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Bioinspired Fluid Dynamic Designs of Vertical-Axis Turbines: State-of-the-Art Review and the Way Forward
Abstract
With the increasing popularity of vertical axis turbines (VATs), researchers are now focussing on their performance improvement. Instead of adopting conventional means of performance improvements such as augmentation techniques and exhaustive parametric design optimization, the bioinspired turbine designs have become a centre of attraction, especially during the last decade. This review article attempts to compile the bioinspired designs belonging to the VATs. Bioinspired designs implemented in Savonius and Darrieus turbines are elaborated besides giving a detailed explanation of the corresponding bio-organism and natural phenomenon. How are the working principles of bio-organisms emulated in the form of fluid dynamic design is explained thoroughly in this paper. The bioinspired designs for VATs are classified pragmatically for the future designs. Research gaps are highlighted for the aspiring researchers, and this is followed by the important strategies and allied challenges.
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This critical review delves into the impact of Computational Fluid Dynamics (CFD) modeling techniques, specifically 2D, 2.5D, and 3D simulations, on the performance and vortex dynamics of Darrieus turbines. The central aim is to dissect the disparities apparent in numerical outcomes derived from these simulation methodologies when assessing the power coefficient (Cp) within a defined velocity ratio (l) range. The examination delves into the prevalent turbulence models shaping Cp values, and offers insightful visual aids to expound upon their influence. Furthermore, the review underscores the predominant rationale behind the adoption of 2D CFD modeling, attributed to its computationally efficient nature vis-à-vis the more intricate 2.5D or 3D approaches, particularly when gauging the turbine’s performance within the designated l realm. Moreover, the study meticulously curates a compendium of findings from an expansive collection of over 250 published articles. These findings encapsulate the evolution of pivotal parameters, including Cp, moment coefficient (Cm), lift coefficient (Cl), and drag coefficient (Cd), as well as the intricate portrayal of velocity contours, pressure distributions, vorticity patterns, turbulent kinetic energy dynamics, streamlines, and Q-criterion analyses of vorticity. An additional focal point of the review revolves around the discernment of executing 2D parametric investigations to optimize Darrieus turbine efficacy. This practice persists despite the emergence of turbulent flow structures induced by geometric modifications. Notably, the limitations inherent to the 2D methodology are vividly exemplified through compelling CFD contour representations interspersed throughout the review. Vitally, the review underscores that gauging the accuracy and validation of CFD models based solely on the comparison of Cp values against experimental data falls short. Instead, the validation of CFD models rests on time-averaged Cp values, thereby underscoring the need to account for the intricate vortex patterns in the turbine’s wake—a facet that diverges significantly between 2D and 3D simulations. In a bid to showcase the extant disparities in CFD modeling of Darrieus turbine behavior and facilitate the selection of the most judicious CFD modeling approach, the review diligently presents and appraises outcomes from diverse research endeavors published across esteemed scientific journals.
As the use of Darrieus turbines in water is becoming increasingly popular in the field of renewable energy, it is essential to explore and evaluate existing research efforts. The situation of the Darrieus water turbine in water still requires further discussion. This paper aims to provide a comprehensive review of optimization methods for Darrieus water turbines, addressing the challenges associated with their efficiency, start-up, and stability. This work summarizes and evaluates the findings of previous studies, focusing on the features of experimental and numerical methods. Influence of geometric parameters, including height-diameter ratio, solidity, torsional angle, and airfoil are also talked into. The existing research adopts solidity values ranging from 0.1 to 0.4, but the design experience is not as extensive as that of the Darrieus wind turbine. Further discussions are still needed on the optimal power coefficient and tip speed ratio of the Darrieus water turbine. The research with a power coefficient ranging from about zero to above the Betz limit needs further summarization. Various optimization strategies, such as multi-turbine arrangement, coupling with Savonius turbines, and blade pitching, are also discussed. By offering insights into the current state of optimization works for Darrieus water turbines, this review aims to facilitate future research, bridge existing gaps in the field, further enrich the utilization of ocean currents, and improve the structure of renewable energy.
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 (); the maximum blade camber () and its location (). 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 (= 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 =1.
Biomimicry, as a field of science, is mainly defined as a solution for design problems inspired by natural models, systems, and elements. For the built environment, using nature as a guide can enhance sustainability or even go beyond that and generate a regenerative approach. This is important in the building sector to evolve towards a sustainable and circular economy and reduce CO2 emissions in terms of energy-use. While several biomimicry-related keywords exist, scholars and practitioners in architecture have given varying interpretations to the term biomimicry depending on the use and goal. There has been increasing interest in biomimicry in architecture (BIA), yet the field has become more fragmented. This study aims to highlight differences and similarities through an extended literature survey and analysis that explores case studies, classification systems, and methodological frameworks related to biomimicry in architecture as a way to contribute to reduce the fragmentation in the field. To provide the necessary context and avoid confusion regarding the many concepts and terms that refer to nature-based design, biomimicry-related keywords and interpretations of the word biomimicry are first clarified. Ultimately, the discussion is an integrative effort at defining the field, and highlights the significance and impact of employing BIA in terms of sustainability and usability, as well as showcasing the opportunities for further research.
The Golden ratio is an irrational number that has a tendency to appear in many different scientific and artistic fields. It may be found in natural phenomena across a vast range of length scales; from galactic to atomic. In this review, the mathematical properties of the Golden ratio are discussed before exploring where in nature it is claimed to appear; beginning at astronomical scales and progressing to smaller lengths, until reaching those of atomic and quantum physics. For each phenomenon discussed, the evidence for the presence of the Golden ratio is assessed. In making such a tour across length scales, it is illustrated just how prevalent this single number is within the natural universe.
