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In this study, the influences of the geometric characteristics of a dimple on the aerodynamic properties of a vertical axis wind turbine are investigated, and its configuration optimization that maximizes the wind turbine's performance is carried out. Three parameters that control the position, size and depth of the dimple are used to parameterize the dimple's configuration and to conduct optimization. With these parameters, when the flows around the wind turbines with various dimples are compared, several meaningful physical phenomena are found. First, a strong association between the position of a dimple and the power coefficient is found where the power coefficient increases as the position of the dimple approaches the trailing edge of the wind turbine blade. Second, high-performance enhancement tends to be achieved when a dimple is small, but this attribution becomes weaker as the dimple moves toward the trailing edge. Furthermore, when the flow of the optimal wind turbine is compared with the one without a dimple, a reduction of the blade wake and delay of the flow separation can be identified. Finally, the shed vortices from the optimal wind turbine blade are weaker and decay faster than the ones shed from the wind turbine without a dimple. [ABSTRACT FROM AUTHOR]

A numerical procedure was developed to determine a variable-pitch law that maximized the performance of a vertical-axis wind turbine (VAWT). The methodology was based on the determination, for each blade, of the angle of attack maximizing the stationary aerodynamic efficiency at prescribed azimuthal positions. The angles of attack were determined by means of a panel method with a low computational effort, and the methodology was implemented in Matlab® software (version R2021a) allowing us to achieve in real time a variable-pitch law suitable for the turbine geometry. The variable pitch law was validated by considering its effect on the torque of a 2D model of an H-Darrieus turbine. U-RANS analyses were carried out with a K−ωSST model and a sliding-mesh technique was used to prescribe the blade motion around the shaft and pitch motion. Results showed how the variable-pitch law delayed the dynamic stall and improved the aerodynamic performance considerably.

To elucidate the wind-direction dependence of the rotor performance in closely spaced vertical-axis wind turbines, wind-tunnel experiments were performed at a uniform wind velocity. In the experiments, a pair/trio of three-dimensional printed model turbines with a diameter of D = 50 mm was used. The experiments were performed systematically by applying incremental adjustments to the rotor gap g and rotational direction of each rotor and by changing the wind direction. For tandem layouts, the rotational speed of the downwind rotor is 75–80% that of an isolated rotor, even at g/D = 10. For the average rotational speed of the rotor pair, an origin-symmetrical and a line-symmetrical distribution are observed in the co-rotating and inverse-rotating configurations, respectively, thereby demonstrating the wind-direction dependence for the rotor pair. The inverse-rotating trio configuration yields a higher average rotational speed than the co-rotating trio configuration for any rotor spacing under the ideal bidirectional wind conditions. The maximum average rotational speed should be obtained for a wind direction of θ = 0° in the inverse-rotating trio configuration. The wind-direction dependence of the rotational speeds of the three turbines was explained via flow visualization using a smoke-wire method and velocity field study using two-dimensional computational fluid dynamics.

Fatigue failure caused by unsteady loads on the shaft is an important limiting factor in the development of large VAWT. This study proposes an active control method that relies on movable mass blocks to reduce the unsteady loads. Then on the basis of the aerodynamic load of the VAWT simulated by computational fluid dynamics, the motion laws of the mass blocks with different masses are investigated. When the mass of mass block is 0.075 kg, the radial load of the shaft can be reduced by 97.1% and the shaft deformation can be eliminated by 99% without affecting the power output of wind turbine. However, further research on the start-up performance of the VAWT indicates that the active control method also prolongs the start-up time. Thus a method of adjusting the moment of inertia of wind turbine with a rotation speed of 15.6 m/s is proposed to save 40.6% of the start-up time with respect to the method used for direct start-up with large moment of inertia of 0.036 kg m2. After the start-up, a transition displacement curve is plotted to aid in the switching design of the operation mode of mass blocks between the start-up phase and the normal operation phase. [ABSTRACT FROM AUTHOR]

This study investigates the elastic response characteristics of a floating wind turbine (FOWT) with two vertical-axis wind turbines (VAWTs), called double-rotor VAWTs. The model consists of two VAWTs mounted on a single semi-submersible floating structure and employs a single point mooring, which allows the FOWT to always self-align with the wind. Usually, a coupled analysis of the wind turbine and floating structure is used in the design of FOWTs; however, there is no coupled analysis available for VAWTs. In this study, we attempted to combine the wind turbine design software “QBlade” and the coupled wind turbine/floating body analysis code “UTWind” as one of the methods of coupled analysis of a VAWT and a floating body. Numerical simulation results were compared with experimental results using an elastic model scaled down to 1/100 of its actual model to determine the motion response and cross-sectional bending moments. The experimental results showed that the thrust of the VAWT had a particular influence on the cross-sectional forces and motion response between the two VAWTs. For cross-sectional forces, all results showed similar trends. Overall, the results of UTWind for double-rotor VAWTs are reasonable. It was also found that the pitch motion must be accurately reproduced to improve the accuracy.

