Two-bladed Turbine Research Articles

Aerodynamic Analysis of a Two-Bladed Vertical-Axis Wind Turbine Using a Coupled Unsteady RANS and Actuator Line Model

Close-packed contra-rotating vertical-axis turbines have potential advantages in wind and hydrokinetic power generation. This paper describes the development of a numerical model of a vertical axis turbine with a torque-controlled system using an actuator line model (ALM). The developed model, coupled with the open-source OpenFOAM computational fluid dynamics (CFD) code, is used to examine the characteristics of turbulent flow behind a single two-bladed vertical-axis turbine (VAT). The flow field containing the turbine is simulated by solving the unsteady Reynolds-averaged Navier-Stokes (URANS) equations with a k - ω shear stress transport (SST) turbulence model. The numerical model is validated against experimental measurements from a two-bladed H-type wind turbine. Turbine loading is predicted, and the vorticity distribution is investigated in the vicinity of the turbine. Satisfactory overall agreement is obtained between numerical predictions and measured data on thrust coefficients. The model captures important three-dimensional flow features that contribute to wake recovery behind a vertical-axis turbine, which will be useful for future studies of close-packed rotors with a large number of blades.

Parameter Analysis of Savonius Hydraulic Turbine Considering the Effect of Reducing Flow Velocity

The Savonius-type hydraulic turbine, mainly known for its good self-starting properties and simple structure, not only has energy capturing characteristics but also has a certain effect on flow velocity reduction. Aside from ensuring energy capture efficiency, studying the effects of parameters on the flow velocity reduction capacity is of great significance for the protection of mariculture, as it can reduce the damage to cages and fishes. In this study, a computational fluid dynamics method was carried out to investigate the hydrodynamic characteristics and variations in the wake of a turbine. The accuracy of the simulation results was verified by experimental comparison. Firstly, the velocity contours and vectors were studied in detail to reveal the mechanism of the flow velocity reduction effect. Secondly, the velocity attenuation coefficient and relative attenuation length were formulated by the variation rule of the velocity field to evaluate the turbine reduction strength and range. Finally, the power coefficient was considered to predict the performance of a turbine under different tip speed ratios, overlap ratios, blade curvatures, and blade numbers. The results showed that the turbine had an obvious flow velocity reduction effect in the rear “sword”-shaped area, where the velocity field distribution had a certain regularity. In addition, by comprehensively comparing the simulation data, it was found that the respective effect trend of tip speed ratio, blade number, overlap ratio, and curvature on the turbine’s energy capture and flow velocity reduction characteristics was basically the same. Considering the effect of reducing flow velocity, a two-bladed turbine with a blade curvature of 0.8 and an overlap ratio of 0.15 is the optimal configuration.

Experimental Investigation of a Downwind Coned Wind Turbine Rotor under Yawed Conditions: Preliminary Results

The growing number of Floating Offshore Wind Turbine (FOWT) concepts that utilize a single point mooring and therefore rely on the self-alignment capabilities of the wind turbine (e.g. SCD nezzy or SelfAligner by CRUSE Offshore) demands an extension of the simulation methods used for their development. A crucial issue for these concepts is the accurate prediction of forces and moments, which contribute to the self-alignment. In contrast to the well-studied behaviour of torque and thrust, yaw moment and lateral forces on a rotor under yawed conditions have not been in focus of previous experimental tests for the validation of aerodynamic simulation tools. In the present work, a model turbine equipped with a 6-axis force/moment sensor to capture the complete load on the rotor is presented. A detailed study of the two-bladed model turbine’s aerodynamic behaviour under yawed conditions was carried out within a range of yaw angles between -55 to + 55° with steps of 1 – 2.5°.

A new approach for comparability of two- and three-bladed 20 MW offshore wind turbines

Wind energy plants are subject to a vast optimization process with a multitude of design parameters. While one specific three-bladed turbine type dominates the on- and offshore market, two-bladed turbines offer promising, but not yet quantified, potentials for cost savings during manufacturing, erection, and maintenance, offshore. Nevertheless, the comparability of two- and three-bladed turbines is challenging, causing an ongoing discussion within research and industry about which alternative to prefer. A new approach could be to reduce the important changes made by maintaining the exact same energy yield. This results in an increase of design changes of the two-bladed rotor, due to slightly (about 2%) longer blades to counterbalance the inevitable losses of aerodynamic efficiency. Nevertheless, it has the great advantage that subsequent comparisons of loads, masses and costs do not have to be associated with a loss of power. This could serve as a solid basis to allow drawing conclusions and considering differences in dynamical loads, masses and costs more directly and hopefully more expediently, when comparing two- and three-bladed wind turbines.

