In pursuit of energy harvesting from ocean waves, our recent progress on studying wave interaction with a lift-type rotor is discussed in this paper. The particular focus is on the characterization of the rotor's unidirectional responsiveness in waves. The rotor consists of six hydrofoil blades in two sets. One blade set has three blades laid out as a vertical-axis wind turbine of the Darrieus type. The other blade set has three blades configured like a Wells turbine. In combination, the formed rotor can be driven by flows in any direction to perform a unidirectional rotation about its vertically mounted shaft. This unidirectional responsiveness of the rotor also holds in waves, making the rotor an effective device for wave energy conversion. For the parametric study of the rotor, hydrofoil blades using different cross-sectional profiles and chord lengths have been employed to configure the rotor. The rotor was then tested in a wave flume under various wave conditions in a freewheeling mode. Experimental results were analyzed and discussed. The yielded research findings will greatly enhance the fundamental understanding of the rotor performance in waves and effectively guide the prototype rotor development for practical applications.

There has been a resurgence of interest in the development of small-scale vertical-axis wind turbines (VAWTs) in the past few decades as is evident from the plethora of published scientific work. This attention may be attributed to the desperate need for cheaper, cleaner, and off-grid mechanisms for generating electric power. In such hasty scenario, VAWTs are being considered as one of the potential solution for small-scale power generation. Among the VAWTs, H-type Darrieus rotors have undergone extensive exploration. In this review work, an attempt has been made to assemble all the major areas of research done in the field of H-type Darrieus rotor development. These areas include the aerodynamic models, computational fluid dynamics (CFD) methods and turbulence models, self-starting and dynamic stalling behavior, blade-vortex interaction and wake studies, and blade designs and optimization studies, besides topics of special interest like blade curvature effects and skewed flows. Overall, the work gives a comprehensive review of the state-of-the-art technology of the H-type Darrieus rotor. Finally, recommendations have been made for each of the areas keeping in view of the technological development.

The Tesla turbine built originally in 1913 by Nikola Tesla is an impulse and passive turbine. These characteristics drew the attention for its potential application for space cycle thermoelectric conversion and the possibility to improve cycle efficiency. The performance of a 70-mm rotor Tesla turbine will be presented and discussed. The turbine used saturated steam as the working fluid for experiments. The pressure range considered was between 300 to 700 kPa.

The fluid–structure interaction (FSI) is generally addressed in multimegawatt wind turbine calculations. From the fluid flow perspective, the semi-analytical approaches, like actuator disk (AD) model, were commonly used in wind turbine rotor calculations. Indeed, the AD model can effectively reduce the computational cost of full-scale numerical methods. Additionally, it can substantially improve the results of pure analytical methods. Despite its great advantages, the AD model has not been developed to simulate the FSI problem in wind turbine simulations. This study first examines the effect of constant (rigid) cone angle on the performance of the chosen benchmark wind turbine. As a major contribution, this work subsequently extends the rigid AD model to nonrigid applications to suitably simulate the FSI. The new developed AD-FSI solver uses the finite-volume method to calculate the aerodynamic loads and the beam theory to predict the structural behaviors. A benchmark megawatt wind turbine is simulated to examine the accuracy of the newly developed AD-FSI solver. Next, the results of this solver are compared with the results of other researchers, who applied various analytical and numerical methods to obtain their results. The comparisons indicate that the new developed solver calculates the aerodynamic loads reliably and predicts the blade deflection very accurately.