The presence of power augmentation effects, or synergy, in vertical-axis wind turbines (VAWTs) offers unique opportunities for enhancing wind-farm performance. This paper uses an open-source actuator-line-method (ALM) code library for OpenFOAM (turbinesFoam) to conduct an investigation into the synergy patterns within two- and three-turbine VAWT arrays. The application of ALM greatly reduces the computational cost of simulating VAWTs by modelling turbines as momentum source terms in the Navier–Stokes equations. In conjunction with an unsteady Reynolds-Averaged Navier–Stokes (URANS) approach using the k-ω shear stress transport (SST) turbulence model, the ALM has proven capable of predicting VAWT synergy. The synergy of multi-turbine cases is characterized using the power ratio which is defined as the power coefficient of the turbine cluster normalized by that for turbines in isolated operation. The variation of the power ratio is characterized with respect to the array layout parameters, and connections are drawn with previous investigations, showing good agreement. The results from 108 two-turbine and 40 three-turbine configurations obtained using ALM are visualized and analyzed to augment the understanding of the VAWT synergy landscape, demonstrating the effectiveness of various layouts. A novel synergy superposition scheme is proposed for approximating three-turbine synergy using pairwise interactions, and it is shown to be remarkably accurate.

The methodology and results of parametric studies of vertical-axis wind turbines (VAWTs) based on three-dimensional aerodynamic calculations are presented. For a model wind turbine with three twisted blades, the dependence of the torque on the wind speed, the speed of rotation of the turbine, and on the variation of geometric parameters that determine the design of the turbine is studied. Estimates of the amplitude of pulsations of the torque of the wind turbine are obtained, depending on the specified parameters. [ABSTRACT FROM AUTHOR]

In this work, two-dimensional models of Savonius rotors are simulated using OpenFOAM® in order to predict the aerodynamic performance of small-scale vertical-axis wind turbines. The results are reported analyzing the aerodynamic performance and forces acting on the rotors. Power coefficient, (C P) , is compared with experimental data for each operation point, and for three different geometries. Simulations with first- and second-order discretization schemes are carried out and compared, both quantitative and qualitative. Since usual grid dimensions result not to be suitable for simulations of Savonius rotors, an analysis of different domains is performed and compared. Finally, a set up for computational fluid dynamics simulation of two-dimensional Savonius rotors is proposed. The fluid–rotor interaction is analyzed and the vortex shedding is correlated with C P values and wake description. [ABSTRACT FROM AUTHOR]

Recently, wind farms consisting of clusters of closely spaced vertical-axis wind turbines (VAWTs) have attracted the interest of many people. In this study, a method using a wake model to predict the flow field and the output power of each rotor in a VAWT cluster is proposed. The method uses the information obtained by the preliminary computational fluid dynamics (CFD) targeting an isolated single two-dimensional (2D) VAWT rotor and a few layouts of the paired 2D rotors. In the method, the resultant rotor and flow conditions are determined so as to satisfy the momentum balance in the main wind direction. The pressure loss of the control volume (CV) is given by an interaction model which modifies the prepared information on a single rotor case and assumes the dependence on the inter-rotor distance and the induced velocity. The interaction model consists of four equations depending on the typical four-type layouts of selected two rotors. To obtain the appropriate circulation of each rotor, the searching range of the circulation is limited according to the distribution of other rotors around the rotor at issue. The method can predict the rotor powers in a 2D-VAWT cluster including a few rotors in an incomparably shorter time than the CFD analysis using a dynamic model.

The phase-synchronized rotation of a pair of closely spaced vertical-axis wind turbines has been found in wind tunnel experiments and computational fluid dynamics (CFD) simulations. During phase synchronization, the two wind turbine rotors rotate inversely at the same mean angular velocity. The blades of the two rotors pass through the gap between the turbines almost simultaneously, while the angular velocities oscillate with a small amplitude. A pressure drop in the gap region, explained by Bernoulli’s law, has been proposed to generate the interaction torque required for phase synchronization. In this study, an analytical model of the interaction torques was developed. In our simulations using the model, (i) phase synchronization occurred, (ii) the angular velocities of the rotors oscillated during the phase synchronization, and (iii) the oscillation period became shorter and the amplitude became larger as the interaction became stronger. These observations agree qualitatively with the experiments and CFD simulations. Phase synchronization was found to occur even for a pair of rotors with slightly different torque characteristics. Our simulation also shows that the induced flow velocities influence the dependence of the angular velocities during phase synchronization on the rotation directions of the rotors and the distance between the rotors.

This article presents a study of modeling and optimization for the dynamic performance of wind turbine composite material blades and investigates the effects of composite material stacking sequence in addition to some design parameters such as twist angle (ɸ) and aspect ratio (AR) on the whole wind turbine performance. The two-stage Savonius rotor VAWT composite blades are designed and simulated within the solidworks simulation 2020 package. Modified mechanical parameters are introduced to improve the scalability, reliability, and accuracy of the developed models. The lamination plate theory is used to compute the equivalent mechanical properties for each composite blade. The finite element analyses (FEAs) are conducted to investigate the dynamic characteristics (frequency and associated mode shapes) of wind turbine models. Taguchi tools such as analysis of variance (ANOVA), signal-to-noise (S/N) ratio and additive model were employed to evaluate and obtain the significant factors and determine the optimal combination levels of wind turbine design parameters. Mathematical modeling based on response surface methodology (RSM) has been established. The analysis of results shows that the aspect ratio with a contribution of 48.08% had the dominant impact on the rotor performance followed by the stacking sequence and twist angle. [ABSTRACT FROM AUTHOR]

