This paper deals with an aeroelastic problem that consists into self-sustained, pitch-heave oscillations of an elastically-mounted airfoil. Such oscillations of an airfoil could be used in order to ...develop a novel, fully-passive hydrokinetic energy flow harvester that is relatively simple from a mechanical point of view. Indeed, the motion of such an airfoil emerges as a result of the fluid-structure interaction between the flow, the airfoil and its elastic supports, and is sustained through a net transfer of energy from the flow to the structure. In this numerical study, the OpenFOAM-2.1.x CFD toolbox is used for solving the aeroelastic problem. Through unsteady two-dimensional viscous simulations at a Reynolds number of 500,000, the fully-passive turbine is optimized and investigated to develop a better understanding of the physics at play. Following a gradient-like optimization of the turbine, two-dimensional efficiencies as high as 34% have been obtained, and two fundamental mechanisms have been found to be very beneficial for enhancing the performances of the turbine: the adequate synchronization between both degrees-of-freedom, and the nonsinusoidal shape of the pitching motion.
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•Comprehensive assessment of ALM compared to blade-resolved CFD for twin Darrieus turbines.•ALM successfully captures the efficiency increase resulting from the close deployment of ...the rotors.•The instantaneous angle of attack sampled from the ALM aligns with that of the blade-resolved CFD.•POD-based wake analysis reveals that the reproduced ALM results are sensitive to the TSR.•The ALM simulation time is two orders of magnitude less than that of the blade-resolved CFD.
The deployment of multiple closely-spaced Darrieus turbines has recently gained interest in the academic and industrial sectors due to their potential to increase power output and wake recovery. However, the computational cost of accurate three-dimensional, blade-resolved CFD analyses rapidly becomes unfeasible as long as multiple rotors are added, putting emphasis on the need for a more practical, yet accurate, simulation method. Based on recent assessments, the Actuator Line Method (ALM) has been shown to represent the most interesting solution for Darrieus turbines’ simulations. However, since the blade-flow interaction is not solved, no evidence was available to date on whether ALM can capture all the physical phenomena related to turbine mutual interaction.
In this study, the reliability and limitations of the ALM in simulating closely-spaced Darrieus turbines were assessed using two pairs of turbines with varying geometrical and operational characteristics, focusing on turbine loads, local flow field, and wake development. The results demonstrate that the ALM is effective for simulating twin-rotor Darrieus turbines, particularly at medium and high tip-speed ratios, providing satisfactory predictions of instantaneous blade loads and capturing the increase in torque due to the mutual interaction between adjacent rotors. The ALM is also able to accurately reproduce the flow field across the twin rotors, as demonstrated through Proper Orthogonal Decomposition (POD) analysis of the wake flow field. With an average simulation time of 7 h/rev/core, compared to 508 h/rev/core for blade-resolved CFD, this study provides evidence that the ALM is a low-cost and reliable tool for simulating closely spaced Darrieus turbines, thus representing the most suitable tool to develop new VAWT applications like floating wind turbines or hydrokinetic installations.
Utilization of electrical energy is the key to economic growth and improvement in people׳s living standards, especially in developing countries. The growing demand for electrical energy and the ...environmental effect of fossil fuel usage are the main topics driving us towards renewable technology. Hydropower, mainly hydrokinetic energy technology, is a well-known source of renewable energy. This study was carried out to present the potential of hydrokinetic energy in the world and Malaysia. Relevant research literature, based on developments, applications, design, operation, efficiency as well as different hydrokinetic technologies have been reviewed. This work critically considers the main characteristics of Malaysian current and water depth and the challenges associated with enhancing the efficiency of hydrokinetic turbines and providing electricity to remote areas with access to running water but little electricity. This paper will aid researchers to identify areas that need to be improved, as well as encourage public bodies to implement proper energy policies regarding hydrokinetic energy technology usage in rural areas with low-speed currents.
Summary
Increased demand of energy leads to exploration of new sources of energy. In the last few years, hydrokinetic energy technology emerged as an important area in the field of renewable energy ...generation. Various hydrokinetic turbines have been studied and used to harness the hydrokinetic potential. Among all hydrokinetic turbine technology, cross‐flow hydrokinetic turbine is considered as the most suitable approach for the riverine system. There are various configurations of cross‐flow hydrokinetic turbine exist for hydrokinetic energy extraction. A number of numerical and experimental studies were carried out on the performance enhancement and design optimization of different configurations under variable operating conditions. Under the present paper, review of different rotor for the configurations of the cross‐flow hydrokinetic turbines are discussed. The paper will be useful to understand the cross‐flow hydrokinetic technology in order to explore the methods for selection, performance enhancement, and design optimization of cross‐flow hydrokinetic turbines.