The morphology, physiology and behavior of marine organisms have been a valuable source of inspiration for solving conceptual and design problems. Here, we introduce this rich and rapidly expanding field of marine biomimetics, and identify it as a poorly articulated and often overlooked element of the ocean economy associated with substantial monetary benefits. We showcase innovations across seven broad categories of marine biomimetic design (adhesion, antifouling, armor, buoyancy, movement, sensory, stealth), and use this framing as context for a closer consideration of the increasingly frequent focus on deep-sea life as an inspiration for biomimetic design. We contend that marine biomimetics is not only a “forgotten” sector of the ocean economy, but has the potential to drive appreciation of non-monetary values, conservation and stewardship, making it well-aligned with notions of a sustainable blue economy. We note, however, that the highest ambitions for a blue economy are that it not only drives sustainability, but also greater equity and inclusivity, and conclude by articulating challenges and considerations for bringing marine biomimetics onto this trajectory.
The present work proposes an artificial neural network (ANN) to analyze vertical axis wind turbines of the Savonius type. These turbines are appropriate for low wind velocities due to their low starting torque. Nevertheless, their efficiency is too low. In order to improve the efficiency, several modifications are analyzed. First of all, an innovative blade profile biologically inspired is proposed. After that, the influence of several parameters such as the aspect ratio, overlap, and twist angle was analyzed through a CFD (computational fluid dynamics) model. In order to characterize the most appropriate combination of aspect ratio, overlap, and twist angle, an artificial neural network is proposed. A data set containing 125 data points was obtained through CFD. This data set was used to develop the artificial neural network. Once established, the artificial neural network was employed to analyze 793,881 combinations of different aspect ratios, overlaps, and twist angles. It was found that the maximum power coefficient, 0.3263, corresponds to aspect ratio 7.5, overlap/chord length ratio 0.1125, and twist angle 112°. This corresponds to a 32.4% increment in comparison to the original case analyzed with aspect ratio 1, overlap 0, and twist angle 0.
Dynamic stall flow conditions affect the performance of most Vertical Axis Wind Turbine (VAWT) designs; however, the associated losses are especially relevant in smaller turbines, such as those typically used in urban environment applications. The work described in this paper experimentally evaluated the effectiveness of leading-edge protuberances in controlling the aforementioned stall behavior. A numerical study was first performed to define the leading-edge geometry to be subsequently tested in the wind tunnel. A custom experimental setup was also developed for this purpose. The wind tunnel measurements of the modified turbine showed significant performance gains over the baseline and a considerably improved self-starting behavior. The power coefficient increase was between 46% and 20% for wind velocities ranging from 5.5 m/s to 9 m/s, respectively. Nevertheless, the tip-speed ratio characteristics of the studied turbine were not meaningfully affected by the use of leading-edge protuberances.
Wind energy is gaining special interest worldwide due to the necessity of reducing pollutant emissions and employ renewable resources. Traditionally, horizontal axis wind turbines have been employed but certain situations require vertical axis wind turbines. With a view to improve the efficiency of a vertical axis wind turbine Savonius type, the present work proposes a bioinspired design blade profile relying on the Fibonacci spiral. This shape is repeatedly presented in nature and thus it leads to a bio-inspired blade profile. A numerical model was carried out and it was found that the Fibonacci shape improves the performance of the original Savonius shape, based on semicircular blade profiles. Particularly, the Fibonacci blade profile increases around 14% the power in comparison with the Savonius blade profile. Besides this comparison between Savonius and Fibonacci, a research study was carried out to improve the efficiency of the Fibonacci turbine. To this end, the effect of several parameters was analyzed: number of blades, aspect ratio, overlap, separation gap, and twist angle. Improvements on the average power greater than 30% were obtained.
Although drag driven wind turbine is regarded as an efficient rotor for low wind speed regions, design reconfiguration is a continuous process in order to improve the performance of the rotor. The main governing factor that influences the performance of the rotor is the blade morphology. Hence, this paper presents a proposed nature-inspired design approach for the development of drag-driven wind turbine blade morphology. The design approach framework comprises three main elements, namely image processing, geometrical analysis and bio-hybridization. The proposed bio-hybridized design consists of a blade mainframe curve inspired by nautilus shell and barnacle on the blade surface. It is found that integration of barnacle geometries on the surface of the blade has affected the performance of the rotor. Result shows that the peak Cm is at λ = 0.55 for experimental and CFD is Cm = 0.238 and Cm = 0.253 respectively. T he proposed design resulted in experimental and numerical Cp = 0.113 and Cp = 0.127 respectively at 7 m/s and λ = 0.7. The presented design technique with appropriate design bio-element provides a systematic method for engineers to model wind turbine blade morphologies.
In recent days, enhancement of Vertical Axis Wind Turbines (VAWTs) by mitigating flow deteriorating effects like dynamic stalling, unsteady wake is given great importance. The following article focuses on implementing four different tubercles on the blades’ leading edge and studying its performance and flow characteristics using CFD techniques. Results indicate that the addition of tubercles generated counter-rotating vortices and delayed flow separation and helped control dynamic stalling. Between azimuth angles 70°–160°, the flow was seen to separate only along the trough regions of the blade and remained attached along the peak regions, thus providing more torque and power. In addition to the enhancements in the flow characteristics, a 28% increase in power coefficient was observed for the optimal configuration at the optimal tip speed ratio. Additionally, a 14% increase in maximum lift generated by the blade was observed. Preliminary aeroacoustics analysis revealed a 12% and 20% decrease in the noise emissions along the blade tip and mid-plane of the turbine, respectively. Hence, it can be shown that tubercles effectively control dynamic stall, reduce noise emissions, and increase the power output of VAWTs.