A simple model for accounting for tower mass when estimating the first-mode eigenfrequency of a semi-guy-wired tower has been derived. This extends previous work where an analytical model of the semi-guy-wired tower of a 200-kW vertical-axis wind turbine was developed. The model was primarily used to estimate the eigenfrequencies as a result of adding guy wires to a free-standing tower (thus creating a semi-guy-wired setup). However, a weakness with the model was that the tower mass was accounted for in a rough way that essentially ignored the guy wires, which gave a larger-than-necessary error. In this work, an effective top mass, that takes into account the tower mass and the constraints from the guy wires, is derived to achieve a higher accuracy when estimating the first-mode eigenfrequency. This, together with the earlier models, gives a more complete method to estimate the eigenfrequencies for a semi-guy-wired wind turbine. [ABSTRACT FROM AUTHOR]

Recent attempts to discover energy–efficient and cost–effective power generation systems unleash tremendous capabilities of urban rooftops vertical–axis wind turbines (VAWT). Their advantages of Omni–directionality, low noise and less maintenance cost allow direct integration to urban neighborhood having unstable wind conditions. Despite continuous effort to investigate performance under a range of operating conditions, aspects such as Turbulent Intensity (TI) and ground clearance remain relatively less explored. Such effects originate either due to sharp topographical variations or placement of the turbine blades in proximity to the ground. In the present work, we perform a parametric study to quantify the performance of VAWT under various levels of TI and ground clearance. We conduct high fidelity Computational Fluid Dynamics (CFD) simulations using ANSYS Fluent and k − ε turbulence model. The H-type VAWT is employed having rated power of 1.5 k W , diameter (D) of 2.5 m chord length (c) of 0.2 m and operates at prescribed wind speed (U 0) of 12.0 m / s and Tip Speed Ratio (TSR) of 1.5–4.5. We determine the performance of turbine under four levels of TI i.e., 0 % , 5 % , 15 % 25 % at five ground clearance levels of 1.0 c , 2.5 c , 4.0 c , 7.5 c and 10.0 c. The results show a performance loss of 30.10%, 20.65%, 10.65% at turbine clearance heights of 1.0 c , 2.5 c , 4.0 c respectively. The height of 7.5 c yield higher and more consistent performance under given operating conditions. The results for induced turbulence identify a decrease in the performance up to 45.42 % corresponding to 25 % TI level. • Influence of ground clearance and turbulent intensity on power performance of rooftop VAWT is stud-ied.. • High _delity CFD simulations via full-scale geometry of H-type VAWT are conducted. • For H-type rooftop VAWT, increases in the optimal ground clearance obtains a higher torque. • The high turbulent intensities alleviates the performance deterioration. [ABSTRACT FROM AUTHOR]

There is still discrepancy regarding the verification of CFD U-RANS simulations of vertical-axis wind turbines (VAWTs). In this work, the applicability of the Richardson extrapolation method to assess mesh convergence is studied for several points in the power curve of a VAWT. A 2D domain of the rotor is simulated with three different meshes, monitoring the turbine power coefficient as the convergence parameter. This method proves to be a straightforward procedure to assess convergence of VAWT simulations. Guidelines regarding the required mesh and temporal discretization levels are provided. Once the simulations are validated, the flow field at three characteristic tip-speed ratio values (2.5 - low, 4 - nominal and 5 - high) is analyzed, studying pressure, velocity, turbulent kinetic energy and vorticity fields. The results have revealed two main vortex shedding mechanisms, blade- and rotor-related. Vortex convection develops differently depending on the rotor zone (upwind, downwind, windward or leeward). Finally, insight into the loss of performance at off-design conditions is provided. Vortex shedding phenomena at the low tip-speed ratio explains the loss of performance of the turbine, whereas at the high tip-speed ratio, this performance loss may be ascribed to viscous effects and the rapid interaction between successive blade passings.

This paper compares three different numerical models to evaluate their accuracy for predicting the performance of an H-rotor vertical-axis wind turbine (VAWT) considering the influence of struts. The strut of VAWTs is one factor that makes the flow feature around the turbine more complex and thus influences the rotor performance. The focus of this study is placed on analyzing how accurately three different numerical approaches are able to reproduce the force distribution and the resulting power, taking the strut effect into account. For the 12 kW straight-bladed VAWT, the blade force is simulated at three tip speed ratios by the full computational fluid dynamics (CFD) model based on the Reynolds-averaged Navier–Stokes (RANS) equations, the actuator line model (ALM), and the vortex model. The results show that all the models do not indicate a significant influence of the struts in the total force over one revolution at low tip speed ratio. However, at middle and high tip speed ratio, the RANS model reproduces the significant decrease of the total tangential force that is caused due to the strut. Additionally, the RANS and vortex models present a clear influence of the struts in the force distribution along the blade at all three tip speed ratios investigated. The prediction by the ALM does not show such distinctive features of the strut impact. The RANS model is superior to the other two models for predicting the power coefficient considering the strut effect, especially at high tip speed ratio.

Vertical-axis wind turbines (VAWTs) are being reconsidered as a complementary technology to the more commercially used horizontal-axis wind turbines (HAWTs) because of their economical installation and maintenance. The selection of the blade numbers is one of the crucial concerns for VAWTs. This study focuses on the effects of the blade numbers on the fatigue lives of VAWT tower bases subjected to wind loading. Three straight-bladed VAWTs, with the same solidity ratios but different blade numbers, varying from two to four, were designed. The aerodynamic loading incurred by the VAWTs was computed using the corrected double-disk multistreamtube (DMS) model. The dynamic equations of the turbine systems were solved using the explicit central difference method. Then, a fatigue assessment model, including the crack-initiation and crack-propagation stages, was developed for the turbine tower bases. The results indicate that the three- and four-bladed VAWTs always presented better performances than the two-bladed VAWT in terms of the fatigue life. Moreover, increasing the number of blades from two to three improves the fatigue life of the tower base more than increasing it from three to four at lower wind speeds, while the latter is the more effective way to improve the tower-base fatigue life at higher wind speeds.