Present review deals with various types of configurations for cross‐flow hydrokinetic turbine. These configurations have certain pros and cons. Therefore, the combination of lift and drag types of configuration were suggested by various investigators, and it has been found that the proposed rotor configuration has the characteristics of both (lift and drag) configurations and reduces drawbacks of each other.
Large Eddy Simulations (LES) are performed on a Vertical Axis Hydrokinetic Turbine (VAHT) at two different solidities, so as to enable a more complete physical description of the flow than for ...classic statistical calculations. To analyze the turbine performances, local quantities are defined to evaluate the contribution of the different turbine elements to the global VAHT power coefficient. For a deeper analysis of the major losses, the real turbine is also compared with an ideal turbine composed of only three infinite blades. It is observed that the ideal turbine with the lower solidity provides the best performance, but the losses due to the blade tips and the arms strongly increase for the real turbine at the same solidity. Consequently, for the considered real turbine, there is no clear gain to decrease the solidity. Simulations of the ideal turbine are performed for various solidities at their optimal Tip Speed Ratio (TSR) to study the evolution of optimal power coefficient as a function of solidity. A maximum power coefficient is obtained for a small value of the solidity. This is explained because the optimal TSR of this optimal solidity leads to angles of incidences on the blade which avoid a penalizing dynamic stall phenomenon but are high enough to produce an important positive torque The design of an efficient turbine has then to limit losses, to be able to use small solidity and then to avoid dynamic stall phenomenon by having a high optimal TSR.
•LES of VAHT are performed for two different solidities.•Major losses are evaluated by comparison with ideal turbines.•The optimal power coefficient is evaluated as a function of solidity.
The effect of hub length and axial location on the performance of shrouded hydrokinetic turbines has been investigated using axisymmetric actuator disk simulations. Five systems with different hubs ...(or without a hub, for reference) are considered with a common shroud design. Flow separation on the hub was found to be detrimental to performance, and to be sensitive to rotor loading, leading to unacceptable off-design performance. When the thrust coefficient, CT, is below the optimal value of 8/9, increasing CT promotes flow separation on the central hub; continued increases beyond this value facilitate reattachment. Flow separation on the hub was avoided altogether when most of the hub was placed upstream of the rotor plane—contrary to the convention for unshrouded turbines—where the shroud geometry lends a favorable pressure gradient. This latter design achieved performance in close agreement with the open-centered design, which is recommended when feasible to avoid the risks of hub-flow separation. With a central hole diameter of 0.228D, where D is the rotor diameter, a penalty of <5% in optimal performance was found compared to an actuator disk spanning the entire shroud throat. Lastly, a discussion of the relationship between total system thrust and power extraction is offered.
•Hub-flow separation can be avoided by mounting the hub upstream of the rotor.•Using downstream hubs, flow separation causes unacceptable off-peak performance.•An open-centered rotor can achieve similar peak performance to the best hub design.
Shrouded wind and hydrokinetic turbines are being widely studied nowadays. Although actuator disk studies have concluded that the addition of a diffuser around a rotor can improve the Power ...Coefficient, not many designs have realized those benefits when compared to standard high performance bare turbines, as shown by Nunes et al. (2020) 1. One reason for low power coefficient on several designs is massive flow separation on the hub surface due to high adverse pressure gradient inside the diffuser, resulting on low mass flow capture and, hence, poor performance. This work presents a novel design methodology for shrouded rotors, which takes into consideration the influence of the entire linear cascade on each annular section. Also, the blade root is left unloaded to guarantee that no boundary layer separation occurs on the hub surface by allowing a layer of energized fluid to bypass the rotor. A turbine modeled by this method has been numerically studied and is shown herein to deliver a peak power coefficient of 0.415 normalized by the diffuser's largest cross sectional area.