Abstract Nautilus vertical axis wind turbine is a new breed which is being designed and developed through bionics and geometry, this kind of wind turbine is not only novel in appearance, and stable in structure, but also has excellent aerodynamic performance. Based on this type of wind turbine, combined with its structural characteristics, this paper designed 20 prototypes by continuously adjusting the structural parameters of the spiral blades. The performance of these prototypes is compared by simulation, the optimal structure of this kind of wind turbine is selected. This optimized wind turbine has a strong ability to collect wind energy and can obtain higher wind energy utilization in a wider range of wind speed, the maximum wind energy utilization rate reaches 32.842%. Then the wake characteristics are analysed, the reasonable arrangement is designed. This paper also built an equal scale reduced version of the real object using a 3D printer, and assembles it with a small disc type electric machine to construct a vertical axis wind power generation apparatus. Finally, experiments are conducted in a real environment to analyse the rotational speed and voltage waveform of this apparatus under different operating conditions experimentally.
Hydropower is one environmentally friendly option for meeting these energy demands. Because such technologies do not require a weir or a dam, they may be used with minimal environmental effect. The drag-based Savonius turbine is characterized by working at low-flow speeds which fits well the natural water streams. In spite of its simple and robust design, the Savonius turbine suffers from low efficiency and high static torque fluctuations. In view of this, CFD-based optimization is applied to a Savonius-type hydrokinetic turbine inspired by a couple of swimming Koi fish. The two main design parameters; overlap ratio and gap ratio are used in this paper to define the gap between the blades. The maximum power coefficient can be obtained by employing an evolution strategy optimization technique over the constructed metamodel. The results revealed that the optimal design for the bio-inspired Savonius hydrokinetic turbine corresponds to overlap and gap ratios of 0.2085 and 0.0057, respectively. The optimal configuration generates a maximum power coefficient of 0.2521, which is 17.6% higher than that for a conventional shape at the same tip speed ratio. The new bio-inspired design performs better than the conventional Savonius hydrokinetic turbine for the considered operating range.
In order to improve the efficiency of the Savonius type vertical axis wind turbine, the present work analyzes an improvement based on an innovative rotor geometry. The rotor blades are inspired on an organic shape mathematically analyzed, the Fibonacci’s spiral, presented in many nature systems as well as in art. This rotor was analyzed in a wind tunnel and through a CFD model. The power coefficients at different tip speed ratios (TSR) were characterized and compared for the Savonius turbine and two versions using the Fibonacci’s spiral. One of the proposed geometries improves the performance of the Savonius type. Particularly, the optimal configuration lead to an improvement in maximum power coefficient of 14.5% in the numerical model respect to a conventional Savonius turbine and 17.6% in the experimental model.
Exploitation of the main renewable energy sources (hydraulic, wind and solar radiation) in different installations involves the use of considerable surfaces, whether they are located on land or on water, having a negative impact on the environment. With the aim to dispose this deficiency, an installation that captures and integrates the three renewable resources has been proposed and patented. Therefore, their simultaneous exploitation in order to obtain electricity has the highest efficiency per unit area. In the laboratory, an installation was built and tested that integrates all three sources of renewable energy: the movement of water currents from flowing waters, the movement of air currents and solar radiation. Improvements that have been made to this pilot installation regarding the efficiency increasing of blades will be presented in the present communication, being analyzed the performance of using the profile blades with geometry inspired by nature.
The aim of this paper is to investigate the power extraction performance of three biologically inspired vertical axis wind turbine. In this study, the proposed turbines are simulated in 2D in Ansys using URANS approach based on two equation turbulent transport model-SST. The turbines are simulated under the similar solver and numerical configuration. FVM is adapted to analyse the turbines assisted by sliding mesh method (SSM) under non-conformal mesh configuration. The proposed turbine is analysed in terms of moment and power coefficient. Design 1 and Design 2 indicated promising result for a feasible and practical wind turbine. However, Design 3 indicated poor performance in power coefficient at high RPM value.
While biofouling is known to degrade the performance of marine energy conversion systems, prior experimental work has not explored this topic for cross-flow turbines. Here, we present experiments that investigate the impact of biofouling on turbine power output and structural loads. Using additive manufacturing, a three-dimensional scan of a barnacle was patterned onto the surface of turbine blades at three sizes and number densities, representing the progression from initial colonization to maturity. The impact of barnacles on turbine power output was found to be substantial and, for the most severe cases of fouling, the turbine does not produce power at any rotation rate. Conversely, barnacle fouling was found to have minimal impact on structural loading. To maintain generation capacity over extended periods, these results highlight the importance of antifouling coatings and proactive blade cleaning.
This paper discusses the historical evolution of the lift-type vertical axis wind turbine (VAWT) since its inception in the early 1930s. The VAWT received almost no development until the early 1970s when engineers in Canada and the USA began to study the VAWT design as a source of generating electricity. Many curved-bladed VAWTs were tested and are reviewed in this paper. Separately, the straight-bladed VAWT was developed by researchers in the UK and is also discussed. This review highlights that the VAWT received very little engineering development in the 1990s due to dominance of the HAWT in the wind energy market. Although, the literature also indicates that the VAWT has entered a Renaissance period whereby the VAWT is now being adopted for the offshore floating wind turbine industry. The latest developments in the floating VAWT technology and attempts to commercialise this technology have been documented.