We present computational flow analysis of a vertical-axis wind turbine (VAWT) that has been proposed to also serve as a tsunami shelter. In addition to the three-blade rotor, the turbine has four support columns at the periphery. The columns support the turbine rotor and the shelter. Computational challenges encountered in flow analysis of wind turbines in general include accurate representation of the turbine geometry, multiscale unsteady flow, and moving-boundary flow associated with the rotor motion. The tsunami-shelter VAWT, because of its rather high geometric complexity, poses the additional challenge of reaching high accuracy in turbine-geometry representation and flow solution when the geometry is so complex. We address the challenges with a space–time (ST) computational method that integrates three special ST methods around the core, ST Variational Multiscale (ST-VMS) method, and mesh generation and improvement methods. The three special methods are the ST Slip Interface (ST-SI) method, ST Isogeometric Analysis (ST-IGA), and the ST/NURBS Mesh Update Method (STNMUM). The ST-discretization feature of the integrated method provides higher-order accuracy compared to standard discretization methods. The VMS feature addresses the computational challenges associated with the multiscale nature of the unsteady flow. The moving-mesh feature of the ST framework enables high-resolution computation near the blades. The ST-SI enables moving-mesh computation of the spinning rotor. The mesh covering the rotor spins with it, and the SI between the spinning mesh and the rest of the mesh accurately connects the two sides of the solution. The ST-IGA enables more accurate representation of the blade and other turbine geometries and increased accuracy in the flow solution. The STNMUM enables exact representation of the mesh rotation. A general-purpose NURBS mesh generation method makes it easier to deal with the complex turbine geometry. The quality of the mesh generated with this method is improved with a mesh relaxation method based on fiber-reinforced hyperelasticity and optimized zero-stress state. We present computations for the 2D and 3D cases. The computations show the effectiveness of our ST and mesh generation and relaxation methods in flow analysis of the tsunami-shelter VAWT. [ABSTRACT FROM AUTHOR]

Analyzing or optimizing wind farm layouts often requires reduced-order wake models to estimate turbine wake interactions and wind velocity. We propose a wake model for vertical-axis wind turbines in streamwise and crosswind directions. Using vorticity data from computational fluid dynamic simulations and cross-validated Gaussian distribution fitting, we produced a wake model that can estimate normalized wake velocity deficits of an isolated vertical-axis wind turbine using normalized downstream and lateral positions, tip-speed ratio, and solidity. Compared with computational fluid dynamics, taking over a day to run one simulation, our wake model predicts a velocity deficit in under a second with an appropriate accuracy and computational cost necessary for wind farm optimization. The model agreed with two experimental studies producing percent differences of the maximum wake deficit of 6.3% and 14.6%. The wake model includes multiple wake interactions and blade aerodynamics to calculate power, allowing its use in wind farm layout analysis and optimization. [ABSTRACT FROM AUTHOR]

In this study, computational fluid dynamics (CFD) is employed to evaluate the influence of surrounding buildings on the performance of a roof-mounted, 2-bladed Savonius vertical-axis wind turbine (VAWT). The latter is planned to be located in the Bovisa Campus of Politecnico di Milano. In the present work a preliminary simulation campaign has been conducted, explicitly depicting the surrounding area and employing an advanced Reynolds-averaged Navier-Stokes (RANS) model. This closure is suitable for Atmospheric Boundary Layer (ABL) simulation, reliably reproducing the various ground roughness elements and employing a Building Influence Area (BIA) for a more accurate representation of the disturbed flowfield. After considering twelve main wind directions, the resulting velocity profiles are extracted and used as inlet conditions for a second session of simulations, related to the wind turbine. The final goal is to reproduce the effect of the surrounding buildings and to accurately forecast the energy production of the machine. This is a relevant aspect of the increasingly topical framework of smart city, implying the exploitation of wind energy. Outcomes indicate that the resulting energy production of the machine remarkably departs from ideal conditions and that accounting for the surrounding topography becomes an aspect of great relevance. • CFD study on influence of urban environment on Savonius wind turbine performance. • Atmospheric turbulence model proposed and validated over experimental data. • Flowfield in Bovisa Campus studied considering 12 wind directions. • Wind profiles extracted and employed for subsequent wind turbine simulation. • Energy production of the machine forecasted and compared with ideal conditions. [ABSTRACT FROM AUTHOR]

In this work, a new airfoil shape optimized for vertical-axis wind turbine applications is proposed. Different airfoil shapes have been analyzed with JavaFoil, a panel method software. Then, the results from the analysis have been used to optimize the performance of the new airfoil shape. Afterward, Computational Fluid Dynamics (CFD) simulations of the proposed airfoil, UO-17-LDA, are run for different angles of attack to provide insight into the flow field and the mechanisms related to this increase in performance. The UO-17-LDA airfoil presents a high lift-to-drag ratio and a delayed stall angle with respect to the original FX-63-137 airfoil, making it suitable for vertical-axis wind turbine applications. This increase in performance has been verified by comparing two VAWT designs with the original and the proposed airfoil using a double-multiple streamtube model. Finally, the practicality of JavaFoil for the comparison of different airfoil geometries has been verified, as it is capable of obtaining results for a wide number of flow conditions in small computational times and with a user-friendly interface. Nevertheless, the results diverge from the actual solution for high angles of attack (beyond stall). Hence, the time and effort required to perform CFD simulations is justified to gain insight into the actual behavior of a particular airfoil, as well as to obtain a richer analysis of the flow field and the mechanisms related to the airfoil performance. [ABSTRACT FROM AUTHOR]