Drift stability of HyStOH semi-submersible supported by airfoil shaped structures

This study presents the results of the German research project HyStOH funded by German Federal Ministry of Economic Affairs and Energy (BMWi). The project consortium of German universities, wind turbine designers, wind farm developers and certification bodies design a novel semi-submersible steel structure with a single point mooring and self-aligning capabilities. The tower, which carries a 6 MW two-bladed downwind turbine, has an airfoil shaped cover, which supports the self-alignment of the full structure towards the main wind direction. The downwind operating wind turbine in combination with the lift force generating tower enables a passive yaw system acting against hydrodynamical impacts from waves and currents. Simulations in time domain applying the fully coupled aero-hydro-servo-elastic code Bladed_4.8 demonstrate the drift sensitivity and the self-aligning capabilities of the HyStOH design. The hydrodynamic coefficients for the simulations have been adjusted by calculations with an 1:1 model of the panel code panMARE, developed by Technical University of Hamburg-Harburg. The motivation of this analysis is to capture the complex coupled motions of an innovative FOWT by numerical simulation tools [1]. The analysis presents dynamic simulations of the HyStOH design operating in turbulent wind and irregular sea state with special focus on the yaw drift behaviour of the FOWT. The sensitivity study of yaw drift is based on numerous variations of wind-wave-current misalignments during normal operation. The results of the simulations demonstrate the weathervane capabilities of the airfoil shaped structures.

Parametric analysis of model Savonius hydrokinetic turbines through experimental and computational investigations

The drag-based Savonius hydrokinetic turbine (SHT) has an enormous potential for small-scale power generation from free-flowing water and it can be deployed especially at sites remote from existing electricity grids. These turbines can be installed in waves, tides, ocean currents, natural flow of water in rivers, manmade channels and irrigation canals to produce power. The performance of a SHT are highly influenced by its design parameters such as blade profile, number of blades, overlap ratio and aspect ratio. Although, over a period of nine dacade since its invention, serveral studies have been carried out, however, no particular concencus on the optimum design of SHT is arrived. In view of this, in the present investigation, as attempt has been made to parametrically evaluate the performance of the SHT through experimental and computational fluid dynamics (CFD) analysis. The SHT under investigation has been developed in-house. Initially, a comparison of performance between two- and three-bladed SHT with conventional semicircular blades has been carried out experimentally where their maximum power coefficients are found to be 0.28 and 0.17, respectively at their corresponding tip-speed ratios of 0.84 and 0.67. Further experiments with a two-bladed SHT turbine with elliptical blades have shown its inferior performance as compared to the two-bladed semicircular SHT. The reason behind the enhanced performance of the two-bladed semicircular SHT is then analyzed through two-dimensional CFD simulations. Finally, the experiments are conducted at various immersion levels, where the performances of the SHTs are found to degrade with a decrease in immersion. However, the two-bladed semicircular SHT maintains to have a better performance than the others.

ORTHOGONAL TURBINE IN A STREAM OF RESTRICTED CROSS-SECTION

The paper discusses the problem of using the energy of rivers without creating the dams and flooding vast areas and notes that there is a progress in the implementation and development of the ideas of patents in 1925, 1931 (the speed of the workers turbine blades is higher than flow rate). The paper gives the diagrams of the new turbines of this type, for example a balanced 6-tier single-vane turbine, turbine-spiral, a balanced two-bladed turbine. Moreover, the paper deals with the features free-threaded orthogonal turbines in the streams of the limited width and depth. The most important characteristic of a turbine is the turbine's power factor that is equal to the ratio of the energy of the rotating turbine to the kinetic energy of the flow in the current tube passing through the turbine circuit. There is a possibility of a significant increase in the power of the turbine in comparison with the conditions of use unlimited streams. The increase in turbine power in a straitened flow is associated with an increase in the flow velocity in the turbine on the approach to the rear section of the blades’ track. It is set the requirements of the turbine parameters for maximum power at a given water flow and the permissible level rise in the river. These requirements relate to the certain rules for selecting the number of blades (and shading) of the turbine, taking into account the permissible increasing in the water level (backup) in front of the turbine. The paper notes the turbines instability at low speed of rotation, describes a turbine design modification that eliminates this drawback. Modification of the high-speed orthogonal turbines is the use of accelerating blades with a cup-shaped cross-section, placed on the route within a diameter 2 times smaller than the diameter of the main (working) blades of the smoothly streamlined profile. It is concluded that all considered variants of turbines for streams with limited cross-section, the design of the blade system may be made rigid, which eliminates the single central shaft (axle), replacing it with a reference semishafts.