Background: This paper proposes a Nautilus isometric spiral vertical axis wind turbine, which is a new structure, and its aerodynamic performance and power generation performance need to be analyzed. Methods: A 3D model of the wind turbine was built and its aerodynamic performance was analyzed. Then the wind turbine power generation and grid-connected simulation platform was built by MATLAB/SIMULINK, and its power generation performance and subsequent grid connection were studied. Results: The basic parameters of the wind turbine were obtained. In order to improve efficiency, parameters such as pressure, torque, wind energy utilization rate and relative velocity of wind turbines with different blade numbers and different sizes were compared. In addition, by building a simulation platform for the power generation control system, the power generation characteristics and grid connection characteristic curves of the generator were obtained. Conclusions: When the number of blades is three and the ratio between the ellipse major axis and minor axis of the blade inlet is 0.76, the best efficiency of the wind turbine can be obtained. Application of the power generation control system used in this paper can achieve grid-connected operation of this wind turbine. It also confirmed that the Nautilus isometric spiral wind turbine has good performance and is worthy of in-depth research.
The fluid dynamics of flapping foils are reviewed in this article. A very wide range of researches are conducted for the two-dimensional flapping foil which has a relatively simple geometry. However, for a three-dimensional foil, the aspect ratio and shape take effects and completely distinct fluid dynamics are revealed compared with the two-dimensional one. This review gives a summary on the experimental techniques and numerical methods used in the researches on the fluid dynamics of flapping foils. The effects of some key parameters including Reynolds number, reduced frequency, flapping amplitude and three-dimensional effect on the fluid dynamics of flapping foils are reviewed. The researches focusing on the wake structures, aero/hydrodynamic characteristics and energy harvesting efficiency are discussed. Finally, some conclusions are drawn and potential future research directions are recommended.
This paper presents mesh and time-step dependence study of newly designed drag type vertical axis wind turbine. Ansys FLUENT a commercially available CFD solver was used to perform CFD numerical study on the drag type wind turbine. In computational analysis, 2D models was simulated under unsteady flow fields using SST k-ω to achieve stabilized numerical convergence. The model was analyzed at static and dynamic mode, where sliding mesh technique was used to analyze the turbine in dynamic mode. Three main parameters were taken under careful consideration: mesh resolution, turbulence model and time-step. Dimensionless quantity such as moment and drag was used in mesh sensitivity study for both static and sliding mesh. A small discrepancy in results of 2D sliding mesh result at different time-step and mesh resolution was observed. The generated results showed good agreement between fine and medium mesh with trivial dissimilarities in the initial initialization. In time-step dependency study for static mesh, dt=0.0002 time-step size was chosen for economical computational cost.
In the pursuit of exploiting wind energy, the drag-based Savonius wind rotors have been extensively used in the low wind velocity regions. These type of rotors, as small-scale stand-alone systems, are characterized by their direction independency, ease of fabrication and the absence of yaw-mechanism. Over the past few decades, various blade profiles/shapes and augmentation techniques have been developed to improve the performance of the Savonius rotors. In recent times, in the quest of further improvement, the nature-inspired (such as golden spiral) and the biomorphic (such as fish-ridge) blades have also been evolved. Some researchers have also studied bio-inspired blade shapes (such as sea-pen) to utilize the organism’s strategy of optimized vortex induced feeding. In the present paper, the starting torque characteristics of a sea-pen bladed Savonius rotor is explored experimentally. The rotor blades are derived from the reported data of the Orange sea-pen (Ptilosarcus gurneyi). Wind tunnel experiments are conducted in the range of 5 m/s to 8 m/s. In order to have a direct comparison, the starting torque characteristics of a conventional semicircular bladed rotor is also obtained in the same range of velocities. The present investigation demonstrates the enhancement of starting torque characteristics of the sea-pen bladed wind rotor at specific rotor angles.
For harvesting wind energy, especially at low-velocity regions, Savonius vertical-axis wind rotors are usually preferred. Among other variants of the vertical-axis wind rotors, the Savonius rotor has become an attractive candidate as a small-scale stand-alone system due to its direction independency, ease of fabrication and the absence of yaw-mechanism. The present paper attempts to study the effect of corrugated blades on the performance of a conventional Savonius rotor. The corrugation is a bio-inspired concept derived from the dragonfly wings. As reported in literature, the corrugation applied in the chord-wise direction of the airfoil improves the lift characteristics. This lift improvement is caused by the trapped vortices inside the corrugation that promotes a low-pressure region over the leading edge and suction surface of the airfoil. This concept of corrugation and its effect has been applied to the blades of a conventional Savonius wind rotor to improve its torque characteristics. In this paper, a rotor with corrugated semicircular blade profiles is studied by two-dimensional (2D) transient numerical simulation in ANSYS FLUENT using shear stress transport (SST) k-ω turbulence model. The simulations are conducted in the range of 5 m/s to 7 m/s to suit the lower wind velocities of the Savonius rotor. The torque and power coefficients of the corrugated semicircular-bladed rotor are calculated at the rotating conditions. Further, the pressure and local torque distributions over the corrugated blade surfaces are obtained and analyzed to understand the torque mechanism of the rotor with corrugated blade profiles. In order to have a direct comparison, the study has also been carried out for semicircular-bladed rotor without corrugation.
The eminent energy crisis and high emission of fossil fuels provide thrust for developing renewable energy-based technologies. Wind and hydrokinetic energies are the most promising renewable energy resources for electric power generation to meet the growing energy demand. The vertical-axis hybrid turbine, which combines the features of good starting characteristics of the Savonius turbine and the operational efficiency of the Darrieus turbine, can serve as a viable option for power generation. A variety of configurations of the hybrid turbine are possible based on several design parameters such as the relative position of the Darrieus and Savonius rotors, overlap ratio, solidity ratio, blade or buckets shape and radius ratio, attachment angle and others. To some extent, the influence of these parameters on the hybrid turbine performance has been investigated through experimental and numerical studies by considering a number of physical and computational models. In most of the findings, the range of maximum power coefficient values is recorded between 0.08 and 0.51. Though individual vertical axis turbines have been widely studied and reviewed, similar review papers on hybrid turbines are scarce. This paper brings out significant developments that have taken place in the area of hybrid turbines, identifies the operating parameters, highlights the challenges related to rotor/turbine aerodynamics, modelling, simulation, testing methodologies. Based on these discussions, the strategies for future hybrid turbine designs are presented.