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A diffuser-augmented hydrodynamic turbine (DAHT) is composed of three main components: a diffuser, a rotor, and a support structure. The design of the support structure plays a crucial role in the ...performance of the DAHT. It has a significant influence on both the rotor and diffuser performance. However, research on this specific topic is currently limited. In this study, the influence of various parameters such as support distance, the number of columns comprising the support structure and the aspect ratio of support section on the performance of the DAHT is analyzed using blade-resolved Computational Fluid Dynamics (CFD) numerical simulations. Reynolds-Averaged Navier-Stokes (RANS) equations are performed and the k-ω SST turbulence model is utilized to predict the performance of the DAHT. The accuracy of the CFD model is verified by experiments. The results indicate that support structures may significantly reduce the acceleration ratio and power coefficient of the DAHT. With a column number of 3, a support distance of 0.1 and an aspect ratio of 1, the acceleration ratio decreases by approximately 22% and the power coefficient decreases by approximately 26%. It is recommended to choose a front support with a larger aspect ratio and maintain an appropriate support distance.
•A blade-resolved Computational-Fluid-Dynamics model of a diffuser-augmented hydrokinetic turbine is established.•Flume experiment is carried out to verify the CFD model.•Support distance, the number of columns and the aspect ratio on the diffuser-augmented hydrokinetic turbine are evaluated.•Suggestions on support structure are put forward.
A systematic review and analysis of the literature of diffuser-augmented horizontal-axis turbines are presented. A collection of 155 articles in the area is analyzed and classified. The work sample ...is divided into 16 main research branches for discussion. Performance assessment metrics are proposed based on power coefficient and tip-speed-ratio, to quantify and compare all diffuser-augmented turbines in a unified, meaningful manner. Design suggestions for the development of diffuser-augmented turbines are pointed out based on the analysis of 73 cases. A power coefficient assessment on the work sample presented that, in 58% of the cases, the diffuser-augmented turbines surpassed the power coefficient of scaled bare turbines of the same diameter. A tip-speed-ratio assessment presented that almost 90% of the diffuser-augmented turbines developed a narrower operational interval. Five high-performing diffuser-augmented turbines are discussed, highlighting their methodologies and contributions. Caution is advised when coupling a diffuser to a bare turbine with an already high power coefficient; the diffuser-augmented turbine, especially in those cases, should be designed employing a simultaneous diffuser-rotor optimization.
•Systematic analysis of the current literature on diffuser-augmented horizontal-axis turbines is made.•A collection of 155 articles is dived in 16 research branches and discussed.•A diffuser-enhancement parameter is defined and used to compare the effectiveness of diffusers.•In 58% of the DAHT analyzed, the diffuser increases the Cp when compared to a bare turbine scaled to the diffuser diameter.•Diffuser-augmented horizontal turbines tend to display narrower Cp versus TSR curves compared to its bare version.
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•Insight on the key factors affecting accuracy of ALM for VAWTs.•Frozen ALM + blade-resolved CFD to understand current limitations.•Kernel shape/size do not affect significantly ...accuracy.•Mesh size in the wake is key for a good resolution.•Dynamic stall modeling is the key factor determining accuracy.
Darrieus vertical-axis turbines are known for their complex aerodynamics connected to the continuous change in the angle of attack experienced by the blades, which often exceeds the static stall limit. Low fidelity tools such as the Blade Element Momentum Theory have been shown lately not to provide sufficient levels of accuracy, while the medium-fidelity Actuator Line Method (ALM) has been increasingly applied to Darrieus rotors. In this method, the blade-flow interaction is modeled as an equivalent momentum loss calculated introducing equivalent aerodynamic forces into the computed Computational Fluid Dynamics (CFD) domain. This strongly reduces the computational cost in comparison to blade-resolved CFD, allowing ALM to be used in three-dimensional problems, e.g., multiple rotors, floating offshore, etc. While several corrections and guidelines have been recently proposed to tailor ALM to Darrieus turbines, issues are still open on how to improve accuracy.
The present study aims at assessing to what extent the three main factors of the ALM theory, namely the quality of input polar, the dynamic stall modeling, and the force insertion in the domain, influence the overall accuracy of the method. In particular, this unprecedented understanding is enabled by the novel use of a “frozen ALM”, i.e., an ALM method fed by the aerodynamic forces calculated by blade-resolved CFD, which allowed to separate the contributions coming from airfoil performance analysis and force projection in the domain. Based on the results, three main important conclusions are drafted out: i) for high and medium tip-speed ratios, provided that the aerodynamic forces are correct, the ALM method is able to generate extremely accurate solutions of the flow field, almost equivalent to blade-resolved CFD; ii) the relevance of the kernel’s shape and smearing function is largely overestimated and current knowledge is adequate for the model to be set; iii) a better dynamic stall model is indeed the real key factor that could lead to an improvement of the ALM accuracy.
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