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Journal ArticleDOI

Modulated circulation control around the blades of a vertical axis hydrokinetic turbine for flow control and improved performance

TL;DR: A kinematics-based method is described, through which the pitching motion of the blades varies in such a way that the flow is controlled around them as well as across the rotor, to derive the blade pitching laws being compatible with the unsteady Kutta-Joukowski condition.
Abstract: The circulation control around the Darrieus turbine blades is considered as a significant practical solution so as to improve the machine's performance. This paper describes a kinematics-based method, through which the pitching motion of the blades varies in such a way that the flow is controlled around them as well as across the rotor. For this purpose, the potential flow approach is used to derive the blade pitching laws being compatible with the unsteady Kutta-Joukowski condition at the trailing edge of the blade. By means of this, a constant circulation around the blade is ensured. Preliminary CFD investigations have evidenced the operating conditions at which the constant-circulation framework may be applied to turbulent flows in order to control the vortex shedding and avoid blade-vortex interactions. Nonetheless, a strict application of this concept prevents the turbine from harvesting the energy. As an alternative strategy, the application of a locally variable-circulation is proposed in order to enhance the power output without compromising on flow control. Three major accomplishments of the variable-circulation-based blade pitching proposition are the following: a. flow control around the turbine blades with definite patterns of vortical structures, b. consistent increase in the turbine's coefficient of power, and c. widened operating range of the turbine.
Citations
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Journal ArticleDOI
25 Sep 2019-Energies
TL;DR: In this paper, an up-to-date review of the potential, technologies, prototypes, installed capacities, and projects related to ocean renewable energy including wave, tidal, and thermal, and salinity gradient sources.
Abstract: This study, firstly, provides an up-to-date global review of the potential, technologies, prototypes, installed capacities, and projects related to ocean renewable energy including wave, tidal, and thermal, and salinity gradient sources. Secondly, as a case study, we present a preliminary assessment of the wave, ocean current, and thermal gradient sources along the Brazilian coastline. The global status of the technological maturity of the projects, their different stages of development, and the current global installed capacity for different sources indicate the most promising technologies considering the trend of global interest. In Brazil, despite the extensive coastline and the fact that almost 82% of the Brazilian electricity matrix is renewable, ocean renewable energy resources are still unexplored. The results, using oceanographic fields produced by numerical models, show the significant potential of ocean thermal and wave energy sources in the northern and southern regions of the Brazilian coast, which could contribute as complementary supply sources in the national electricity matrix.

40 citations

Journal ArticleDOI
TL;DR: In this paper , a new turbine with semi-cylinder deflectors was introduced to make feasible power extraction from this low-speed tidal current, and the results showed an increase in power coefficient (C P ) to 0.38 at an optimum TSR of 0.68 for two-blade Savonius rotor with and without deflectors.

7 citations

Journal ArticleDOI
28 Oct 2021
TL;DR: In this article, a comparison between the sliding mesh technique and the chimera mesh technique is made in order to establish their advantages and disadvantages in the two-dimensional simulation of vertical axis turbines.
Abstract: The application of Computational Fluid Dynamics (CFD) to energy-related problems has increased in the last decades in both renewable and conventional energy conversion processes. In recent years, the application of CFD in the study of hydraulic, marine, tidal, and hydrokinetic turbines has focused on the understanding of the details of the complex turbulent flow and also in improving the prediction of the performance of these devices. There are several complexities involved in the simulation of Vertical Axis Turbine (VAT) for hydrokinetic applications. One of them is the necessity of a dynamic mesh model. Typically, the model used in the simulation of these devices is the sliding mesh technique, but in recent years the fast development of the overset (also known as chimera) mesh technique has caught the attention of the academic community. In the present paper, a comparison between these two techniques is done in order to establish their advantages and disadvantages in the two-dimensional simulation of vertical axis turbines. The comparison was done not only for the prediction of performance parameters of the turbine but also for the capabilities of the models to capture complex flow phenomena in these devices and computational costs.

5 citations

Journal ArticleDOI
TL;DR: It is observed that the uncertainty in the turbine's rotation is more influential on the power output compared to free-stream velocity, and the sensitivity of the turbine’s performance to these input uncertainties is quantified using probability methods, including response surface, PDF and CDF.