For the purpose of harvesting wind energy from low wind velocity regions such as north-east India, the Savonius-type drag-based vertical-axis wind turbine (VAWT) can be a potential candidate due to direction independency, absence of yaw mechanism, small-scale stand-alone systems and ease of fabrication due to absence of airfoil-shaped blades. Based on the aforementioned merits, it is decided to conduct a performance improvement task by developing a novel blade shape bio-inspired from the polyp leaf of the orange sea-pen (Ptilosarcus gurneyi). The similarities between flow physics of the Savonius rotor and the feeding mechanism of the sea-pen are discussed. The procedure of shape extraction for novel blade is narrated and the rotors are fabricated. Wind tunnel experimentation and 2D numerical simulations are performed for preliminary performance assessment of the rotors. The performance potential of the novel blades is assessed in comparison with the semicircular blade. The result indicated the possibility of the performance improvement in case of novel bio-inspired blade.KeywordsVertical-axis wind turbine Savonius wind rotorBio-mimicryOrange sea-pen
Wind energy is employed as an effective source for catering to the increasing energy demands and depleting
fossil fuels, while also counteracting their adverse environmental effects. VAWTs have numerous advantages
over HAWTs. But, the relatively low power coefficient of VAWTs in comparison to HAWTs restricts their utili-
zation. Bio-inspired leading-edge tubercles appear to be one of the potential design changes for enhancing the
power performance of VAWTs. This study systematically investigated the relative influence of tubercle design
variables and their geometrical trend subjected to power performance by employing a hybrid Design of Exper-
iments (DoE) approach and Response Surface Methodology (RSM) rather than choosing random values of tu-
bercle variables. Furthermore, the conflict existing in the literature about the improved or degraded power
performance of VAWTS with tubercles is resolved by evaluating the power performance of VAWT at on-design
and off-design conditions. For the computation of aerodynamic forces, unsteady Computational Fluid Dynamics
(CFD) was utilized with a 4-equation transition SST turbulence model. Tubercle amplitude influences the
aerodynamic efficiency of VAWTs relatively more than its wavelength. It is evident from the results that energy is
harnessed efficiently at on-design conditions considering lower tubercle amplitude and higher wavelength.
However, the reverse trend of variables is observed under off-design conditions. At on-design conditions, tu-
bercles degraded the VAWTs performance in comparison to baseline VAWT configuration by a minimum of
13.55% among the 14 hybrid DoE generated cases, while the performance was enhanced by a maximum of 55%
at off-design conditions of VAWT.
The use of metropolitan wind power by small-scale wind turbines has become an emerging technique to reduce the battle among growing energy needs. However, the available technical designs are not yet adequate to develop a reliable and distributed wind energy converter for low wind speed conditions. The Savonius wind turbine rotor, or simply Savonius rotor, seems to be particularly promising for such conditions, however, it suffers from low power coefficient. The blade profile/shape is an important aspect of designing the Savonius rotor. In this context, the use of optimization techniques (OTs) along with soft-computing techniques (SCTs) can significantly help to arrive at the intended design parameters. The selection of rotor blades developed through OTs and SCTs can significantly improve the rotor performance. This review study aims to summarize the OTs and SCTs used till date in the blade design of Savonius rotors.
The energy crisis followed by severe environmental concerns has led mankind to develop renewable energy sources. Recently, the development of hydrokinetic technology is gaining research interest to tap the kinetic energy available in water flowing streams. Among all hydrokinetic turbine configurations, the lift-based vertical axis hydrokinetic turbine (VAHT) is found to be efficient at a high tip speed ratio, rotation independent of flow direction, and entails an unsubmerged generator unit. In this study, an attempt is made to review in detail the performance comparison of straight-bladed with helical-bladed lift-based VAHTs and the influence of design and operating conditional parameters. Furthermore, the potential research areas that need to be addressed in future studies are also found and presented. The review would be helpful for academic researchers in understanding the impact strength of performance parameters and efficiency enhancement techniques and inspire industries to commercialize the product in the existing market conditions.
Inspired by the polyp leaf of the Orange sea-pen (Ptilosarcus gurneyi), a novel blade shape of the Savonius vertical-axis wind rotor is developed. The similarities between the aerodynamic and the hydrodynamic aspects of the Savonius rotor blade profile and the sea-pen leaf are reviewed, and an appropriate analogy is thereby established. The shape of the sea-pen leaf is then extracted to fabricate the rotor blades. The performance of this sea-pen bladed rotor is evaluated in a low-speed subsonic wind tunnel at different wind velocities. The two-dimensional (2D) numerical analysis is also performed to support the experimental findings and to study the influence of blade shape on the pressure and the torque distributions of the rotor. The novel sea-pen bladed rotor, having lesser material requirements, is seen to demonstrate higher performance than that of the conventional semicircular bladed rotor in the tested range of low tip-speed ratio.