Technological advancement is specifically identified by the usage of energy. The energy requirement is increasing because of the exponential population rise, increased industrial activities, and subsequent accelerated economic activities in both urban and suburban areas. The traditional power sources are becoming unsustainable as energy demand and induction of new sources for augmenting the energy need are lopsided. Additionally, traditional energy sources cause pollution, natural hazards, and more importantly, it is uneconomical. Due to these reasons, it becomes compelling to look for alternative sources of energy. Hydropower generation is reliable, non-polluting to a large extent, and a cheaper source of electrical energy. However, the conventional large hydropower projects, especially with reservoir components, are being opposed worldwide for social, environmental, economic, and safety reasons. Therefore, electricity production from free-flowing water may present a viable choice. Here, the free-flowing river water current is used to drive vertical-axis turbines preferably, Savonius rotors which are ideally built for wind energy conversion systems (WECS). The rotor is directly coupled to electric generators, and the scheme presents a typical variable voltage and variable frequency system similar to that of WECS except that the working force is due to water rather than wind. The use of the Savonius rotor for hydrokinetic power generation is uncommon; however, increased exploitation of this methodology may help in augmenting future energy need. This paper reviews the Savonius rotor and its possible application for hydrokinetic power generation; the merits and demerits of such schemes are clearly outlined.

To address the problem of the imminent energy crisis, pollution from fossil fuels, and global warming, it is necessary to incorporate renewable technologies. In that context, the drag-based Savonius wind turbine has tremendous potential to extract wind energy and can be operated as a standalone system at remote areas where the conventional electricity cannot be provided. The present study primarily focuses on the performance evaluation of a conventional semicircular-bladed Savonius rotor with capped vents (CVs) or nozzle chamfered vents. The rotor blades having vent ratios of 7%, 14%, and 21% are tested in a wind tunnel, and subsequently, their performances are compared with a rotor without CVs under identical test conditions. Computational fluid dynamics (CFD) simulations have also been carried out to compliment the surprising experimental results and also to analyze the flow physics around the rotor blades. From the understanding of torque distribution, it has been noticed that the performance of the rotor with CV deteriorates compared with the conventional semicircular-bladed rotor. The vents are found to decrease the positive torque and increase the negative torque by disturbing the pressure distribution of the conventional semicircular-bladed Savonius rotor.

This study investigates the performance of microjets for load reduction on the NREL-5 MW wind turbine and identifies optimal system parameters. Microjets provide blowing normal to the blade surface and can rapidly increase or decrease lift on a blade section, enabling a wind turbine to respond to local, short-term changes in wind condition. As wind turbine rotors become larger, control methods that act on a single blade or blade section are increasingly necessary to reduce critical fatigue and extreme loads. However, microjets require power to operate, and thus, it is crucial that the fatigue reduction justifies any energy input to the system. To examine the potential for fatigue reduction of a range of potential microjet system configurations, a blade element momentum (BEM) code and a flow energy solver were used to estimate the energy input and the change in primary fatigue metrics. A parametric analysis was conducted to identify the optimal spanwise position and length of the microjets over a range of air mass flow rates. Both active and passive air supply methods were considered. A passive microjet system applied to the NREL 5-MW rotor produced a 3.7% reduction in the maximum flapwise root bending moment (FRBM). The reduction in the peak bending moment increased to 6.0% with a 5 kPa blower that consumes approximately 0.1% of the turbine output power. The most effective configurations placed microjets between the blade midspan to three-quarters span. Load reduction was achieved for both active and passive modes of air supply to the microjet system.

Brimmed-diffuser augmented wind turbines (B-DAWTs) can significantly increase the performance of the rotor. Multirotor systems (MRSs) have a lot of merits such as significant saving mass and overall cost of the wind turbine system. In the present research, B-DAWTs are studied in a MRS. In wind tunnel experiments, the power output and aerodynamics of three B-DAWTs placed in close vicinity have been investigated. The results show a significant increase of up to 12% in total power output of the MRS with B-DAWTs compared to the sum of the stand-alone (SA) same turbines. The accelerated gap flows between B-DAWTs in a MRS cause lowered pressure regions due to vortex interaction behind the brimmed diffusers and draw more wind into turbines.