3 citations

Journal ArticleDOI
TL;DR: In this article, a new flapping foil harvester with circulation control was proposed and the effects of different parameters on its energy-extraction capacity to improve efficiency and achieve lowest cost.
Abstract: Oscillating motion, an effective way to harvest energy, has gradually become a hotspot in bionic motion research in recent years. Means of improving the energy-extraction efficiency of a flapping foil harvester have long been a focus of researchers. This paper proposes a new flapping foil harvester with circulation control and explores the effects of different parameters on its energy-extraction capacity to improve efficiency and achieve lowest cost. Setting the injection ports on the upper and lower surfaces near the trailing edge of the foil and implementing injection control during motion, the effects of the location of the injection port, pitching amplitude, momentum coefficient, reduced frequency, and jet mode on the circulation control flapping foil are systematically investigated under the condition of a Reynolds number of 13,800. The results show that circulation control can enhance the energy-extraction efficiency of a flapping foil across a wide range of parameters, in which the location of the injection port and momentum coefficient have the most obvious influence on efficiency, followed by pitching amplitude and reduced frequency. In addition, the jet mode is a crucial factor affecting net efficiency. Relative to the constant mode, the triangular mode of circulation control has the lowest energy consumption, and the net energy-extraction efficiency reaches up to 38.77% under a reduced frequency of 0.12, which is 22.24% higher than that of the plain flapping foil.

3 citations

References
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Journal ArticleDOI
TL;DR: In this article, the main aerodynamic models that have been used for performance prediction and design of straight-bladed Darrieus-type VAWTs are presented. But, the authors did not discuss the performance of these models.
Abstract: Since ancient past humans have attempted to harness the wind energy through diversified means and vertical axis wind turbines (VAWTs) were one of the major equipment to achieve that. In this modern time, there is resurgence of interests regarding VAWTs as numerous universities and research institutions have carried out extensive research activities and developed numerous designs based on several aerodynamic computational models. These models are crucial for deducing optimum design parameters and also for predicting the performance before fabricating the VAWT. In this review, the authors have attempted to compile the main aerodynamic models that have been used for performance prediction and design of straight-bladed Darrieus-type VAWT. It has been found out that at present the most widely used models are the double-multiple streamtube model, Vortex model and the Cascade model. Each of these three models has its strengths and weaknesses which are discussed in this paper.

485 citations

Journal ArticleDOI
TL;DR: In this article, the authors show that the fractional power loss increases from 1/3 to 2/3 as the fraction of the channel cross-section spanned by the turbines increases from 0 to close to 1.
Abstract: There is an upper bound to the amount of power that can be generated by turbines in tidal channels as too many turbines merely block the flow. One condition for achievement of the upper bound is that the turbines are deployed uniformly across the channel, with all the flow through them, but this may interfere with other uses of the channel. An isolated turbine is more effective in a channel than in an unbounded flow, but the current downstream is non-uniform between the wake of the turbines and the free stream. Hence some energy is lost when these streams merge, as may occur in a long channel. We show here, for ideal turbine models, that the fractional power loss increases from 1/3 to 2/3 as the fraction of the channel cross-section spanned by the turbines increases from 0 to close to 1. In another scenario, possibly appropriate for a short channel, the speed of the free stream outside the turbine wake is controlled by separation at the channel exit. In this case, the maximum power obtainable is slightly less than proportional to the fraction of the channel cross-section occupied by turbines.

382 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigate several possible methods for redirecting turbine wakes, including some existing and some novel approaches, and compare their ability to redirect turbine wakes and their effects on turbine power capture and structural loads using the high-fidelity wind plant simulation tool Simulator for On/Offshore Wind Farm Applications.

346 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigated the variations in loads and moments on the turbine as well as the experienced angle of attack, shed vorticity and boundary layer events (leading edge and trailing edge separation, laminar-to-turbulent transition) as a function of pitch angle.

262 citations

Journal ArticleDOI
TL;DR: In this paper, a combined experimental and numerical investigation is carried out to study the performance of a micro vertical-axis wind turbine (VAWT) with variable pitch, which involves a low aspect ratio (AR) three straight blades with struts.

185 citations