Modal dynamics of structures with bladed isotropic rotors and its complexity for two-bladed rotors

Abstract. The modal dynamics of structures with bladed isotropic rotors is analyzed using Hill's method. First, analytical derivation of the periodic system matrix shows that isotropic rotors with more than two blades can be represented by an exact Fourier series with 3/rev (three per rotor revolution) as the highest order. For two-bladed rotors, the inverse mass matrix has an infinite Fourier series with harmonic components of decreasing norm; thus, the system matrix can be approximated by a truncated Fourier series of predictable accuracy. Second, a novel method for automatically identifying the principal solutions of Hill's eigenvalue problem is introduced. The corresponding periodic eigenvectors can be used to compute symmetric and antisymmetric components of the two-bladed rotor motion, as well as the additional forward and backward whirling components for rotors with more than two blades. To illustrate the use of these generic methods, a simple wind turbine model is set up with three degrees of freedom for each blade and seven degrees of freedom for the nacelle and drivetrain. First, the model parameters are tuned such that the low-order modal dynamics of a three-bladed 10 MW turbine from previous studies is recaptured. Second, one blade is removed, leading to larger and higher harmonic terms in the system matrix. These harmonic terms lead to modal couplings for the two-bladed turbine that do not exist for the three-bladed turbine. A single mode of a two-bladed turbine will also have several resonance frequencies in both the ground-fixed and rotating frames of reference, which complicates the interpretation of simulated or measured turbine responses.

Effect of the number of blades and solidity on the performance of a vertical axis wind turbine

Two, three and four bladed ϕ-shape Vertical Axis Wind Turbines are simulated using a free-wake vortex model. Two versions of the three and four bladed turbines are considered, one having the same chord length as the two-bladed turbine and the other having the same solidity as the two-bladed turbine. Results of the two-bladed turbine are validated against published experimental data of power coefficient and instantaneous torque. The effect of solidity on the power coefficient is presented and the instantaneous torque, thrust and lateral force of the two-, three- and four-bladed turbines are compared for the same solidity. It is found that increasing the number of blades from two to three significantly reduces the torque, thrust and lateral force ripples. Adding a fourth blade further reduces the ripples except for the torque at low tip speed ratio. This work aims to help choosing the number of blades during the design phase of a vertical axis wind turbine.

水平軸型風車特性の数値解析

It is difficult to predict only by a wind tunnel experiment, because there are the problems such as the scale effect with respect to the actual wind turbine performance. Therefore, the practical use of CFD that the enough validation was made by experiments becomes more and more important in future. In this study, in order to validate an analysis precision of CFD, the results of two-bladed and three-bladed wind turbines are compared with experiments. The accuracy of the performance prediction was investigated. The possibility that the errors overlooked by the experiments and the unprecedented phenomenon are found has been shown. In addition, for two-bladed and three-bladed wind turbines both, the predictions of the thrust coefficients and the power coefficients are in agreement with those of experiment. The effectiveness of the performance prediction by CFD has been shown.

Description and Computer Modeling of a Ball-and-Socket Hub That Enables Teetering for Three-Bladed Wind Turbines

A horizontal axis wind turbine with a ball-and-socket hub is disclosed. The hub enables horizontal axis turbines with two or more blades to teeter in response to wind shear gradients. Computer modeling was done using existing and modified fast code in order to compare the new hub design with existing designs. Results show that a three-bladed turbine with the ball-and-socket hub provides very significant reductions in out-of-plane bending loads applied to the main shaft in comparison to a three-bladed turbine with a rigid hub. Results also show that the new hub design provides significant reductions in the out-of-plane loads applied to the blades. A blade fatigue study using a rainflow counting of multi-axial torque contributions at the blade root was performed in order to assess the impact of these reductions, and results show that the three-bladed turbine equipped with a ball-and-socket, teetering hub provides for very significant reductions in lifetime blade damage in comparison to existing wind turbine designs due to a combination of factors. The first factor is that teetering largely eliminates the cyclic variations in out-of-plane torque on the blades that are observed with rigid hubs. Here, the fatigue study shows that the three-bladed wind turbine with a teetering hub provides for an approximate sixfold reduction in lifetime blade damage in comparison to a three-bladed turbine with a rigid hub. The second factor is that the addition of a third blade reduces the load on each blade by one-third. Here, the fatigue study shows that a three-bladed turbine with a teetering hub provides for an approximate fourfold reduction in lifetime blade damage in comparison to a two-bladed turbine with a teetering hub.

