design of experiments is used to generate blades with different tubercle amplitude and wavelength that are then simulated by a computational fluid dynamics (CFD) analysis. The resulting power and annual energy production (AEP) are compared with the baseline values noticing a positive effect of tubercles on the power at high wind speeds.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051207-051207-11. doi:10.1115/1.4042642.

A design concept for a wind turbine blade with an adaptive twist transformation is presented. The design improves partial-load wind capture by adapting the twist distribution in relation to wind speed. Structural adaptability is enabled by actuating a series of compliant sections that are mounted on a relatively rigid spar. The sections are assumed to have a unique stiffness that is achievable through additive manufacturing technology. The authors' prior work employed an aerodynamic model to establish the theoretical blade twist distribution as a function of wind speed. The work in this paper focuses on a method to optimize the stiffness of each blade section that has been previously defined. A mathematical model is proposed to support design optimization. The model is parameterized in terms of actuator locations and the torsional stiffness ratios of each blade section. These parameters are optimized to allow the blade to adapt its twist distribution to match the prescribed configurations. The optimization is completed using a weighted-least squares approach that minimizes the error between the theoretical and practical design. The selected solution is based upon the configuration that maximizes production. Weights are assigned to bias the performance of the blade toward different operating regimes. Our results indicate that quadratically penalizing twist angle errors toward the blade tip increases power capture. A Rayleigh distribution is used to create three sets of wind data, which vary in average speed. These sets of data are used to evaluate the performance of the proposed blade and design technique.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051208-051208-9. doi:10.1115/1.4042967.

An improved diagnostic mass-consistent finite element model (FEM) has been developed to construct 3D wind fields over irregular terrain. Instead of using constant Gauss precision moduli over the whole domain in the existing mass-consistent models, the improved mass-consistent model adopts different Gauss precision moduli based on the terrain topography gradient associated with atmospheric boundary conditions. These terrain sensitive moduli resolve wind flows over large topographical obstacles more accurately than constant Gauss precision moduli. In this study, a terrain following mesh generator is developed based on digital elevation data from the U.S. Geological Survey, and the data linked to the modified mass-consistent FEM model. The improved model is validated and verified using a benchmark study for flow over a semicylinder. The model is then used to re-examine 3D wind fields previously simulated for the Nellis Dunes area near Las Vegas, NV. Results show that the improved mass consistent modeling system shows better agreement with the recorded meteorological tower data than the previous results obtained using constant moduli.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051209-051209-12. doi:10.1115/1.4042968.

It is desired, through this work, to investigate in detail the scenario that takes place behind a single wind turbine unit by focusing on three parameters; average axial wind velocity component, velocity deficit, and total turbulence intensity. The testing was done at mainstream velocity, U, of 5.2 m/s, u and v velocity components were captured by x-probe dual-sensor hot wire anemometer. A massive amount of point data was obtained, which then processed by a matlab script to plot the desired contours through the successive transverse sections along the entire length of the test section. By monitoring the previously mentioned flow parameters, the regions of low velocity and high turbulence can be avoided, while the location of the subsequent wind turbine is selected. The estimation of the distance, at which the inlet flow field will restore its original characteristics after being mixed through the rotor blades, is very important as this is the distance that should separate two successive turbines in an inline configuration wind farm to guarantee the optimum performance and to extract the maximum power out of the subsequent array of turbines. It is found that the hub height axial velocity recovery at six rotor diameters downstream distance is only 82%. This fact means that the power extraction out of the downstream turbine in an inline configuration wind farm is only 55% of the upstream turbine if the same free stream velocity and blade design are adopted.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051210-051210-11. doi:10.1115/1.4042732.

Icing of wind turbine blades poses a challenge for the wind power industry in cold climate wind farms. It can lead to production losses of more than 10% of the annual energy production. Knowledge of how the production is affected by icing is of importance. Complicating this reality is the fact that even a small amount of uncertainty in the shape of the accreted ice may result in a large amount of uncertainty in the aerodynamic performance metrics. This paper presents a numerical approach using the technique of polynomial chaos expansion (PCE) to quantify icing uncertainty faster than traditional methods. Time-dependent bi-dimensional Reynolds-averaged Navier–Stokes computational fluid dynamics (RANS-CFD) simulations are considered to evaluate the aerodynamic characteristics at the chosen sample points. The boundary conditions are based on three-dimensional simulations of the rotor. This approach is applied to the NREL 5 MW reference wind turbine allowing to estimate the power loss range due to the leading-edge glaze ice, considering a radial section near the tip. The probability distribution function of the power loss is also assessed. The results of the section are nondimensionalized and assumed valid for the other radial sections. A correlation is found allowing to model the load loss with respect to the glaze ice horn height, as well as the corresponding probability distribution. Considering an equal chance for any of the ice profiles, load loss is estimated to be lower than 6.5% for the entire blade in half of the icing cases, while it could be roughly 4–6 times in the most severe icings.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051211-051211-8. doi:10.1115/1.4042971.
OPEN ACCESS

Brimmed-diffuser augmented wind turbines (B-DAWTs) can significantly increase the performance of the rotor. Multirotor systems (MRSs) have a lot of merits such as significant saving mass and overall cost of the wind turbine system. In the present research, B-DAWTs are studied in a MRS. In wind tunnel experiments, the power output and aerodynamics of three B-DAWTs placed in close vicinity have been investigated. The results show a significant increase of up to 12% in total power output of the MRS with B-DAWTs compared to the sum of the stand-alone (SA) same turbines. The accelerated gap flows between B-DAWTs in a MRS cause lowered pressure regions due to vortex interaction behind the brimmed diffusers and draw more wind into turbines.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051212-051212-8. doi:10.1115/1.4042916.