Icing limits the performance of wind turbines in cold climates. The prediction of the aerodynamic performance losses and their distribution due to ice accretion is essential. Blade element momentum (BEM) is the basis of blade structural studies. The accuracy and limitations of this method in icing condition are assessed in the present study. To this purpose, a computational study on the aerodynamic performance of the full-scale NREL 5 MW rotor is performed. Three-dimensional (3D) steady Reynolds-averaged Navier–Stokes (RANS) simulations are performed for both clean and iced blade, as well as BEM calculations using two-dimensional (2D) computational fluid dynamics (CFD) sectional airfoil data. The total power calculated by the BEM method is in close agreement with the 3D CFD results for the clean blade. There is a 4% deviation, while it is underestimated by 28% for the iced one. The load distribution along the clean blade span differs between both methods. Load loss due to the ice, predicted by 3D CFD, is 32% in extracted power and the main loss occurs at the regions where the ice horn height exceeds 8% of the chord length.

It is desired, through this work, to investigate in detail the scenario that takes place behind a single wind turbine unit by focusing on three parameters; average axial wind velocity component, velocity deficit, and total turbulence intensity. The testing was done at mainstream velocity, U ∞ , of 5.2 m/s, u and v velocity components were captured by x-probe dual-sensor hot wire anemometer. A massive amount of point data was obtained, which then processed by a matlab script to plot the desired contours through the successive transverse sections along the entire length of the test section. By monitoring the previously mentioned flow parameters, the regions of low velocity and high turbulence can be avoided, while the location of the subsequent wind turbine is selected. The estimation of the distance, at which the inlet flow field will restore its original characteristics after being mixed through the rotor blades, is very important as this is the distance that should separate two successive turbines in an inline configuration wind farm to guarantee the optimum performance and to extract the maximum power out of the subsequent array of turbines. It is found that the hub height axial velocity recovery at six rotor diameters downstream distance is only 82%. This fact means that the power extraction out of the downstream turbine in an inline configuration wind farm is only 55% of the upstream turbine if the same free stream velocity and blade design are adopted.

Momentum models or streamtube models represent one of the fundamental approaches in modeling the aerodynamics of straight bladed vertical axis wind turbine (SB-VAWT) of Darrieus type. They are based on momentum (actuator disk) theory and widely used in performance evaluation of VAWTs. In this short review, the authors have strived to compile the basic momentum models that have been widely assumed in the literature for design and performance estimation of SB-VAWTs of Darrieus type. A comprehensive demonstration of the formulation needed for the implantation of these models is also proposed. Three streamtube models are investigated in this paper, namely, the single streamtube (SST), the multiple streamtube (MST), and the double multiple streamtube (DMST) models. Each of these models has it merits and demerits which are also thoroughly discussed in this review.

A multirotor system (MRS) is defined as containing more than one rotor in a single structure. MRSs have a great potential as a wind turbine system, saving mass and cost, and showing scale ability. The shrouded wind turbine with brimmed diffuser-augmented wind turbines (B-DAWT) has demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about 2–5 compared with a bare wind turbine. In the present research, B-DAWTs are used in a multirotor system. The power output performance of MRSs using two and three B-DAWTs in a variety of configurations has been investigated in the previous works. In the present study, the aerodynamics of an MRS with five B-DAWTs, spaced in close vicinity in the same vertical plane normal to a uniform flow, has been analyzed. Power output increases of up to 21% in average for a five-rotor MRS configuration are achieved in comparison to that for the stand-alone configuration. Thus, when B-DAWTs are employed as the unit of a MRS, the total power output is remarkably increased. As the number of units for an MRS is increased from two to five, the increase in power output becomes larger and larger. This is because that the gap flows between B-DAWTs in a MRS are accelerated and cause lowered pressure regions due to vortex interaction behind the brimmed diffusers. Thus, a MRS with more B-DAWTs can draw more wind into turbines showing higher power output.