Wind tunnel tests have been performed of individual and paired H-type Darrieus vertical-axis wind turbines. The turbines in the paired configuration are closely spaced, at 1.2 and 1.3 rotor diameters shaft to shaft, and are counter-rotating. Two directions of rotation were studied, one where the facing (inner) blades move along with the incoming flow, and one where the facing blades move against the wind. The wind tunnel tests confirm a net increase in the power coefficient of the paired configuration compared with twice the power coefficient of the individual turbine. We found average relative increases in the power coefficients between 13% and 16%, which is consistent with numerical studies available in the literature. • Wind tunnel experiments of a pair of closely-spaced VAWTs exhibit a power increase. • Power increase depends on inter-turbine distance and direction of rotation. • Power increase persists to off-design tip-speed ratios. • These experimental findings confirm numerical simulations available in literature. [ABSTRACT FROM AUTHOR]

As the wind power market grows rapidly, the importance of technology for real-time monitoring and diagnosis of wind turbines is increasing. However, most of the developed technologies and research mainly focus on large horizontal-axis wind turbines, and research conducted on small- and medium-sized wind turbines is rare. In this study, a novel low-cost and real-time health monitoring and diagnosis system for the small H-type Darrieus vertical axis wind turbine is proposed. Turbine operating conditions were classified into parked/idle and power production. In the case of the power production condition, abnormality diagnosis was performed using key monitoring parameters, including vibration, fundamental frequency, the bending stress of the tower and generator vibration. The turbine abnormalities were diagnosed in two stages by applying the alert and alarm limits, determined by referring to international standards and material properties and the long-term measurement data together.

Wind energy is widely recognized as a key element of the worldwide effort to limit greenhouse gas emissions. As compared with the general population, environmental activists have a much higher level of knowledge, interest, and capacity to affect the final outcome of a proposed wind turbine facility. To explore how their opinions on wind energy, particularly on vertical‐axis wind turbines, differ from the general public, we administered the same online experimental survey to a general population sample of adult Californians and to a self‐selected sample of online energy and environmental activists. We find that support for wind energy increases with the degree of environmental activism and engagement. The general public prefers vertical‐axis wind turbines in open spaces, away from one's residence. Location and price sensitivity, however, are weaker among activists. Among activists, attitudes about specific vertical‐axis wind turbine technologies are more crystalized and less susceptible to the information effects except on the topic of minimizing bird deaths. Highlights: Attitudes toward wind energy differ among environmental activists and general public.General public prefers vertical‐axis wind turbines (VAWTs) in open space, further from one's residence.Location and price sensitivity are weaker among activists.The potential of minimizing bird deaths increases support for VAWTs. [ABSTRACT FROM AUTHOR]

The paper presents the results of a computational study on the aerodynamics and the performance of a small-scale Vertical-AxisWind Turbine (VAWT) for distributed micro-generation. The complexity of VAWT aerodynamics, which are inherently unsteady and three-dimensional, makes high-fidelity flow models extremely demanding in terms of computational cost, limiting the analysis to mainly 2D or 2.5D Computational Fluid-Dynamics (CFD) approaches. This paper discusses how a proper setting of the computational model opens the way for carrying out fully 3D unsteady CFD simulations of a VAWT. Key aspects of the flow model and of the numerical solution are discussed, in view of limiting the computational cost while maintaining the reliability of the predictions. A set of operating conditions is considered, in terms of tip-speed-ratio (TSR), covering both peak efficiency condition as well as off-design operation. The fidelity of the numerical predictions is assessed via a systematic comparison with the experimental benchmark data available for this turbine, consisting of both performance and wake measurements carried out in the large-scale wind tunnel of the Politecnico di Milano. The analysis of the flow field on the equatorial plane allows highlighting its time-dependent evolution, with the aim of identifying both the periodic flow structures and the onset of dynamic stall. The full three-dimensional character of the computations allows investigating the aerodynamics of the struts and the evolution of the trailing vorticity at the tip of the blades, eventually resulting in periodic large-scale vortices. [ABSTRACT FROM AUTHOR]

Abstract High-rise buildings in urban environments provide tremendous opportunities for harvesting wind energy. For example, a through-building channel is beneficial for wind speed enhancement and thus wind power generation. In this study, we investigate the performance of a Savonius-type vertical-axis wind turbine (VAWT) placed in a confined long channel, based on computational fluid dynamics (CFD) simulations. Two-dimensional (2D) simulations are conducted using ANSYS Fluent, employing the shear-stress transport (SST) k-ω turbulence model. Incoming wind velocity at the channel inlet is U o = 4.05 m/s, corresponding to a Reynolds number Re = 1.0 × 105, based on U o and the diameter (D) of the wind turbine. The channel is 45 D long. Three different widths, 2 D , 3 D and 4 D , of the channel together with different wind turbine locations in the channel are considered. The performance of the Savonius-type VAWT in the long channel is discussed, taking into account the contribution of pressure drop towards power generation, and is compared with that in open space. Results show that the power output (P out) of the Savonius-type VAWT placed in the channel can be significantly increased by 200% in the 2 D -wide channel compared with that in open space. Furthermore, a higher optimum tip-speed ratio (TSR) is observed for the wind turbine located in the channel than in open space. A comprehensive comparison is made between the wind turbine in the channel and that in open space. Finally, the maximum power coefficient of a Savonius-type VAWT in the channel is predicted based on a correction to that of a propeller-type horizontal-axis wind turbine (HAWT) in a long channel. Highlights • The performance of a Savonius-type vertical-axis wind turbine (VAWT) placed in a confined long channel is numerically investigated. • Power output of the Savonius-type VAWT placed in the channel can be significantly increased by 200% in the 2 D -wide channel compared with that in open space. • Higher optimum tip-speed ratio is observed for the turbine located in the channel than in open space. [ABSTRACT FROM AUTHOR]