This paper uses the Particle Image Velocimetry technique through wind tunnel experiments to investigate the spanwise velocity and vortex characteristics of a straight-bladed vertical axis wind turbine (VAWT) and to verify the reliability of the numerical simulation model. The aerodynamic forces are obtained by a multiport pressure measurement device. To assess the flow field of VAWT with bionic blades, this paper compares results between numerical simulations and experiments. Different flow field characteristics of wind turbines have been found in the baseline blades and bionic blades. The research shows that the bionic blade can suppress the influence of the tip vortex on the velocity of the cross-section z/(H/2) = 1.0, and the wake recovery velocity of VAWT with bionic blades is lower than that with baseline blades. By comparing the tangential force coefficients of the blades, it has been found that the tangential force of the bionic blades increases in azimuth angle regions 30°<θ < 116° and 193°<θ < 301°. It has also been proven that the aerodynamic performance of the bionic blade is slightly better than the baseline blades at TSR = 2.19. This study provides a better understanding of the development of aerodynamic forces and vortex characteristics through torque and velocity measurements.
The utility of small wind turbines (SWTs) covering horizontal and vertical-axis types as off-grid, standalone, and decentralized energy supplement systems has gained market attention. Such turbines primarily operate at low Reynolds number ( Re) and low tip speed ratio ( λ) conditions. Under such circumstances, the design, development, and testing of SWTs have become a tedious task, mainly due to the lack of precise aerodynamic knowledge of SWT. The present article reviews the fundamental aspects of SWT, including airfoil selection criteria, blade design, and aerodynamic improvement through passive flow control and augmentation techniques. The article also reports several classes of potential airfoils that can be employed in the design of SWTs. The airfoils considered operate mainly in the range of Re = 0.3 x 10 ⁵ to 3 x 10 ⁵ and λ = 0.5 to 6. Besides the classical approach, the article showcases the prospects of several bioinspired profiles/shapes that are meant for SWTs operating at low Re and λ conditions. Towards the end, various design constraints and applicability of SWTs are summarized.
Vertical axis wind turbines (VAWTs) are advantageous than horizontal axis wind turbines (HAWTs) in terms of portability and ease of application in urban areas and turbulent environments. Though VAWTs have lower efficiency than HAWTs in ideal low-turbulence wind environments, in high-turbulence and directionally shifting wind circumstances, VAWTs function more smoothly and produce higher energy than HAWTs. However, their low efficiency, self-starting capabilities and inability to generate continuous positive torque are the main downsides. To harness maximum energy from flowing streams, augmentation techniques can be deployed for VAWTs, which is the subject of the present work. The methods that are tried include modification of inlet flow path, modification in blade profiles and several other innovative designs such as cowling technique and INVELOX profile. These new design methods are mainly focused on reducing negative torque and increasing the inlet velocity of the wind. Research has shown that there is a substantial increase in the power coefficient (CP) and the output power of different VAWTs by incorporating augmentation systems.
A new blade configuration is proposed to further increase the performance of a Savonius rotor through a sequence of unsteady Computational Fluid Dynamics (CFD) simulations. The blade is made by a multi-curve and auxiliary profiles for a reduction of the negative drag on the rotor. The flow aspects around the new blade are analyzed and quantitatively compared with that of the conventional and other blade configurations. The results imply a dependency of the rotor performance on the blade shape, demonstrating an appropriate configuration that produces the highest coefficient of torque CT and power Cp. The newly optimized configuration is recognized with the peak of Cp at a tip speed ratio (TSR) of 1.5, which is more than two times higher than the conventional one. This makes the Savonius rotor being better applicable to the urban environment. Importantly, this blade significantly increases the Cp by 6.3% at TSR < 1.0, known as a typical working condition in rural areas. The present results thus point out a feasible solution for powering the poor households with no access to the grid and reducing the harmfulness to the environment, with high efficiency in wide operating conditions over the previous designs.
This paper explores the function approximation characteristics of Artificial Neural Network (ANN) by implementing it on the vertical-axis Savonius wind rotor technology. In this regard, a suitable experimental dataset documented in literature is exploited to train the ANN comprising the rotor performance as output and 11 different design and operating parameters as input with the help of MATLAB R2020b software. Multiple ANN models are trained by varying the number of hidden neurons which are then evaluated based on their estimation error and correlation coefficient (R) as decision criteria. The optimum ANN architecture demonstrates R ≈ 98 0.96 and 0.98 for the testing and training datasets, respectively. Further, in the quest of finding the optimum performance from the entire power curve of the rotor, the Golden Section Method (GSM) is linked with the trained ANN model. Using these soft computing techniques, a parametric study is carried out to understand the dependency of rotor performance on their design and operating parameters. At the end, a graphical interface is developed as a product so as to allow the user to predict the performance of the new rotor designs intuitively.
This experimental study is focused on quantifying the effect of trailing edge flexibility on the performance of a three straight bladed vertical axis wind turbine, with a chord-to-diameter ratio c/D=0.16 at a moderately high Reynolds number (based on diameter) ReD=4⋅105. The blades consist of NACA-0015 profiles that are fixed with a pitch angle β=6∘ toe-out, and allow interchangeable trailing edges in the last 17% of their chord length. The research presented here provides a proof of concept for the improved performance of vertical axis wind turbines, due to the effect of flexibility at the trailing edge of their blades. We show that blades with semi-flexible trailing edge, can extend the range of rotor operating regimes, leading to an increase of approximately 10% in the performance of the turbine. An excess of flexibility results in diminished efficiencies.