Wind energy has had a major impact on the generation of renewable energy. While most research and development focuses on large, utility-scale wind turbines, a new application is in the field of small wind turbines for the urban environment. A major design challenge for urban wind turbines is the noise generated during operation. This study examines the power production and the noise generated by two small-scale wind turbines tested in a small wind tunnel. Both rotors were designed using the blade-element momentum theory using either the NREL S823 or the Eppler 216 airfoils. Point noise measurements were taken using a microphone at three locations downstream of the turbine: 16% of the diameter (two chord lengths), 50% of the diameter, and 75% of the diameter. At each location downstream of the turbine, a vertical traverse was performed to analyze the sound pressure level (SPL) from the tip of the turbine blades down to the hub. The rotor designed with the Eppler 216 airfoil showed a 9% increase in power production and decrease of up to 7 dB(A).

The elliptical-bladed Savonius wind turbine rotor has become a subject of interest because of its better energy capturing capability. Hitherto, the basic parameters of this rotor such as overlap ratio, aspect ratio, and number of blades have been studied and optimized numerically. Most of these studies estimated the torque and power coefficients (C T and C P ) at given flow conditions. However, the two important aerodynamic forces, viz., the lift and the drag, acting on the elliptical-bladed rotor have not been studied. This calls for a deeper investigation into the effect of these forces on the rotor performance to arrive at a suitable design configuration. In view of this, at the outset, two-dimensional (2D) unsteady simulations are conducted to find the instantaneous lift and drag forces acting on an elliptical-bladed rotor at a Reynolds number (Re) = 0.892 × 10 5 . The shear stress transport (SST) k–ω turbulence model is used for solving the unsteady Reynolds averaged Navier–Stokes equations. The three-dimensional (3D) unsteady simulations are then performed which are then followed by the wind tunnel experiments. The drag and lift coefficients (C D and C L ) are analyzed for 0–360 deg rotation of rotor with an increment of 1 deg. The total pressure, velocity magnitude, and turbulence intensity contours are obtained at various angles of rotor rotation. For the elliptical-bladed rotor, the average C D , C L , and C P , from 3D simulation, are found to be 1.31, 0.48, and 0.26, respectively. The average C P for the 2D elliptical profile is found to be 0.34, whereas the wind tunnel experiments demonstrate C P to be 0.19.

Wave rotor with pressure exchange function can be attempted to improve refrigeration performance. The objective of this paper is to verify the feasibility of the method by thermodynamic and experimental analysis. First, a refrigeration process which contains wave rotor pressurization was established. Then, a thermodynamic model which reflects the refrigeration process was designed. The thermal performance was researched under various key parameters. Finally, based on the novel wave rotor refrigeration platform, the experimental work was carried out, and the effects of main parameters of the device were systematically studied. The results showed that it was feasible to enhance the coefficient of performance (COP) by using pressure exchange characteristic of wave rotor. The COP could be improved substantially at relatively small expansion ratio. Under the design point, more than half of the pressure energy could be restored. The performance curve of the novel equipment was also obtained. Enhancing the isentropic efficiency of expansion is the effective means to improve the COP and σ of the system. This paper was designed in a way that contained a novel equipment to enhance the COP of wave rotor refrigeration.

The design and development of wind turbines is increasing throughout the world to offer electricity without paying much to the global warming. The Savonius wind turbine rotor, or simply the Savonius rotor, is a drag-based device that has a relatively low efficiency. A high negative torque produced by the returning blade is a major drawback of this rotor. Despite having a low efficiency, its design simplicity, low cost, easy installation, good starting ability, relatively low operating speed, and independency to wind direction are its main rewards. With the goal of improving its power coefficient (C P ), a considerable amount of investigation has been reported in the past few decades, where various design modifications are made by altering the influencing parameters. Concurrently, various augmentation techniques have also been used to improve the rotor performance. Such augmenters reduce the negative torque and improve the self-starting capability while maintaining a high rotational speed of the rotor. The C P of the conventional Savonius rotors lie in the range of 0.12–0.18, however, with the use of augmenters, it can reach up to 0.52 with added design complexity. This paper attempts to give an overview of the various augmentation techniques used in Savonius rotor over the last four decades. Some of the key findings with the use of these techniques have been addressed and makes an attempt to highlight the future direction of research.