Abstract Evolutionary based Genetic Algorithms (GAs) were employed to optimize the blade shape of a Savonius-type vertical-axis wind turbine (S-VAWT) as well as the shape and the location of one doubly-curved deflector placed upstream the returning blade of an S-VAWT. To this end, GAs were incorporated into computational fluid dynamics (CFD) simulations. Significant improvement in performance was observed for the S-VAWT with optimal blades and that with the optimal deflector. The time-averaged power coefficient of the S-VAWT with optimal blades can be increased by up to 34% over that of the S-VAWT with semi-circular blades. For the S-VAWT with the optimal deflector, its time-averaged power coefficient can be boosted by 95%, compared with that without a deflector. The underlying mechanisms responsible for the performance improvement are discussed based on numerical field data from the CFD simulations. [ABSTRACT FROM AUTHOR]

Abstract The performance of multiple vertical-axis wind turbines (VAWTs) can be greatly enhanced when they are placed close in some configurations where positive interaction or coupling occurs. Based on a systematic study using computational fluid dynamics (CFD) simulations, we investigate the positive coupling of two co-rotating Savonius-type VAWTs. A wide range of configurations are covered, i.e., P/D = 1.2−3.0 and α = -90°−90°, where P is the distance between the turbines' centres, D is the diameter of turbine rotation, and α is the incident angle between the line connecting the turbines' centres and incoming wind's direction. It is found that the time-averaged power coefficient of the upstream turbine can be significantly improved by up to 60% at (P/D, α) = (2.2, 80o), compared to that of a single, isolated turbine. For the two-turbine system, the overall power coefficient can be increased by 40% over that of a single, isolated turbine. Typical configurations corresponding to different positive couplings of the two turbines are identified. The behaviour of torque on the individual turbines and the associated flow structures are discussed. [ABSTRACT FROM AUTHOR]

To investigate the optimum layouts of small vertical-axis wind turbines, a two-dimensional analysis of dynamic fluid body interaction is performed via computational fluid dynamics for a rotor pair in various configurations. The rotational speed of each turbine rotor (diameter: D = 50 mm) varies based on the equation of motion. First, the dependence of rotor performance on the gap distance (gap) between two rotors is investigated. For parallel layouts, counter-down (CD) layouts with blades moving downwind in the gap region yield a higher mean power than counter-up (CU) layouts with blades moving upwind in the gap region. CD layouts with gap/D = 0.5–1.0 yield a maximum average power that is 23% higher than that of an isolated single rotor. Assuming isotropic bidirectional wind speed, co-rotating (CO) layouts with the same rotational direction are superior to the combination of CD and CU layouts regardless of the gap distance. For tandem layouts, the inverse-rotation (IR) configuration shows an earlier wake recovery than the CO configuration. For 16-wind-direction layouts, both the IR and CO configurations indicate similar power distribution at gap/D = 2.0. For the first time, this study demonstrates the phase synchronization of two rotors via numerical simulation.

In this work, the aerodynamic performance and optimization of a vertical-axis wind turbine with a high tip-speed ratio are theoretically studied on the basis of the two-dimensional airfoil theory. By dividing the rotating plane of the airfoil into the upwind and downwind areas, the relationship among the angle of attack, azimuth, pitch angle, and tip-speed ratio is derived using the quasi-steady aerodynamic model, and aerodynamic loads on the airfoil are then obtained. By applying the polynomial approximation to functions of lift and drag coefficients with the angle of attack for symmetric and asymmetric airfoils, respectively, explicit expressions of aerodynamic loads as functions of the angle of attack are obtained. The performance of a fixed-pitch blade is studied by employing a NACA0012 model, and influences of the tip speed ratio, pitch angle, chord length, rotor radius, incoming wind speed and rotational speed on the performance of the blade are discussed. Furthermore, the optimization problem based on the dynamic-pitch method is investigated by considering the maximum value problem of the instantaneous torque as a function of the pitch angle. Dynamic-pitch laws for symmetric and asymmetric airfoils are derived.

The main purpose of this work is to analyze the usefulness of the active cell model (ACM) developed by the author of this article to estimate the flow field around a single-bladed vertical-axis wind turbine (VAWT) with the Darrieus-type rotor. The obtained flow velocity fields were compared with the experimental values taken from the literature available on the Internet. Additionally, the flow fields around the rotor and the aerodynamic forces were determined using the following approaches: the 𝑘-𝜀 RNG turbulence model, the scale-adaptive simulation (SAS) and the laminar model. The velocity profiles behind the turbine rotor obtained with all numerical approaches are consistent with the experiment. The aerodynamic blade loads obtained using numerical methods also appear to be satisfactory.