The flow-induced rotation of the modified Savonius rotor, for which the blade consists of a semicircular profile and an elliptical shape, is studied using a series of unsteady computational fluid dynamics (CFD) simulations. The present study first concentrates on the validation of the numerical scheme against Blackwell's experimental data of the conventional rotor. The computed flow physics around the modified rotor with the same diameter is then analyzed and compared with that of the conventional rotor during one rotation cycle. As the result, the modified rotor is outperforming the conventional one but keeping its unique features. The modified rotor offers exceeding performance at a tip speed ratio (TSR) greater than 0.8. The new peak of the power coefficient Cp is reached at TSR = 1.4 which is a typical operating condition of the wind turbine in urban areas. The remarkable finding is that the suppression of the flow separation on the blade is an effective way to improve the rotor's aerodynamic performance. As expected, the additional elliptical profile plays a key role in increasing the positive torque and in preventing the flow separation on the blade, especially at high TSR > 0.8. Finally, this study points to not only advances the fundamental understanding of flow mechanism around the rotor but also proposes good practical energy harvesting application in urban environments.
This paper investigates the effect of bionic airfoil on the aerodynamic characteristic and flow field of an H-type vertical axis wind turbine (VAWT). The pressure acting on the blade surface and flow field around the blade were predicted at different tip speed ratios (TSRs) and the angles of attack were predicted by numerical simulation. As a result, it was showed that the maximum value of lift force coefficient was 0.723 for baseline airfoil at the angle of attack 12° and was 0.743 for bionic airfoil at the angle of attack 14°. When the angle of attack was equal or greater than 14°, the static bionic airfoil showed better lift characteristic. Moreover, the maximum value of power coefficient occurred at TSR = 2.58 for the simulation, but the maximum value of torque coefficient was identified at TSR = 2.19. Meanwhile, the growth rates of the power coefficient were 7.02% at TSR = 1.38, 7.35% at TSR = 2.19, and 3.42% at TSR = 2.58 for simulation between baseline blades and bionic blades. The power performance of VAWT improved in dynamic stall for bionic airfoil by delaying stall on the blade's surface and promoting the laminar-to-turbulent transition to improve the power performance of wind turbine at the low TSRs for the mainstream wind velocity of 8.0 m/s. The research results were helpful to predict the performance of VAWTs under various wind environments and to improve the aerodynamic performance of VAWTs.
Vertical axis wind turbines are receiving renewed attention due to the challenges faced by horizontal axis wind turbines in recent years. To achieve commercial applications, a key aspect is to improve the relatively low power coefficient of vertical axis wind turbines. Among several factors that affects output efficiency, the three-dimensional tip loss effect is essential because each vertical axis wind turbine blade has two tops (e.g., the H-type wind turbine). Although several blade tips have been designed, a comprehensive and fair analysis is necessary to find a suitable one. Therefore, in this paper, a total of 20 blade tips have been analyzed using the three-dimensional computational fluid dynamic method. The results indicate that the improved winglet and endplate we proposed are relatively optimal when the synthetic performance of aerodynamic efficiency, structural loads and start-up torque is considered. However, the improvement mechanisms of these two tips are different. Based on the performance curves and the fluid fields at specific tip speed ratios, we find that the endplate increases the blade torque in the upwind region by reducing the spanwise flow, but its negative effect in the downwind region partially counteracts the improvement. For the improved winglet, it reduces the tip loss in the upwind region and suppress separation in the downwind region at the appropriate tip speed ratio, thus significantly improving its performance in one rotor revolution. The winglet also contributes to the reduction of rotor thrust. In addition, the influence of the blade tip on wake is not remarkable and is mainly concentrated in the tip area affected by the tip vortex.
The airfoil shape in the turbine blades is responsible for lift generation in horizontal axis wind turbine (HAWT). However, the main problem is the occurrence of stalls on the blade after a certain angle of attack. It is noticed in the literature that vortex generator, tubercle, micro cylinder, spherical ball, etc., can enhance the momentum transfer in the wind turbine blade and can help in increasing lift production. Simultaneously, these modifications help in delaying the stall occurrence on the blade. In the present study, an attempt is made to review the different modifications done on the leading edge of the HAWT blade using tubercles and their effects on aerodynamic performances. From the study, the following significant findings are summarized with respect to the performances of HAWT with leading-edge tubercles: (i) blades with tubercles on the leading edge will have superior performance in the post-stall regime, (ii) tubercles with a smaller amplitude and lower wavelength will produce higher lift and lower drag in the low wind speed condition, whereas, at higher wind speed, tubercles with higher amplitude and larger wavelength perform better, and (iii) tubercle blade will have a stable and smooth performance in varying wind speed conditions, producing higher torque and power at low wind speed. Finally, some crucial scopes for future research for further developing the HAWT with tubercle blades are delineated.
This paper has attempted to investigate the aerodynamic characteristics of the straight-bladed vertical axis wind turbine (VAWT) with bionic blades for better application of bionic blades to VAWTs. The study was based on the computational fluid dynamics numerical simulation (CFD numerical simulation) technique to investigate the differences of the power and tangential force coefficients of the wind turbine between the bionic blades and standard blades at three different tip speed ratios (TSRs = 1.38, 2.19, and 2.58) for the mainstream wind velocity of 8.0 m/s. The results showed that the average power coefficient enhancing effect of the bionic blade increases as the TSR increases, and the average power coefficient of the bionic blade is higher than that of the standard blade mainly because of the high wind energy utilization in the wave trough section. Moreover, the power coefficients of the wave crest sections for bionic blades are better than that of the standard blades at the downstream region of 200° < θ < 260°. This research provides a reference for the use of bionic blades in the straight-bladed VAWT.