Turbine inlet temperatures (TIT) of 1500–2000 K have become a sort of standard for most modern advanced applications. First-stage blades are obviously the most exposed components to such hot gases, and thus they need proper cooling. In the preliminary design of the blades and their cooling system, designers must rely on simple models that can be further refined at a later stage, in order to have an approximate but valuable set of guidelines and to reach a feasible first-order configuration. In this paper, a simple lumped thermodynamic model of blade cooling is proposed. It is based on mass/energy balances and heat transfer correlations, and it predicts a one-dimensional temperature profile on the blade external surface along the chord for a given gas temperature profile, as well as the required cooling air flow rates to prevent blade material from creep. The greatest advantage of the model is that it can be easily adapted to any operating condition, process parameter, and blade geometry, which makes it well suited to the last technological trends, namely, the investigation of new cooling methods and alternative coolants instead of air. Therefore, the proposed model is expected to be a useful tool in the field of innovative gas turbine cycle analysis, replacing more computationally intensive and very time-consuming models.

In this study, we present the results of a two-dimensional fluid-dynamic simulation of novel rotor geometry with spline function which is derivative of the traditional S-shape Savonius blade. A computational fluid dynamic (CFD) analysis is conducted using the Spalart–Allmaras turbulent model, validated using experimental data released by Sandia National Laboratory. Results are presented in terms of dimensionless torque and power coefficients, assuming a wind speed of 7 m/s and height and rotor diameter of 1 m. Furthermore, analysis of the forces acting on the rotor is conducted by evaluating frontal and side forces on each blade, and the resultant force acting on the central shaft. A qualitative representation of the vorticity around the traditional and spline rotor is shown to prove that the novel blade allows less turbulent flow through the rotor.

Wind energy technology is facing new challenges due to the increment in rotor diameter. Nowadays, several studies focus on the development of new flow control methods for load alleviation, in order to increase the lifetime of the blades. This paper describes a shape morphing-based method for smart blades. The study includes an aerodynamic model with a computational search algorithm to find the optimal C p . A section with shape morphing technology was developed to prove the performance of the method. The smart blade prototype section incorporates a novel structure with a flexible skin and a compliant mechanism. This deformable structure achieves the required displacements for different NACA profiles through camber morphing. In this way, the efficiency and the load variations are improved. The compliant mechanism has to be as light as possible and it has to be competitive in cost. In order to achieve these limitations, different actuating mechanisms were evaluated. Among different possibilities, servo actuators presented higher load/weight capabilities and the required displacement ratios to cover the entire deformable range. The airfoil is modified according to the wind condition and the wind speed is the input variable for controlling the actuators displacement. The control algorithm has a very high frequency response; in this way, the blade profile can be modified in a shorter time and it can respond to high wind velocity variations. Therefore, a deformable section improves the overall performance of wind turbines since it increases power and extends the lifetime of the blades.

In the current experiments, two identical wind turbine models were placed in uniform flow conditions in a water flume. The initial flow in the flume was subject to a very low turbulence level, limiting the influence of external disturbances on the development of the inherent wake instability. Both rotors are three-bladed and designed using blade element/lifting line (BE/LL) optimum theory at a tip-speed ratio, λ , of 5 with a constant design lift coefficient along the span, C L = 0.8. Measurements of the rotor characteristics were conducted by strain sensors installed in the rotor mounting. The resulting power capacity has been studied and analyzed at different rotor positions and a range of tip-speed ratios from 2 to 8, and a simple algebraic relationship between the velocity deficit in the wake of the front turbine and the power of the second turbine was found, when both rotors have the coaxial position.