The purpose of this study is the analysis of flow around a one-bladed Darrieus-type wind turbine using computational fluid dynamics (CFD). The rotor geometry consists of a NACA 0015 airfoil with chord length of 0.15 m. Numerical simulations are performed using ANSYS Fluent, employing laminar model and two turbulence models: SST k-ω and RNG k-ε. The obtained numerical results of unsteady aerodynamic blade loads are compared with available experimental results from literature. Computed aerodynamic characteristics of normal and tangential forces comply with the experiment results. The RNG k-ε turbulence model has a good accuracy in determining aerodynamic blade loads for the upwind and downwind parts of the rotor. The laminar model and the SST k-ω turbulence model slightly overestimate the tangential aerodynamic blade loads at the downwind part of the rotor. An averaged wind turbine velocity profile computed at one rotor radius downstream of the rotor has a Gaussian shape. The steady-state airfoil characteristics are computed for the Reynolds number comparable to the Reynolds number of a moving blade employing the SST k-ω and RNG k-ε turbulence models and using the same computational grid as for unsteady simulations of the rotor. [ABSTRACT FROM AUTHOR]

NENUPHAR aims at developing the next generation of large-scale floating offshore vertical-axis wind turbine. To challenge the horizontal-axis wind turbine, the variable blade pitch control appears to be a promising solution. This article focuses on blade pitch law optimization and resulting power and thrust gain depending on the operational conditions. The aerodynamics resulting from the implementation of a variable blade pitch control are studied through numerical simulations, either with a three-dimensional vortex code or with two-dimensional Navier-stokes simulations (two-dimensional computational fluid dynamics). Results showed that the three-dimensional vortex code used as quasi-two-dimensional succeeded to give aerodynamic loads in very good agreement with two-dimensional computational fluid dynamics simulation results. The three-dimensional-vortex code was then used in three-dimensional configuration, highlighting that the variable pitch can enhance the vertical-axis wind turbine power coefficient (Cp) by more than 15% in maximum power point tracking mode and decrease it by more than 75% in power limitation mode while keeping the thrust below its rated value. [ABSTRACT FROM AUTHOR]

The paper presents the results of a computational study on the aerodynamics and the performance of a small-scale Vertical-Axis Wind Turbine (VAWT) for distributed micro-generation. The complexity of VAWT aerodynamics, which are inherently unsteady and three-dimensional, makes high-fidelity flow models extremely demanding in terms of computational cost, limiting the analysis to mainly 2D or 2.5D Computational Fluid-Dynamics (CFD) approaches. This paper discusses how a proper setting of the computational model opens the way for carrying out fully 3D unsteady CFD simulations of a VAWT. Key aspects of the flow model and of the numerical solution are discussed, in view of limiting the computational cost while maintaining the reliability of the predictions. A set of operating conditions is considered, in terms of tip-speed-ratio (TSR), covering both peak efficiency condition as well as off-design operation. The fidelity of the numerical predictions is assessed via a systematic comparison with the experimental benchmark data available for this turbine, consisting of both performance and wake measurements carried out in the large-scale wind tunnel of the Politecnico di Milano. The analysis of the flow field on the equatorial plane allows highlighting its time-dependent evolution, with the aim of identifying both the periodic flow structures and the onset of dynamic stall. The full three-dimensional character of the computations allows investigating the aerodynamics of the struts and the evolution of the trailing vorticity at the tip of the blades, eventually resulting in periodic large-scale vortices.

In this paper, a dynamic stall control scheme for vertical-axis wind turbine (VAWT) based on pulsed dielectric-barrier-discharge (DBD) plasma actuation is proposed using computational fluid dynamics (CFD). The trend of the wind turbine power coefficient with the tip speed ratio is verified, and the numerical simulation can describe the typical dynamic stall process of the H-type VAWT. The tangential force coefficient and vorticity contours of the blade are compared, and the regular pattern of the VAWT dynamic stall under different tip speed ratios is obtained. Based on the understanding the dynamic stall phenomenon in flow field, the effect of the azimuth of the plasma actuation on the VAWT power is studied. The results show that the azimuth interval of the dynamic stall is approximately 60° or 80° by the different tip speed ratio. The pulsed plasma actuation can suppress dynamic stall. The actuation is optimally applied for the azimuthal position of 60° to 120°.

This work presents an optimized design of a dynamic rotor vertical-axis wind turbine (DR VAWT) which maximizes the operational tip-speed ratio (TSR) range and the average power coefficient (Cp) value while maintaining a low cut-in wind velocity. The DR VAWT is capable of mimicking a Savonius rotor during the start-up phase and transitioning into a Darrieus one with increasing rotor radius at higher TSRs. The design exploits the fact that with increasing rotor radius, the TSR value increases, where the peak power coefficient is attained. A 2.5D improved delayed detached eddy simulation (IDDES) approach was adopted in order to optimize the dynamic rotor design, where results showed that the generated blades’ trajectories can be readily replicated by simple mechanisms in reality. A thorough sensitivity analysis was conducted on the generated optimized blades’ trajectories, where results showed that they were insensitive to values of the Reynolds number. The performance of the DR VAWT turbine with its blades following different trajectories was contrasted with the optimized turbine, where the influence of the blade pitch angle was highlighted. Moreover, a cross comparison between the performance of the proposed design and that of the hybrid Savonius–Darrieus one found in the literature was carefully made. Finally, the effect of airfoil thickness on the performance of the optimized DR VAWT was thoroughly analyzed.