This paper addresses the application of artificial neural network (ANN) and genetic expression programming (GEP), the popular artificial intelligence and machine learning methods, in order to estimate the Savonius wind rotor's performance based on different independent design variables. Savonius wind rotor is one of the competent members of the vertical axis wind turbines (VAWTs) due to its advantageous qualities such as direction independency, design simplicity, ability to perform at low wind speeds, potent standalone system. The available experimental data on Savonius wind rotor have been used to train the ANN and GEP using MATLAB R2020b and GeneXProTools 5.0 software, respectively. The input variables used in ANN and GEP architecture include newly proposed design shape factors, number of blades and stages, gap and overlap lengths, height and diameter of the rotor, free stream velocity, end plate diameter and tip speed ratio, besides cross-sectional area of wind tunnel test section. Based on this, the unknown governing function constituted by the aforementioned input variables is established using ANN and GEP to approximate/forecast the rotor performance as an output. The governing equation formulated by ANN is in the form of weights and biases, while GEP provides it in the form of traditional mathematical functions. The trained ANN and GEP are capable to estimate the rotor performance with R2 ≈ 0.97 and R2 ≈ 0.65, respectively, in correlation with the reported experimental rotor performance.
A detailed numerical study has been carried out to investigate the effects of leadingedge protuberance as a novel flow-separation control-technique for vertical-axis-windturbine (VAWT). The aerodynamic performance of a stationary protuberanced blade and an associated H-type Darrieus VAWT made of three protuberanced blades were investigated using unsteady Reynolds-Averaged-Navier-Stokes (URANS) and Implicit Large Eddy Simulation (ILES) methods. The current study involves a comprehensive set of five different types of sinusoidal leading-edge protuberances with three different wavelengths and amplitudes. It is found that this passive flow-control-method can favourably change lift and drag at stall and post-stall regions, which is of high importance for VAWTs. Static stall variation can be changed from leading-edge-stall type to trailing-edge-stall type. This is attributed to a “bi-periodic” phenomenon over the blade suction side and a reduced turbulence kinetic-energy production. Protuberance amplitude was found to be more important than the wavelength in improving aerodynamic performance. Of all the tested cases, the protuberance with amplitude of 1% chord-length (c) and wavelength of 2.5%c behaves the best. The leading-edge protuberance can significantly increase VAWT power-coefficient at low tip-speed-ratios, which are dominated by post-stall conditions. The dynamic stall was delayed and the coherent spanwise wavy flow-structures on the blade suction-side were observed.
The field of biomimetics attempts to inspire and integrate the morphology and function of biological organisms into the design of human-made technology. In that organisms have been able to adapt through evolution, they have performed the “cost-benefit analysis” to solve a variety of problems of concern to humans and can potentially improve technologies. One example of a structural adaptation that can improve the aero/hydrodynamic performance of wing-like designs is based on the flippers of the humpback whale. The humpback whale is able produce small radius turns with its elongate, high aspect ratio flippers. This whale differs from related species in using maneuverability to capture prey. Maintenance of lift throughout a turning maneuver requires a modification of the wing-like flippers. The flippers possess rounded bumps, called tubercles, along the leading-edge. Empirical and computational studies have demonstrated that the tubercles passively modify the flow over wing-like structures. The flow between the tubercles produces counter-rotating vortices in a sacrificed separation that helps to energize the flow over the tubercles. The flow pattern over a wing induced by the tubercles increases lift, delays stall, and maintains low drag post stall. The tubercles have applications for aircraft wings, rudders, dive planes, skegs, sailboat masts, stabilizers, truck mirrors, bicycle wheels, rotor blades, propellers, compressors, pumps, fans, and tidal and wind turbines.
Background：The wind turbine is divided into a horizontal axis and a vertical axis depending on the relative positions of the rotating shaft and the ground. The advantage of the choke fan is that the starting torque is large and the starting performance is good. The disadvantage is that the rotation resistance is large, the rotation speed is low, the asymmetric flow occurs when the wind wheel rotates, the lateral thrust is generated, and the wind energy utilization rate is lowered. How to improve the wind energy utilization rate of the resistance wind turbine is an important issue to be solved by the wind power technology.
Objective：The nautilus isometric spiral wind turbines studied in this paper have been introduced and analyzed in detail, preparing for the further flow analysis and layout of wind turbines, improving the wind energy utilization rate of wind turbine wind turbines, introducing patents of other structures and Output characteristics of its generator set.
Method：Combined with the flow field analysis of ANSYS CFX software, the numerical simulation of the new wind turbine was carried out, and the aerodynamic performance of the new vertical axis wind turbine was analyzed. The mathematical model and control model of the generator were established by the maximum power control method, and the accuracy of the simulation results was verified by the measured data.
Results：The basic parameters of the new fan tip speed ratio, torque coefficient and wind energy utilization coefficient are analyzed. Changes in wind speed, pressure and eddy viscosity were investigated. Three-dimensional distribution results of wake parameters such as wind speed and pressure are obtained. By simulating the natural wind speed, the speed and output current of the generator during normal operation are obtained.
Conclusion: By analyzing the wind performance and power generation characteristics of the new wind turbine, the feasibility of the new wind turbine is determined, which provides reference and reference for the optimal design and development of the wind turbine structure.
A numerical study was carried out to investigate the effects of a Gurney flap on the aerodynamics performance of the NACA 0018 aerofoil and an associated three-blades rotor of a H-type Darrieus wind turbine. The flow fields around a single aerofoil and the Vertical Axis Wind Turbine (VAWT) rotor are studied using URANS. The height of Gurney flap ranges from 1% to 5% of the aerofoil chord length. The results show that the Gurney flap can increase the lift and lift-to-drag ratio of the aerofoil as associated with the generation of additional vortices near the aerofoil trailing edge. As a result, adding a Gurney flap can significantly improve the power coefficient of the VAWT at low tip speed ratio, where it typically gives low power production. The causing mechanism is discussed in detail, pointing to flow separation and dynamic stall delay.