Self-healing wind turbine blades offer a substantial offset for costly blade repairs and failures. We discuss the efforts made to optimize the self-healing properties of wind turbine blades and provide a new system to maximize this offset. Copper wire coated by paraffin wax was embedded into fiber-reinforced polymer (FRP) samples incorporated with Grubbs' first-generation catalyst. The wires were extracted from cured samples to create cavities that were then injected with the healing agent, dicyclopentadiene (DCPD). Upon sample failure, the DCPD and catalyst react to form a thermosetting polymer to heal any crack propagation. Three-point bending flexural tests were performed to obtain the maximum flexural strengths of the FRP samples before and after recovery. Using those results, a hierarchy of various vascular network configurations was derived. To evaluate the healing system's effect in a real-life application, a prototype wind turbine was fabricated and wind tunnel testing was conducted. Using ultraviolet (UV) dye, storage and transport processes of the healing agent were observed. After 24 h of curing time, Raman spectroscopy was performed. The UV dye showed dispersion into the failure zone, and the Raman spectra showed the DCPD was polymerized to polydicyclopentadiene (PDCPD). Both the flexural and wind tunnel test samples were able to heal successfully, proving the validity of the process.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051213-051213-8. doi:10.1115/1.4043326.

This study investigates the performance of microjets for load reduction on the NREL-5 MW wind turbine and identifies optimal system parameters. Microjets provide blowing normal to the blade surface and can rapidly increase or decrease lift on a blade section, enabling a wind turbine to respond to local, short-term changes in wind condition. As wind turbine rotors become larger, control methods that act on a single blade or blade section are increasingly necessary to reduce critical fatigue and extreme loads. However, microjets require power to operate, and thus, it is crucial that the fatigue reduction justifies any energy input to the system. To examine the potential for fatigue reduction of a range of potential microjet system configurations, a blade element momentum (BEM) code and a flow energy solver were used to estimate the energy input and the change in primary fatigue metrics. A parametric analysis was conducted to identify the optimal spanwise position and length of the microjets over a range of air mass flow rates. Both active and passive air supply methods were considered. A passive microjet system applied to the NREL 5-MW rotor produced a 3.7% reduction in the maximum flapwise root bending moment (FRBM). The reduction in the peak bending moment increased to 6.0% with a 5 kPa blower that consumes approximately 0.1% of the turbine output power. The most effective configurations placed microjets between the blade midspan to three-quarters span. Load reduction was achieved for both active and passive modes of air supply to the microjet system.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(5):051214-051214-10. doi:10.1115/1.4042914.

Improvement of the aerodynamic performance for cambered airfoils with leading-edge slots is investigated in this work. This concept is proven both computationally and experimentally in recent years. Five design variables of interest are slot's length, slot's width or thickness, inlet angle, exit angle, and the vertical position. The objective is to perform design of experiment and optimization studies on these variables and evaluate the behavior of the objective functions, namely lift and lift over drag ratio (LoD), within the appropriate ranges of the independent variables. Simulations are mainly carried out at the Reynolds number of 1.6 × 106 and the angles of attack (AoA) of 6 deg for NACA 4412 airfoil. However, some of the analyses are repeated at Reynolds number of 3.2 × 106 and AoA of 0 and 8 deg to show the scalability of the results. Results indicate that the proper selection of three of the design variables, i.e., length, inlet angle, and vertical position, can have a significant impact on both lift and LoD, while the other two variables seem less influential. For the combination of the operating conditions and the values of the design variables considered in this investigation, a LoD improvement as large as 11% is observed.

Commentary by Dr. Valentin Fuster

Discussion

J. Energy Resour. Technol. 2019;141(5):055501-055501-7. doi:10.1115/1.4042447.

The exploitation of wind turbines in complex terrain has recently been growing. The comprehension of wind flow, especially in the downstream area, is by itself a challenging task in complex terrain: even more so, it is difficult to account for the mixing between terrain effects and the wake interactions between nearby turbines. Efficiency is one of the simplest and meaningful metrics for quantifying the impact of wakes on wind farm production, but its definition is well established basically only for offshore wind farms. In this work, the definition of wind farm efficiency is, therefore, discussed, based on the critical points arising in complex terrain, where there can be at the same time a considerable variation of free wind flow along the layout and a directional distortion of the wakes, induced by the terrain. In this work, operational data of a test case wind farm sited in a very complex terrain, featuring 17 multimegawatt wind turbines, are elaborated and inspire a discussion and a novel definition of efficiency, that restores in the complex terrain case the meaning of the efficiency.

Commentary by Dr. Valentin Fuster

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