Experimental Study for the Single-stage and Double-stage Two-bladed Savonius Micro-sized Turbine for Rain Water Harvesting (RWH) System

Abstract This paper examines the performance of the single-stage and double-stage two-bladed micro-sized turbine for the Rain Water Harvesting (RWH) System. RWH system is a method of collecting rain water and storing this free rain water into a tank before reused it for a particular purpose. In this paper, the performance study was focused on the electrical generation capability when Savonius turbine is used. The Savonius turbines were designed and built using an aluminium sheet with an aspect ratio (AR) of 1.8, with height (H) of 8 cm and diameter (D) of 4.5 cm. In this study, performance is compared between the single-stage and the double-stage designs in terms of electrical power generation when the incoming water flows from the water tank passing the micro-Savonius blades in the pipeline. From the finding, it was found that the designed and the built systems have good performance in terms of producing a constant voltage and current. The single-stage produced better performance than double-stage rotor, where power can be generated almost double. Rotor can rotate up to 1280 rpm when single-stage two-bladed Savonius rotor is used, compared to the double-stage rotor. The single-stage two-bladed Savonius micro-sized turbine also capable to light the 0.3 watt LED lamps.

Teeter end impacts: analysis and classification of most unfavourable events

Two-bladed wind turbines are recently being discussed more often as the question arises for the most suitable offshore turbine concept. Regarding this turbine concept, a solution is required for the more challenging dynamics. A teetered hub has often been a load reduction concept of two-bladed turbines. During normal operation, a teetered hub eliminates the hub bending moment coming from unequal blade loading. But looking at extreme load cases, the teeter end impact is a major problem. The teeter end impact is quite often described as the occasion that destroys the load-reducing advantage of the teeter mechanism. The turbine must be designed to withstand the loads from the teeter impacts leading to additional weight, which is actually supposed to be reduced by using the teeter hinge. Although the teeter end impact is often described as a kind of ‘killer aspect’, a more detailed analysis and quantification of its nature is not given in open literature. This paper will do an analysis and quantification of the loads coming from teeter end impacts using an existing teetered turbine, the Controls Advanced Research Turbine 2 (CART2). First, there will be a look at analytical teeter equations to get an overview of the basic parameters leading to teeter movement. Then, teeter end impact behaviour will be analysed using aeroelastic load simulations of the CART2 according to International Electrotechnical Commission (IEC) 61400-1 edition 3. For each design load case, the most significant teeter response will be examined. A classification of teeter end impacts will be extracted from the simulation data. Results will be compared with a rigid turbine in order to get an evaluation of how severe teeter end impacts are, compared with extreme loads of a rigid turbine. Additionally, these results will be compared with modified teeter parameters of the CART2. These are the introduction of pitch–teeter coupling, a reduced free teeter angle and a different Lock number. It will be shown to what extent these parameters may reduce the intensity of teeter end impacts. Results show that it is worth discussing teetered turbines as an alternative to today's three-bladed turbines. According to this study, teeter end impacts need not be regarded as completely intolerable, and there are several turbine parameters that have a significant influence on them. Copyright © 2015 John Wiley & Sons, Ltd.

Wind tunnel experiments of a newly developed two-bladed Savonius-style wind turbine

Wind tunnel experiments have been conducted with a newly developed two-bladed Savonius-style wind turbine specifically meant for a small-scale energy conversion. This novel shape of the turbine blade is evolved from a series of experiments with different types of blades in the recent past. The developed two-bladed turbine is tested in an open type test section and its performance is assessed in terms of power and torque coefficients. Experiments have also been conducted with other standard blades such as semi-circular, semi-elliptic, Benesh and Bach types in order to have a direct comparison. In this study, all the reported experimental data are inclusive of wind tunnel blockage corrections. Further, the effects of Reynolds number on the dynamic and static characteristics are also discussed. The present investigation demonstrates a gain of 34.8% in maximum power coefficient with the newly developed two-bladed turbine.

Relevance of aerodynamic modelling for load reduction control strategies of two-bladed wind turbines