This paper presents a numerical workflow designed and developed for autonomous synthesis of vertical axis wind turbine (VAWT) blades for maximum annual energy production at a specified location given by specified wind speed distribution and prescribed tip speed ratio. The workflow can synthesize shapes of both classical VAWT designs: Darrieus and Savonius rotors. This is achieved using a novel shape parameterization scheme based on B-splines which represents a compromise between shape generality and the multitude of shape variables. The developed computational framework enables the optimizer to synthesize and evaluate a variety of geometrically and even topologically different shapes such as the Darrieus and Savonius types. Moreover, the workflow can invent (i.e. numerically generate without a resembling initial shape) new generic shapes for custom operating conditions. Both single wind-speed and systems related to real-site operating conditions specified by a given distribution of wind speeds are considered. The developed workflow consists of efficient geometry parameterization, a genetic algorithm based optimizer and a computational fluid dynamics based simulator. The rather promising results of custom-shaped vertical axis wind turbines for maximum annual energy production at a given specific site are presented using a set of case studies. [ABSTRACT FROM AUTHOR]

The goal of this paper is to increase the efficiency of using alternative energy sources, namely: to analyze the ways of controlling the angular position of the wind turbine blade. The object of analysis are the wind turbines with a vertical axis of rotation, the subject of analysis is justification for the use of adjustable blades in a vertical-axis rotor. Critical analysis of various methods blade angle control of the wind turbine has been carried out and task of active and passive control systems design for the vertical-axis wind turbine rotor blade angular position has been solved. [ABSTRACT FROM AUTHOR]

Noise levels emitted from a 200 kW H-rotor vertical-axis wind turbine have been measured using a microphone array at four different positions, each at a hub-height distance from the tower. The microphone array, comprising 48 microphones in a spiral pattern, allows for directional mapping of the noise sources in the range of 500 Hz to 4 kHz. The produced images indicate that most of the noise is generated in a narrow azimuth-angle range, compatible with the location where increased turbulence is known to be present in the flow, as a result of the previous passage of a blade and its support arms. It is also shown that a semi-empirical model for inflow-turbulence noise seems to produce noise levels of the correct order of magnitude, based on the amount of turbulence that could be expected from power extraction considerations. [ABSTRACT FROM AUTHOR]

Large eddy simulation (LES) is coupled with a turbine model to study the structure of the wake behind a vertical-axis wind turbine (VAWT). In the simulations, a tuning-free anisotropic minimum dissipation model is used to parameterise the subfilter stress tensor, while the turbine-induced forces are modelled with an actuator line technique. The LES framework is first validated in the simulation of the wake behind a model straight-bladed VAWT placed in the water channel and then used to study the wake structure downwind of a full-scale VAWT sited in the atmospheric boundary layer. In particular, the self-similarity of the wake is examined, and it is found that the wake velocity deficit can be well characterised by a two-dimensional multivariate Gaussian distribution. By assuming a self-similar Gaussian distribution of the velocity deficit, and applying mass and momentum conservation, an analytical model is developed and tested to predict the maximum velocity deficit downwind of the turbine. Also, a simple parameterisation of VAWTs for LES with very coarse grid resolutions is proposed, in which the turbine is modelled as a rectangular porous plate with the same thrust coefficient. The simulation results show that, after some downwind distance (x/D≈ 6), both actuator line and rectangular porous plate models have similar predictions for the mean velocity deficit. These results are of particular importance in simulations of large wind farms where, due to the coarse spatial resolution, the flow around individual VAWTs is not resolved. [ABSTRACT FROM PUBLISHER]

A possible way to capture the kinetic energy available in the wind and then convert it into a useful form of energy is to use wind turbines. Out of many kinds of wind turbines, a lot of in-depth studies have been conducted to improve the performance of the drag-based vertical-axis wind turbine called Savonius rotor. This kind of vertical-axis wind turbine usually suffers from poor energy conversion efficiency, particularly for its conventional shape (thin half-cylinders). In the current work, a bionic blade shape of the Savonius-type wind turbine rotor inspired by sandeels (also called sandlances) is proposed, combined with an optimization step in order to maximize performance. Sandeels are slim elongated fish that often swim in vast shoals. The bionic blade is described by two principal geometrical parameters, both divided by the blade length (d); the maximum blade camber (f m a x) and its location (X f m a x ). The optimization relies on evolutionary algorithms to maximize the average power coefficient of the Savonius rotor with sandeel-inspired blades when varying the two design parameters. A total of 128 individuals have been evaluated numerically using a series of two-dimensional transient simulations with the software Star-CCM+. In comparison to the conventional rotor with two arc-type semi-circular blades, the optimal design with sandeel-inspired blades exhibits a considerable increase in turbine efficiency of up to 9.21% at the design tip speed ratio (λ = 0.8). Investigating the performance over the entire operating range results in a maximum power coefficient of 0.255 by the optimal rotor, leading to a gain of about 10.58% with respect to the standard rotor at λ =1. [Display omitted] • A geometrical analogy between sandeels and blade profiles is suggested for Savonius wind turbines. • Two main design parameters (maximum camber value and its location) are considered. • Evolutionary strategies are considered to maximize the average power coefficient of Savonius rotor. • The optimal design with sandeel-inspired blades showed an enhancement of 10.58% in performance at λ =1. [ABSTRACT FROM AUTHOR]