A new load reduction concept is being developed for the two-bladed prototype of the Skywind 3.5MW wind turbine. Due to transport and installation advantages both offshore and in complex terrain two-bladed turbine designs are potentially more cost-effective than comparable three-bladed configurations. A disadvantage of two-bladed wind turbines is the increased fatigue loading, which is a result of asymmetrically distributed rotor forces. The innovative load reduction concept of the Skywind prototype consists of a combination of cyclic pitch control and tumbling rotor kinematics to mitigate periodic structural loading. Aerodynamic design tools must be able to model correctly the advanced dynamics of the rotor. In this paper the impact of the aerodynamic modelling approach is investigated for critical operational modes of a two-bladed wind turbine. Using a lifting line free wake vortex code (FVM) the physical limitations of the classical blade element momentum theory (BEM) can be evaluated. During regular operation vertical shear and yawed inflow are the main contributors to periodic blade load asymmetry. It is shown that the near wake interaction of the blades under such conditions is not fully captured by the correction models of BEM approach. The differing prediction of local induction causes a high fatigue load uncertainty especially for two-bladed turbines. The implementation of both cyclic pitch control and a tumbling rotor can mitigate the fatigue loading by increasing the aerodynamic and structural damping. The influence of the time and space variant vorticity distribution in the near wake is evaluated in detail for different cyclic pitch control functions and tumble dynamics respectively. It is demonstrated that dynamic inflow as well as wake blade interaction have a significant impact on the calculated blade forces and need to be accounted for by the aerodynamic modelling approach.Aeroelastic simulations are carried out using the high fidelity multi body simulation software SIMPACK. The aerodynamic loads are calculated using ECN's AeroModule and NREL's BEM code Aerodynl3.

0520 水平軸型風車と透過性円盤の遠方後流特性について

We dealt with the wake of a permeable disk with different solidity as a simple model to investigate an essence of wake of a wind turbine. Velocity defect at the center of the streamwise cross section of the wake was measured at each downstream position by hot-wire anemometer to characterize the wake. Variations of the velocity defect obey power law in far wake regions and the variation of the corresponding index numbers with respect to the solidity has the local maximum. It suggests there is a regime change on the vortex structure. We also investigated the wake of a model of two-blade turbine. Here the variations of the velocity defect show different tendency in far wake regions.

Montagem e testes de uma bancada para medição de desempenhode aerogeradores em escala

O presente trabalho consistiu na montagem e testes, na disciplina Energia Eólica do curso de Engenharia Mecânica, de uma bancada cujo objetivo é o levantamento de curvas de potência para diferentes pás de aerogeradores, com torção e comprimento da corda calculados pelo método do momento do elemento de pá (BEM). Desta forma as pás foram projetadas e construídas, o aerogerador foi montado e teve levantado seu gráfico de desempenho, que apresenta o coeficiente de potência gerado no eixo versus a velocidade específica na ponta da pá (lambda). Para a realização dos experimentos foi utilizado um ventilador de coluna com potência de 200 W, que gerou uma velocidade média do vento de 3,8m/s, medida com anemômetro portátil. Os testes foram realizados com uma turbina bipá em escala de 1:4, projetada e construída com o perfil S809, que obteve coeficiente de potência máximo de 20,57%. A curva de desempenho da turbina foi traçada, demonstrando sua boa adequação a projetos de aproveitamento de energia eólica.

Comparative study of the performances of twisted two-bladed and three-bladed airfoil shaped H-Darrieus turbines by computational and experimental methods

The vertical axis H-Darrieus turbines are simple in construction, self-starting, omni-directional, and self-regulating at all wind speeds. A comparative study of the performances of twisted two-bladed and three-bladed airfoil shaped H-Darrieus turbines was made. The models of the turbines were designed, fabricated, and tested in a subsonic wind tunnel. The power coefficients (C p ) and torque coefficients (C t ) were evaluated for ten numbers of height-to-diameter (H/D) ratios. Both C p and C t of the two-bladed turbine were higher than the three-bladed turbine. The maximum C p for the two-bladed turbine was 0.237 and that for the three-bladed turbine was 0.151. The performances of the turbines were also compared computationally through 2D steady-state CFD simulations using Fluent 6.2 software. The agreement between the computational and experimental C p was within ± 5.86%. The contour plots of static and dynamic pressures and velocity magnitudes were generated for chord Reynolds number of 8.5 × 10 4 . At any position on the blade, dynamic pressure, velocity and also the velocity gradient between the ends were higher for the two-bladed turbine than the three-bladed turbine.

Vortex motion in doubly connected domains

The unsteady two-dimensional rotational flow past doubly connected domains is analytically addressed. By concentrating the vorticity in point vortices, the flow is modelled as a potential flow with point singularities. The dependence of the complex potential on time is defined according to the Kelvin theorem. The general case of non-null circulations around the solid bodies is discussed. Vortex shedding and time evolution of the circulation past a two-element airfoil and past a two-bladed Darrieus turbine are presented as physically coherent examples.

Vortex capturing vertical axis wind turbine

An analytical-numerical study is presented for an innovative lift vertical axis turbine whose blades are designed with vortex trapping cavities that act as passive flow control devices. The unsteady flow field past one-bladed and two-bladed turbines is described by a combined analytical and numerical method based on conformal mapping and on a blob vortex method.